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MG Midget and Sprite Technical - Tyre circumfrence measurements.

I've restarted this thread under another title, because it's not about phones.

It's about getting the most accurate measurement of the circumfrence of an inflated tyre, mounted on a wheel, under load, and in motion, for the purposes of the determining the most accurate method of ascertaining the TPM for a speedomoter.

Here are all the previous relevant postings. Hope nobody objects. :)


Posted 16 July 2012 at 11:22:51 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

Which measured circumfrence? Static unloaded circumfrence, loaded slow rolling circumfrence, or dynamic loaded rolling circumfrence?

There's a marked difference in the unloaded and loaded circumfrence. Take a wheel off and roll it one revolution, measuring the points between start and finish. Now do the same with the wheel on the car. The difference in distance is likely to be in the order of 1.5 to 2 inches.(165/70/13 at 26.5 psi).

Now Continental tyres have a table that states their dynamic rolling circumfrences are measured at circa 37MPH. So clearly even moderate speeds are significant.

Of course, whilst the mechanical oddometer can be made to be quite accurate, the eddy current speedometer, esp one 50 years old, is not likely to be as accurate. So maybe a certain % error in the tyre measurement is irrelevant anyway. But when you get one re-calibrated it also gets a service, and the moving parts get to be as good as new again. So whatever tolerance it was capable of when new, will be again.

And this may be unfair, but I'm not entirely sure if the calibrators take these factors into consideration. So I want to do the calc's myself and see how much % difference it makes to the turns per mile(TPM).

Edit. Peter, no it doesn't.

Look at a tyre face on when loaded. The "effective" circumfrence is reduced by the deformation of the tyre under load.

Posted 16 July 2012 at 11:06:52 UK time
Peter B, Wiltshire, United Kingdom View vehicle profile

Apart from any speed related changes, surely the tyre circumference will remain the same whether loaded and deformed, or not?
Posted 16 July 2012 at 10:35:21 UK time
Guy Weller, Cumbria, UK

Hi Lawrence,
My (untypically) short posts were a sort of challenge to open up this question of rolling radius. Because although I understand the issue of how and where you measure the radius of the tyre; to the flat bit, actual static radius or dynamic radius; I have never quite got my head around what happens in the real world situation.

We all know that if a wheel rotates at high speed it stretches and has a bigger circumference. And this is critical on land speed record cars which is why they have solid tyres or disc wheels. I am not sure that this factor is too significant on our cars at normal road speed. Maybe a bit, but not much!

But what about the flat bit at the bottom (i.e. the reduced radius) Surely it remains with a flat bit there when the car is travelling at, say 60mph, else there would be minimal footprint. The tyre continually flexes as the flat bit moves around the tyre. But you cannot use this radius measurement to the ground with Pi to calculate the distance travelled on one revolution, 'cos it won't give you the true circumference, as the thing is a distorted circle.

So the question remains, why not just use a measured circumference? That must give the distance travelled per revolution (noting issues like tyre slippage etc)
Posted 16 July 2012 at 10:05:32 UK time
Malcolm Le Chevalier, Aberdeen, UK View vehicle profile

Aye, no worries Lawrence, saw that.

I have ended up with the funny image of one of my university tutors, when we were doing some exam prep writing the letters ATFQ on the board then jumping up and down shouting "ANSWER THE FLIPPING QUESTION". Nut case!

Malcolm
Posted 16 July 2012 at 10:04:40 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

This is Pirellis definition of "Rolling radius"
The radius of a rigid wheel which covers the same distance covered by a tyre, performing an equal number of turns. It has an intermediate value between the free radius and the load radius and varies for a given tyre according to the type of road and load, the pressure and the speed."
Posted 16 July 2012 at 09:56:03 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

Malcolm, see my post, I was too long typing to beat you with my reason for the thread.
Posted 16 July 2012 at 09:53:40 UK time
Malcolm Le Chevalier, Aberdeen, UK View vehicle profile

I didn't mean for mentioning OBD to lead to talking of speedo speed to lead to this, the OBD thing was an aside to my actual answer. As a sprite, as far as I am aware doesn't have a diagnostics socket! :-D

Radius/circumference, they are directly proportional so does it really matter? Static radius is probably easier to work out as it is tyre width x tyre profile + half rim size.

Not really sure how this all helps the original question tho!

Malcolm
Posted 16 July 2012 at 09:53:30 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

That's the question. Take a tyre and fit it to a wheel. Inflate it. Measure the radius from the centre of the hub to the extreme of the tread. That is the unloaded radius. Measure the circumfrence and it will be the same as if you calculate it using pi and the radius you just took.

Now fir the wheel to the car and measure the radius, The radius is now smaller, because of the weight of the axle. If you use that radius to calculate the circumfrence, it will be incorrect, because the tyre is deformed. So roll the car along 1 revolution and measure the circumfrence again. Then you can calculate the rolling radius.

But if the car is going at speed, the tyre will have different measurements.

Quite apart from tyre pressures, measuring the radius/circumfrence of the tyre is more complicated than first sight suggests.

I should have started a different thread in the tech section.

Anyway, I wanted to get an accurate speed measure, and then knowing my axle and speedo gear ratios, work backwards to calculate the exact turns per mile to get my speedo calibrated, rather than let them calcualte it for me.
Posted 16 July 2012 at 09:43:40 UK time
Guy Weller, Cumbria, UK

But why radius, not circumference?
Posted 16 July 2012 at 09:35:16 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

Guy, you seem to have a habit of preempting me.:)

I didn't start this thread idly. It was for a reason, and that reason involved rolling radius.

There is static radius, rolling radius, and also dynamic radius. It alters the circumfrence of the tyre, and hence the speedo reading. So when getting a speedo calibrated, you have to quote the correct radius.

But which do you quote, and do the calibrators adjust for it.

A tyre "grows" a little at speed, but Speedy cables for example only ask for the static radius, which will give different result to using the rolling or dynamic radius.
Posted 16 July 2012 at 09:21:42 UK time
Guy Weller, Cumbria, UK

Now there's an interesting reference.
What exactly is a "rolling radius" ?
Posted 16 July 2012 at 08:36:54 UK time
Anthony Cutler, Hereford and Worcester, United Kingdom

Use the 'get directions' options from Google Map - will likely already be installed on any Android SP.

You'll need to turn on GPS and 'Mobile Data' (aka GPRS or UMTS/HSPA). The SP battery won't last long.

I use a USB charged plugged into the power socket (aka cig lighter!).

