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MG Midget and Sprite Technical speedometer drive vs wheel revolutions BBS discussion at MG-Cars.net
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MG Midget and Sprite Technical - speedometer drive vs wheel revolutions
| God evening gentlemen!
I need to have my speedometer repaired, since the odo meter does not work anymore. At the same time I will have it recalibrated to suit my new Ford T9 gearbox speedometer drive ratio, Speedy Cables will do it for me. To calibrate it, Speedy Cables needs informations from me about how many revolutions the drive makes when turning the driving wheels six times. Can I do this while having the car jacked up? Or do I have to wait until it's on the ground again, so I can push it around? The reason for the question is, that in the past, when having a car jacked up, I have learned that when rotating one of the wheels, the opposite wheel will turn the other way around, if not locked, due to the plain nature of the differential. Will this work, or will the gearbox outlet shaft only rotate truely, if rotating both wheels simultanously? Now what I mean? Jan |
| Jan Kruber |
| Jan, Have you printed off the form on the Speedy Cables website? If so, follow the instructions exactly, complete the form and send it off with your speedometer. The more accurate you are with the figures they have requested, the more accurate your speedometer will be after they return it to you. I completed the same exercise as you a couple of months ago ago and have now received my speedometer back. According to the sat-nav it is now spot on. Regards Robert |
| w r holmes |
| If you lock one wheel and rotate the other wheel 12 times, it will produce the same result as both wheels turning together 6 times. |
| Dave O'Neill 2 |
| Why complicate things? What is wrong with carrying out their instructions? |
| w r holmes |
| W R I read the instructions a while ago, refeshed it right now. Sure it's better to do as Speedy cables write, but as I'm very delayed in my projects, I was just looking for a short cut, so I could have the speedometer recalibrated and fitted again when the car is lowered again and the wheels hit the ground for a test spin. Dave Thanks for the info. |
| Jan Kruber |
| Jan, Fair enough, you want a shortcut and Dave has come up with a solution - why not check with Speedy Cables? A word of advice - insist that your query is answered by a qualified member of staff. Regards Robert |
| w r holmes |
| Jan, according to my mate Speedicables were anything but speedy. Why not give them and JDO and their unhelpful ways (in my experience) a miss and contact the often recommended on here Richard Jenkins instead and see what system he uses. Richard Jenkins Unit 49h Pipers Road Park Farm Industrial Estate Redditch B98 0HU email woodykeys67@msn.com phone 07747 357044 |
| Nigel Atkins |
| Nigel Dropped Richard an email. Jan |
| Jan Kruber |
| Jan, What rear axle ratio do you have? With this information I can give you a good idea what the drive turns will be for 6 wheel turns. After that whoever is doing the calibration will need to know the tyre size, to work out the tpm (turns per mile) that speedo needs to be calibrated to. What size tyres are you using? To be completely sure you do need to do the 'Speedy Cables' process on the car. Richard |
| Richard Wale |
| Why is one never asked to wrap a string around the circumference of the tyre, in the middle of the tread, and then measure the string to get the distance travelled per wheel revolution? Seems so obvious. |
| GuyW |
| Jan, let me know how you get on with Richard Jenkins as I must one day get around to having mine done (I just read off the rev counter at the moment). David Smith, along with others, recommend Richard Jenkins and a couple of posters have quite recently used his services and given good reports back. |
| Nigel Atkins |
| Guy, Is the 'rolling circumference' the same as the 'measured circumference', and does it vary with speed very much? It would vary with load and air pressure. Richard |
| Richard Wale |
| when I was getting my numbers together for Richard, I emailed Yokohama comps dept with the circumference and tpm questions, prompt answer next working day, they are very helpful. |
| David Smith |
| Richard, I have long puzzled about that. Clearly, with the flat bit of tyre in contact with the road, the radius to that point is less than that of an unloaded tyre. But the tyre distorts in advance and after the contact point so you cannot use the normal 2PiR equation. But surely all of the continuous tyre circumference must contact the road at some point of a revolution, so unless there is tyre spin, then the circumference should be equal to the forward motion of the car. Consider taking tyre distortion to the limit, what happens with a tracked vehicle!
