Wheel size for AutoX?

[runbikerun]

Just out of curiousity is there a preferred wheel size that all of you autoxers use? Is it simply lighter is better? Or are there other factors you look at?
Thanks

[alvarospatchez]

Generally lighter is better, however with autoX you will want width as well. You want to maximize the contact patch so it's a balance to get the most out of it.

[mynameisphunk]

Depends on what class you're running. If you're running stock class, the wheel can't be any wider than a factory wheel - so 6". Is the car you're running stock?

[teamkitty]

Piggyback question: the stock steelies are 15x5.5, so does that mean if you're running a base model in a stock class with aftermarket rims, that you have to stick to 5.5-inch width? Or can you upgrade to the stock width of the sport model without breaking the rules. Will anyone care that you are "cheating" by bumping the width up to a still relatively skinny 6 inches?

[Type 100]

Originally Posted by alvarospatchez

Generally lighter is better, however with autoX you will want width as well. You want to maximize the contact patch so it's a balance to get the most out of it.

Interesting.

A related question:
Rules permitting, is there any benefit to running a larger diameter wheel in autocross (e.g. moving up to 16" from the stock 15")? I hear "less sidewall squirm" being bandied about for larger wheels with thinner tire sidewalls, but does that really impact one's autocross time? Or is lighter wheel+tire weight more desirable?

[cab0053]

Originally Posted by Type 100

Interesting.

A related question:
Rules permitting, is there any benefit to running a larger diameter wheel in autocross (e.g. moving up to 16" from the stock 15")? I hear "less sidewall squirm" being bandied about for larger wheels with thinner tire sidewalls, but does that really impact one's autocross time? Or is lighter wheel+tire weight more desirable?

wouldn't increasing wheel diameter increase distance covered per rotation?

say the actual diameter of the 15" wheel and with the tire is 19" (I really don't know the actual diameter)
so 19*3.14= 59.66"

and if 16" wheel is 20.25" (the 15:19 is 79:100)
20.25*3.14= 63.63"

That's a 4" increase.

now, there are 15840 inches in a 1/4 mile.

with the 15" wheel (59.66) that's 265.5 rotations
with the 16" wheel (63.63) thats 248.93 rotations

the 16" aprox does 6% less rotations than the 15"

I don't know the ratio of the hp: weight of mass rotating,but it's something like: for every x# lost of the thing that rotates (wheels, axels), y hp is gained. I think its like 4#lost=1 hp

so, if ssr type c's 15" +45 offset weighs 8# and the 16" version is 11.3#, that's 6.6# increase in the wheels. plus the tires.

so is the 6% less rotations worth the aprox2.5hp loss?

I'm no math guy, but sometimes I really like doing this stuff, even though I'm not sure about it.


after rereading the comment I quoted, I discovered I completely skipped the comment about the thinner side walls for better handling in autocross.

dammit.



I'm not letting all this math go to waste.

[gwai-lo]

get some ls meshes and use those. thats what i run. 195/50 or 205/50

[alvarospatchez]

It's been a year or 2 since I autoX'ed but I do not believe anyone will balk if you used the 6" wide rims on a base Fit. I think that falls into the gray area in the rule book. A lower profile tire does have some benefits over a higher one. There will be less sidewall flex. However if you do have a taller sidewall, running higher tire pressures can compesate somewhat. It's not uncommon for autoX'ers to run 45 psi or even higher. I used to run at 45 psi. It takes some trial and error to find out what's best for your set up.

[Type 100]

Different strokes for different folks then. Thanks!

Never had any autoX experience, just trackdays...so all this is pretty new to me. Didn't know 45psi tire pressure is common :P

[alvarospatchez]

yeah really high pressures are common. Tire heat is not a concern with autoX like it is with track days. I've done loads of both.

[ptt127]

If you haven't autocrossed before, I would highly recommend running at least a few events before making any changes to the car. You really don't need anything other than for your car to be in safe operating condition to learn a lot and have a great time, but the rules and class structure are such that making tiny modifications can throw you into a class that your car is wholly underprepared and/or uncompetitive for.

If you do some events, you'll learn a lot about rules and classes and then be able to come up with a good plan for your car before you start spending money. And honestly, a very good plan might be to do nothing for a while. Autocross is tremendous fun, but it requires the driver to hone skills that may not get much use on the street. Even the most naturally gifted driver (and face it, aren't we all ) can take a while to get used to it. One of the most common mistakes for people starting is focusing on the car, when you will quickly see that the best thing to work on is the nut behind the wheel.

