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MG Midget and Sprite Technical - Correct tyre pressures.

Hi all,
Can anyone tell me what the correct front & rear pressures for :
Michelin Classic 165 / 70 R13 79T are?
I cannot find this info anywhere.
Cheers colin
colin frowen

technically no 'correct' pressure as low-profile tyres weren't around when BL sold our cars. Try 24F - 28R and adjust from there.
David Smith

What he said. I run my 175 70 13s at 26 all round, if that helps. Adjust till it feels right :)
Rob Armstrong

I'd suggest as it all epends on your suspension and what feels good for you to start at 24 front and 26 rear and as Rob and Dave have put increase by 2 psi increaments each time until you've gone too far and then drop back down 1 or 2 psi to what is best

you need to stick to one a reliable gauge and take the pressures when tyres are not warm

don't use forecourt gauges they are usually inaccurate
Nigel Atkins

Rob. Do you think I'm running my 175/70x13 too soft at 22 front and 24 rear. It feels OK, but I don't want to get premature wear.

Bernie.
b higginson

I've often wondered about this.

If you increase tyre size, is there a formula to follow for the amount of air you should inject?

I've always gone to some degree by the look of the footrint the tyres make when viewed from the front. That and the compromise of getting even wear across the tyres. Too much air wears the centres, and too little wears the outers.

I've always found that 23 in the fronts is good on 165/70/13s, 22 feels "heavy". I've stuck to 25/6 at the rears.
Lawrence Slater

Go as high as feels comfortable!

Modern tyres do not have the old problems of wear in the middle if set high (in fact to high is only over the max inflation value on the side of the tyre)

The structure of a modern tyre keeps the surface flat even on very high pressures.

3bar felt to high with my hard suspension.
2.2bar is good on all surfaces where 2.5 is fine on nice tarmac.

Though UK roads are sometimes a bit more uneven I've never run lower that 2 bar on my UK trips
Onno K

don't start with that filthy bar stuff :)

my gauge only shows psi

I personally think the differences are more noticable if you start soft say 22/24 then increase as I've put by 2 psi increaments until you go beyond optimum then drop back down

if you've got adjustable damper you can also balance between the two

personally for UK road use I've always found the whole cars better set up on what those a lot younger than me would consider soft

I had a standard set up 1.6 MX-5 with a low hard set up 1.8 up my tail pipe as we went up the uneven single track hill road to Tan Hill so I just dropped a gear and soon left him behind as his car couldn't ride the road at a higher speed

a mate lent me and pre-set his digital tyre pressure gauge which after pumping up the tyres and driving a short distance I realised was set to bar and not psi as I'd thought
Nigel Atkins

24F/28R
Guy

This website doesn't agree with you Onno.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=1
Lawrence Slater

Or this picture from a current webasite, and major tyre seller.


Lawrence Slater

Been thinking about this.

Take a bicycle tyre. What's the tyre pressure?
Take a motorbike tyre. What's the tyre pressure?
Take a car tyre. What's the tyre pressure?

The bigger the tyre gets, the lower the air pressure gets.

When you put air in a tyre, you measure the air pressure. But what you are actually doing, is filling the tyre with enough air to support the weight of the car. So there must be a formula, related to the internal volume of the tyre, that could tell you exactly how much air you need.

So, if you go for a bigger sized tyre, that has a bigger internal volume, you would need to lower the air pressure. But if you go for a bigger size tyre, with a lower profile, that could result in a smaller internal volume, and would need an increase in air pressure.

So, if you were to somehow work out the internal volume of each size of tyre, and compare each, as a % of eachother, then you could increase or decrease the psi, as a %.

How does that sound?
Lawrence Slater

Good "rule of thumb", but I doubt it would give you an exact definitive answer. There are too many other factors involved.
Similar issue, I guess, but narrow high pressure tyres also exert a far higher pressure on the ground than fat low pressure tyres. The pressure on the footprint of a racing bike tyre is much, much higher than that of a heavy forestry extractor on wide soft tyres, despite it weighing several hundreds of times as much! Maybe the same thing.

Interesting thought though.
Guy

Lawrence

it's not the volume of the tyre, but the size of its contact patch that's the key to your proposition. The bigger the contact patch (=more square inches) then the less pressure (pounds per square inch) is needed to support a given weight. That's why when a tyre is under-inflated it has that squashed look - it settles down until the increase in area of the contact patch balances the weight of the car and the air pressure in the tyre.

For a normally inflated tyre, the two parameters which determine the contact patch size will be width and to a lesser degree the diameter.

Tyre pressure affects grip on the road through minimising tread shuffle. Shuffle is made worse by overloading the tread blocks (hence the advantage of wider tyres which reduce the unit load in the tread blocks) or by increased tyre case deformation unnder latteral loading.

As the pressure in a tyre increases, the contact patch gets smaller (generally reduces grip) but tyre case rigidity increases (generally increases grip). The "right" pressure is when these two opposing tendencies have the best mix. The point at which this happens will vary depending on the tyre, the car, its load and the type of use.