OBD speed depends on tyre rolling radius; IMO on a straight road with no incline, GPS speed will be much more accurate.

A
Posted 15 July 2012 at 22:48:34 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

Thanks all.

I finally downloaded speedview.
http://tinyurl.com/86ntxm9.

I reckon it should be pretty easy to use, even by a pre stone age phone hater. :)

I'll take it for a drive and see how it performs.

Here's a publicity pic of the interface.




Posted 15 July 2012 at 22:14:54 UK time
Guy Weller, Cumbria, United Kingdom

Just waking you up from your trance Malcolm - we all knew what you meant!
Posted 15 July 2012 at 22:02:23 UK time
Malcolm Le Chevalier, Aberdeen, UK View vehicle profile

On Doard Biagnostics of course. Sorry, should be OBD!

When I see the letters o, b and d together my brain thinks they should be arranged as odb as I spend a lot of time at work dealing with .odb files (Abaqus output data base files).

Sorry for the confusion

Malcolm
Posted 15 July 2012 at 21:56:32 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

I've found one called speedview lite. I'm trying to download it, but it's taking longer than walking in front of the car with an oil lamp at 3 mph lol.

I think I'd better face it. I'm old hahahaha.
Posted 15 July 2012 at 21:52:40 UK time
J Southwood, Norfolk, United Kingdom

I use an app called My Tracks that tracks my route when cycling. It uses GPS and overlays onto google maps and has an option to display the current speed. Never used it in a car though.
Posted 15 July 2012 at 21:49:35 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

ODB? Old daft bugger?

I understand a little of that Malcolm, thanks, I think lol. :)
Posted 15 July 2012 at 21:42:07 UK time
Guy Weller, Cumbria, United Kingdom

ODB Malcolm?
Posted 15 July 2012 at 21:10:17 UK time
Onno K, Netherlands View vehicle profile

Pete
Get an Iphone.
It is so populair because anyone can get it to work!
Even my 64 year old dad got the hang of it in 10 minutes!
Posted 15 July 2012 at 21:03:31 UK time
Pete Ottewell, Staffordshire, United Kingdom View vehicle profile

No point looking my way Lawrence. My wife has a smart phone neither of us are smart enough to work out how it works. We have to rely on our 4 year old Nephew to help us. Not sure if he will understand your question but I'll ask him lol. The wife has a sat nav though, I might just see how accurate my speedo is though, good idea.
Posted 15 July 2012 at 21:01:42 UK time
Malcolm Le Chevalier, Aberdeen, UK View vehicle profile

Try an app such as Torque (great ap if your car has an ODB socket and you buy a bluetooth ODB dongle... but that is a bit of an aside) or getting the navigation addon to google maps. Then it works just like a sat nav.

Malcolm

Edit: The torque app can even plot your journeys on google maps and show your speed ever second of your journey! if you are into that kind of thing... ha ha
Posted 15 July 2012 at 20:49:45 UK time
Lawrence Slater, Kent, United Kingdom, slaterlpj@btconnect.com

Are there any experts on here that are into phones, and how get them to read road speed from the built in GPS functionality of a Samsung Galaxy SIII?

It's not my phone btw, I just want to borrow it to check the accuracy or otherwise of the speedo in the Sprite. I could use a stop watch and road markers, but I thought I'd drag myself into the 2000's and get modern :)

So can anybody tell me how to set it up, or what to download if needed?
Lawrence Slater

"There's a marked difference in the unloaded and loaded circumfrence. Take a wheel off and roll it one revolution, measuring the points between start and finish. Now do the same with the wheel on the car. The difference in distance is likely to be in the order of 1.5 to 2 inches.(165/70/13 at 26.5 psi)."

Are you saying that you have actually done this yourself, or are you quoting this from somewhere else?

When calibrating my bike computer for the car I was faced with the issue of entering the tyre circumference.
I measured it by wrapping a tape around the tyre, off the car. I also measured it by marking a point on the tyre wall with chalk, making a corresponding mark on the road and pushing the car through 20 wheel revolutions. Calculations from both measurement coincided exactly. (+/- 0.5cms)

I puzzled over this issue of the reduced radius to the ground with the weight of the car. But believe it cannot be used to calculate the rolling circumference as the tyre is no longer a true circle.

I accept that the tyre deforms with rotational speed and this will increase the circumference a bit, although I still doubt that it is really significant. There will also certainly be more variation through tyre wear over its life.
Guy Weller

Yup, I've done it Guy. I was quite surprised at the difference. Deflate a tyre, and you'll get a similar effect. Although not that great, it is significant in terms of speedo and oddometer accuracy.


Forgot to add. My measurements for the off car and on car test were. 67.25" on car, and 68.8" off car.
Lawrence Slater

I got no difference at all!

Make a mark on the tyre surface. Measure around the circumference of the wheel. Roll the wheel forward by one revolution and that mark is the tyre circumference further forward. If the tyre doesn't slip on the road then how can it not be the same as the distance travelled on the road?

If the tyre had a very deep open tread pattern (deep tread blocks) the blocks will deform at the contact patch which could account for some variation, but what then if you were doing this with slicks?

(PS you should have edited your cut and paste!)
Guy Weller

Lawrence,
I know you've seen these pages and may have read the relevant part/s on the page/s but just as a reminder and for others the link - http://www.carbibles.com/tyre_bible_pg4.html

and remember there are minor differences in the same stated sizes of tyres bewtween models of tyres
Nigel Atkins

Yup thanks nigel, There's a useful description of the circumfrence issues there. Here's a quote from there.

"Straight from the manufacturer, a tyre is circular and it's outer diameter (OD) is based on this unladen, perfectly circular condition. Because tyres deform under load (flatter on the bottom than they are on the top when taking the weight of a car), the vertical radius of a tyre under load is not half the diameter. It's typically about 44% of it (from the centre of the wheel to the road). In addition, a tyre doesn't roll like a solid wheel; the steel or fabric belt 'rolls along the ground' like a caterpillar tank tread does. Because of this, the rolling circumference of the tyre isn't quite what you'd expect. The closest approximation for this value can be calculated by taking the outer diameter, subtracting twice the tread depth and multiplying by PI. Roughly speaking, that's 0.96 x OD x PI."

Guy, my test was, take wheel off car. Mark tyre wall, and ground with chalk. Roll one revolution, mark floor at the chalk mark on the tyre wall.

