Sorry guys, i got a quick question about rims - Page 2

**mhale71** · 3-17-08, 23:58

Quote:

Originally Posted by ragincanadian4

Rrrrrrrrriiiight.

Biffmeistro · 3-18-08, 0:30

It is just a matter of car weight and tire PSI.

Say you have a 3000 pound car, and your tires have a pressure of 30 pounds per square inch.
Logically, your car would have a total contact patch of 100 square inches, or 25 square inches per tire. It doesn't matter how wide the tires are, that just determines the shape of the patch. A wide tire doesn't magically make you require a total of 150 square inches at 30 pounds per square inch to hold up 3000 pounds.

**daevilone** · 3-18-08, 5:48

^^^^ yeah when sitting and thinking about it I agree with you, assuming the sidewall of the tyre is the same (design, build, thickness). Given a thinner and/or harder sidewall I reckon this changes stuff.... but for all things equal, only changing width, I agree.

**Evert** · 3-18-08, 7:51

I still don't get it.
If wheel diameter and tire pressure are equal then how the shape of contact patch of wider tire can possibly be shorter? Also, what you're saying basically is that in terms of traction it doesn't matter if you're tires are 175mm wide or 225mm wide, they both still have equal size contact patch i.e. friction surface?

Biffmeistro · 3-18-08, 11:19

The shape of a tire is cylindrical, correct? And were it to be of infinite hardness, a cylindrical object only touches a flat plane on a single line. A line the width of the cylinder.

However, tires are not infinitely hard, they deform. They squish flat at the point of contact with the road.

Now, the tire HAS to have at least one dimension of it's squishing that is the width, because even if it didn't squish at all, it would still have a contact patch that is merely it's width, and no length.

Now, let us assume a five inch wide economy tire. It's holding up that same 3000 pound car, and it has that same 30psi. (3000 pounds/30 pounds per square inch = 100 square inches. 100 square inches/4 tires = 25 square inches per tire)
The tire has to have a contact patch of 25 square inches, and we know one dimension of that square is it's width, which is five inches. Therefore the length of the patch is 5x5 inches. 25 square inches. (25 square inches/5 inches = 5 inches)

Now let us assume a wide, high performance tire of 10 inches wide. Once more, we know one dimension of the contact patch is it's width, 10 inches. Therefore, we know that the length of the contact patch is 2.5 inches. (25 square inches/10 inches = 2.5 inches)

So the 5 inch wide tire has a 5x5 patch, while the 10 inch wide tire has a 10x2.5 inch patch. Same square inches, but the wide tire just has a wider, shorter patch, while the economy tire has a narrower, longer patch.

Same contact patch size, because there is no way to make 30psi need more than 100 square inches to hold up 3000 pounds. A stiff sidewall can assist in holding up the car, but that would decrease the size of the patch, not increase it.

I admit there are plenty of variables that I didn't take into account, such as sidewall thickness, or the fact that tires aren't perfectly cylindrical. Those, however, would only result in
a: a reduction in patch size, as the sidewall gets stiffer, or
b: distortions of the shape of the patch, while maintaining the patch size.

The primary bonus of the wider tire, though, is lateral stability. In drag cars, it's an ability to run a lower than normal PSI without compromising the stability of the tire. (Drag radials are usually run at 15 PSI)

Hope that was helpful!

**daevilone** · 3-18-08, 19:18

Quote:

Originally Posted by Evert

I still don't get it.
If wheel diameter and tire pressure are equal then how the shape of contact patch of wider tire can possibly be shorter? Also, what you're saying basically is that in terms of traction it doesn't matter if you're tires are 175mm wide or 225mm wide, they both still have equal size contact patch i.e. friction surface?

with the same psi, straight line tracion doesnt change. cornering though is where you see the benefit of a wider tyre.

This is with removing a lot of variables though... I reckon that instability in the car/surface/etc would mean that a wider tyre ends up giving better traction off the line because it can cope with those instabilities better.

**ragincanadian4** · 3-18-08, 21:55

Your mom's a tire.

