Do wide tires get better traction than narrow tires?

[buterworth]

I've heard that wider tire doesn't mean more grip or more traction.
But why powerful cars ,especially RWD cars, always use very wide tires on the driving wheels?

[XKRacer]

IMO hell yeah.......If you have a powerful car (RWD) with skinny tyres, it will just sit there spinning away until it meets the rims and starts eating its way through those too

Most of the cars I do have 285's on the back to give that bit of extra grip particularly when you are talking BHP of 400+

Of course there are down sides, drag, aqua plane, tram lining, road noise but on a track most of those dont apply, I would put happy put up with a bit of drag for more grip.

I will be using Yokos on my track car 285/40/18 rear and 245/45/18 front but then I gotta hold onto a possible 700bhp

[jdd]

I’d be interested to see if anybody can respond with a technical answer to this question.
You would think there would valid performance reasons but when you’re talking about contact pressure on the road surface for a given area it really doesn’t change as the weight of the car doesn’t change (ignoring aero).
There are reasons why narrower tyres are better in some scenarios (eg snow), but wider tyres have a number of disadvantages, some of which are mentioned above.

[Chris Y]

I think it's to do with the 'stiction' of the tyres, and basically the amount of rubber contacting the track/road. More rubber down gives the tyre more area to flex, and that's what generates the grip (and heat).

Possibly.



I seem to recall there was some research into not just the size of the contact patch, but also the *shape*. I.e. a wide, small circumference tyre produces an oval contact patch, which may have been better than a more circular patch.

[phoenix]

As technical as I can get.....

The contact patch on a wider tyre will be shorter but wider for the same load and tyre pressure, but the contact area will be the same size for both tyres.
A wider tyre has a larger contact patch only if the tyre pressure is lower than in the narrower tyre. This is an important fact to remember.

With a larger contact patch the contact pressure per square unit is lower than with a narrower tyre. The narrower tyre also has a contact patch which is long and thin - i.e. it extends round the rim of the tyre, thus increasing the amount of effective tread area in that direction. In snow (and mud even) the higher contact pressure and the longer contact patch helps the tread pattern penetrate the road surface, compacting the snow/mud into the tyre tread and thus increasing the grip; in the same conditions a narrow tyre with no tread would spin like crazy, of course.

As said above, with the correct inflation pressure the larger contact patch on a wider tyre means a lower contact pressure. This means that the tyre can suffer more load added to it before is loses grip than a tyre with a smaller contact patch. This extra load, in racing, comes from braking, cornering forces and to a lesser extent acceleration loads. Of course, you might expect a tyre with a larger contact patch to last longer than a narrow tyre, because of the lower loads per square unit, so read on.......

Because the relationship between the grip level given by a tyre and the load on the tyre is not a linear in the performance envelope of the tyre we are interested in - i.e. right at the limit - there is no simple formula to explain clealry grip versus load. A chap called Pacejka did his best to model this relationship mathematically and his maths has been used in simulators and computer games. If you are into very complicated maths, look him up!

But the real benefit of wider tyres is that because the contact patch is larger, the inflation pressure is lower, allowing the rubber to deform and follow more closely the small variations in the track surface, and a softer rubber compound can be used to give the same rate of wear without the temperature in the tyre increasing to a level where the rubber deteriorates. The same loads on the same rubber compound in with a smaller contact patch would over-heat, which first loses grip and then destroys the tyre tread completely.

Softer rubber has a higher coefficient of friction, and that is where the man gain in wider tyres comes from. But the setting of tyre pressures in wider tyres becomes critical, as that is what balances the size of the contact patch and hence the pressure per square unit of the contact patch with the road, and the shape of the contact patch.

Too much pressure in the tyre and the contact area shrinks, which loses grip. Too little pressure in the tyre and the shape of the contact changes for the worse, which again causes a loss of grip.

Tyres and grip are very complicated subjects. Please consider what I have written as only a very simplistic overview of what is going on between your tyres and the track.

[BertMk2]

There are all sorts of things to factor in here: tyre width (obviously), power, weight of the vehicle, torque, suspension set up, type of surface being driven on (ice tyres for example are very narrow to concentrate the weight over a small area).

Just bolting on a wider tyre isn't necessarily going to make you go quicker - the car will need to be set up to work with those tyres, if you've got wider tyres but can't get them up to temperature then you're not going to gain anything. Also an increase in tyre grip can lead to problems with transmission and driveshafts as suddenly the load going through these is increased too.

[tristancliffe]

My understanding of an aspect of it:

The theory that a wider tyre won't give more grip is because basic mechanics of smooth body friction (what we are taught in schools) doesn't use contact area as part of the equation. F=mu*r, where F is the force due to friction, mu is the coefficient of friction between the two surfaces and r is the load normal to the contact patch. On this basis it would be impossible to turn, accelerate or brake at more than 1g.