Fit wheel back to car. Same tyre wall chalk mark adjcent to same 1st mark on floor. Roll car forward one revolution. Make new mark on floor adjacent to tyre wall chalk mark. Compare difference between the 2 second chalk marks. I'll double check, but I'm certain I didn't make a mistake. Logic dictates that the effective circumfrencre is less when loaded.
Lawrence Slater

"(PS you should have edited your cut and paste!)"

Yup your right. It is a bit difficult to follow. :(. Please feel free to edit and re-post. :)
Lawrence Slater

In essence, the same test then, although I measured around the tyre off the car with a linen surveyor's tape, and did the laden on the car test through 20 wheel revolutions for accuracy.

I see from the car bible pages he says that the effective rolling circumference is calculated on 96% of the outside diameter. But that the static laden radius is typically only 44% of the true radius. So using the static laden radius would give a very different figure.
Guy Weller

OK, I accept it is different, although when I did my measurements it wasn't! Maybe I have my tyre pressures too high!

I still have difficulty in mentally understanding how the difference arises though as even if the tyre does have a flat section (footprint) the same length of rubber must pass across the road surface as goes around the wheel circumference. Unless the tyre slips.
Guy Weller

Guy,
note: >>The closest approximation . . .<<

allowing for slight variances of actual tyres sizes and perhaps wear and inaccurances at the speedo I think a close figure or small round up or down would be ok

my reconditioned speedo was quite accurate, more so that my wife's then new mordern car's speedo, but that was with the previous oversized 155/80 tyres so it would have been very slightly inaccurate with the standard 145/80 tyres
Nigel Atkins

I think it's a bit like a bike chain, it stays the same length what ever the tension or gear you select, isn't it?.
Pete Ottewell

Guy,
>>a tyre doesn't roll like a solid wheel; the steel or fabric belt 'rolls along the ground' like a caterpillar tank tread does.<<

see if you can find a slo-mo of a large dragster tyre doing a burn-out or just leaving the start line, the large size of tyre and low pressure will highlight/exaggerate some of the effects
Nigel Atkins

Nigel,
The point isn't to do with speedometer gearing. Though getting this measurement accurate does impact on that.

I accept that there is a difference between the rolling circumference and the true tyre circumference.

I ma still not clear why rolling radius is a term used when rolling circumference is the more useful measure.

But for all this the bit that my brain cannot grasp is this:
Make a mark on the tyre, across the tread. Set the tyre (laden if you like)on the road and make a corresponding mark on the road. Roll it forward one revolution. The mark on the tyre moves around the outside of the tyre by the circumference of the tyre. But this apparently differs from the distance that the tyre moves forward on the road. If the tyre doesn't slip, where has the extra distance gone / come from? ! ?? !!!!
Guy Weller

er, my usual non-tech, non correct technical or scientific thinking and terminology or jargon now -

the mark remains in contact with the road longer if the tyre is squashed

you measure when the mark returns to 6 o'clock but the mark contacts the road at say 10 passed 6 and would leave contact at say 10 to 6
Nigel Atkins

Lawrence & Guy,

While I do not have a scientific answer for you, I offer you this twist. My wife and I participate in our Car Club organized (TSD) Historic Rallye's. Interestingly, we (usually) have a check point approximately 10 miles from the start which is used to calibrate distances for the entire event. If we "zero" our odometer at the start we can then compare our "indicated" milage to the official mileage at the 10 mile check point. If we are over/under the Rallye Masters figure we then have a correction factor to use for the event to keep us on course, and schedule. I was always under the impression early (bias-ply) tyres would expand with temperature, and this first check point allowed for that factor. Maybe not?

Regards,

Larry C. (radial tyres these days)
Larry C '69 Midget

I'm not reading that lot but if you want the TPM for a given tyre, simply phone the manufacturer's UK office and ask them. They are used to having to provide this info for racers...
David Smith

Thanks David, that's essentially what the table on the continental tyres page was saying, and they do the test at circa 37mph. Good idea, I will ring them.

Meanwhile, Guy. The tyre deforms as it revolves. The side wall pushes out and the circumfrence is pushed in. So in effect the effective circumfrence reduces to much nearer that of the section height(side wall height).

Look at this face-on picture.

This is at the point of the tyre under load, and flat on the ground. For the rest, the remainder of the tyre is at it's "normal" shape/circumfrence. Imagine overinflating to such an extent that the tyre didn't deform, and thus acted much more like a rigid disc. That would get closer to the unloaded circumfrence, as the laden radius would also be closer to the unladen radius.

Then imagine underinflating to the extent that the tread section of the tyre was almost a rim level. The effective tyre circumfrence would then be closer to that of the wheel rim, because the deformed rubber has been pushed inside. It hasn't gone anywhere, just been displaced.


Lawrence Slater

"I think it's a bit like a bike chain, it stays the same length what ever the tension or gear you select, isn't it?"
Hi Pete. No, it's different.

If you cut a slice off a section of circle, and replace it with a straight line, and then measure the whole way around, the straight section shortens the whole.

That's what happens to the car tyre as it gets deformed and flattened on the ground under load.

Conversely, when travelling at sufficient speed, the tyre tries to reform, or if you like deforms less, and hence the dynamic circumfrence is different to the slow or static circumfrence under load.

The key question is, how much?

so calling the makers and asking for the true TPM is probably the way to find out.
Lawrence Slater

Time out :(

So calling the makers and asking for the true TPM is probably the way to find out, but it would be nice to be able to calculate or guesstimate it for myself. :)
Lawrence Slater

Lawrence,
That point is already covered. The carbibles site gives a figure of 96% of the static (undeformed)

I understand that (and how) a tyre deforms. Its the old joke about the puncture that is only flat at the bottom. But I still wonder where the actual circumferential length is lost to. (you cannot illustrate this with cross sectional diagrams). If, for example the tyre were a steel belted one you then have a strip of steel around the full circumference of the tyre. It doesn't matter how much that belt deviates from a true circle, it still remains a belt of steel of the same length. You could form it into a triangle or a square or any other polygon but you still have the same length of steel.
Guy Weller

Guy you are ignoring the third dimension... think about it. The steel is actually a series of wires, which bend and flex easily.
David Smith

I was actually thinking of the tyres with a steel band. But the same applies. The wires may bend and flex easily, but how does the wire change in length?
Guy Weller

Guy, it doesn't change it's length, it changes it's shape. Do you accept that an inflated tyre has a greater circumfrence than a deflated tyre?

I've been looking for some text on this, there must be a concise explanation somewhere on tyre dynamics.

