DarkStar66

I couldn't believe one of the the replies to a letter in Autocar (25/1/05).

Someone pointed out that there wasn't much difference in braking distances according Autocar stats. In fact they say that a LR Discovery TDV6 HSE (2718kg) stops in less distance than a Porsche Carrera GT (1472kg)!!!

The reply was that the Discovery's braking performance is very impressive and it's probably a good thing that all cars stop roughly in the same distance as otherwise it would cause more motorway pile-ups.

All cars stop in roughly the same distance!!! Have Brembo and Alcan been pulling the wool over our eyes or is the Autocar editor talking complete b0ll0x to cover up some serious errors in their famously accurate stats?

Or maybe they mixed up the stats between the Porsche Carrera GT and the Cayenne?

AvalancheS8

It depends what speed the braking distances are from though. Ultimate retardation will always be limited by the grip of the tyres, which doesn't vary with speed, and will probably not vary all that much from car to car when talking about road cars. Most cars can reach the limit of the tyre's adhesion under braking from low speeds, using the stock, weedy brakes, but not so many could stop from 120 mph, on the limit of tyre adhesion all the way to standstill, and once you repeat that test a few times the number that can do it will fall even further. The advantage of big brakes is primarily that they can absorb and dissapate more energy, more quickly, so they stand up far better to hard stops from high speeds and repeated heavy applications. If a car can lock it's wheels with the brakes then you can't improve the maximum peak braking available by changing only the brakes. You can probably improve the feel and fade resistance, and performance at high speeds, but not the peak braking available, that is the point at which the wheels lock.

So, if a big heavy 4x4 can lock up it's (probably very large) tyres on a single stop from relatively low speeds, it should be able to produce quite a short stopping distance. After all, although there is lots of weight to stop, it also has plenty of weight available to push the tyres onto the road for friction. It will have put a huge amount of energy into the brakes doing it though, so if you did repeated tests, it would probably fade terribly badly.

DarkStar66

You make some good points. It depends on how much grip the tyres can generate but I still can't see a Porsche taking longer to stop than a big 4X4.

It was 70mph to 0mph BTW.

dazc

A big 4x4 will not stop quicker than a Porsche, ever.
That is why the stopping distances have not changed on the highway code even though braking standards generally have improved, simply because 4x4's braking distances are so poor compared even to an average road car.
You can't change the laws of physics. 2 tons to stop in the same distance as 1 ton = twice as much braking effect. Its never going to happen. Daz

PS: Alcan recycle tin cans.
Alcon make brakes. ;-)

DarkStar66

Do you think there is a connection?

AvalancheS8

Thing is, the laws of physics say that the friction force is simply the co-efficient of friction multiplied by the force pushing the surfaces together, so if the 4x4 has the same rubber compound in the tyres, and therefore the same co-efficient of friction as the Porsche, then having twice the weight allows it to produce twice the friction force at the tyres.

It's then just a case of F=ma, where F=force, m=mass and a=acceleration. Since F and m have both doubled, a is the same, so the potential maximum retardation is the same in both cases.

BUT, I agree, in practise it's not quite so simple and the 4x4 will very likely take longer to stop, probably due to the fact that it doesn't have the same tyre compound as the Porsche, the suspension doesn't do such a good job of keeping all 4 tyres on the road and working hard, and it's not going to be easy to produce brakes for the 4x4 that are probably smaller diameter than the Porsche ones, cost less, and can still produce twice the braking force without fading. If it was, they'd be fitted to the Porsche....

On a single stop though, a modern, quality 4x4 with very good brakes and good suspension set up might get quite close to the Porsche.

The other thing that occurs is that the Carrera GT will very likely have brakes that work best hot, whereas the 4x4s will be best cold, so if they did 1 stop, or let the brakes cool between the Carrera will be giving it's worst performance and the 4x4 it's best. I wouldn't be surprised if doing several successive hard stops would bring the distance down each time for the Porsche as the brakes bedded in and heated up.

Tim-H

I have an old lwb Isusu Trooper, I have also had land rovers , and most of the rest of those type of things, ( all old crap for towing ), I find with them that it is not the grip / brakes, ( put 3 ton on the back of the trooper , still stops well ) It is the high center of gravity , they stop to a certain extent but once the weight transfer goes "over the top" instead of pushing the tyre into the tarmac it starts to pull it up and forwards, it "skates" along ( hmm how do you describe it, ? feels like you are going fall on your face?? :-)
Where as in the sccob the extremly low center of gravity is one of it`s best design`s, feels like the car is going to bury itself in the tarmac,
Have a good one
Tim