Put your registration into someone like Black Circles' website, and it'll find the correct width/diameter/load/speed rating for your car.

Being simple-minded, I would suggest that there is no single brand of tyre that is the only correct choice for your vehicle. As with oil, several brands will be suitable provided you get the correct size and rating. You would need to be a specialist driver of some sort to tell the difference between those. Usually, as with many things, you get what you pay for in terms of performance and durability.

And are the mixed brands on rear axle OK, or should I ditch the Avon (good tread) and have 4 matching?

If the amount of tread is the same as the other and the tread pattern is similar, just leave it on.

When you change . it's up to you.

But as many here will say, that patch of rubber is your contact with the road, buy the best you can.

I believe best practice is to fit identical brand and specification on each axle.

Edited by FP on 25/09/2020 at 15:49

As was pointed out, if there were a claim. claims assesor will be looking for a reason not to pay.

I think you misunderstood what i was saying. The legal issues was regarding having tyres with a lower speed rating than your car is capable of doing. But that isn't the case here, a V rating (up to 149mph) is absolutely fine for a 130mph car.

I don't believe having that higher load rating would have any legal implications, they are not massively over specified after all.

Maybe different if you had tyres for a 3.5 tonne van on it!

*Don't* under-pressure them to 'compensate' for the stiffer sidewall. The tyre walls don't carry the load, the air in the tyre does.

No it doesn't. The air just allows the tension in the sidewalls to carry the load.

www.millersville.edu/physics/experiments/059/index...p

Air inside the tyre doesn't carry any load.

On the road, the bottom sidewalls are compressed and bulge outwards. This stretches the sidewalls at the top and generates tension in them . Now these stretched sidewalls pull the beads upward and the beads of tyre attached to rim pull the rim upwards. So, wheels are not supported from bottom , they are hanged by top part. Easy to understand when you see a flat tyre pumped up.

Yes you need the tensile strength in the sidewall. The pressured tyre is a system under preload which depends both on the air pressure and the casing tension (including tension in the tread) which maintains that pressure and the tyre shape. You need both.

I wasn't giving a physics lesson, I'm neither a physicist or a teacher. It's a shame we can't quiz your source because I can't see how the bead can pull the wheel rim anywhere or how the rim can be "[hung] by the top part"- it isn't glued on and there's no mechanical purchase there, the vertical component he refers to is pulling against the tensile force in the bead wire not against the wheel rim - often there is actually lubricant between the two. Without that opposing force the tyre would pop off, not lift the wheel up.

I don't think the man on the Clapham omnibus would take issue with the idea that a car on pneumatic tyres rides on a cushion of air.

Try taking the air out of the tyre and see what happens to the ground clearance.

Even your man says "The tensile stress in the sidewall... is related to the pressure...in the tire". As indeed it would be because the sidewall has stretching forces in it from holding the air in the tyre!

He's made a simple job very complicated. Yes there are forces in the sidewall but we (unless we are tyre designers) don't need to know what they are. In any case any vertical force exerted by the bead on the top of the wheel would be cancelled by the opposing force on the bottom of the wheel.

Just consider the contact patch.

For a 2000lb car (we're using psi) the load on each wheel is 500lb (average). As the car is going neither up nor down, and the only part of the tyre in contact with the ground is the tread, the force the tread applies to the ground is 500lb. That is spread over the contact patch. If you know the weight on the wheel and the excess pressure as read by a tyre gauge (note not absolute pressure) in the tyre you can work out the approximate size of the contact patch.

This seems to work quite well.

Here's a link to a site that has a number for the contact patch of a Crown Victoria with tyres at 32psi

vehicledynamics.com/the-tire-contact-patch/

They don't give the weight of the car but google says it's 4129lbs.

Average load per tyre is therefore 1032.25lb. At 32psi that suggests a contact patch of 1032.25/32=32.26 sq in.

In fact the contact patch is 36.75 sq. in. per tyre, which therefore exerts about 28.1psi on the road.

(Yes there are no doubt different weights, pressures and tyre sizes but the result is not an order of magnitude different if you fiddle about with the assumptions)

Back to the higher load tyres on the Astra. It has often been suggested, counter intuitively, that extra load (XL) tyres need slightly more than standard pressure to achieve the right shape with lower load than they were designed for. There is a rule of thumb method of working out how much this is using the maximum allowed pressure for the tyre, but the adjustment needed is usually too small to worry about.

I'm not sure I've thought about this enough particularly the relationship between the tyre pressure and the ground pressure but I don't think your man's ideas are very helpful.

P.S.

I suppose the wheel might be said to 'hang' from the bead but not from the top - it sits on the lower bead.

Edited by Manatee on 27/09/2020 at 13:47

It's still the air wot does it:)