Lawrence Slater

In the picture, the centre of the tyre is pushed into the space inside the walls of the tyre, because the air pressure isn't great enough to keep it pushed out. Thus reducing the circumfrence of the tyre. If you increase the amount of air in the tyre, or lighten the load, the tyre would return to shape and the circumfrence increase.

In the picture the centre of the tyre is no longer in contact with the road, the outsides are, and they form the effective circumfrence. The circumfrence depends on load and air pressure and so changes accordingly.


Lawrence Slater

Ok, l have got it.
Nothing to do with Lawrence's section drawings. They just show how the side walls flex in an underinflated tyre - that isn't at question. And they are diagramatic anyway, they don't behave quite like that in real life!
And you can't tell how long something is by looking at a single section.
Guy Weller

Come on then Guy. Share the secret. :)

Btw, I think the drawings do explain it. The sidewalls moving out, allow the centre of the tyre to move into the interior, thus reducing the circumfrence.

But I'm all ears for a better or truer explanation. :)
Lawrence Slater

Hear's a better picture from the Continental Technical data book 2008-2009.

Much clearer picture of the reduced circumfrence under load.

Width expands and diameter and hence circumfrence reduce.




Lawrence Slater

I rang continental tyres and asked for the size specifications. Unfortunately the only person i could get a conversation with, whilst working in the tech department, was clearly not technical. She is sending me a technical doc that she says will contain the information. Hope so.

Meanwhile, I downloaded the latest Continental tyre handbook. Here's a quote.

"The rolling circumference is the distance covered by a point on the circumference when the tyre revolves once at 60 km/h (37 mph)."

That's the figure I'm trying to find out. I guess it would be pretty much the same for all new road tyres of a particular size, irrespective of make.
Lawrence Slater

Got this table from the continental PDF download.

Rolling circumfrence of a 165/70/13 at 37mph is 1714mm plus 1.5% or minus 2.5%




Lawrence Slater

I rang michelin technical and was given the same figures, same tolerances, so must be tyre industry standards.

Rolling circunfrence for:
165/70/13 is 1714mm
175/70/13 is 1757mm
185/70/13 is 1800mm

Plus 1.5% or minus 2.5%

Lawrence Slater

The wheel when unloaded is not distorted so all distances the wheel travels when on the car are based in its distorted (squashed) dimentions. As only the radius is measureable that is squashed it is based on the squashed (loaded) radius. The unloaded diameter and circumference are irrelevant. Its the circumference you would have for a solid wheel the radius of the squashed (loaded) side that counts. So all calculations have to be based on the loaded radius as that is the only measurement available to work from for the loaded wheel. As this can change depending on tyre pressures, car weight etc. it is an estimate that is used for each 'standard' vehicle when coming off the line with its standsrd tyres, weights etc. that is used to calibrate a standard speedo. All mods and loads etc. affect this loaded wheel dimension and I presume that is why all speeds are allowed + or minus 10% in law for speedo performance when being tested etc.
What I want to know is will we get done if the speedo doesn't work and we use gps on a phone and an officer finds our speedo is not working proper. As I understand it he can do you for it can't he?

Simples ----
Dave Squire - Notts

So my unloaded (off car), 165/70/13 tyre, rolling circumfrence is 1747.52 millimetres

Loaded (on car) static rolling is 1708.15 millimetres

Dynamic rolling at 37mph is 1714 millimetres.

I assume it grows more at higher speeds, but for the purposes of speedometer calibration the 37mph figure would be the one to use.
Lawrence Slater

By the way Lawrence, how are you calibrating the speedo appart from the loaded v unloaded stuff? It would be good to know how to as it were.
Dave Squire - Notts

Lawrence,
I think we were beating different dead donkeys. I did say that I accepted that the laden and the unladen circumferences were different. What I have been pursuing is where that different length is absorbed in the tyre construction, and how this could be in the case, for example, of a steel belted tyre.
Guy Weller

Hi Dave, I think you'd get nicked for looking at your phone instead of the speedo, unless you have a heads up display. Then you might be able to argue that you have an electronic portable speedo?

I'm not calibrating my own speedo, I just want to check in advance the correct tpm, before I send it to speedycables or richfield or other. There is a bit of a discrepancy in the way the various companies ask for you data. As they only ask for a static rolling circumfrence, I wanted to know how much of a factor speed would introduce (dynamic rolling circumfrence). Now I know, and will calculate how much of a difference that will make. Not much by the look of it, but I am the worlds worst -- sometimes-- for details that don't matter. :) But I like details. :)

Guy, I agree. It is a conundrum sort of. I know what you are saying, the tyre can't change it's length.
Lawrence Slater

Dave, "Its the circumference you would have for a solid wheel the radius of the squashed (loaded) side that counts"

Not as straightforward as that. For Lawrence's tyre size the Continental Tyres table gives a loaded radius of 255. If you simple use that as the radius figure then using 2Pi R gives a rolling circumference of 1602, well below their quoted figure of 1714.

The rolling circumference is due to circumferential distortions in the tyre casing, tread blocks and temperature changes in the tyre. Its not directly calculated from the loaded (static) radius.
Guy Weller

Guy: Yes, its dynamic and always changing as the car tyres move through complex changing conditions such as pressure (ie temp) and weight (apparent depending on cornering G per wheel) changes, just trying to simplify, (its the ex teacher in me *?xl£@*s - just don't go there!).
If anyone ever feels I am droning on like a teacher by the way tell me to shut up. I will be suitably chastised and try to become 'normal' again. (Appologies to any long serving teachers, we need them, its just me!)

Lawrence: Shame about the calibration. My VDub mate tells me calibration costs loads so they use gps when on holiday (they go camper convoy, man in front checks speed, take in turns etc. to spread cost of data connectivity and make sure they are in speed limit proper).
I agree it would be an augument by the way so I am hoping mine works fine when off the axle stands. Lots of bits seem to be coming alive and OK so a reasonable bet.
Its just the rot thats the problem really, just as well I am liking metal bashing, cutting and clamping (the weldings the easy bit).

Cheeers, Dave
Dave Squire - Notts

Dave calibration is circa 70 quid plus vat plus your initial postage and packing to whoever you chose.

So if I tow a mate in a VW camper van, I always know the correct speed. lol.
Lawrence Slater

I fitted my midget with an electronic programmable speedo from Speedy Cables. In the instructions it tells you to mark the tyre with chalk and mark the ground, roll forward one turn and mark the ground again. Then measure the distance between the marks and apply it to a formula that takes into account the rear axle ratio and programme that figure into the speedo.