Of course if you are going very fast there may be some tyre slippage, but conversely, if you are going very, very, very fast then the tyre will baloon outwards and the circumference will increase. |
| GuyW |
| Richard I have the 3.9 ratio diff, according to the stamping on the housing at least. I have some AVON 165 / 13 tires on the rims. I'm in contact with Richard, I have just asked him if he can recalibrate out of some "static" informations from me or if we have to do the "Speedy Cables" method. Nigel, I will :-) |
| Jan Kruber |
| Thanks Jan. BTW, and I'm (we) not laughing at your excellent written English but we bury our heads in the sand over here rather than hiding them in the bush (that would be more in Arie's line of thought). |
| Nigel Atkins |
| I noted that too Nigel, but decided not to comment as l thought it was a much better and more descriptive metaphor for the modern age. After all, who has easy access to that much sand these days? |
| GuyW |
| Jan,
To check the rear axle ratio, mark the pinion flange so that you can count the turns, lock one rear wheel (it must not turn whilst doing this check) and turn the other one exactly 2 complete turns, whilst counting the turns that the pinion flange makes. It should be 3.9!! Using the most common T9 speedo drive ratio of 22:8 (2.75:1), and the 3.909:1 diff ratio, the speedo drive will run at 1.421 turns for each wheel turn, so for 6 wheel turns, the speedo drive will turn 8.53 turns. If the car is on the ground, it will move ~36.75ft or ~11.2m along the ground, which is another measurement that is used to calibrate the speedo. With 165/80x13 tyres (are they '80' section tyres?) they rotate at ~862 revolutions per mile, multiply that by the rear axle to speedo drive factor (1.421), and the speedo TPM (turns per mile) is 1225. Will be interesting to see what the Speedy Cables method produces? Richard |
| Richard Wale |
| Nigel
Allthough we have a lot of sand here in Denmark we have allways hidden our heads in a bush if nescessary :-) It was just a risky translation - I'm aware that it's not allways possible to directly translate sayings. .....which reminds me of a little story. In the end of the 1970ies I had a Triump 750 Bonneville and hang a lot around in Triumph in Copenhagen. Now, we have a saying here in Denmark, that we use if there are no problems, which translatet to English would sound like this: "No cow on the ice"! When the two Triumph guys was on a course on the Triump factory in Meriden, they frequently commented "no cow on the ice" to underline when there were no problems in a given situation. The Meriden guy's just liftet their eyebrows not understanding a word of what those silly Danish guys just said. But when the Triumph guys had heard the sentence one time to many they just had to ask what nonsense the danes were saying. The Danish guys explained what the saying stood for in Denmark: "Well, if there are no cow on the ice there are no problems - however if the cow actually was on the ice, we would have a problem, right Sir? The legend says that there could e be heard an "aaahh" from the Triumph guys :-) :-) Jan |
| Jan Kruber |
| Has this thread been edited? I cannot see where Jan has buried his head in a bush. |
| Nick and Cherry Scoop |
| Nick, Yes it looks like something has changed as I recall reading Jan having posted that earlier on, maybe though it was on the wishbone thread and not this one. Currently it is in Jan's penultimate post in the wishbone thread. |
| David Billington |
| Yes, thread hopping involved. It's only to be expected at this time of the year. |
| GuyW |
| Guy;
>>>so you cannot use the normal 2PiR equation You've made my brain turn to jelly trying to ponder this on a Sunday evening(!) New cars registered since 2014 have be fitted with a Tyre Pressure Monitoring System (TPMS). Many of these systems simply use the ABS sensors on each hub and compare the rotational rates of each wheel. It appears that these systems are capable of detecting a change of only a couple of lbs psi. This must mean that it is the distance of the centre of the wheel to the road surface (the rolling radius) that dictates the turns/mile and not the circumference of the tyre? Jim |
| J Smith |
| Ooops, sorry for the thread hopping......... |
| Jan Kruber |
| Jim, that explanation of TPMS sounds flawed to me as if it was that simple then you would show low pressure on one side when going round tight corners as the outside wheels will be going faster than the inside ones. Trev |
| T Mason |
| I think the method of pressure sensing that Jim describes is correct. Maybe they work on an average over a longer distance, to get around the cornering effect. They must also be pretty sensitive to rolling radius changes as the typical BMW ones are designed to work with run-flat tyres.
