How to PROPERLY select and size TIRES for PERFORMANCE

[fatalelement]

Originally Posted by Locust

18x12" wheels with 215/40R18 tires.

#stancenation

/thread

[Locust]

Originally Posted by fatalelement

Attachment 119931

That one is such a classic.

[campy]

So you're telling me I can't just go with 18x9.5 all around on my otherwise stock car and call it a day?

[DarkSunrise]

Originally Posted by fatalelement

Attachment 119931

Haha that was perfect.

[SVLTE]

This is definitely gospel for track driving, but does it hold any water for autocross? If it did, it seems the ideal setup in CS would be the 205/45/17 RE71R since it has a 7" tread width. I haven't seen anyone claiming to be running that or the 215, just 225 and 245.

[Skurj]

Originally Posted by strat61caster

I can't imagine how bad the understeer would be if that was the only change off the showroom floor on my car. I probably wouldn't have bothered modifying it and just sold it for something else...

My car came off the showroom floor with 225's front and 255's rear on staggered wheels... That first track day was hoooorrrible... talk about understeer..

Ok that was over 2 years ago...

I now run them the other way around... 255 on the front, 225 on the rear..


Actually come to think of it... both my cars came off showroom floor staggered... (only one came that way from the factory) they now are both square for trackdays at least.

[strat61caster]

Originally Posted by SVLTE

This is definitely gospel for track driving, but does it hold any water for autocross? If it did, it seems the ideal setup in CS would be the 205/45/17 RE71R since it has a 7" tread width. I haven't seen anyone claiming to be running that or the 215, just 225 and 245.

Per the article:

On the other hand, if a tire is too wide for a given wheel width, the sidewall ‘bulges”. This usually results in sloppy handling characteristics, vague steering feel, excessive tread squirm, and the carcass of the tire can also distort and have uneven pressure across the surface of the tire.

The sloppiness is an easy hit to take for a tire that makes larger contact with the ground, the flex means that the tire rolls around and complies continuing to make contact with the ground, couple it with the flex generating additional heat so the tire 'hooks up' faster and that leads you to what is observed on the scene right now.

[ame="https://www.youtube.com/watch?v=pQTFJg7R1KQ"]Critical Velocity Scuff - YAW - Tire deflection - YouTube[/ame]


[ame="https://www.youtube.com/watch?v=sjx8WFXHNLA"]Team VCMC FR-S - AutoX Video 3: Rear Suspension, Subframe, Tire (HD) - YouTube[/ame]

[OkieSnuffBox]

Originally Posted by SVLTE

This is definitely gospel for track driving, but does it hold any water for autocross? If it did, it seems the ideal setup in CS would be the 205/45/17 RE71R since it has a 7" tread width. I haven't seen anyone claiming to be running that or the 215, just 225 and 245.

Typically no. At Auto-X speeds you aren't as worried about rolling resistance and aero, however, the car is typically constantly in transition and can use the extra width.

On a road course, you spend alot of time going straight where the extra width is slowing you down between corners.

The Miata guys have even done testing showing Hoosiers (while having a higher apex speed) will actually lose a few mph at the end of the straight to something like an NT-01 (same width for both) which entered the straight slower.

And that's purely on compound.

[CSG Mike]

Originally Posted by OkieSnuffBox

Typically no. At Auto-X speeds you aren't as worried about rolling resistance and aero, however, the car is typically constantly in transition and can use the extra width.

On a road course, you spend alot of time going straight where the extra width is slowing you down between corners.

The Miata guys have even done testing showing Hoosiers (while having a higher apex speed) will actually lose a few mph at the end of the straight to something like an NT-01 (same width for both) which entered the straight slower.

And that's purely on compound.

Also, note that Hoosiers are WAY, WAY lighter than a NT01 of the same size, and weight is being reduced from the highest point of inertia.

[AreteAuto]

When I switched to some lighter wheels, and 225 PSS, I lost a couple mph at the end of the back straight at road atlanta. But the rest of the course they were much much quicker than the stock setup of course.

[philooo]

I ran my car this weekend back to back with a 245 BFG R1 tire setup then later on 225 AD08, and really I can barely tell the difference.

Yes there is a slight edge to wider tires, but if you don't plan on driving your car 10/10 on the track you will be more than fine.

I have a full cage in my car and still I am not looking to drive it 10/10 unless I was racing, which I am not..yet ..

I would love to see real comparison on the same tire make/model and different size, especially 225 on 8" rims and 245 on 9" rims.

I bet they even out, faster turns on 245 and faster straight on 225. I think 225 may be more fun all around.

[strat61caster]

Originally Posted by philooo

I ran my car this weekend back to back with a 245 BFG R1 tire setup then later on 225 AD08, and really I can barely tell the difference.

Yes there is a slight edge to wider tires, but if you don't plan on driving your car 10/10 on the track you will be more than fine.

I have a full cage in my car and still I am not looking to drive it 10/10 unless I was racing, which I am not..yet ..

I would love to see real comparison on the same tire make/model and different size, especially 225 on 8" rims and 245 on 9" rims.

I bet they even out, faster turns on 245 and faster straight on 225. I think 225 may be more fun all around.

Road and Track did it with 215 and 235 Dunlop ZI SS, it's exactly as you predict lateral grip vs. acceleration and straightline speed.

http://www.roadandtrack.com/car-cult...ransformation/

Around a track (I believe some iteration of streets of willow, can't tell) the 215's were faster than the 235's.

[redlined600]

Originally Posted by philooo

I would love to see real comparison on the same tire make/model and different size, especially 225 on 8" rims and 245 on 9" rims.

I bet they even out, faster turns on 245 and faster straight on 225. I think 225 may be more fun all around.

Likely course dependant

[SVLTE]

I appreciate the input on my AutoX question! I'll be sticking with 225's.

Trackwise, I've been running some 17x9 Rota Titans w/ 245/40 PSS. I enjoy driving on the setup, but they are most certainly overkill on my C-street prepped car. I've got those wheels up for sale and plan on dropping to a 17x8 with either 215/45 or 215/40 AD08R's. Has anyone else run a 215/40? My local track has a lot of elevation change and the gearing advantage seems like it would be a welcome one. There are two tricky turns, both are tight, uphill increasing radius right handers. Staying in third puts me out of the power band, but downshifting to 2nd requires an extremely awkward shift back to third before the wheel is straight again.