This is a complex subject, it is not only to do with the theoretical grip provided by the tyres, it is also to do with centre of gravity, weight transferance, and the ability of the chassis (suspension etc) to maintain an optimum contact between the tyres and the road.
Well, this was my thinking before I read the article - it must be a very complex subject. But the conclusion to the article showed that it was actually very simple. All car/tyre combinations gave almost identical levels of grip on corners. Run of mill family saloons and expensive sport cars - identical.
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>>Run of mill family saloons and expensive sport cars - identical.>>
Pull the other one...:-)
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What\'s for you won\'t pass you by
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Not many people cornering on dry roads get anywhere near the ultimate grip of the tyres (unless they're in the process of having a crash!). While a family saloon may have the same ultimate grip as a "sports car" on the same tyres (as they have the same size contact patch of rubber on the road), a car such as a Lotus Elise has a much lower centre of gravity and stiffer, better damped suspension.
Being driven at speed it will roll less and be less affected by imperfections in the road, so will feel safer and inspire more confidence, and therefore allow higher cornering speeds, before the driver "bottles it"!
Cornering on a test track until the cars lose traction is a different ball game to driving on UK roads.
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I don't think this can be true, as I remember that the Alfa SZ had fantastic levels of cornering grip meaning that, at the time, it could corner better than any other car (in terms of the number of g's before breaking away). If there was no difference, how could one be better?
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OK - first of all, look at the Suzuki Liana on Top Gear. Frequently, during cornering, it's up on 3 wheels. So there's less grip and more likelihood to slide.
Secondly, it's not about how much grip they tyre has on the road, it's about how much force the car puts on those tyres to make them want to slide in the first place.
The first thing you can do is to stiffen the chassis to stop the Liana thing happening.
Then you can lower the centre of gravity to stop it rolling and pulling 2 of the wheels off the road.
Then you can balance the weight of the car properly (they don't make supercars mid-engined for nothing) which stops too much g-force pushing on the front or rear wheels, keeping them all in check for longer.
A Porsche 911 (well an old one) will understeer like mad because there's no weight over the front wheels to make them grip. It will also flip around under tight cornering as the weight of the engine behind the back wheels will put more Gs on them and swing them around.
There are many aspects to car handling aside from the tyres.
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Life is complex; it has real and imaginary parts.
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The first thing you can do is to stiffen the chassis to stop the Liana thing happening.
BTCC cars do that three wheel thing all the time - come to think of it, so do F1 cars. Not sure if chasis stiffening is the answer to one wheel up - could be just the opposite.
HV
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Not many people cornering on dry roads get anywhere near the ultimate grip of the tyres (unless they're in the process of having a crash!). While a family saloon may have the same ultimate grip as a "sports car" on the same tyres (as they have the same size contact patch of rubber on the road), a car such as a Lotus Elise has a much lower centre of gravity and stiffer, better damped suspension. Being driven at speed it will roll less and be less affected by imperfections in the road, so will feel safer and inspire more confidence, and therefore allow higher cornering speeds, before the driver "bottles it"! Cornering on a test track until the cars lose traction is a different ball game to driving on UK roads.
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But are you saying that fitting wider tyres would give more grip in corners? I always understood that contact area was the same no matter what size of tyre was fitted to a given car - it is only the shape of that patch changes. Is this wrong?
I am sure that a test track is different from everyday driving, but it does provide a controlled environment. And the controlled environment showed that grip in corners was unnafected by tyre type.
I agree that a car with good feedback will give you much greater confidence in any circumstance, but the original article was relating to grip in corners - which was found to be pretty constant across all cars/tyres.
I'm not sure that center of gravity would come into it at the limit of tyre grip. A bus might topple before a lotus, but the tyre will lose grip at a certain point - low centre of gravity can't move that point, can it?
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From what I remember of the article the cars were driven in a circle at increasing speeds until they lost grip and broke away.
The variety of vehicles tested all broke away at a fairly consistant value of about 0,9g. The exception was the Porsche 911 where the tail broke first, and by applying power to keep the front "pushed" on track, a figure of about 1,1g was recorded.
Tyres need a load applied to grip. At "normal" road speeds the only load is the weight of the car. Cornering centrifugal force "throws" this weight towards the outside of the bend. When the value of this weight transfer approaches the weight of the car, it will lose grip.
This is assuming a dry, smooth road.
The results of the test seem to bear this out.
As a point of interest, a F1 car, at 120mph will produce a downforce of of about 2000kg. Add this to its weight of 600kg and it should, at that speed, be able to produce a cornering capability of over 4g
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From what I remember of the article the cars were driven in a circle at increasing speeds until they lost grip and broke away. The variety of vehicles tested all broke away at a fairly consistant value of about 0,9g. The exception was the Porsche 911 where the tail broke first, and by applying power to keep the front "pushed" on track, a figure of about 1,1g was recorded. Tyres need a load applied to grip. At "normal" road speeds the only load is the weight of the car. Cornering centrifugal force "throws" this weight towards the outside of the bend. When the value of this weight transfer approaches the weight of the car, it will lose grip. This is assuming a dry, smooth road. The results of the test seem to bear this out. As a point of interest, a F1 car, at 120mph will produce a downforce of of about 2000kg. Add this to its weight of 600kg and it should, at that speed, be able to produce a cornering capability of over 4g
So I wasn't imagining things then.
