Safe Speed Forums

I've read quite a few articles about new on the rear and though, of course, the tyre industry want to sell tyres I can't see them suggesting anything that the ambulance chasers could jump on. And anyway won't you get more miles out of new on the rear route?
There is method in this madness. You put a lot more heat into the fronts - espec. FWD - and the deeper the tread the more heat as the rubber moves about. It's quite possible to cook new tyres so they're never quite as good as the manufacturer intended. A part worn rear will have been through lots of gentle heating cycles and being part worn you won't be getting as much heating from the blocks do the rumba. They'll wear slower as well as a consequence..

As for dry grip on worn tyres many saloon race series have a road tyre everyone must use and for dry races they have the new tyres shaved to around 3mm for the reasons above. Much less than this and you're onto the carcass compound.

Barkstar

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Why don't you replace the back ones as well.

Maybe I'm just too naive or not cynical enough but is it fair to say that the tyre companies just recommend whatever sells them more tyres? They are all working on run-flats which will (one day, I'm sure) mean they only get to sell 4 tyres per new car instead of five!

Besides that, I'm not sure about buffed-down tyres for race cars being as a result of wanting to minimise heat build-up. I think it's more to do with minimising tread block distortion so that they feel more precise. A couple of heat-cool cycles seem to bring tyres out at their best and then subsequent ones all seem to harden the rubber further and reduce grip. Generally in clubman racing, a new set of tyres for practice and then using the same tyres for the race seem to give best results. After three or four races, it doesn't matter how much tread is left on them, they just don't seem to grip well at all.

Have you seen the price of runflats? Not to mention the fact that when one collects a puncture you (obviously) have little choice but to drive home on it, which then generally renders it as scrap, so they then get to sell you another double price tyre instead of a £10 repair...

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It's a bit of both. You'll get more feel with less tread to move about but as important, it not more so, if you run full treads you'll overheat the tyres - were talking road tyres here - and quite easily kill them in a lap. It's way too much of a generalisation to say a clubman gets the best results using a new set for practice and then racing on them.

Barkstar

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I'm not sure if this is something you are aware of, although some of your replies have touched on it, but cars steer because of the braking effect of increased friction against the road when you turn the wheels. This is why low profile tyres give a better steering response, becaus the lower profile is innately stiffer (there is less sidewall to flex), and so less of the braking effect is lost in things like flexing and heat generation.

If your new tyres have either a lower profile, or a stiffer compound than the old, they will create the effect you describe, and i dont think there is anything you can do to get rid of it. It should be possible to check by lowering the pressure in your new tyres and seeing if that reduces the effect.

On the subject of aquaplaning,i have seen an article about low profiles being less efficient in the rain than normal tyres-unfortunately i cant remember where, but the argument is that because they are generally wider, there is less pressure on the contact patch of the tyre.

Lawman

The new tyres on rear idea is clearly the best default choice in a one-size-fits-all world.

We don't live in a one-size-fits-all world though so lets leave room for personal preference where people know enough about the subject to express a preference.

Now, in a vain attempt to bring things back on topic.



Is it just more bite or some kind of delayed reaction?

Didn't you have a similar problem before?

I'm tempted to say the new tyres simply have better response but if it's catching you out then maybe they aren't biting initially as the cornering forces build something is acting as if to apply more lock, similar to the way passive rear steer systems work.

The effect has gone (I think) after about 3,000 miles. Possibly I'm subconsciously 'driving around it', but I don't think so. I think it's still more nervous / less stable in a high speed straight line (which is really the main reason that I DON'T think I'm driving around it).

The 'delayed reaction' theory is closest to the way it felt.

Possibly it was simply 'sharp shoulders' on the new front tyres.

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Paul Smith
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Is it? New on the rear means the certainty of longer wet braking distances with only the increased risk of oversteer.

New on the rear is clearly best in a litigious world. But we know well that the law and safety are drifting off in opposite directions. When you 'can't stop in time' it's always the driver's fault. But when the car 'does something funny' then there's the possibility of blame.

But I can't say which 'one size fits all' recommendation I'd make for safety reasons. I don't have enough information about the numbers caught out by oversteer, or the severity of their crashes.



I don't recall posting about any similar problems, or experiencing any in the last few years. But over the years I've experienced various effects due to worn bushes including rear steer / passive steer behaviours.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
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eh ?

I think I know where he's coming from but "braking effect" is an odd term to use. Maybe he means that a tyre with zero rolling resistance would also generate zero grip?

Is it the case that for a wheel to be generating "grip" (in any direction) it MUST be sliding relative to that surface? That can't quite be right because otherwise everyone who parked their car on a hill in gear with the handbrake on would find it at the bottom the next morning but I think it's true to say that (static or "limiting" friction exceeded) there needs to be some sort of slip angle before "grip" can be generated?

yes to get force you need some kind of slip, either longitudinal, lateral or combination of both.

(so how come cars with ackerman geometry get round corners then?
)

Nah, surely not. Non-elastic systems provide force without slip (think rack railway). So slip arises because of the force.

i.e. force gives rise to slip.

If slip gave rise to force, then we'd be forever slipping and never getting the force.

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Paul Smith
Our scrap speed cameras petition got over 28,000 sigs
The Safe Speed campaign demands a return to intelligent road safety

ummm.. arent tyres an 'elastic system' ?

Couldn't agree more, speking as someone who had a competition licence before I was old enough to drive on the roads, and used to race Karts, I was brought up with oversteer.

As a rule though, I think campers tend to understeer, sometimes visciously so as did my first camper, a T3 VW conversion.

But in a camper, you tend not to corner fast enough for it to matter, even though I know following car drivers sometimes get frustrated by how much I slow down for corners in the van.

'cause they're both slipping!

Actually, a funny but true story on this one. YEars ago, I worked for a sports car manufacturer. One day, having removed the one-piece front end and whilst standing on top of the engine (for a reason I can't remember!) whilst someone turned the steering, I thought I noticed it had NEGATIVE Ackerman! On checking it out, we found that (quite by accident), it DID! As the car handled just fine, we left it like that and a long time later, I learned that it was a fairly common trick to get a car to "turn-in" a bit better. Get the front tyres to "fight" each other so that they are both generating some lateral grip before the car starts ti turn (as it were) by having some static toe and then as you start to wind the lock further on, the outside tyre "wins" all of a sudden and the car turns in a bit more crisply.

I'm by no means a ride & handling expert but I think that's how it worked!