The law of physics says friction force depends only on coefficient of friction and weight and not on surface area.
But it is often said tyre width has a contributing factors in traction.
Do wider tyres provide better traction? How does it tally with physics?
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I'll leave this for somebody with specialised knowledge to answer. What I do know is that the car I have ever driven in snow, by miles, was our Saab 96 which had quite narrow tyres.
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You are confusing sliding friction with static friction
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Grand prix tyres are wide.
Edited by Hamsafar on 14/12/2008 at 20:05
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Narrow tyres are better in snow as they distribute the car's weight over a smaller surface area. Thus the force exerted on the ground will be greater at any given point on the contact patch of the tyre. It is the opposite of this that explains why people wear tennis rackets on their feet to stop them sinking into the snow. A car with skinny tyres tends to slice through the snow, whereas the garden-rollers on a porsche will tend to slide along the suface, like an eskino with tennis rackets!
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Do wider tyres provide better traction?
Without going into the physics of it, they must provide better cornering traction otherwise Formula 1 cars would be runnning around on bike wheels.
But that is only half the story as most F1 races are run in almost perfect dry conditions - does anyone know if wet F1 tyres are the same dimensions as slicks ?
If you look at the ice racers in France, they run on wheels and tyres similar to the space savers supplied on modern cars and most manufacturers, when specifying winter tyres, recommend the smallest wheel size and narrowest tyre width for that model.
Also, consider the aquaplanning formula which is the square root of the tyre pressure x 9 to give the speed in knots when the tyre will aquaplane. No mention of tyre dimensions there, just pressure for traction or loss there of.
So, while a fat tyre may well give better cornering, it doesn't guarantee the best all round traction.
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Been my experience that there's no more or less actual surface area of contact or any more ultimate grip when wider/low profile tyres are used.
Wider low profile tyres do roll less on bends and corner better, but they do seem more prone to be skittish when wet and certainly when cold conditions apply.
Maybe its me, but as a general feeling i find that when the tyre reaches profile proportions where comfort is lost, that seems to be about the same point where the tyres become skittish when wet.
Thats been my experience of running various sizes and types on same model cars over a number of years.
If you look at the footprint of a normal narrow tyre it will be squarish or if very narrow oblong along the tread, where the footprint of the wide tyre will be oblong across the tread, but both of the same surface area.
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A wider tyre will give more rolling resistance than a skinny, when you get down to low-low profiles (35s), not only will it tramline badly, but also the rubber compounds used on low-low is very soft, so don't expect more than 10 000 miles, especially on a powerful FWD, with a heavy right foot!
As many boy racers have discovered, to their detriment, a low-profile and wider tyre often does interesting stuff round corners - grips TOO well, and rather than the car going into a semi-slide, ends up rolling over!
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snipquote
Do wider tyres provide better traction? How does it tally with physics?
The law of physics on friction only applies to rigid bodies on rigid surfaces, e.g. steel railway wheels on steel rails (I know these deform at the contact point, but not enough that the load x coefficient law is not near enough correct).
Pneumatic tyres do not follow the simple load x coefficient law; I'm sure some backroomers can expound on slip angles etc, the maths is a bit advanced for me!
Edited by Dynamic Dave on 15/12/2008 at 00:19
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As somebody that no longer knows anything at all...
it's my understanding that the compounds on wide tyres are much softer than narrower tyres, and this softness is responsible for the increased grip in dry/reasonable conditions.
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There are a few confusing and difficult aspect to tyre / road friction - in fact, as soon as you find yourself trying to evaluate the properties of anything made of rubber, you face a challenge!
The complicating factors are:
1) The contact area is determined by the vertical load divided by the tyre pressure
2) On a given car, at a given tyre pressure, changing the width of the tyre, changes the *shape* of the contact patch, not the area
3) Even if the area did change, as the tyres are flexible, the rubber conforms to the surface, the usual area independence of friction is challenged a bit.**
4) The shape of the contact patch determines allows the shape of the friction ellipse to be tweaked - a wide tyre can produce greater side force, while a thin tyre, more traction.
5) The detailed mechanism by which rubber grips the road is more complex than in most materials, there is the more usual temporary chemical bonding (which you lose in the wet), and the grip provided by the rubber's hysteresis.
** The area independence rule of friction hides quite a lot of detail. For most solid materials, the actual area of contact is tiny, and consists only of asperity to asperity contact. Kinematically, you only need a small number of asperities to touch to support the interface, and making the interface apparent contact area larger won't change the actual contact area.
To see this, imagine standing atop a rigid board supported by rough gravel. There would probably be a dozen points of contact. Even if you doubled the area of the board, it would still only touch at the same number of contact points.
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What is the formula for calculating traction?
4x4 nerds often say wheel slip occurs when torque is greater than traction. Consequently, 4WD cars have twice the traction than 2WD cars so they lose traction later than 2WD's.
When we say torque greater than traction , what unit we use to measure traction? Torque is measured in "Nm" - is traction also measured in same unit? How?
Very confusing!
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I think I have found answers of part of my questions :)
Tractive Force = uW [u = coeff of friction, W = mg]
Propulsive force = Torque * radius of wheel
If propulsive force > tractive force, wheel slip will occur!
In a 2WD car, if torque is say 400 Nm and wheel radius is 0.25 m, the propulsive force at each wheel (2 in this case), (400/2)/0.25 = 800 N
For 4WD car, propulsive force on each wheel (4 here) will be, (400/4)/0.25 = 400 N
So for 4WD vehicles, it is much less likely that propulsive force will be more than tractive force, so wheel slip is less likely than 2WD cars.
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The gearing between the flywheel and the road wheels is missing here. The logic still seems to work though.
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The gearing between the flywheel and the road wheels is missing here.
Similarly, when people rave about the engine output torque of diesels compared with that of petrols. The relevant parameter is torque at the wheels, and the higher gearing of diesels gives a lower proportion of engine torque at the wheels. If only motoring journalists would learn this.
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Manufacturers specify narrow tyres/small wheels for use in the snow,not just to cut thro' the snow but to allow the fitting of snow chains without destroying the wheel arches.
Edited by jc2 on 15/12/2008 at 09:01
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