Physics ITT

[AngelinIsRich08]

For balance.

[yakev724]

You are correct in assuming that friction is independent of surface area, but there are several obvious (and dozens of complicated not-obvious) factors which favor a wider tire.

Formula-wise, we're concerned with force, which as you correctly stated F=m*g*(mu), and also pressure, which is P=F/A. As we increase tire width (and contact area), pressure (on the rubber) decreases.

This is key--imagine a car on bicycle wheels and tires. Since there is almost no surface area between them and the road, the pressure on the rubber is immense. For them to act as closely to regular tires, the rubber must be made in a way to withstand this pressure in both lateral (acceleration/braking) and transverse (turning) directions. It will (if at all possible) be hard as plastic, provide poor adhesion, and be useless when hitting a bump or debris as a small obstruction can easily encompass the entire contact area and cut off friction to the entire wheel.

Now looking at normal tires--our contact area is many times greater and thus pressure on the rubber has decreased. The tire can now be softer and made to withstand much less pressure as the force per given patch size in contact with the road is many times less than our bicycle tires.

F1 cars' tires deal with forces much greater than our road cars. Therefore, a wider tire decreases pressure on the material and allows for it to be softer and more forgiving. Too wide a tire faces detrimental factors such as poor turn stability and difficulty in maintaining an even pressure on the road from the center to the outer edge.

In sum, wider = greater contact area --> less pressure on rubber (allows for/with trade-off of greater wear) softer rubber --> better adhesion --> more grip

[infinitekidM2C]

I think i actually understand this crazy russian bastard.

[scollins]

Race tires go way beyond friction in order to provide traction. How do we describe race tires? "Sticky" comes to mind. What does that mean? The road and tire are actually sticking together briefly, in which case surface area does matter (adhesion).

[ideliver]

http://www.tirerack.com/tires/tirete....jsp?techid=10

the difference is that the wider tire has a shorter and stiffer sidewall which quickens steering response and increases cornering stability

also consider the weight of the tire/rim....unspung weight is a killer

[Serious]

Wider tires do not necessarily increase contact patch area, and any increase in contact patch area due to increased with is small compared to total area. The normal force on the tire surface is equal given cars of equal mass & equal distribution, but remember than a tire is in general elastic and will deform when a load is applied. So with a skinny tire you get a contact patch closer to a square shape and with a wide tire the contact patch will be shaped more like a rectangle lengthwise across the tire surface. The actual area of the two contact patches is very similar.

The real benefit to wider tires encompass two aspects. The first is heat management, this is by far the biggest defining factor to the performance capability of the tire; too cool or too hot and the tire simply won't perform. Go examine even a street tire after it comes off the track, it has the consistency of chewed gum or an eraser, not the hard surface we are used to seeing on the street.
The reason a wider tire is preferred is because it has more overall tread surface area and thus dissipates heat more efficiently making it more difficult to overheat the compound.

The other major factor is slip angle, slip angle is the angle measured between the contact patch and the rest of the tire tread surface as the tire turns. As the wheel turns the area of the tire that is not in contact with the road is actually twisted away the wheel, the angle of this deflection is called the slip angle.



Video illustrating this here:

Slip angle is important because it defines the available cornering force the tire can generate, as slip angle is actually proportional to cornering force (for a while). With a wider contact patch more lateral force is required to deform the tire beyond it optimum slip angle vs a slimmer contact patch. The optimum slip angle will be defined by the tire construction.

It should also be apparent than that as you increase the weight or force on the tire you increase it's potential cornering force, which is why downforce is so highly coveted in top level motorsport.

[Maestro]

^^^ Very interesting video...

Everyone presented lots of good information about tires, however, none of it provided the physic answer. The reason being there is no simple answer, in a perfect world with perfect engineering and perfect materials the tires may not need to be wide or narrow. As it was pointed out the reason is a lot more complicated than what is taught in a basic physics class.

The real answer is there is no physic reason, it is call engineering trade offs, for all the various reasons that everyone above pointed out. Honestly, it would be better not to have wider tires all that means is you have more surface area between the tire and ground which would slow you down and required more power to move the car.

I can tell you since I have done the experiments in engineering school, surface area does have an affect, because for a fixed weight, and fixed coefficient of friction, an item with less surface area is easier to move or get moving than an item with a larger area, why, because of other forces they come into play beyond friction like inter molecular bonding.

If you do not believe me, then why is it very hard to pull two items which are highly polished and flat apart, or take two flat surfaces and put water between them and try pulling them apart.

