Does anyone use 15in wheels for auto-x? My bro gave my the tires he had on his si and they're a lot better than my re92's. I saw some people had trouble with this size on a GD. Let me know. Thanks.

 

seems a bit small for a car with as much mass as the GD, what size are the tires (height and width)? I'd recomend going bigger, especially for autox

 

the tires are yoko 195/60/15. I was going to put them on a set of rims just for auto-x. I know it doesn't seem right to go down to 15's, but its cheaper and most likely lighter. My main concern is whether or not the rims will clear my brakes.

 

There are only a few 15" wheels that will clear the GD brakes. (Compomotive, Speedline being two).

That said, you want as wide a contact patch you can get...and optimally a stiff sidewall for auto-x. Deceasing the contact patch using a 195/60-15 tire is definitely going in the oposite direction and I wouldn't recommend it. (You'd be pissed at the lack of grip and you'll have even more plow) Try finding a set of WRX 16" wheels (which are light anyways) and put some 215/45-16 on them. Both Hoosier (Stock) and Falken (Azenis for STS or STX) make that size.

--KC

 

Doh! I thought they were 45s. OK. Point still stands... go with 16s.

 

Actually, a guy that runs in the KC SCCA auto-x events had (has, maybe?) an RS that he ran. He suggested that I get 15 inch wheels for my car, saying that using the smaller size would enable the turbo to play more of a part on those short straightaways.

He also told me that good, wide tires will be more helpful than the 15" wheels. He runs Falkens, but said that Kumhos are pretty good too.

And, on a lighter note, at the final event for the year he was driving his new Forester XT (with Rota wheels and Falkens) and beat every single Cobra that ran.

(no substitute for cubic inches my eye. hahahahahaha)

 

The RS can run 15" naturally. The WRX has larger front calipers and that's why it's difficult to run 15" wheels on a WRX.

Also, if you're going to think about a 15" for the WRX (ie: Compomotive), remember to verify the load rating of the tire you're going to use. Not many 15" tires are made to handle the 3K weight of a WRX under high load. This is also another big reason to go 16", with just a lower profile.

--kC

 

15's might work. but try to get a tire that will make the overall diameter of the wheel + tire less than the stock diameter. why? to improve gearing. the smaller the diameter of the wheel + tire, the faster the car will accelerate which will allow the car to rev up quicker. so now you'll be at a higher RPM during a corner. means you can hit boost sooner and get to the next corner sooner.

cliff notes : aim for smaller diameter tires for autoX.

 

Can you go into more about the diameter thing? I was under the impression that the mass moment of inertia of the wheels pertained more to the acceleration of the car.

 

The mass moment of inertia in getting the wheels to turn is a pretty small factor when compared to the moment between the axle and the contact patch, which is where the power is put to the ground.

Think of it this way, 1 lb-ft of torque will provide one pound of force at the edge of a 12" radius wheel, but it will provide 2 pounds at the edge of a 6" radius wheel.

It's kind of the same idea (working the opposite direction though)to why a long lug wrench gives you more nut tightening force, but in that case you are providing the linear force (in pounds) to a longer moment.

Now we aren't talking about 2:1 factors in wheel size, but the basic principle holds.

Your correct in noting that MMI of the wheel would be the trade off, but not as much as you might think, as lower profile tires will weigh less, somewhat offsetting the larger diameter of the wheel itself.

Remember moment and torque are both "force times distance" functions

 

15 inch wheels help because for the car to go the same speed, the engine RPM's have to be higher as compared to stock diameter.

example :

wheels with a total diameter of 18 inches. on a WRX. if the wheel makes one revolution, the car will have travelled 2 * Pi * 18 inches.

wheels with a total diameter of 17 inches. if the wheel makes one revolution, the car will have travelled 2 * Pi * 17 inches.

so for the smaller wheel to travel the same distance, the engine has to go through more revolutions.

this actually means that the engine is having to work more to get the car to move the same distance. but the benefit is that at any given speed, the engine RPM's will be slightly higher with a smaller diameter wheel+tire combo. this is good for the WRX since staying is boost is very important.

this generally only applies to AutoX type situations where all the corners are tight and generally low speed corners. this allows the engine to be in the 'sweet spot' during most of the course.

i hope i've explained this properly. because it's very clear in my head.

 

You almost got it... I fixed a few things. You could have a 17" wheels that have a tire with an overall diameter shorter than some 15s or 16's with tires on them. Total diameter is measured on tires, becuase, natually they extend farther out compared to the wheel. The 17" or 18" measurements are the actual wheel diameters... the tires ad another couple inches to the total diameter of the wheel/tire combo.

Miata.net has a really good tire calculator on it to play around with tire sizes.

WHEEL= that thing you put your TIRE on.

 

yeah. i meant the total diameter meaning wheel + tire. i was just too lazy to type "wheel + tire" throughout the post. so i just said 'wheel'