3-18-08, 5:48	#18 (permalink)
daevilone Join Date: Jun 2007 Location: Canberra, Australia Age: 21 iTrader: (1)	^^^^ yeah when sitting and thinking about it I agree with you, assuming the sidewall of the tyre is the same (design, build, thickness). Given a thinner and/or harder sidewall I reckon this changes stuff.... but for all things equal, only changing width, I agree.
	-- '92 2G -- KLZE, MTX, ebay headers, CAI w/ apexi pod, front and rear strut bars, Whiteline 18mm rear sway bar, camber bolts, 17*7 5ZIGEN FN01R-C's with 225/45 Federal 595 RS semi slicks, JSpecs with 6000k HID's, audio stuff... to come: exhaust, Jspec fogs

3-18-08, 7:51	#19 (permalink)
Evert Join Date: Sep 2002 Location: Tallinn, Estonia Age: 29 iTrader: (0)	I still don't get it. If wheel diameter and tire pressure are equal then how the shape of contact patch of wider tire can possibly be shorter? Also, what you're saying basically is that in terms of traction it doesn't matter if you're tires are 175mm wide or 225mm wide, they both still have equal size contact patch i.e. friction surface?
	Mazdayasna, the worship of Mazda

3-18-08, 0:30	#17 (permalink)
Biffmeistro Join Date: Oct 2007 Location: Fort Wayne, IN, USA Age: 21 iTrader: (0)	It is just a matter of car weight and tire PSI. Say you have a 3000 pound car, and your tires have a pressure of 30 pounds per square inch. Logically, your car would have a total contact patch of 100 square inches, or 25 square inches per tire. It doesn't matter how wide the tires are, that just determines the shape of the patch. A wide tire doesn't magically make you require a total of 150 square inches at 30 pounds per square inch to hold up 3000 pounds.

3-18-08, 11:19	#20 (permalink)
Biffmeistro Join Date: Oct 2007 Location: Fort Wayne, IN, USA Age: 21 iTrader: (0)	The shape of a tire is cylindrical, correct? And were it to be of infinite hardness, a cylindrical object only touches a flat plane on a single line. A line the width of the cylinder. However, tires are not infinitely hard, they deform. They squish flat at the point of contact with the road. Now, the tire HAS to have at least one dimension of it's squishing that is the width, because even if it didn't squish at all, it would still have a contact patch that is merely it's width, and no length. Now, let us assume a five inch wide economy tire. It's holding up that same 3000 pound car, and it has that same 30psi. (3000 pounds/30 pounds per square inch = 100 square inches. 100 square inches/4 tires = 25 square inches per tire) The tire has to have a contact patch of 25 square inches, and we know one dimension of that square is it's width, which is five inches. Therefore the length of the patch is 5x5 inches. 25 square inches. (25 square inches/5 inches = 5 inches) Now let us assume a wide, high performance tire of 10 inches wide. Once more, we know one dimension of the contact patch is it's width, 10 inches. Therefore, we know that the length of the contact patch is 2.5 inches. (25 square inches/10 inches = 2.5 inches) So the 5 inch wide tire has a 5x5 patch, while the 10 inch wide tire has a 10x2.5 inch patch. Same square inches, but the wide tire just has a wider, shorter patch, while the economy tire has a narrower, longer patch. Same contact patch size, because there is no way to make 30psi need more than 100 square inches to hold up 3000 pounds. A stiff sidewall can assist in holding up the car, but that would decrease the size of the patch, not increase it. I admit there are plenty of variables that I didn't take into account, such as sidewall thickness, or the fact that tires aren't perfectly cylindrical. Those, however, would only result in a: a reduction in patch size, as the sidewall gets stiffer, or b: distortions of the shape of the patch, while maintaining the patch size. The primary bonus of the wider tire, though, is lateral stability. In drag cars, it's an ability to run a lower than normal PSI without compromising the stability of the tire. (Drag radials are usually run at 15 PSI) Hope that was helpful!