FACT ......

a 315 tyre has EXACTLY the same size contact patch as a 295 tyre

It's only the shape that changes

You will NOT get a bigger footprint with 315's

 

I have 11.5 inch Champions, so I can run a huge variety of sizes. They also weight about 18-19lbs if my memory serves. I like the forgestars and almost put them on my dd, but I would hold out for a used high-end set of truly forged rims, if I were in your shoes.

Tom

 

Please elaborate. It doesnt make sense to me.

 

The contact patch equals the weight of the car on that particular corner divided by the pressure in that tire. For example, of your 3400 lb car weight is equally divided at your tires are at 30 psi then each contact patch is 28.3 sq inches.

I can't tell you how the shapes differs ... I'd only be guessing.

 

I can assure you that if all things are equal and I were to put a 275 tire on the left rear and a 315 tire on the right rear that my contact patch would be greater on the 315 tire......

 

This site appears to agree:

http://performancesimulations.com/fa...on-tires-1.htm

with a hypothesis (I read it quickly, so don't quote me) that the contact patch pressure is lower than the tire pressure.

 

I disagree with this. The link in the previous post also disputes this.

20mm is 20mm. Using your logic, there would be no reason to not run 275s instead of 295s. Thus a 275 would also be the same as a 315. Sorry, but the math just isn't there for me.

 

I went from factory 11" to Forgestar 12" (lighter) so really not much weight difference, Forgestar calculated 36mm offset, wasn't enough, ended up running 7mm spacers (which I wasn't happy doing) running 315's from 295

 

I am also running the F14 Forgestars. I had the same concern with offsets. I stuck with the 11" rears and still have 315s with no issues and no spacers.

Love your wheels!

 

I run 11.5 rears seems to be a good size between the 11 and 12 inch options...

 

Will do - You'll have to bear with me though
It takes a little while to explain - as it doesn't make any sense what so ever

BUT - It is 100% fact



I can assure you this is not the case
I thought exactly the same as you too, and it took me ages to get my head around it

It took me ages to properly understand it
I'll try and explain it, but it's difficult to word it correctly


Also - a 275 tyre will have the same size contact patch as a 315 tyre

 

Any thoughts on a 10" for the rear? I'm looking at the OZ Alleggerita wheels and they seem to be only available in a 10 or 12 (18 rims). For the 10", Tirerack recommends the same tire size that I currently have on some 11s (295/30R18).

 

So if the contact patch doesn't change regardless of tire width, then why have tires made at all that are over 185 or so?

 

Tyre width is a secondary factor of grip

Wider Tyres can and usually will have more grip that a narrower tyre - BUT the actual contact patch remains unchanged in size

I'm writing something up at the minute which should explain things a little better

 

The wider tire gives you a wider contact patch, but the length of the contact patch get shorter. The area stays the same.

The reason we get wider tires is for cornering traction. The wider contact patch increases our cornering grip with a minimal change to straight line traction (it's not a one to one trade off).

Again, the size (area) of the contact patch doesn't change but the shape does. A longer contact patch gives more straight line traction and a wider contact patch give more cornering traction.

Of course there are still diminishing returns. If the width of the tire gets too small relative to the contact patch needed for the given weight and air pressure, then the sidewalls of the tire are forced to carry a percent of the weight of the car. At this point you are losing contact patch area and traction will suffer.

Basically I don't think you will see a noticable difference in straight line traction from a 295 to a 315. You will notice more cornering traction with the 315 versus the 295. Of course a 225 is not wide enough to provide the contact patch needed with a 3,500 car running approximately 33psi of air pressure (it might be good for gas mileage though).

Later, Steve