tyre rating dilemma

Joe_Horner

Well, part of the reason for putting thse on the front was that those old Pug diesels aren't exactly great on grass, but it goes off road fairly regularly. Extra tread on the driven wheels does help in that situation!

I also know I'll be replacing the others in a month or so because one's down to about 2.5mm, so then the new ones will be on the back

Ultrasonic

molerat wrote: »

Both my local National Tyres and Ford dealer asked why I wanted the new ones put on the back as surely I need the better grip on the front !

A Ford dealership I can kind of understand, but I'm pretty stunned that someone working for National Tyres would say that! Not that I have a high regard for them or anything, but the one thing they should know something about is tyres.

I'm please to say the local independent where I last got tyres fitted didn't question my request for the new tyres to end up on the rear.

Joe_Horner

Ultrasonic wrote: »

But the rear wheels don't have to lock for the lateral grip of the rear tyres to be exceeded, just think of a car going around a corner with no brakes involved. For the current example, braking and thus throwing the weight distribution forwards makes a loss of control of the rear more likely without the wheels locking.

No, they don't have to lock.

But you won't spontaneously lose lateral grip "in a straight line", even allowing for weight transfer. With a loaded estate there's still going to be half a ton or more holding the rears on the road even under severe braking.

Ultrasonic

Joe_Horner wrote: »

But you won't spontaneously lose lateral grip "in a straight line", even allowing for weight transfer. With a loaded estate there's still going to be half a ton or more holding the rears on the road even under severe braking.

As you acknowledged above there are other factors that would cause the car to deviate from a straight line. The weight transfer can also be significant (see e.g. here).

My point was that the shifting weight distribution improves front tyre grip and reduces rear tyre grip. This is relevant as you were trying to draw conclusions about the relative performance of your part worn premium tyres and new budget tyres. The problem is they were not being tested equally.

Your new budget tyres may be great, or terrible, but I don't think we can really conclude anything either way. The result with the new tyres on the rear could have been better, or worse, nobody knows.

Joe_Horner

Yes, weight transfer can be significant. But, if you notice, in the link you give, even with their extreme example (few cars will achieve 1g braking forces in practice), there was still 1000 lbs of weight on the rear tyres. That's more than enough to prevent them sliding sideways as long as the rear wheels haven't locked.

The situation is different if you brake hard in a corner because the lateral forces are already significant. But when driving in a straight line there will only be maybe a few 10s of kg of lateral force caused by irregularities in the road. If that wasn't so, cars would fall off the side of straight, dry, roads all by themselves. Which they don't.

The simple fact is that car brake systems are designed to lock the front wheels first if the tyres are similar. They do this by reducing the rear braking force in proportion to the weight transfer, so that the braking force applied to the fronts is a greater proportion of the weight they're carrying. Having better tyres on the rear will increase that effect, having less grip on the rear will reduce it.

ONLY if the tyres on the rear have significantly less natural grip than the fronts (or if there's a fault with the braking system, which there isn't on my car) will the rears lock first under straight braking. That's how they're designed simply because, as soon as a tyre starts to lock, it loses virtually all of it's grip and any slight lateral force will then move it sideways.

With front wheels locking that doesn't matter so much because the (still gripping) rears will drag it into line and the car will stay straight. With rear wheels locking, it will matter because momentum will start to move the rear towards the front, causing it to swing out even further, creating more lateral force, causing it to swing out further and so on.

Ultrasonic

[quote=Joe_Horner[/quote]ONLY if the tyres on the rear have significantly less natural grip than the fronts (or if there's a fault with the braking system, which there isn't on my car) will the rears lock first under straight braking. That's how they're designed simply because, as soon as a tyre starts to lock, it loses virtually all of it's grip and any slight lateral force will then move it sideways.[/quote]

So if I've got this right you think that the only way your car could have behaved in the way it did is if the rear wheels locked before the fronts? I'm not fully convinced of that, but even accepting it I still don't think this makes your case for enabling you to compare the tyre's performance.

Am I right to assume that your argument is that a tyre with less grip will cause a wheel to lock sooner? I think that makes sense. The issue then is that this grip is a due to a combination of the tyre and the weight exerted on it. The latter is significantly different front-to-rear when braking in a straight line, and so concluding that the tyre alone is responsible for the lack of rear control is not possible IMHO, even allowing for:

Joe_Horner wrote:

The simple fact is that car brake systems are designed to lock the front wheels first if the tyres are similar. They do this by reducing the rear braking force in proportion to the weight transfer, so that the braking force applied to the fronts is a greater proportion of the weight they're carrying. Having better tyres on the rear will increase that effect, having less grip on the rear will reduce it.

That may well be how the car was designed to behave when new, on a flat road and for a particular loading of the car, but neither of us know how your (far from new albeit excellently maintained) car actually behaved during your test.