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MG Midget and Sprite Technical - Tyre pressures

chaps.... 145/13 radials on wires and everything else on the car is standard..as per the book...which indicates 22 psig on the front..but...when running at this I have found that the outer edges of the treads are wearing faster than the centre suggesting that this pressure is too low. Modern tyre construction and materials are clearly different to that from the 60s so what have others been running? I will be trying 25 psig and keeping a close eye on the wear rate...
David Cox

Could it be excessive toe-in rather than tyre pressure?
GuyW

If it's the inside and outside edges, then it does sound like under inflation.

How accurate is your gauge?
Dave O'Neill 2

Guy..toe is is parallel.. again as per book. Had it checked thoroughly this morning by old school methods and old school tester...telescopes and mirrors ..witchcraft to me!
David Cox

Dave..the chap who did the tracking has his guages calibrated so I'm happy they are as stated...
David Cox

... a more interesting question is how much do tyre pressures increase after a good blat? something to measure tomorrow...
David Cox

David,
if it's just the outboard edge of the front tyres, rather than both outer edges of each front tyre, then it could be as Guy puts, or other suspension issue.

If it's both edges of each tyre then it's under inflation.

You're quite right that things have changed since the Driver's Handbooks were written although it's always a good place to start and reference from.

Tyres of the same size vary from model to model let alone manufacturer to manufacturer so I always suggest -
• add 2-6 psi above what DH has, I suggest in stages of +2 psi each time until too firm then drop back 2 psi - but it all depends on your tyres and suspension set up
- this would also go with tyre wear.

Tyre Bible has-
"Shoulder Wear - Both Shoulders wearing faster than the centre of the tread
. Under-inflation
. Repeated high-speed cornering
. Improper matching of rims and tyres
. Tyres haven't been rotated recently"

"One-sided wear - One side of the tyre wearing unusually fast
. Improper wheel alignment (especially camber)
. Tyres haven't been rotated recently

http://www.carbibles.com/tyre_bible_pg3.html

I've adjusted my tyres pressures and have run at +2psi with previous tyres but I'm back to as per book with my present set. I have "One side wear and think it's down to camber and tyre but it might also include too much toe-in I've yet to have any adjustment made despite rotating my tyres recently. Some, including Tyre Bible author originally, dispute the need or any advantage of rotating the tyres, I've no evidence but think if you carry a spare it would make sense to get some wear out of it in rotation at least.
Nigel Atkins

I took so long to type my post (plus batteries went flat) that I've missed several posts, hope this catches up

From the good book (1973 version)-
"It is recommended that for sustained speeds at near the maxximum the above tyre pressures are increased by 4 lb/sq. in. (.28kg./cm3)."

So race track stuff nowadays.
Nigel Atkins

Also add summer tyre performance deteriorates at below 10c.

A lot of high speed cornering and braking will increase the heat in the tyres.

Also accuracy of the tyre gauge and ambient temperate when the reading is taken, many commercial use gauges are way out, such as petrol stations, etc..
Nigel Atkins

<< A lot of high speed cornering and braking ..>
guilty as charge, m'lud
Thanks for that reference to carbible, Nigel, will make interesting reading I'm sure
David Cox

Well if the tracking is correct, then at least its good to have eliminated that possibility. It wasn't clear to me that the inner edge of the tyres were also wearing fast. That's why that was my first suggestion. Toe-in is a specific that can be easily verified whereas the various permutations of pressure, tyre tread and suspension can open up lots of variables.
GuyW

Sounds like a classic case of under inflation.
This site
http://www.tyresafe.org/check-your-pressures/

gives pressures for the rubber bumper MkIV Midget as 26.11psi front and 23.21psi rear. Not sure my gauge is accurate to two decimal places(!) but I run 26psi front and 24psi rear on a MkIII RWA with 155/80/13 tyres with so far, even tread wear.
Jeremy MkIII

Thanks for everyone's input. With tracking OK I'll run with 25 psig and keep looking ....
Of course, after a good hard drive this pressure will increase anyway..and if it's a hot day with a rough road surface I have a heavy passenger and....
aaaaargh!
David Cox

you want a bit more in the back for neutral handling; original factory pressures are 24F / 28R
David Smith

My driver's handbook says 22F /24R for radials.
Dave O'Neill 2

I run a '69 MK III on wires at 22 and 24 which I find to be a good compromise between current (rough) roads ride and performance. No noticeable wear issues although I only do c3000 miles p.a. It's interesting that my "good book", 1969 version, says 22 and 24 even for "sustained speeds in excess of 80-85m.p.h". but the supplement to same book says for GAN5 "....sustained speeds at or near maximum vehicle weight........increase by 4lbs/sq. in".
The original '69 book emphasises the impact of high speeds whereas the supplement introduces the impact of weight.
Ray Rowsell

..thanks for everyone's input to this....I suspect that the "true" pressure will never be known and I suspect the true answer is to start with your gauge reading 22 psig front and keep a close eye on wear..then adjust accordingly, making a note of what your tyre pressure gauge is indicating for satisfactory ride/handling/wear etc and keep it there.

Bit of a b*gg*r this retirement...when I was working I had so many other things to worry about instead.....
David Cox

yup Dave & Ray remind me that I was using the higher recommended pressures as I was commuting up and down the M4 to Brentford, and this was back in the 80's when you could do 75-80 all the way...
David Smith

DavidC,
you've got it in your post as far as I'm concerned at least, set at 22F & 24R to start with and see how you go.

Tyres vary so much, some seem to wear more on the edges than others, some have crazing on the sidewalls very soon others not so much. Even if the model name remains the same where it's made can vary over the years.

In case it's not obvious there are four pages to the Tyre Bible and the general Car Bible site could take up your retirement time. Bear in mind his approach is from a more modern car point of view and as all of us has opinions but the site is crammed with info. -
http://www.carbibles.com/

Tyres are a very complex component much overlooked by most drivers, just think about F1 tyre testing days and how many tyres and tyre specialists they have with them.

I'm surprised one of the racers didn't give a rough figure for increased pressure after a run - what figure did you come up with?
Nigel Atkins

...some simple calculations [assuming no change in volume] have given a change from 22 psig at 25 C to 26.4 psig at 30 C.... so there you go. You pays yer money and takes yer choice! Like everything else, the moment you start to ask questions you almost wish you hadn't started....
Spridget World is grand, ain't it?!
David Cox

Tyres ... simple calculations ... assuming - you've got a retirement hobby now! :)

Out of non-scientific interest, curiosity (nosiness) what tyres (make/model/year) have you got?

(See page 1 for Wheel & Tyre Bible for location of manufacture date of tyre.)
Nigel Atkins

Nigel..the tyres have gone.. sorry..but they were only a couple of years old. Firestone.
David Cox

Fair enough, thanks for the reply, so you're starting all over again.
Nigel Atkins

yep..the wear was unacceptable. Back to square one...
David Cox

Cold weather plays havoc on tire pressure

I'm constantly having to add air to the tires on my truck ... esp if the temps are big swings in range

60*F to 10*F and I'll lose 10psi over night easily, I'm not sure where the air goes, but it bleeds out somehow

So if you have some brutal temp changes in the weather then that could be the culprit for loosing a couple psi in a .tire or 2


Prop
1 Paper

Prop,
If the air is damp and cold then ice can form inside the tyre valve and as it expands it lifts the valve off its seating and let's the air seep out. Worse if you don't use the valve caps which can easily go missing on a work truck.
GuyW

This thread was discussed between 17/01/2018 and 19/01/2018

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