Tyre width and Grip

[NICAT]

In simple way, if we increase tyre contact area, then the pressure per mm2 would be reduced. So the friction force per mm2 reduced.
But most Race cars have wide tyres. (Likely to have best possible grip)
Is it because, even if there is enough friction force, sometime the rubber may not have enough strength and would tear. So if there is more contact area, then more rubber will have more overall strength thus we will achieve more breakforce ?

Is there any other reason why wide tyres have better grip ? (If they actually have)

And in wet conditions, may narrow tyres have better grip ? Such as by being able to have better asperity contact with the asphalt ?

 

[JLTD]

More contact area on a wider tire, more gripping surfaces with siping. Your analysis of pressure for mm^2 is correct, but the increase in surface contact area is a larger increase in friction than what you are losing (up to a point and dependent on rubber compound).

In rain as long as the grooves in the tires are deep enough, same thing applies. Once a tire begins to get towards slicks, then rain traction goes away rapidly.

 

[AuthorEditor]

Here's the answer as to why wider tires offer better grip: https://www.physlink.com/education/askexperts/ae200.cfm

 

[Quattro Pete]

Originally Posted by NICAT
In simple way, if we increase tyre contact area, then the pressure per mm2 would be reduced.

Theoretically, wider tires don't necessarily increase contact area. They mainly change your contact patch shape. With wider tires, the contact patch becomes wider, but shorter.

https://www.roadandtrack.com/car-cu...you-knew-points-of-contact-tire-patches/

https://www.tirerack.com/tires/tiretech/techpage.jsp?techid=10

 

[maxdustington]

You want to run the narrowest snow tires you can get for max PSI.

That's also the reason the best bicycle for winter riding is a road bike or another bike that uses 700c tires. PSI is important when you need to cut through debris to get to the road surface.

 

[Cujet]

Originally Posted by AuthorEditor
Here's the answer as to why wider tires offer better grip: https://www.physlink.com/education/askexperts/ae200.cfm


While that's a good answer, it does not match real track-day experience. The link claims "Among tires of the same type and composition, here is no appreciable difference in 'traction' with different widths" That is simply not so.

It's clear that larger sized tires of the exact same compound, sidewall and manufacture perform better on the track. In fact, the difference can be so significant in certain forms of spec racing, the single change to a wider tire can make the difference between last and first!

Furthermore, it has little to do with a track days heating tires. As the performance increase is noticed equally on very short autocross tracks.

This is why those who race "stock" class often pick the model of car with the widest/biggest tire options.

It's also why real world empirical experience matters. Drag racers worked through the issues of HP and traction and refined the art.

 

[Cujet]

The proper answer is simple. As the pressure on the tire rubber increases, the rubber's coefficient of friction decreases. This is due to the molecular nature (polymer chains) of tire rubber compounds.

 

[meep]

^^^ Interesting statement. So coef of fric varies based on Force_Normal? I thought coe of fric was usually static, while force of friction would vary based on normal force? I could see coeffecient of F varying as a function of deformity and the above premise of siping/patch size/shape... that would all make sense. I'm just confused by the molecular nature of rubber part.

How does the molecular nature of the rubber change the coef of friction based on pressure?

 

[Dave9]

Originally Posted by maxdustington
You want to run the narrowest snow tires you can get for max PSI.

That's also the reason the best bicycle for winter riding is a road bike or another bike that uses 700c tires. PSI is important when you need to cut through debris to get to the road surface.

This is untrue and a dangerous mistake. 700c is only wheel diameter, both it and other diameter sizes come in different widths while if you were referring to skinny road tires they don't have the tread pattern needed for traction in snow. You want wide tires aired DOWN to lowest tolerable PSI, with a fairly open lug pattern.

If you don't have that, a simple snow/ice rut in a road can cause a wipeout, let alone times you won't have any chance of cutting through the snow to reach pavement, are always riding on top of the snow. Granted the larger the wheel the "slightly" more that will help but there are tires designed for winter so it's not some hack of just airing up a tire meant only for bare road contact. This is common knowledge among winter cyclists despite being a different choice than for 4 wheel automobiles that have different stability dynamics than a bicycle.

 

[bullwinkle]

It depends on what you're trying to accomplish-for a performance or track vehicle that never (or seldom) sees rain, you want the widest, softest compound tire available. As the weather gets worse (rain-snow-ice-glare ice), narrower & firmer with more sipes, silica rubber, and studs are better. For example, the Cherokee in my sig has P235/75R15 Yokohama all terrains for summer tires, which will slip dangerously in the first few minutes of a summer rain shower, and P215/75R15 Conti Extreme WinterContact studless winter tires-which are night & day better in winter weather. Bicycles are a different animal, cars & trucks aren't wiping out due to balance loss (although the XJ with it's 3 inch lift has tried!).

 

[NICAT]

Originally Posted by bullwinkle
track vehicle that never (or seldom) sees rain, you want the widest, softest compound tire available.

May be i missed the point.
Do you mean in hot summers, softest tire is better ? winter tyres are the softest as far as i have seen. So they are better for summer ?
Please clarify it.

 

[Cujet]

Originally Posted by meep

How does the molecular nature of the rubber change the coef of friction based on pressure?



This is my understanding and not a scientific explanation.

Rubber has elasticity. This is due to cross linking. A cross-link is a bond that links one polymer chain to another.

Rubber can deform and return to it's original shape. For our discussion, the limit is about 3x it's original size and shape. This is due to the nature of the crosslinked polymers.

We could say that the static frictional coefficient of a car tire is 0.72, and that actually matches how well older, well tuned cars performed. However, it's known that the organization of polymer chains changes as rubber stretches and/or as rubber is placed under heavy loads. A small contact patch of tire rubber actually deforms the polymer chains sufficiently to change the frictional properties.


It's also why a grossly overweight vehicle has significantly more difficulty stopping, even with good brakes.

 

[Quattro Pete]

Where is CapriRacer when you need him?

It would have helped if OP posted this in the correct forum.

 

[userfriendly]

I live in deep snow country. The notion that skinny tires work better than fat ones on snow and ice is just not true.

 

[Quattro Pete]

Originally Posted by userfriendly
I live in deep snow country. The notion that skinny tires work better than fat ones on snow and ice is just not true.

I guess it depends on if your intent is to cut through snow and reach the pavement or ride on top of snow. For a low ground clearance vehicle, it may be better to stay on top of snow so that your bumper doesn't turn into a snow plow.

 

[Garak]

We also have to remember here that our coefficient of friction depends upon the surface in use. Collision analysis tables show some of the trends for different surfaces with the same tires.

 

[userfriendly]

Cutting through the snow to reach pavement is an interesting concept. Cutting through snow to reach the rail doesn't always work with locomotives at 17 tons per wheel.

 

[Silk]

Rally cars use wider tyres for a sealed rally, narrow rally tyres for gravel.

 

[dailydriver]

Originally Posted by Silk
Rally cars use wider tyres for a sealed rally, narrow rally tyres for gravel.


...and small bore motocross motorcycle width tires with MUCH metal in them for snow/ice (Swedish Rally)

For tarmac, they have to have some small amount of tread cut into them, even for bone dry tarmac, according to the current rules, whereas they were allowed to run full on slicks back in the Group B (and I believe the immediately following Group A) days.