wanna911

Was going to pose that question next.

Nordschleife

as long as the tread is not distorted, the width of the contact patch is determined by the width of the tread. How it stays in contact with the ground will be affected by the width of the rim, inflation pressure and sidewall characteristics.

the length of the contact patch is determined by how hard the tyre is inflated.

Over/under inflation chasnges the length of the contact patch. Extreme over inflation can cause the tread to distort so that the contact patch width is reduced, taken to the extreme the tyre would take on a spherical shape with a miniscule contact patch tangentical to a solid sphere.

R+C

wanna911

Yes but if you have the same type of tire, but put a 245 as opposed to a 225 tire with the same PSI in them, does the contact patch grow?


I would say yes if for no reason than treads (I would assume) are based on percentage of tire width so when you make a bigger tire of the same sort, you make the tread bigger as well, unless I am mistaken here.

Nordschleife

Depends on tyre pressures - you can over or under inflate the tyre and change the area. Because the weight of the car remains almost constant 245 vs 225, the amount of compression of the tyre will be different and the pressure per square mm will change. So, in an ideal world you would run a slightly different tyre pressure.....

If Danni Minogue wears stiletto heeled shoes on a wooden floor she will make dents in the floor, but put Beth Ditto in a pair of wedges and she wont leave a mark

R+C

Willard Bridgham 3

Physical effects with liquids (air, water and etc) do not scale.

The real question is does stick improve with larger contact patch and there are elastic effects to consider, ie, the change in tire size will change the aspect ratio and moment of inertia and stiffness. Tires are not perfectly elastic, ie, not homogeneous and isotropic.

Is this a problem in statics where things do not move or in dynamics where things do move?

Bonster

Okay . . . I'm no physics genius, but this doesn't require much of that anyway. You said all things remain the same except for the width of a tire. So the profile and circumferance of the tire in question are the same. Okay . . . so say, for the sake of argument, we have a 205-50-15 tire versus a 195-50-15 tire. Obviously the 205 width tire has more VOLUME, and therefore it has MORE contact patch. See? Simple.

95m3racer

Tires are not an easy topic, but you can start to edumacate yourself with this book:




I went to a seminar given by Paul Haney, small group of fsae students, and while there is no "secret" to tires, there is a lot of knowledge on how tires work...why they work is something thats not been exactly discovered yet lol Sort of a black art to say the least.

Professor Helmüt Tester

As soon as you said...

...then I knew this would devolve into a totally theoretical discussion that would have no contact with the realities of tires. It will certainly fascinate some, but in my world -

1. The air in the tires does not hold the car up. If it did, then why is it if you set tire pressures for X with the car jacked up...and they're still X when the car is dropped on the ground ?

2. Read #1 again. Repeat every half-hour.

The weight of the car is supported by the sidewalls. All the air does is force the carcass of the tire to conform to some shape. Variances in sidewalls thickness and stiffness, contact carcass thickness and stiffness (in more than one axis) and a whole bunch of 'black-art' items related to tire construction and performance relegate us to the roles of 2nd graders discussing particle physics.

But that's just my opinion.

Nordschleife

but remember to get the truck under the low bridge, we let the air out of the tyres....

R+C

171mph

Wider tires change the shape of the contact patch. It changes the shape from long and skinny to short and wide. There is an advantage to this.The sidewall of the tire must bend a little to create the contact patch.Because the sidewall has less bend, less of the car's weight is being supported by the sidewall. Instead, it is being supported by the tire's air pressure, which is constant among the entire contact patch. Since the even distribution of weight among the tire's surface will allow the tire to function better, the wider contact patch has more overall grip.

Bill Verburg

ALL other things being equal the area of the contact patch is the same.

The shape is different.

The wider tire has a shorter but wider contact patch. This is desireable for a performance car because it runs at a lower slip angle than the longer narrower patch of the narrower tire.

The lower slip angles allows the tire to generate more cornering force and run cooler.

In addition the slip angle vs lateral force curve is non linear and varies according to the shape and construction of a tire. The wider tire has a curve that is more favorable for generating lateral forces.

wanna911

AHA! This is the answer I have been looking for and while in theory it may seem possible, in at least one tire test done by a manufacturer, it has been proven not to be so.


http://www.performancesimulations.co...on-tires-1.htm

Bill Verburg

ALL other things being equal
They rarely are when comparing 2 different tires

amaist

Are tires like spherical cows?

Dan in Florida

Hi Mark,

Good analogy. Mark Donahue had it right. Dan