But real life isn't "Smooth Bodies" (except, arguably, on catwalks). The tyres are not merely using tangential friction, but digging into the surface of the road and pushing directly against it. There is also a chemical bonding function that nobody ever goes into any detail about and just assures us it happens (if anyone knows more about the actual chemical bonds that occur I'd love to know more in detail).

As such, more area DOES give more grip.

The shape of the contact patch defines the directions in which the grip can be generated. A wide, small diameter tyre will have a wide, thin contact patch, and will provide grip in a different way to the same area that is narrow and thick (if you see what I mean), although I think it's unfair to suggest that wide&thin = lateral and narrow&thick = longitudinal. But F1 cars use wide tyres when not regulated away from them, whilst dragsters use narrower taller tyres to get good straight line performance. Obviously drag, wear, heat management and so forth will play their parts in the choice too.

But essentially, and ignoring variables that could make the opposite true if you wanted it to, like silly camber angles, wider tyres give more grip.

[dtype38]

Originally Posted by tristancliffe

There is also a chemical bonding function that nobody ever goes into any detail about and just assures us it happens (if anyone knows more about the actual chemical bonds that occur I'd love to know more in detail).

Good question tristancliffe, and I can't imagine why it's such a secret . There are obviously chemical bonds between the long chain carbon molecules that make up the rubber, and the different layers of rubber compound within a racing tyre are bonded together. Put that idea together with the something like say, the near instant rubber bonding qualities of cyanoacrylate glues, and I guess it doesn't take much imagination to envisage producing rubber with some "glue like" properties. It would be like attempting to get the tyre to literally "glue" itself to the road as it made contact. The obvious trick would be to control this to avoid too much drag or tyre wear, but with the budget in F1 to play with you probably come up with something effective. Actually, if you made this extra adhesion property temperature sensitive then it could make it only become active when you got your tyres hot.... but get them too hot and they might stick too well and pull chunks of rubber off the surface. I wonder if that's what's now called "graining".

[JohnD]

And, that tracks can be 'green', actually new, or not used recently, been rained on, etc. As a race or race meeting goes on, the tiny layer of rubber accumulates on the tarmac and changes the adhesion, presumably as tyre and track rubber stick together as dtype suggests.

John

[XKRacer]

Wow And there was me trying to keep it simple

[JamesH]

An acquaintance fromm a while back, who now works in F1, told me the story of a BTCC car (or whatever it was then) being wheeled in to the garage one evening on its slicks. In the morning they tried to jack it up but the wheel stayed on the ground - its just stretched the shocks/spring. Eventually they pinged off. They had effectively glued themselves to the floor. They took the paint off....

[Adam43]

Originally Posted by phoenix

The contact patch on a wider tyre will be shorter but wider for the same load and tyre pressure, but the contact area will be the same size for both tyres.

I thought that it only got shorter if you increased the diameter? (pressure notwithstanding).

Originally Posted by phoenix

With a larger contact patch the contact pressure per square unit is lower than with a narrower tyre.

A similar situation to disc and pad contact area for bigger brakes.



When increasing tyre size consider the wheelwidth size too, this can effect the contact width too.

[JamesH]

Presumably it gets to a point where the contact pressure per sq unit is so low, that in fact the tyre doesn't generate enough friction heat to start to work properly?

[greenamex2]

One benefit is wider tyres offer better heat managment. So you can have more stable performance and/or use a softer compound in the same situation.

[phoenix]

Originally Posted by JamesH

Presumably it gets to a point where the contact pressure per sq unit is so low, that in fact the tyre doesn't generate enough friction heat to start to work properly?

Precisely what happens to an F1 car behind the pace car.....

[phoenix]

Originally Posted by Adam43

I thought that it only got shorter if you increased the diameter? (pressure notwithstanding).

I assure you it's a fact...

[gttouring]

racecar engineer had an article on this i believe some years back...
so far all comment so good, there is something about acceleration (not lateral grip where width has the edge) a narrow tyre will do better, hence the ballon induced behavior of Drag tyre construction..
but i still don't "Get it" even though i understand what is being said.
it might be magic

[Adam43]

Originally Posted by phoenix

I assure you it's a fact...

Thinking about this I didn't mean increase the diameter, that would actually work the other way. Anyway.

I'm not disagreeing, I don't know either way. I'd like to see how that works. Intuitively it doesn't seem right. Why would increasing the width decrease the contact patch.
Two tyres has (twice the) contact area than one tyre. Why isn't having a tyre twice the width similar to two tyres? (I can see why it would deviate a little due to edge effects, but not decrease).
Got a diagram?

Edit...
Ah because of the weight.
http://autospeed.com/cms/title_Tyres...5/article.html
I'm in the zone now. As you said a wider tyre can run cooler and allow a softer compound.

[dtype38]

Phoenix said that if you increase the width of the tyre, the contact patch gets shorter (i.e. in the direction of the tread), he didn't say it necessarily gets smaller. Remember that is also gets wider, so I think in theory, for the same weight of car, tyre pressure and construction, the actual area of the contact patch should stay roughly the same size.