I am not saying I am right, but I find it hard to get past the idea that the rubber of the tyre tread that forms the circumference of the tyre and is in contact with the road surface, isn't also the length of tarmac traversed in one revolution. Allowing for a bit of give-and-take for slippage between the two surfaces and assuming most cars don't indulging in Colin Macrae style tyre smoking all the time! |
| GuyW |
| Trev, As a former software engineer I would expect the algorithm that detects lower tyre pressure would be far more sophisticated than picking up short term differing rotation rates such as when cornering but would pick up a general trend in the faster rotation of one tyre to another indicating lower pressure on the faster rotating side. |
| David Billington |
| On this question of theoretical tyre circumference as against actual distance covered per revolution, this is the answer I gave back in 2012 to the thing that is now puzzling me!
<<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>> Answered my own question 6 years before I asked it! Proof that time isn't necessarily linear or uni-dimensional! |
| GuyW |
| Guy,
Earlier you said this: "But the tyre distorts in advance and after the contact point....." Quite right too. Think of the distortion as a pair of bulges, which after all are easily discernible to the naked eye on low ground pressure tyres (tractors etc), especially radials (vs crossplies) owing to the softer sidewalls. Consider that the lengths of those radii that intersect the "bulges" will be greater than the free radius of the unloaded tyre. I haven't done the calculations but it seems to me those longer radii would yield momentarily greater theoretic "effective circumferences" just as the shorter radii intersecting the contact patch yields a momentary smaller theoretic circumference, or rather a series of them. This may go a long way to accounting for the discrepancy you note between the rolling circumference and the free circumference, although undoubtedly the tread movement you mentioned contributes a share. Sunday evening musings..... |
| Greybeard |
| Jan, it was me that started the thread hopping so you don't need to apologise, thread drift is nothing new. I can't say I fully go along with that saying but I am now thinking of the poor bovine beasts of your country.
As for TPMS I can guarantee to you that the systems are not all that sensitive, my wife realised there was something wrong with one of her rear tyres well before the warning came up. I'd imagine the sensors relate to the suspension as there's nothing to wheel or tyre. I blame the tardiness of the TPMS for costing a new tyre on one with plenty of life left on it and for giving me a bad back that caused my current knee problem. I'd imagine by its operation that Microsoft might have been involved with the TPMS on my wife's car. Guy, I can't remember (or understood) the explanation of the moving tyre circumference but it did involve something like to do with being like a tracked vehicle. |
| Nigel Atkins |
| David, no doubt you are right. I have not had a car with the system so have no idea how quick they react to pressure loss. I was expecting it to be instant but I guess there is no need for it to be that quick. Trev |
| T Mason |
| Interesting topic. I had no idea about TPMS, so I did a bit of reading. For example: https://www.motoringassist.com/motoring-advice/car-maintenance/wheels-tyres-and-brakes/guide-tyre-pressure-monitoring-sytems-tpms/ In the last few years I've been doing a good bit of tyre-wrestling, but I've never come across any sensors in the wheel. Probably a good thing - most likely I'd have broken them. Presumably the indirect type tied into the ABS is more common then? |
| Greybeard |
| My experience of TPMS is with several VW-Audi group cars. Unfortunately, I have had 'tyre pressure loss' warnings on several of them, but have never found anything amiss. It's a bit like the boy who cried wolf. I've had so many false alarms that I'm now inclined to ignore any future warnings. |
| Dave O'Neill 2 |
| I may be wrong but I seem to remember reading somewhere that some of them have a sensor in the valve which is then transmitted through bluetooth to the display. They may be an aftermarket system or I may have imagined it. Trev |
| T Mason |
| TPMS is now an MOT fail if they do not work, had to have one changed cost £90.00 its part of the valve , found that one was more sensitive than the other 3,a temp changes over night could upset it. |
| mark heyworth |
| Trev I believe there are systems like that. I think Renault used to use them. Probably Nissan, too. I had a Qashqai, briefly and that displayed the pressure for all four tyres. |
| Dave O'Neill 2 |
| At £90 thats more expensive than a decent tyre on a lot of run of the mill cars, all for something thats completely unnecessary because people cant be bothered to do basic checks these days.