This I did, and once on the road I checked it against my TomTom - guess what? It's spot on. So the pushing the car forward and measuring does work, but the whole concept is very confusing to an old buffer like me. You can see that the radius is reduced by the flattening of the bottom of the tyre. And yet there is a finite amount of rubber round the wheel that all must touch the ground and doesn't shrink or stretch by much in normal use, so surely the circumference remains the same. Doesn't it? I haven't tried the chalk mark and measurement technique with the wheel off the car and unloaded. Must do that.
Mike Howlett

Mike the "effective" circumfrence does change. static, rolling and dynamic have different measurements.

There is no extra length to disappear it seems to me. It's an inflatable. When fully inflated and not under load, the radius is one measurement, but under load and when revolving at speed, the radius is a different measurement. See dave's explanation below. I reckon that's about spot on.

Also, the circumfrence is a product of the radius, not the other way around. So if the radius changes, so must the circumfrence.

How much was the electronic speedo, what does it look like, and where do you mount the sensor?
Lawrence Slater

Ah, so someone is on the same wavelength as me!

Lawrence, this is the bit that you are ignoring, as it doesn't quite fit your concept.

"there is a finite amount of rubber round the wheel that all must touch the ground and doesn't shrink or stretch by much in normal use"

Mike says it better than me!
Guy Weller

Ah, yes but no but. :)

That bit of rubber that is in contact with the ground, is closer to the hub, when it revolves around and comes into contact with the ground. Or to put it another way, the radius has reduced at the point of contact, and hence the effective circumfrence. The rubber is still all there, it's just changed shape.
Lawrence Slater

It's the relationship (distance) between the centre of the halfshaft(hub) and the point of contact of the tyre with the road that's important. Not the full rotational circumfrence of the tyre, which as you say, if it has a steel strap in it, must remain the same.

You have to stop thinking of it as a circle. It's not a circle, when it's mounted on a car-- and under a load. It's a deformed circle, and that's where the apparent disappearing circumfrence goes to.
Lawrence Slater

Lawrence,
I think we may be agreeing, but still seeing it differently!

All that matters is how far forward the car travels in one rotation of the wheel. That is what you need for your speedometer calibration, not the radius (which is only useful for calculating the circumference of a true circle, which you agree this isn't). It could be running on hexagonal wheels, or triangular ones, it still would only matter what the total length of the perimeter of the shape is.

The fact that there is a circle with a flat on the bottom cannot change the physical distance around the tyre, unless the rubber etc changes composition and "flows" in some way. In the extreme case of a steel belted tyre this is hard to imagine .

We know that the laden tyre appears to be in the shape of a circle with a chord across one edge, and that the chord is clearly a "short cut" when compared to the arc of the circle that it cuts off. (A to B on my sketch below). But this then contradicts my earlier statement about the material of the tyre cannot just disappear!

The answer is I think in my sketch shown by the blue line. When the tyre is laden it flattens at the bottom, but this footprint extends forwards and rearwards as well, and it also distorts the tyre in front of and behind the footprint at C and D. That is where the difference between the geometric chord and the arc is displaced to. The shape is no longer a circle with a chord. It is a sort of arch shape (exaggerating here)

Now, last point. In a perfect theory this explanation would result in no change in the rolling circumference of the tyre. That fits well enough if the tyre were a steel belt in direct contact with the road. But it isn't. It has a depth of rubber casing and tread. The tread has blocks and grooves which move and displace as the tyre rotates and is deformed in contact with the road and it is this that makes the difference between the static circle of the tyre and the real life laden tyre on the road at 37 mph.




Guy Weller

Mike said:
'and doesn't shrink or stretch by much in normal use,'
and that is the flaw in the argument.
The tyre changes both shape and size to a MUCH greater degree than most people realise. Bar room logic (the forum is the equivalent of the pub isn't it) never did win over knowledge and research and understanding.
Start with reading up on the use of pneumatic tyres in early LSR attempts... in one case the tyres ballooned so much the ride height was several inches higher than designed, air got under the car, the rest is history (and so, nearly, was the driver)..
David Smith

Thanks David, I'm searching, but meanwhile I found this on Wiki.

"Performance characteristics
The interaction of a tire with the pavement is a very complex phenomenon. Many of the details are modeled in Pacejka's Magic Formula. Some are explained below" ---

"Centrifugal growth
A tire rotating at higher speeds tends to develop a larger diameter, due to centrifugal forces that force the tread rubber away from the axis of rotation. This may cause speedometer error. As the tire diameter grows, the tire width decreases. This centrifugal growth can cause rubbing of the tire against the vehicle at high speeds. Motorcycle tires are often designed with reinforcements aimed at minimizing centrifugal growth."

Read the whole thing here. http://en.wikipedia.org/wiki/Tire

Lawrence Slater

Lawrence: Good luck with the camper tow, them VDub surfers are always claimin they have no money. I can think of a lot of things to do with a camper rather than tow it, (and not strane the midget), all unrepeatable here.
In the scheme of things £70 is not that bad unless you can do it yourself.

Mike: I like it, means if we change the rims / tyres / take off the the big rubber bumpers etc. it can be recalibrated. Lovely.
Dave Squire - Notts

David,
I was going to agree with you about research but then realised I don't have to - :) - all sources of info need checking and cross referencing, if this place is only 'pub wisdom' level then would that include all your posts - or just all of mine :)

and I did suggest to Guy that if he found a slo-mo of a dragster tyre it might help him understand

Dave,
AFAIK and it could be wrong - for the MoT the speedo just needs to be marked for 30 mph no test for accurancy, mines 25% slow, or is it 20% and 25% margin, anyway it's noticibly slow

not sure if 10% error is still the standard - in Terry Horler's book it has that for the German (IIRC) market the speedo could only be 5% error
Nigel Atkins

More tyre geometry from
http://en.wikipedia.org/wiki/Tire_code#Tire_geometry

"When referring to the purely geometrical data, a shortened form of the full notation is used. To take a common example, 195/55R16 would mean that the nominal width of the tire is approximately 195 mm at the widest point, the height of the side-wall of the tire is 55% of the width (107 mm in this example) and that the tire fits 16-inch-diameter (410 mm) wheels. The code gives a direct calculation of the theoretical diameter of the tire. For a size shown as "T/A_W" use (2×T×A/100) + (W×25.4) for a result in millimeters or (T*A/1270)+ W for a result in inches. Take the common example used above; (2×195×55/100)+(16×25.4) = 621 mm or (195×55/1270)+16 = 24.44 inches. ----

---- The diameters referred to above are the theoretical diameter of the tire. The actual diameter of a specific tire size can only be found in the TRA Yearbook or the manufacturer's data books. ----"

"Note that the tire's cross-section and diameter are always specified when measured on a wheel of a specified width; different widths will yield different tire dimensions."