Those of us who can remember competing on the high profile tyres of times past will recall that the trick then was to use much higher tyre pressures than is the case now, as the gains made via increased case stability far outweighed losses from a theoretically smaller contact patch when cornering. I say "theoretically" because the case distorted so much at (say) 30 psi under race conditions that the contact patch was centred on the tyre shoulder with the sidewall providing a fair bit of it, with the result that most of the tread was in the air. One production car championship winner here used 60 psi in the outside front tyre when the class was restricted to road tyres only!



Paul Walbran

Hi Guy,

Complex stuff. Take tubless solid bicycle tyres. Obviously they have a very high internal pressure. But how much pressure is transmitted to the ground?
Lawrence Slater

The pressure transmitted to the ground is nothing to do with the air pressure in the tyre. It is simple maths of the the weight of the bike and rider divided by the tyre footprint (which is very little on a racing tub. Or in the case of the forestry extractor, the heavy weight of the machine and the tree it is handling, divided by the very considerable footprint of its multiple low ground pressure tyres (or tracks).

The air pressure in the tyre controls how much the tyre deforms at its contact point with the ground, given the force applied to it. With a racing bike this needs to be absolutely minimal, whilst on the forestry vehicle it can deform a great deal, and the tread pattern is designed to actually grip soft surfaces better as the blocks move and "grasp" the ground surface.
Guy

The rigidity brought about by very high pressure reduces drag (not as much energy going into deforming the tyre) but of course then you run into the grip trade-off. For a bike, especially a racing bike, minimising drag is much more important that getting traction, as the amount of traction available even with hard pressures is usually well more than ever required.
Paul Walbran

Cheers Paul.

Nothings ever simple is it :)
Lawrence Slater

"Nothings ever simple is it :)"
Be boring if it was ... just think of all the questions we wouldn't need to ask on the BBS for a start :-)
Paul Walbran

I race the TF on 165/70/15's. I use 40 psi in the dry, so that the car doesn't ride on the sidewalls. You know the old racer trick (for road tyres)? After a racing speed lap or two you look at the area of tread that comes around onto the sidewall: if it's not scuffed, the pressure's too high or you're not trying hard enough. If it's scuffed nicely down to the bottom of the tread, it's pretty right. If the whole sidewall is scuffed, you've fallen over.

However, that's for when you're at the limit - don't try this at home, kids. In the wet I use not much more than my 26/28 road pressures.

PS Sorry about the image. Next time I'll show the midget.


D A Provan

Onno wins.
Laurence's "picture from a current website" is what bias ply tires do, but radials behave as Onno said.

Go here for a discussion, including a summary of the measurements of tread shape vs pressure I made last week:
http://www.mgexperience.net/phorum/read.php?41,2007018,page=2

FRM
FR Millmore

Hi FR.
I'll make the following points. :)

1) I wasn't aware that I was in a competition with Onno.
2) I didn't say Onno was wrong. I simply pointed to two sources that disagreed with Onno, and left it for conclusions to be drawn.
3) The picture isn't mine as you put it. It belongs to kwik fit. Are you are saying that kwikfit are giving out misleading information by posting pictures of crossplies, when they are no longer on sale? They might be. They don't have the best reputation, but I think that might be a tad unfair. I'm not sufficiently knowledgeable to determine if those are radial or crossply tyres. If you say they are, I'll take your word for it.
4) The original post was about road use, not racing, as far as I understand it.
5) Here's another picture. This is from the oft-quoted, and unchallenged here at least, car/tyre bibles website.



Lawrence Slater

I am pretty certain that you are right Lawrence. This is what I have always understand and found to be the case in practice. Over inflated tyres wear prematurely in the centre of the tread and under inflated ones wear towards the shoulders. I am talking here of normal road use rather than under racing conditions when other factors come into play.

At one stage the marketing people made a great thing of "steel belted radials" giving a more ridged flat footprint area. But given that the steel belts were exceedingly thin in order to flex as the tyre rolled, it seems unlikely that could have accounted for what Fletcher is stating so authoritatively and with such conviction. I am not convinced. Over inflation of my radial tyres definitely causes them to wear faster in the centre of the tread.
Guy

That's my experience too Guy.
Lawrence Slater

to be fair (to all)

the picture that L posted from Car Bibles also has the following wording above:

'Your tyre wear pattern can tell you a lot about any problems you might be having with the wheel/tyre/suspension geometry setup. The first two signs to look for are over- and under-inflation. Whilst this used to be a problem on older tyres, modern radials have much stiffer carcasses but even so, you might still be able to spot the following:'

plus

'Here's a generic fault-finding table for most types of tyre wear if you can spot them' - giving a table of causes of different wear patterns

you can read more here -: http://www.carbibles.com/tyre_bible_pg3.html/
Nigel Atkins

This thread was discussed between 30/03/2012 and 10/04/2012

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