Tyre width vs grip

[Oversize]

With the discussion that started under the Red Bull thread, I thought I should create one on the subject that's been raised. Apparently grip vs width behaves in an inverse curve and I'm trying to determine the peak of the curve depending on weight and pressure....

This thread on another forum blows me away and it just goes to show that it's not a black or white answer. Sections have been written by what appears to be a tyre engineer.

http://www.gtr.co.uk/forum/119227-tyre-width-vs-car-weight-vs-grip-levels.html

I do know some experts in the field and I might ask some questions to see if any conclusion can be made.

 

[Oversize]

Funny that at the end of the thread one member decided to put exactly the same size tyres on a Skyline that I was going to put on Red Bull. Being that a Skyline must weight far less than the MB and I assume the Nissan still would have grip, I'd say the tyres would be fine on a 6.9.

I'm trying to get my head around some of the theory.... If one assumes that the wider a tyre, the more the weight of the car is distributed, it's logical that the force applied to a given area will be reduced. Could it be that it gets to a point where there's no force applied to a given area?? I doubt it. If this was true and you found tyres wide enough to go beyond this limit, then it'd be impossible to keep the car still as it'd keep sliding around!!!!! WTF?? I think that yes, the force applied to a given area may be reduced, but the overall downward force remains exactly the same....

There's two planes of forces that're can be applied to a tyre. Acceleration / braking and left / right.

Think about a drag car and how it grips. What factors determine grip? If width has nothing to do with grip, then why don't they just fit cheap 205s??

High speed cornering grip is dependant upon downards force vs sideways force. If sideways force is larger than downward, then the tyre has to lose grip (stating the obvious)...

My head hurts!

 

[Styria]

Mark, so does mine ! Talking about overtyring, which is easy to do. One sometimes has to ask themselves as to at what stage does one gain benefit from larger, lower profile tyres, and also if circumstances under EVERYDAY normal driving conditions will provide one with the desired ,and additional, roadholding. Personally, I would not want to go any bigger than the tyres on Gleaming Beauty now. I get all the roadholding I want, with the sort of driving I do... and talking about overtyring - I used to have a Husqvarna 420 with a huge Metzeler rear tyre - it was great for roosters, but absolutely dominated the handling. You could't slide the bike into a corner, all the tyre wanted to do was to stand up straight. You, Mark, have some difficult choices ahead of you. Have you yet decided in what order you're going to start working on the car ? I can see engine mounts on the menu - 6.9 units are expensive, 450s will fit but do not have the correct rubber density. Keep us posted. Regards Styria

 

[Tony66_au]

Don't forget that Mercedes steering geometry is special too so the bulk of the tyre stays on the road when cornering.............

 

[Oversize]

I'm still thinking this one through.... Firstly what's the best way to define 'grip'? Is it the fastest time through a given corner (or series of corners), highest lateral forces generated (G force), or the fastest 60 foot in a drag race??? In order to find a solution, we need a beginning....

 

[Styria]

Mark, just ask Michel to get involved with this thread. What he can do with a car is no Schumacher's business. I have been with him a few times, and boy when he twists that big toe (as distinct from turning the throttle), you know that something is happening. Regards Styria

 

[Oversize]

There's many tests I'd like to conduct regarding tyre / wheel modifications, because there is very little info available to the public. I believe many common and trendy modifications could lead to an increased safety risk, but without evidence it's all theory. And without resources we'll never know.... Maybe a TV network, motoring magazine, or news corporation could help?

 

[Oversize]

I guess this one's a little too complex for me to give a definite answer. So probably the easiest way to determine the best grip is to look at a current performance vehicle with similar characteristics and more importantly, similar weight.

 

[Oversize]

The current W221 S63 has a kerb weight of 2120kg, tare 2048kg and gross 2670kg. It runs 255/35R20 93Z (f) and 275/35R20 98Z (r).

It's obviously a bit heavier than the 6.9 at 1935kg (kerb) and 2420kg gross. The closest I could find is a S320CDI which has a kerb of 1955kg. It runs 235/55R17 all around.

According to the link below the HSV Grange has some similarities to the 6.9 and note the size of the tyres (245 f, 275 r).....

http://www.automobile-catalog.com/car/2012/1062530/hsv_grange.html

In fact other sites say the Grange is even lighter.... Based on that I still think I'll be getting 235/45R18 (f) on 8" wide wheels and 255/40R18 (r) on 8.5"s. On the other wheel set with the 9.5"s on the rear, I'll try 265/40R18 if I can find them.

 

[Oversize]

A car that would appear to be over-tyred is the C4 Corvette (1984 - 1996). Weighing only 1469kg the factory fitted 255/50R16 front and rear! And you don't see many of them aquaplaning off into the scrub.... In fact some people in the states are fitting 275mm (f) and 315mm (r)!!!!

 

[Afro107]

My opinion:

I would not use modern vs. classic comparison method for tyres.
Modern cars have ABS, ESP and a bucket load of those 2 and 3 letter acronymes that affect the whole handling/grip/tyre game. Also, modern tyres have rubber compounds with better grip per square of contact area, sidewalls are more adaptable to the camber changes too therefore tread have more uniform pressure maintenance... then there are shockers, spring rates and so on. Too many parameters are different on modern cars.

A car that would appear to be over-tyred is the C4 Corvette (1984 - 1996). Weighing only 1469kg the factory fitted 255/50R16 front and rear! And you don't see many of them aquaplaning off into the scrub.... In fact some people in the states are fitting 275mm (f) and 315mm (r)!!!!

There was a Top Gear episode where Richard Hammond drove the latest Corvette. It was quite informative funny and sarcastic about its handling.
I'd scrap Corvette specs as a valid input on multiple grounds.

I'm still thinking this one through.... Firstly what's the best way to define 'grip'? Is it the fastest time through a given corner (or series of corners), highest lateral forces generated (G force), or the fastest 60 foot in a drag race??? In order to find a solution, we need a beginning....

Grip, as I define it, is ability of tyre to resist slipping longitudinally or laterally AND to resist aquaplaning. Efficiency of such ability defines levels of it. I'd not directly link it to any of other vehicle performance measures (e.g. cornering speed) as grip is just one of the factors affecting them.

My rule of thumb is that with modern tyres, good shockers and perfect subframe mounts 10% wider than max originally recommended by AMG is acceptable for modern tyres at the not-so-expensive cost of braking performance and aquaplaning.

Modern tyre in max size recommended by AMG would have, I believe, balanced improvement in all the aspects of the grip. I am sure they did not randomly picked the width size.
Not all the tyres of the same size are the same; being that there's no ABS nor ESP, I'd pick the softest compound tyre with shortest treadwear for maintained grip trough the tyre lifespan.
But all of previous is near pointless if shockers, springs, subframe mounts and to some extent wheel alignment are less than perfect.