That last point is pretty important it seems to me. When the speedo calibrators ask you for the size and make of your tyre, they don't ask you what the width of your wheel is. So they may consult the yearbook themselves to get the dynamic rolling circumfrence, but it may well be wrong if you don't have the standard wheel width that the tyre maker based their tables on.
Lawrence Slater

To all those who are interested, the electronic speedo and rev counter are from Speedy Cables and are digital versions of their Classic range. They don't show the digital ones on the web site so I have attached a photo of the gauges in my car. The digital display at the bottom is the odometer which shows total and trip mileages and is operated by a remote button which I have fitted at the side of the instrument housing, where it is easily reached but out of sight. Of course, the gauges have no warning lights in them, hence the separate ignition and high beam lights on the dash. I find the digital display a bit dim in the daytime, so I wired the internal lamp to the ignition circuit. The lamp is on when ever the car is running and backlights the display. Incidentally, these gauges are much better illuminated than the standard ones.

The sender is a Hall effect magnetic thingy. It looks like a short metal pencil on a bracket and I drilled through the web on the front of the differential case to bolt it to. It's business end is in close proximity to the four bolts that connect the propshaft to the diff and it "counts" the speed with which the bolts are passing it. This also came from Speedy Cables and they give you the formula for calibrating the speedo.

Fitting the tachometer is simply a case of setting dip switches in the back to tell it whether you have 4, 6 or 8 cylinders, and connecting it to the coil. I used the magnolia Telemetrix gauges on my V8 MGB and really like them. As the Midget's original speedo was knackered, the cost of the new gauges was not so great compared with having the old one rebuilt.


Mike Howlett

Lawrence, the data sheet you attached a few posts back did clearly show a variation of the same tyre on different rim widths, and highlighted the specific rim width that was used when they measured the rolling circumference.
David Smith

Hi David, yup I noticed that after I made the comment, I should have edited it to say so.

Mike thanks for that. it looks quite nice to me even though I think prefer analogue for the oddometer. I'll enquire about the cost rather than getting mine calibrated.
Lawrence Slater

I guess, with the wheel/tyre under load (on the car) the radius reduces at the bottom but increases at the top, that would at least take up some of the spare circumference. Effectively the centre of rotation is offset, so the radius from the hub centre to the road surface can't be used to calculate the rolling circumference.
S Holt

Found this on a US site called Tire Rack.
http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=203

Tire Specs Explained: Revolutions Per Mile

Revolutions Per Mile
Revolutions per mile indicates the number of times a tire revolves while it covers the distance of one mile. Depending on the tire manufacturer, revolutions per mile may be either measured in a laboratory or derived from calculations based on their previous test experience.

Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road.

Since the resulting loaded or rolling radius is less than half the tire’s published overall diameter (which would only reflect the tire’s unloaded radius), calculating the tire’s absolute rolling circumference isn’t possible.

Additionally, a tire transitions from an unloaded to loaded state as it rolls, continuously flattening where the tread footprint comes into contact with the road. These continuous transitions result in some tread slippage, again increasing the tire revolutions per mile beyond what simple math would indicate.

Edit.
S Holt, that makes sense. Effectively a smaller tyre, at the lower end of the actual tyre.
Lawrence Slater

Hi Lawrence,
Couldn't get to the link you posted, I'm at sea at the moment and some sites are too much for the system.
We use GPS here to give speed down to a couple of centimetres per second when berthing, so most GPS systems should give a good “reverse computation" for calculating the rolling circumference
A few trials with different Tyre wheel combinations might give us a factor that could be used.
Simon
S Holt

so Guy you were right with your tyre slippage
Nigel Atkins

Hi Simon,
--" I'm at sea at the moment and some sites are too much for the system. We use GPS here to give speed down to a couple of centimetres per second when berthing" --

Sounds interesting. Are you in the Navy or the like?

"A few trials with different Tyre wheel combinations might give us a factor that could be used."

Yup, as long as someone with a good maths brain can put the equation together. :) Any volunteers?

You picked up on the slippage Nigel, it's hard to imagine there being that much at a steady speed, but maybe there is. If I take my static loaded radius and use it to calculate the static loaded circumfrence, I get one figure. But then when I roll the car forward one revolution, and actually measure the rolled circumfrence, I get another figure, that is out by 1.25". I might have made a mistake with my loaded r mesurement, but otherwise I can't see how the measured and calculated figure can be 1.25" different. I'm going to repeat it just to check. Could there be so much slippage do to the "tire transitions from an unloaded to loaded state as it rolls"?

Lawrence Slater

the stated slippage was 'tread slippage' plus we've had >>a tyre doesn't roll like a solid wheel; the steel or fabric belt 'rolls along the ground' like a caterpillar tank tread does.<<

but 1.25" sounds like you've possibly got your figures or maths wrong

'show your workings' :)
Nigel Atkins

"static loaded radius and use it to calculate the static loaded circumfrence"
Lawrence, you cannot use the static radius in the equation 2 Pi R as the tyre is no longer circular when laden.The equation only works for circles!

That is what I have been pointing out. The rolling Circumference cannot be calculated from the static (laden) radius. The Carbibles site gives a typical static radius of as low as 44% of true radius, and gives a rolling diameter of 96%. Your Continental tyres pdf gives a static radius of 255 which if used in 2 Pi R would give a rolling circumference of only 1602, well below their quoted rolling circumference of 1714 (your tyre size)

The difference between rolling circumference and the measured tyre circumference is not calculated from the static radius. It relates to tyre slippage, tread block and tyre casing deformation.
Guy Weller

Thanks Guy for your excellent explanation.
Peter B

I agree Guy, and that's why I said I'm going to recheck the figure. I also agree with what you say about the Continental pdf showings, and the reason for my making that post, was to estblish the exact reasons for the difference.

I should also have posted this.
"The static radius is the distance between the wheel centre and the ground contact patch under MAX. load at the recommended tyre pressure."

"MAX load, at the recommended pressure". So stationary, with the maximum load on the wheel, will naturally distort the tyre sufficiently to make a meaningful calculation of circumfrence, and thus turns per mile, virtually impossible.