So race cars have wide tires to deal with a number of engineering challenges which balances a number of forces of a car going around a track.

BTW, Land Speed cars I believe use very narrow steel or hard rubber tires since they try to minimize contact with the ground and only need to go straight.

[Serious]

Originally Posted by Maestro

I can tell you since I have done the experiments in engineering school, surface area does have an affect, because for a fixed weight, and fixed coefficient of friction, an item with less surface area is easier to move or get moving than an item with a larger area, why, because of other forces they come into play beyond friction like inter molecular bonding.

If you do not believe me, then why is it very hard to pull two items which are highly polished and flat apart, or take two flat surfaces and put water between them and try pulling them apart.

Not sure what you mean here because the surface area of the tire is the entire tread area of the tire, but it seems like you're referring to the contact patch area. I have already stated that due to the elastic deformation properties of road car tire under load the contact patch area is defined by the load on the tire not the tread width.

Water is a viscous fluid if you put it in between two surfaces it is going to act as a lubricant and lower the force necessary to pull them apart.

[BTM]

Short answer: mechanical grip

[ragingclue]

Originally Posted by Serious

Not sure what you mean here because the surface area of the tire is the entire tread area of the tire, but it seems like you're referring to the contact patch area. I have already stated that due to the elastic deformation properties of road car tire under load the contact patch area is defined by the load on the tire not the tread width.

Water is a viscous fluid if you put it in between two surfaces it is going to act as a lubricant and lower the force necessary to pull them apart.

+1 on the contact patch area part. A lot of people don't realize that a wider tire doesn't do anything significant to the size of that patch.

Also, I believe maestro was talking about pulling the two polished items apart with forces pirpendicular to the surfaces applied with water, due to suction.

[Maestro]

Originally Posted by ragingclue

+1 on the contact patch area part. A lot of people don't realize that a wider tire doesn't do anything significant to the size of that patch.

Also, I believe maestro was talking about pulling the two polished items apart with forces pirpendicular to the surfaces applied with water, due to suction.

Exactly, there are more forces than frictional forces you're dealing with when determining the size of the tire to be used on a race car.

BTW it is not suction it is molecular attraction between the free electrons in water and the other materials free electrons.


To your point about contact patch it is another trade off

http://www.tirerack.com/tires/tirete....jsp?techid=10

and add this to the discussion

http://insideracingtechnology.com/tirebkexerpt2.htm
http://www.insideracingtechnology.com/tirebkexerpt1.htm
http://insideracingtechnology.com/booktiredescrptn.htm

This guy will sell you a 288 page book on tires and car dynamic if you're really interested in understanding why race cars have wide tires.

[infinitekidM2C]

Originally Posted by ragingclue

+1 on the contact patch area part. A lot of people don't realize that a wider tire doesn't do anything significant to the size of that patch.

Also, I believe maestro was talking about pulling the two polished items apart with forces pirpendicular to the surfaces applied with water, due to suction.

So assuming you can put anywhere from 245 to 265 tires on ur ride..all else being equal, the 265 wont help performance in terms of not breaking the tires loose on launches/shifts, and turning?

[ragingclue]

Originally Posted by Maestro

BTW it is not suction it is molecular attraction between the free electrons in water and the other materials free electrons.

Cool, thanks for correcting me.

Originally Posted by infinitekid2002

So assuming you can put anywhere from 245 to 265 tires on ur ride..all else being equal, the 265 wont help performance in terms of not breaking the tires loose on launches/shifts, and turning?

They will help, but not because of the size of the contact patch. There are a lot of factors going on there that determine the wider tire holds the road better. Maestro posted a lot of useful links that will explain it better than I could.

[BTM]

Originally Posted by infinitekid2002

So assuming you can put anywhere from 245 to 265 tires on ur ride..all else being equal, the 265 wont help performance in terms of not breaking the tires loose on launches/shifts, and turning?

All things equal (namely alignment, tire compound), 265s won't perform significantly better in a straight line (provided the alignment isn't set up for drag racing), the effect on contact patch when tracking straight is not very large. Negative camber characteristic to BMWs (which is also a function of your alignment) will result in the outside tire becoming flatter (more parallel to the road) while turning, increasing the contact patch in these situations with a wider tire and providing more lateral grip.

[Wolf]

wide tires for tracks... better traction at corners, maintains high speeds through corners.
Narrow tires for drag racing... less friction more speed but might affect the launch of the car badly depending on the type of the tire.