Why are wider tyres grippier?

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ermm, it's blindingly obvious that the force on the tyres is not constant, and fwiw the air pressure is not constant either.

I was assuming the car was at a standstill, it's easier to compare contact areas that way, so the only forces acting on the tyres is from the weight of the car (granted not evenly distributed, the front wheels would normally recieve more), and also that the tyres were inflated to the same pressure.
 
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I'll be honest here and say that I always thought it was down to a bigger contact patch.

At the same pressure on the same size wheel/tyre bar the width you get the same contact patch on a 225 as a 255 for example, as the wider tyre deforms less as the cars weight is spread over a greater area.

simple maths take a 50/50 balanced car of say 1400kg (3086lbs) 771.5 per wheel at 30 psi gives 25.7 square inches footprint per tire. That could be 10"w x 2.57" L or 8"w x 3.21" L

The handling dynamics can be altered by tyre width (narrow fronts turn in more easily etc) but "grip" is ultimately friction which is controlled by weight, tyre compound, tyre tread and road surface. The only really consistent information on tyre width is the use of narrow tyres in the wet , snow , mud to concentrate pressure on the road to sink down to find solid ground rather than float and spin on the soft stuff with a wider tyre.

Side wall stiffness impacts a lot, side loading capability and tyre deformation under acceleration / deceleration

There are many reasons for manufactures choosing certain tire widths and size, it is in fact quite a complex question.

I know little of the topic and from the little investigation I have done there appears to be lots of books on the topic so it does seem to be an involved and debated topic
 
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At the same pressure on the same size wheel/tyre bar the width you get the same contact patch on a 225 as a 255 for example, as the wider tyre deforms less as the cars weight is spread over a greater area.

simple maths take a 50/50 balanced car of say 1400kg (3086lbs) 771.5 per wheel at 30 psi gives 25.7 square inches footprint per tire. That could be 10"w x 2.57" L or 8"w x 3.21" L

Unfortunately you can throw that simple maths out of the window as it assumes the sidewalls are infinitely flexible and offer no support at all. In reality unless you run balloon tyres on your car they do, and low profile tyres have much stiffer sidewalls than ones with higher aspect ratios, so the simple maths goes wrong.

Other issues:

The frictional coefficient of the tyre/road interface is not constant, it reduces the more it's loaded, so keeping the tyre more lightly loaded (with a larger contact patch area) offers more available grip.

Wider tyres of the same rolling radius have to be lower profile, and they tend to be more stable under load. Assuming the suspension is optimised with work with low profile tyres, the contact patch will distort less under cornering with wide, low profile tyres than narrow skinny ones (where you can end up running on the sidewalls if you push hard). This is essentially the slip angle, i.e. the angle between the direction the tyre is pointing and the direction the car is actualy following.
 
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Ah, I'm new to the motors forum, I bet you get this question every fortnight :p

It seems fairly well accepted by the more technical members that the contact patch is pretty much the same, although it doesn't seem especially clear why they're better going round corners. My understanding is that when you turn, the front of the contact patch grips the road, and the side forces acting on it laterally deform the tyre, with the deformation increases towards the back of the contact patch. There's a point where the tyres structure stops it deforming any further, so it can no longer stay in static contact with the road, and so it slips.

In wide tyres, the wider contact patch means that imperfections in the tyre will not as readily make a section slip, so you can get a higher overall grip. On the negative side, this also means that when it slips, it'll slip suddenly. Hope that made sense :)
 
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I quote Carroll Smith:

"The basics are simple. The length of the footprint has a major effect on longitudinal traction - acceleration and braking - which is why drag racing tires are huge in diameter. The width has a major effect on lateral acceleration - cornering force - which is why F1 tires are as wide as the rules allow. The area of the footprint effects both and, in terms of stick, bigger is better."

From Drive to Win.

The man has been racing and engineering race cars for 30+ years.

It helps if you understand how a radial ply tire is constructed compared to an old school cross bias play tire, and to understand what happens to a tire in a corner (a lot. A hell of a lot)
 
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Now this will inevitably result in a rush of people claming that they have more grip because the contact area is bigger, but I'm afraid this isn't the right answer. The real answer is much more complex, I'm interested to what everyone thinks though as there seems to be a range of explanations! So why do you think wider tyres are grippier?

Erm no it isnt:

Place your thumb or one finger onto a carpet, press as hard as you can and try to drag it.
Repeat but not just one finger but your whole hand.

It is much harder to drag with the whole hand, because it has more contact area...
 
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If I had a copy of Tune to Win I could tell you.

However I don't, but I can postulate.

Larger contact patch = more rubber in contact with the road.

Knowing how a radial ply tire works, the sidewall on the outside of the corner flattens (the inside remains almost the same shape), presenting the maximum footpring available to the tire assuming a whole bunch of suspension and alignment factors.

With a lower profile, but wider tire on a larger rim using the same pressure you would on a larger profile tire on a small rim you get much less flex across the whole of the tire.

Useful, because small tires have a tendency to lean over the contact patch with low camber (ie typical camber on a factory standard road car) and end up on the outside of the tire.

Even though the radial ply tire is incredibly flexible, you'll never really utilise the maximum contact patch in a corner on a small tire unless the suspension has been tuned for it.

It helps if you've seen real world tire data taken from a car on track to understand exactly what happens to a tire. The rate at which it gains heat, sheds it and generally gets mutilated in the space of a quarter of a second upon turn in really has to been seen to be believed.

I still need to get a copy of Tune to Win anyway. I highly reccomend his books if you're remotely interested in racing or vehicle dynamics.
 
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Erm no it isnt:

Place your thumb or one finger onto a carpet, press as hard as you can and try to drag it.
Repeat but not just one finger but your whole hand.

It is much harder to drag with the whole hand, because it has more contact area...

I've never said that increased contact area doesn't increase grip, I completely agree with your example, what I'm saying is the contact area DOESN'T change (check this page, about half way down for an explanation).
 
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Yeh I agree, however it doesn't actually explain WHY length is good for longitudinal grip and width is good for lateral.

As I said already in this thread, its because the shape of the contact patch relative to the direction its going.

Look at brake pads, the length of the frictional surface is in the direction of travel. If it was the other way round so the length was orthogonal to the direction of travel I would imagine the brake pads would very quickly overheat despite them being the same size because the greater heat gradiant from front to back and probably more shearing forces acting on the shorter face. Which is why narrow tyres are not so good for cornering as the temperature gradient across the tyre would be greater than that of the wider tyre which would make them go off quicker.
 
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Erm no it isnt:

Place your thumb or one finger onto a carpet, press as hard as you can and try to drag it.
Repeat but not just one finger but your whole hand.

It is much harder to drag with the whole hand, because it has more contact area...

Not a very good example as you can apply far more pressure to your entire hand than you can your finger, and I doubt the frictional coefficient of a finger/carpet interface is well documented anyway.

The reason that tyre width does not appear to obey simplified friction models is that the complexity of the tyre construction, interaction of suspension etc. cause numerous second order effects.
 
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