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[professor]

I read this article and found it very illuminating! It dispelled some of my misconceptions about the presumed "benefits" of wide tires and the assumed correlation with a higher contact patch area they provide. Ceteris paribus, wide tires do not have a larger contact patch; they provide a different shape to it.:iono:

Many of you may already know this, but for those like me that are newbies and want to think scientifically about what sort of tire setup to have this is perhaps one more crucial issue to be aware of.

I would be grateful if someone can suggest whether I have read the science behind this incorrectly! Barring that I summarize this as a rule of thumb - If you want to drive spiritedly do not get the widest possible tire your wheel can take.

[modbargains]

There is a different between "over all grip" and various other set-ups. A wider tire will give you more contact patch and in coverage of area but does not also mean more grip, the grip really lies in the compound of the tire itself. The width of the tire is also relative to the entire wheel and suspension set-up.

Someone please do correct me if I am wrong.

[professor]

Originally Posted by **********s

There is a different between "over all grip" and various other set-ups. A wider tire will give you more contact patch and in coverage of area but does not also mean more grip, the grip really lies in the compound of the tire itself. The width of the tire is also relative to the entire wheel and suspension set-up.

Someone please do correct me if I am wrong.

the bits in bold: no it won't, at least not according to the article! it will only change the shape of the contact patch maintaining area ceteris paribus (holding pressure and other variables constant). that is the most counterintuitive yet brilliantly obvious piece of information in the article!

the bits underlined: +1 i'd like to know this too and especially how camber affects the contact patch (would a 3 degree camber on a 235 convert it to a 225 contact patch - surely there has to be some correlation to this albeit weaker than that!?!). again something nobody seems to have discussed (at least not in layman's terms!).

[Berduderunner]

im pretty sure thats because when u squeeze a wide tire on a smaller rim.. the tire contact patch curves out and is no longer flat to the road.

[WAY]

Prof, that is rubbish. Wider tyre = bigger contact patch with the right tyre pressure, espeically under load. As for camber, a 235 with -3 degrees will have less contact patch than a 225 that is flat to the ground I would imagine. But this obviously changes when it is loaded up.

[WAY]

Originally Posted by Berduderunner

im pretty sure thats because when u squeeze a wide tire on a smaller rim.. the tire contact patch curves out and is no longer flat to the road.

Ah, if that is what the article is refering to, then yes it makes sense...

[professor]

Originally Posted by WAY

Prof, that is rubbish. Wider tyre = bigger contact patch with the right tyre pressure, espeically under load. As for camber, a 235 with -3 degrees will have less contact patch than a 225 that is flat to the ground I would imagine. But this obviously changes when it is loaded up.

not rubbish at all wayne. you ignored my very important ceteris paribus condition and then changed two variables in your response: 1. load and 2. tire pressure. therefore, sorry but my original summary still stands that with those variables unchanged wider tires will not give you a larger contact patch area just a different shape!

if you are correct that 235 with 3 deg camber gives less of a contact patch than a 225 that is eye-opening!! can someone verify that?

[professor]

Originally Posted by Berduderunner

im pretty sure thats because when u squeeze a wide tire on a smaller rim.. the tire contact patch curves out and is no longer flat to the road.

i am not sure that is what the article is saying! what it is saying is that fitting as wide a tire to what the wheel can take is misguided because it compromises the sidewall and thereby cornering ability.

[AussieRacer]

I haven't read the article, but prof you need to remember that during cornering a significant weight transfer occurs which places greater downward pressure on the tyre. That is combined with lateral forces, which will at certain levels will cause a slip angle (contact patch slipping/dragging across the surface). Maximum slip angle is achieved by a bunch of variables, including sidewall flex, tyre pressure, surface grip, rubber compound, tread depth, etc... On a car that has significant weight transfer (often can be described as a very understeery car) the outside tyre simply has too much downward pressure which distorts the contact patch (by reducing width from inside tyre to out), which reduces the slip angle and overloads the tyre very quickly. A wider tyre should maintain the contact patch longer as the sidewalls start to give way under the increased load, and hence have greater stability in the slip angle giving more grip.

Of course the same applies under braking and acceleration.

So whilst a wider tyre may have the same size contact patch by what you are saying, that shape is VERY important!

[professor]

thanks for that aussieracer. you are a bonafide genius! that is what i was looking to get straight in my head!! of course!!!

the lateral vector in turns will traverse a wider contact patch more slowly thereby becoming less likely to distort the perimeter formed at two edges by the side wall. a narrower one will not achieve that, and even latitudanal g-force vectors would be limited by the radius of the wheel and those other variables you mention...

i guess when the ceteris paribus condition i imposed in my proposition is lifted, one really must be aware of all those variables you mentioned and the article actually does cite some of the them!

thanks again.

[WAY]

Prof, you are really funny. This is why you are a Professor. You like the theories, whereas I just like to drive the crap out of my car and let my fingers and bump do the feeling instead of getting into the theory. Of course I also need to understand the basic theory so that I understand what I am feeling, but not the way you get into it. Lol.

[whiplash]

I've always loved this debate! It's come up on every enthusiasts site I've ever seen and no matter what, people will just never agree. If you look at just the pure physics of it, it's pretty clear (I started my professional career as a mechanical engineer with a focus on hydraulics and pneumatics so I love crap like this!). If we assume a 3000 lbs car and put 30 PSI in each tire...it equals an area of 100 square inches of contact patch. It doesn't matter if you have 215's or 275's on the car...it still equals 100 square inches, roughly the size of a sheet of 8.5 x 11 paper. As was mentioned, what changes is the shape of the contact patch.

Like Aussieracer said, there are a lot of variables that come into play...sidewall stiffness being one of the biggest. Our run flats will maintain a fairly consistent contact patch even at zero pressure because of the sidewall but for sake of this thread, I think we need to assume we're talking about non-RFT's.