Now I haven't checked against the max load indicated on my michellins, but I suspect that the load from my offside rear axle on the wheel, is less than the maximum load permissible, and hence I am still surprised that the 2PiR loaded calculation, is so far out from the "static or slow" -(if you can call it that)- rolling circumfrence. That's why I asked, -- "Could there be so much slippage due to the tyre transitions from an unloaded to loaded state as it rolls?"

Now, if the answer to that question is yes, there is that much slippage, then that leads to me to conclude, that for the purposes of re-calibrating the speedo, the static loaded radius, is a meaningless piece of information. And yet, that's one of the questions asked by Speedy cables, in order to get the information they require to calculate the turns per mile for the speedo. They also ask for the "static or slow" rolling circumfrence over 1 revolution. -( Note, I'm calling it "static or slow" rolling circumfrence to distinguish it from the dynamic rolling circumfrence, which is given by the manufacturers as measured at 37MPH.) -

Since those two results are so different, which of those 2 results do they use to calculate the TPM? Do they take an average? If they do, it will be a smaller figure, than the more accurate figure of the dynamic rolling circumfrence taken at 37MPH.

I'll repeat the quote from tireTrack, as I've read similar elsewhere in several places. -- "Tire revolutions per mile cannot be calculated by simple math because the tire tread and sidewall bend and stretch (deflect) when the load of the vehicle presses the tire against the road."

Hence I want to establish the "appropriate" tpm myself prior sending my speedo anywhere for calibration.

And yes, I should telephone or email the calibrators and ask them all this stuff, and I will, but first I want to fully understand the question before I do that.
Lawrence Slater

I cannot see any better way of doing it than your very first method. Mark the tyre wall, roll the car forward one full wheel turn, and measure how far it progresses. Except my method of pushing the car forward by 20 revolutions and calculating from that (greater accuracy).

One probably should go for a 10 mile drive first, to warm the tyre up!

I cannot think of a way of simulating the wheel at high speed!

The calibration by reverse calculation from a GPS device sounds as if it has possibilities.

For all that, I am not that bothered about speedo accuracy. I have only participated in this discussion because I was always puzzled by the use of the term "Rolling Radius" that Anthony used, when it is the Rolling Circumference that matters.

Guy Weller

"that's why I said I'm going to recheck the figure"
I don't see why you need to recheck a figure if you know that the basis of trying to calculate rolling circumference from the static loaded radius is wrong in the first place
Guy Weller

back to basics; the 'effective rolling circumference' i.e the distance travelled in one full turn of a wheel at a set speed IS 2 x pi x the effective rolling radius.
David Smith

Mathematically, that is true. But I don't think that the process involves measuring the "effective rolling radius" in order to work out the rolling circumference. I rather suspect that one works out the rolling radius from a measured rolling circumference, and uses the rolling radius to work out calculations to do with torque figures for the driven wheels.

Another can of worms?
Guy Weller

As I've said before, rather than guess why not ring up and ask them? If I were building a rig for this purpose I'd have something a bit like a treadmill, mount the wheel and give it a predetermined load, drive the wheel, count the turns (Hall effect trigger?) and count the distance on the moving 'road' to 1 mile.
David Smith

Merchant Navy. It's great seeing what people are doing to their cars that I can maybe do to mine on leave. Or not as the case may be!!
Simon
S Holt

To add my 2 pence worth, I changed diff's ages ago, long before GPS & I phones were readily available.
I used an excel spreadsheet (which I would have to dig out) to recalculate the given speed based on RPM vs Diff Ratio. The variables in the calculation were Diff Ratio and Tyre size / Tyre rolling radius. I then marked the speedo accordingly as I had a base to work from i.e the original speedo/rpm vs the re-calibrated Speeds. So as not to ruin the original face I labeled the glass with Letraset (you could use printed stickers etc). Cost nothing and is within a gnats wotsit of being correct. As we all have rev counters we should know just by RPM what speed we are going when in top gear providing you are using BMC running gear & know the rolling radius of the tyre. Obviously this ignores the variables a tyre gets from tread wear,pressures, temperature and centrifugal forces
(ever watched a rear tyre on a top fuel dragster grow on take off!!!)
Ed
Ed H

Guy, my interest in this stems from my speedo being so far out. I'm fed up with relying on the car in front, going slow enough through a radar trap to save me getting a ticket. I only know roughly what my true speed is, and after 10 or so years of putting up with this, I thought I'd get the speedo calibrated (for the T9 box). Then reading the calibrators websites, I realised that the data asked by them is not really a completely accurate way of doing it, so I decided to investigate the smarthphone application to enable me to do the calcs myself.

"I don't see why you need to recheck a figure if you know that the basis of trying to calculate rolling circumference from the static loaded radius is wrong in the first place"

I want to recheck because I still find it hard to believe, that at essentially zero mph, the measured and calculated circumference can be over 1" adrift. I suspect I might have mis-measured the static radius. If not, then it adds further credence to my distrust of the calibrators even being interested in the static figures for calculating the tpm.

"I don't think that the process involves measuring the "effective rolling radius" in order to work out the rolling circumference. I rather suspect that one works out the rolling radius from a measured rolling circumference, --"

I agree Guy. The effective or dynamic rolling radius isn't given. I suppose if put on a rolling road, it might be possible to measure it at various speeds, but even then, if the tyre distortions are that great, you would always get a different calculated rolling circumference. Maybe this could result in a factor X that could be applied to the 2PiR formula to correct it?

Anyway, David, you're right, and I did ring a couple of them. Michellin technical gave the same dynamic rolling circumference as is in the Contentinental table (37MPH). So it must be an industry standard?. Albeit with a -2.5 and +1.5% tolerance.

I don't think a high speed wheel circumference test is needed. Just a middle setting. If the tyre is "effectively" smaller when stationary, and slightly larger at 37mph, then clearly it will be slightly larger again at higher speeds. Hence relying on the turns per mile of the rolling rear wheel to set a speedo, will always result in an inaccurate speedo, -- to some small degree. Hence I think the best measure to use will be the dynamic rolling circumference at 37mph, as given in the table. It must always be a compromise to some extent it seems, unless you use lasers front and rear, or radar, or GPS.

Ed, yup I agree, that would work fine, but unfortunately my rev counter is also out, and I only really use it to tell me the engine is actually running. Lol. I've tried Anthony's Cutlers method of resetting the rev counter, but the adjustment pot is too worn, and I can't get a clean setting. I have a spare rev counter and intend to rebuild it with new electronic components, caps and stuff ---- when I get around to it.

