Query / Advice re: new tyre on Vauxhall Astra

Stooby2

AdrianC wrote:

ABS intervenes when the tyres lock. It releases, then reapplies the brakes. ALL OF THEM. Not just the one wheel that's locked

For very early ABS systems, that would be correct, but they very quickly moved to ABS across the axle. However modern systemsare very sophisticated and do indeed release the brakes on individual wheels, but, momentarily. You can actually feel it pulling the steering slightly if you skid, especially across road repairs or differing road surfaces under different wheels.

A modern ABS system continuously monitors each wheel speed and if one locks, releases the brake and reapplies it again. If the wheel locks again, the ABS releases it, reapplies it. If the wheel rotates, it will maintain the pressure to keep the wheel at maximum braking. And it does that for each wheel, at about 15 times a second, simultaneously.

This is quite a good article on it and explains in much greater depth.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/anti-lock-brake1.htm

AdrianC

Stooby2 wrote: »

For very early ABS systems, that would be correct, but they very quickly moved to ABS across the axle. However modern systemsare very sophisticated and do indeed release the brakes on individual wheels, but, momentarily. You can actually feel it pulling the steering slightly if you skid, especially across road repairs or differing road surfaces under different wheels.

A modern ABS system continuously monitors each wheel speed and if one locks, releases the brake and reapplies it again. If the wheel locks again, the ABS releases it, reapplies it. If the wheel rotates, it will maintain the pressure to keep the wheel at maximum braking. And it does that for each wheel, at about 15 times a second, simultaneously.

So, according to your theory, if you have one side on a very poor surface, and the other on a good surface, the stopping distance will be determined by the friction on the good surface, rather than the poor surface?

Go on, then. Try it.

Stooby2

No, the ABS will react to the wheel that is decelerating in relation to the others. So if one wheel is locked on ice whereas the other is on good tarmac, it will release and reapply the locked brake only. It is not just about shortening braking distance, it is also about allowing the wheel to turn and maintain a level of lateral stability so that a degree of steering can be maintained.

Read the article, Google it and read a few more, it's quite interesting as to how much the ABS system does.

Jamie_Carter

sinbad182 wrote: »

I assume you know more on the subject than me so I'll take your word for it, but the way your average motorists drive around town (ie. on autopilot) I cant help but think the tyre wouldnt make any difference in a freak, random occurrence like that. I'd imagine the sophistication of the cars ABS system would probably be more important.



Have to disagree here - again, looking at your average motorist doing a mixture of pootling round town and the odd dual carriagway/motorway, there is no way they are going to notice any difference between tyre brands. I certainly don't, and im a bit more into cars than some. Of course, this will be different for a real motoring enthusiast, or someone with a performance car, or who drives 'progressively'.



I'd have assumed the same too, so thats pretty shoddy practice!

The tyres can actually make a huge difference to stopping distances and handling (even driving around town).


I did a lot of research before replacing the tyres on my Vectra last year. In the end I went for the best combination of price, grip in the wet, fuel economy, and Auto Express test results. In the end I opted for Goodyear Efficient Grip.


I was amazed by the extra grip in the wet that these tyres provided, over the previous Bridgestones. You could really notice this when pulling out of a junction in a hurry, when previously it would be extremely easy to experience some wheel spin. But with the Goodyears there is virtually none.

Jamie_Carter

Stooby2 wrote: »

For very early ABS systems, that would be correct, but they very quickly moved to ABS across the axle. However modern systemsare very sophisticated and do indeed release the brakes on individual wheels, but, momentarily. You can actually feel it pulling the steering slightly if you skid, especially across road repairs or differing road surfaces under different wheels.

A modern ABS system continuously monitors each wheel speed and if one locks, releases the brake and reapplies it again. If the wheel locks again, the ABS releases it, reapplies it. If the wheel rotates, it will maintain the pressure to keep the wheel at maximum braking. And it does that for each wheel, at about 15 times a second, simultaneously.