Fortunately, you will probably find that people who autocross are very helpful. Talk to folks with experience and similar cars. Seek out the fastest people in your region and ask if you can ride with them. Maybe ask if they can ride with you too. It can be great fun and a real eye opener. Many regions run schools, or host national schools like Evolution which can be of great benefit even to a beginner.

For your reference, the SCCA Solo rules are available online here:

http://cms.scca.com/documents/Solo_R...Solo_Rules.pdf

They are very thick, but section 13 starting on p.59 describes the stock category rules. What is important to note is that the rules state explicitly which modifications can be done, it is never a case of "it didn't say that I couldn't..." Here's a clip from the wording on wheel sizes in stock:

13.4 WHEELS
Any type wheel may be used provided it complies with the following:
it is the same width and diameter as standard, and as installed
(including wheel spacers if applicable) it does not have an offset
more than +/- 0.25 inch from a standard wheel for the car. The
resultant change in track dimensions is allowed.
Wheel spacers are permitted, provided the resultant combination
complies with the offset requirements of this section. Wheel studs,
lug nuts, and/or bolt length may be changed.

So the width and diameter have to be the same, and the offset must be within 1/4" inclusive of any spacers. That means that if the GE Fit Sport wheel is 16 x 6 +53 (is that right?) then the allowed offsets would be 47-59. You could use up to a +64 with a 5mm spacer, but good luck finding one of those

Regarding running the sport width on a base, the rules say that all package conversions must be done as a whole, not in pieces:

Except for modifications authorized below, Stock Category cars must
be run as specified by the factory with only standard equipment as
defined by these Rules. This requirement refers not just to
individual parts, but to combinations thereof which would have
been ordered together on a specific car. Any other modifications or
equipment will place the car in Street Touring, Street Prepared,
Street Modified, Prepared or Modified Categories as appropriate.
Configurations involving damaged parts (e.g., blown fuses) are not
typically authorized by the manufacturer and hence are not allowed.
Option package conversions may be performed between specific
vehicles of a particular make and model, but only between configurations
from within a particular model year. Such conversions must
be totally complete and the resultant car must meet all requirements
of this Section. These requirements are not met by simply
pulling a fuse to disable a feature which distinguishes one model
from another.
Alternate parts listed in a factory parts manual are not authorized
unless their use is specifically referenced in the factory service
manual or in a service bulletin for the specific model.

So technically it is not legal. Since autocross is for the most part a self-policed sport, I would highly recommend that you ask your competitors if they are okay with you running another configuration. There's a good chance they might at the local level, but it is a nice courtesy to ask.

Sorry for the novel, go out there and have fun!

[teamkitty]

ptt127, thank you for that incredible writeup! Autocrossing is definitely something that I want to try my hand at one of these days. In a past job I got to take daimlerchrysler's srt vehicles to various tracks and parking lot set-ups for fun runs, but never for competition. Call me crazy, but I'm thinking that zinging a Fit around the cones will be just as fun as trying to keep a 600-hp viper under control and not spinning out during the same parking lot maneuvers. Might even be faster!

[Type 100]

Originally Posted by ptt127

Sorry for the novel, go out there and have fun!

Hey no problem. I appreciate helpful "novel" posts

I guess the rules and eligibility will depend where you go (I'm based in the Philippines for instance - our stock GD1 14" wheels have +45 offset) but I see the logic.

I'm considering it too, although the tire expense is kind of keeping me back. Thanks! +rep for you

[ptt127]

Originally Posted by teamkitty

ptt127, thank you for that incredible writeup! Autocrossing is definitely something that I want to try my hand at one of these days. In a past job I got to take daimlerchrysler's srt vehicles to various tracks and parking lot set-ups for fun runs, but never for competition. Call me crazy, but I'm thinking that zinging a Fit around the cones will be just as fun as trying to keep a 600-hp viper under control and not spinning out during the same parking lot maneuvers. Might even be faster!

Manufacturer events are a lot of fun and a good reason why you should try autocross. I have been doing it for about 10 years, and always enjoyed flogging the cars with the least power (E Stock '93 MR2 N/A and Street Touring '89 Civic Si) more than the others ('99 BMW Z3 Coupe and '04 S2000).