Meanwhile I confess to have become rather interested in this subject, probably beyond it's worth. So I'm exhausting it to the nth degree. As Ed said, it may not matter a gnats wotsit. Lol.

Then again, not that you needed proof, but just as it is now wholly accepted that physical exercise keeps the body young and extends your life, it is also recognised that mental stimulus keeps the brain from degenerating. Who wants alzheimer's? Maybe chewing the cud on here will keep it at bay. :)
Lawrence Slater

Lawrence, I went for the GPS option - I bought a GPS speedo (and matching 8000rpm tacho) from Speedhut (http://www.speedhut.com/gauge_products-gauge_type-speedometer_GPS.htm) for a few reasons - the speedo drive in my Toyota T50 box is stripped, I've been experimenting with different diff ratios and am planning to try several different wheel/tyre combinations, and even after fixing the speedo drive (which would have required pulling the engine and box - a job I have had more practise at than I would like lately) the speedo would have had to be calibrated for the gearbox, then recalibrated if I changed the diff ratio or dramatically changed the tyre size - GPS seemed easier. I was a bit sad to give up the Smiths gauges, but I think the new gauges look good too, and I was very impressed with the product. Very accurate, excellent visibility, nice included wire harnesses with plenty of length, all around seems like a good product to me. Anyway, I only installed them a week ago, but so far I am extremely happy. You can customise the appearance of the gauges however you like also.




Andrew F

With gauges lit - I asked for the optional dimmer, which they included without charge, and I am glad I did. This is at full brightness for the picture and I have it dimmed down a lot so I have better visibility of the road when driving. Please excuse the crummy camera phone pictures.

By the way, they include an aerial for the GPS that is a small box on a long lead from the gauge. They told me it will work anywhere where it is not under metal, so I have it installed under the carpet on the transmission tunnel behind the gearlever at the moment. Totally out of sight, and seems to work perfectly.


Andrew F

Looks very good. Why did you go for the 260mph one rather than a 120 ? I would expect it to be easier to read.
Guy Weller

I think that's KPH rather than mph guy.

Agreed, they do look nice. I'll check those out too.

What did you do to replace the no charge warning light and the main beam light, or are they enclosed in those 2 greenish dots in the speedo?

Lawrence Slater

Didn't realise that Australia was in metric; assumed that they still measured in proper miles and mph.
Guy Weller

Does seem strange Guy, after all, they do still speak english, ---- after a fashion. ;)
Lawrence Slater

Yes, it's km/h, not mph. The high beam light is included in the speedo - it's the top LED of the 3, and lights up blue. The 2 green LEDs can be used for the blinkers (thats the intention) or for any other warning light. The charge light will need to be replaced, I plan to fit a 'jewel' type to match the other warning lights. I have ordered this, but while waiting for it to arrive the charge light is insulated and taped out of the way under the dash, lights my footwell up nicely when it's lit! I put another ~400km on the car over the weekend, mostly at night, and I'm still very happy.
Andrew F

We went French too, but proper cars still have miles!
Paul Walbran

To be honest, I probably would have fitted another mph speedo, but my wife is not impressed with having to do the conversion in her head. Having learned to drive in my fathers MGA it's natural to me, but she found it very frustrating when she drove the car.
Andrew F

Hi Lawrence,

I worked as a tyre designer for the best part of a decade, but have since escaped... ;)

Rolling circumference and Rolling radius are NOT proportionally related, that is to say that they do not obey 2 * Pi. (In the example below is was more like 6.4 - 6.5, but will vary with different tyres)

SLR is very arbitrary as there seems to be little agreement between different tyre manufacturers as to what load / alignment conditions they are quoted at and as such each do there own thing to varying extents, it is possible there has been some change in this in past 15 years or so and there may be an ETRTO set of conditions they now conform to but I am not sure. So this loaded radius as the name suggests is under static conditions and used to give some idea of the approximate ride height that a vehicle will be at at said pressure / camber.

Rolling circumference is measured dynamically and as you have seen will be quoted at a specific speed / pressure / alignment so therefore it is entirely normal that the quoted rolling circumference is not 2 * Pi * SLR.

Moving on to the dynamic effects such as speed, both dynamic rolling radius and rolling circumference increase with speed, just having a quick look at some measured data, (compared with reference data at 50 km/h);

Rolling circumference grew by; 0.3% at 100 km/h, 0.9% at 150 km/h, 1.7% at 200 km/h, as you can see non linear growth

Rolling radius grew by; 0.2% at 100 km/h, 0.7% at 150 km/h, 1.4% at 200 km/h, again non linear growth

The quoted SLR for this tyre agreed with the rolling radius at 150 km/h.

Unfortunately these figures are not transferable, another tyre with different geometry (aspect ratio, centre of mass of it's cross section) will display different percentage growth.

On any of the race cars I have engineered we tend to go to somewhere with a measured mile / km (usually MIRA), run the car at low speed (minimise any slip effects) and datalog the distance / speed recorded by the vehicle, then modify the rolling circumference held by the data logger until the measured distance logged by the vehicle is in agreement with the measured mile. The vehicle will be at say mid race weight (50% fuel load) and the tyres at a pressure equivalent to the running pressure (heated where possible - which takes thermal effects into account - but tyres cool very quickly in thus test) I do this because the quoted rolling circumference is usually little better than a starting point in most cases.

HTH

Cheers

Spencer






S Deakin

very interesting Spencer

this also hepls explain the difference in tyres states as being the same size

normally these differences are very smalll and not really noticed or noticeable

my last change of tyres on my present Midget even allowing for the change in sidewall ratio was visually noticeable and confirmed when measured

Spencer,
are you able to confirm please, say the radial tyres that would have been original when my car was new where 145/13 - so was the ratio nearer 82% than the later and normally quoted 80%
Nigel Atkins

Yup thanks Spencer. That is very interesting stuff.

So even on a new car with an electronic speedo, the speedo will only really be accurate, if the same tyres fitted by the factory are always used as replacements?
Lawrence Slater

not even then exactly because of the variables but near enough not to worry

plus the digital mile readouts just changes in an instant rather than the gradual rolling of the cogs

when my wifes car shows say 9.4 miles in the Spridget (when it was previously more accurate than now before g/box type change) I could see that the reading was perhaps 9.45 or 9.475 miles by how far the tenth of a mile cog was from central
Nigel Atkins

This thread was discussed between 16/07/2012 and 02/08/2012

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