With F1 cars the downforce plays a huge part in getting them around corners at speed. Not much of that happening in most road cars, hence the consistent figures in the test.
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OK. Consider an extreme example where a Porsche 911 has an engine hanging off the back that's so heavy it lifts the front wheels off the ground. You have no grip and the car can't corner at all.
Now, add weight to the front of the Porsche until the front wheels are just touching the ground. You can corner at really low speeds, but anything over a couple of miles and hour causes it to slide at the front.
Similarly, imagine a 4-door front wheel drive (like an Audi) with the heavy engine hanging over the front wheels. Now the back wheels of the car are in the air. Add weight to the back and what you have is a car that slides from the back at more than a couple of miles an hour.
Most cars are a less extreme example of this phenomenon. One pair of tyres will give out before the other (ie, is the car prone to oversteer or understeer). But a well-balanced car will put the same force on both axles which allows good cornering.
This isn't the be-all and end-all of it, but it's an example of how the car design itself can affect the handling.
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Life is complex; it has real and imaginary parts.
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OK. Consider an extreme example where a Porsche 911 has an engine hanging off the back that's so heavy it lifts the front wheels off the ground. You have no grip and the car can't corner at all.
But I can't think of any car which has got more than 65% of it's weight on the front or rear, so it's nowhere near as extreme as your example. Add a driver and passenger and it's even more neutral.
The weight of the car and weight transfer due to the suspension and anti-roll bars are more significant.
As an aside, tyres need to slide to provide dynamic grip, and this happens at anything over 5mph for the steered wheels. Therefore the tyres are sliding long before you think they are!
What was the question again?
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>> But are you saying that fitting wider tyres would give more grip in corners? I always understood that contact area was the same no matter what size of tyre was fitted to a given car - it is only the shape of that patch changes. Is this wrong?
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The wider a tyre and to a degree the greater the diameter of a tyre then the larger the contact patch.
I'm not sure that center of gravity would come into it at the limit of tyre grip. A bus might topple before a lotus, but the tyre will lose grip at a certain point - low centre of gravity can't move that point, can it?
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The lower the centre of gravity the more even the load (from one side of the vehicle to the other) thus all four tyres are able to do some work, i.e. just before the double decker topples the inside wheels are not gripping at all because there is no weight on them.
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But are you saying that fitting wider tyres would give more grip in corners?
No I wasnt saying that; but I did think a wider tyre had a bigger contact patch, I thought it had the same length patch but wider. I thought the length of contact patch varied with car weight or air pressure in the tyre?. But I dont know, can anyone help on this one?
And the controlled environment showed that grip in corners was unnafected by tyre type.
I wasnt disputing what you said, as I havent read the article. :o) I was just going on to suggest that although a 225/45 tyre may have the same grip on two different cars, that grip is likely to be more usable in a sports car with better handling characteristics.
I mean that even if they have the same grip, on real roads you might be able to comfortably use 90% of that grip in a sports car, but only 75% in a family car, because it doesnt inspire the same confidence.
I used to have a Peugeot 405 Mi16 which had 195/50x15 tyres; it was fast (no sports car) and did have well sorted suspension.
After that car I mistakenly bought a Mk3 Golf 1.8CL, on which the handling was so bad I fitted it with alloys and 195/50x15 tyres. Even on the same size tyres, the soft suspension and vague steering meant you could not corner anywhere near as fast as in the Pug. It would roll and lurch and you had to take corners slower.
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.I used to have a Peugeot 405 Mi16 which had 195/50x15 tyres; it was fast (no sports car) and did have well sorted suspension. After that car I mistakenly bought a Mk3 Golf 1.8CL, on which the handling was so bad I fitted it with alloys and 195/50x15 tyres. Even on the same size tyres, the soft suspension and vague steering meant you could not corner anywhere near as fast as in the Pug. It would roll and lurch and you had to take corners slower.
The 405 was renowned for its handling and the Mi16 even more so, being 4 wheel drive (I think). I remember a edition of Grandstand, where some saloon car racers compared a standard road going model, of the marque that they raced in competition, with their race prepared car. The chap who raced the 405 was very impressed with the bog standard 405's handling. I seem to remember that the Toyota Carina GT came well out of the test too.
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The 405 was renowned for its handling and the Mi16 even more so, being 4 wheel drive (I think).
The standard 405 Mi16 was front wheel drive but peugeot did do an Mi16x4 which was 4 wheel drive. These do handle well as it's extremely difficult to lose grip in them.
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>> The 405 was renowned for its handling and the Mi16 even >> more so, being 4 wheel drive (I think). The standard 405 Mi16 was front wheel drive but peugeot did do an Mi16x4 which was 4 wheel drive. These do handle well as it's extremely difficult to lose grip in them.
Yes my Mi16 was the 2 wheel drive one, and it handled better in the dry than the Audi 80 quattro I had before it. I was advised to avoid the Mi16x4 because IIRC it had rear suspension that was prone to seize up if not maintained carefully.
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Yes the Mi16x4 is a bit temperamental - it shares the rear suspension with the Citreon BX.
www.pughosting.co.uk/forum/showthread.php?t=43707
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