This got me thinking about all the other unnecessary warnings on modern cars that you have to be an idiot not to notice. Things like door open, ice on road, seatbelt, low fluid level etc. All that but I have yet to see one on a car that would be useful, this being low screenwash as where they tend to put the container on a lot of modern cars makes it almost impossible to see how much is in it if it isnt full up. Trev |
| T Mason |
| Trev, you're right about the sensors in unit with the valves. That's the Direct type system shown in the article I posted, which now that I've read it properly, states that the direct system is more common than the indirect. Not as I thought.
Not sure about Bluetooth though - if the article is up to date it uses low-frequency RF, but Bluetooth (AFAIK) is in the 2.4GHz band. Agree completely about the stupid nanny-alarms for things that any half-competent driver should spot immediately, but your idea of a low level monitor for screenwash is excellent. Lancias used to have a button on the dashboard which was a small suction device. It pulled fluid up a tube and the vacuum balance was displayed as fluid level. My Beta had the same idea for engine oil level. Simple, effective and (surprisingly) accurate. |
| Greybeard |
| I have a tyre pressure monitor system on my Saab Aero, it works wirelessly and gives a fault light if initially tyres drop 6psi from standard and another warning below 28psi and a further warning if air is lost suddenly (comes up as a puncture) - it seems to work O.K as I have had all three types of warnings and all were correct - yes I do still check TP's but a lot can happen on long journeys !
The down side is the tyre units battery cannot be replaced and last about 10years, however it seems that if you do not have sensors in all 4 tyres the machine assumes you are running 'Winter wheels and tyres' (good old Swedes) and shuts the system down until wheels with sensors are fitted ! Oh and it does have a warning for low washer fluid of 1 litre and at that state it shuts down the headlight washers (mandatory for discharge type lights) to save fluid for the screen. Hope it never goes wrong as I couldn't fix it - wouldn't know where to start ! |
| richard b |
| I believe many of the 'mainstream' car manufacturers use the ABS sensors for TPMS simply because most of the hardware is already there. (ABS has also been a legal requirement on new cars for many years)
As I understand it, on these simpler systems, the TPMS compares the 'change' in each wheel rotation compared to the other three wheels measured over a given time/distance period. Note that it is the relative change that is measured. EG if you replace a spare wheel with a 'space saver' which may have a different diameter tyre, you must reset the TPMS system so that it builds a new 'map' in it's memory of the new relative rotational rates for each wheel. More expensive systems or indeed 'retrofit' systems use individual pressure sensors and transmitters fitted to each tyre. I assume then that no-one here has yet fitted a TPMS system to their Spridget? :-) Jim |
| J Smith |
| My wife's car (one of the better makes in the VW group, a Skoda) must have the indirect type of TPMS after a very quick look at the start only of the link Grey put up.
By the sound of it the puncture fell in to the blind spot of the system as my wife's electronic driver aids (her brain) detected a possibility of a problem before the car's. The system requires resetting leaving another fall out area for some - "Unfortunately, indirect Tyre Pressure Monitoring Systems are often not very accurate and require the driver to reset the system every time a tyre is inflated." As you can probably guess I read the Driver's Handbook for her car (very unhelpfully not called that) so knew and have reset the system when required so there was no user error in this case. :) |
| Nigel Atkins |
| Thats absolutely crazy that you cant replace the battery in them and you would have to be crazy to fit such a system to a Spridget when the price of one unit would buy you two top notch tyres for it. Trev |
| T Mason |
This thread was discussed between 30/03/2018 and 02/04/2018
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