This is quite a good article on it and explains in much greater depth.

http://auto.howstuffworks.com/auto-parts/brakes/brake-types/anti-lock-brake1.htm

You are quite right. And there are also some systems where the ABS works on wheels that are diagonal to each other on the vehicle. However AdrianC is also correct in stating that stopping distances are determined by the grip between they tyres and the road surface, and not by ABS. In fact a skilled driver can stop in a far shorter distance with ABS turned off. ABS was introduced because many drivers would attempt to steer and brake at the same time. The ABS system would allow the wheels to keep turning so that the driver would still be able to steer whilst braking hard, rather than locking wheels, and just sliding in a straight line.

sinbad182

Jamie_Carter wrote: »

The tyres can actually make a huge difference to stopping distances and handling (even driving around town).


I did a lot of research before replacing the tyres on my Vectra last year. In the end I went for the best combination of price, grip in the wet, fuel economy, and Auto Express test results. In the end I opted for Goodyear Efficient Grip.


I was amazed by the extra grip in the wet that these tyres provided, over the previous Bridgestones. You could really notice this when pulling out of a junction in a hurry, when previously it would be extremely easy to experience some wheel spin. But with the Goodyears there is virtually none.

You have a very different experience to me then - my Insignia had expensive Continentals on it when it was a company car, once I bought it and had to fund replacements myself it has been treated to a beautiful set of Kumho tyres.

I've noticed absolutely zero difference in town, in all braking situations. Perhaps this is because I live in London and cant drive as fast due to permenant congestion, or perhaps you're more tuned in to cars performance than me. Either way, personally, I think paying for expensive tyres on bog standard cars which spend most of their lives farting about town is a waste of money.

My last car was a Lexus IS250 - that had expensive tyres on it, which I was pleased for as it spent a bit of time getting booted on the motorway, and even a couple of 'progressive' drives across Europe! I didn't pay for the tyres, but if I did, in that case I would have paid for high quality ones as it was a fast car that spent time being driven that way on motorways.

Jamie_Carter

AdrianC wrote: »

So, according to your theory, if you have one side on a very poor surface, and the other on a good surface, the stopping distance will be determined by the friction on the good surface, rather than the poor surface?

Go on, then. Try it.

The stopping distance will be determined by the total amount of friction from all 4 tyres. If the tyres on one side are on tarmac, and on the other side they are on mud (with virtually no grip), then the total friction will be almost halved. So the stopping distance will be almost double.

AdrianC

Jamie_Carter wrote: »

The stopping distance will be determined by the total amount of friction from all 4 tyres. If the tyres on one side are on tarmac, and on the other side they are on mud (with virtually no grip), then the total friction will be almost halved. So the stopping distance will be almost double.

Overly simple, I think.

The tyres on the "good grip" side will have the same amount of grip.
The tyres on the "poor grip" side will have much less.

If the theory that ABS only pulses the "poor grip" tyres is correct, then the tyres on the "good grip" side will still have exactly the same maximum braking effort. Sure, they'll be slowing a greater proportion of the car's weight, but that won't have a massive effect on the braking distance, although it will give them much more heat to dissipate.

Jamie_Carter

sinbad182 wrote: »

You have a very different experience to me then - my Insignia had expensive Continentals on it when it was a company car, once I bought it and had to fund replacements myself it has been treated to a beautiful set of Kumho tyres.

I've noticed absolutely zero difference in town, in all braking situations. Perhaps this is because I live in London and cant drive as fast due to permenant congestion, or perhaps you're more tuned in to cars performance than me. Either way, personally, I think paying for expensive tyres on bog standard cars which spend most of their lives farting about town is a waste of money.

My last car was a Lexus IS250 - that had expensive tyres on it, which I was pleased for as it spent a bit of time getting booted on the motorway, and even a couple of 'progressive' drives across Europe! I didn't pay for the tyres, but if I did, in that case I would have paid for high quality ones as it was a fast car that spent time being driven that way on motorways.

Budget tyres can vary massively. Some can be very hard compound, and give very little grip. Others can be very soft compound, and give excellent grip, but won't last very long.


I actually went for what I considered to be the best compromise for my vehicle, at a reasonable price.


I've actually had continentals previously on my old MK2 Astra that were terrible. But I can't remember what model they were.

AdrianC

Jamie_Carter wrote: »

I've actually had continentals previously on my old MK2 Astra that were terrible. But I can't remember what model they were.

I had Conti PremiumContacts on the Saab 900 T16 - very decent grip, far better than the Mich EnergySavers that preceded them, and not far different from the Mich Pilot Primacies before those. But they wore much, much more quickly.

The biggest problem running older performance cars these days is getting good enough tyres in the right sizes.