Here's what I've been wrestling with over the past year or so...

My 2900-lb EP3 has 16" wheels on it now. I'm thinking about a set of race-only wheels & tires (probably for auto-x, since I don't know when I can ever make it out to a track). 15's and 17's can fit, too, and I'm not that interested in 18's since there are so much fewer tire choices.

But anyway...

Given the same overall diameter & tire width, and assuming that weights of each tire-rim combination are basically equal, which is really better? Small-diameter wheels with more sidewall, or large-diameter wheels with less sidewall?

I'm going to leave it at that for now...

 

for autocross ive always been told, the smallest wheel you can fit over your brakes is whats best, the fastest miatas and civics out there are on 13x7+ wheels.

 

A big factor you didn't mention is tire availability. It doesn't matter which has a slight theoretical advantage if you can't get the best tires for it.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxtasy &raquo;</TD></TR><TR><TD CLASS="quote">A big factor you didn't mention is tire availability. It doesn't matter which has a slight theoretical advantage if you can't get the best tires for it.</TD></TR></TABLE>
Good point. I can find Azenis RT-615's, Hankook 212's and some similar tires for my size in 17's, but not much of anything besides that. But, by moving to 15's, I can go with 225/50's and 225/45's and pick between Hoosiers, Victoracers, and RA-1s, among others.

 

But anyway...

Again, given the same tires and widths, is it better to have a shorter sidewall, or a smaller rim diameter?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BarracksSi &raquo;</TD></TR><TR><TD CLASS="quote">smaller rim diameter</TD></TR></TABLE>

 

Smaller tire + wheel diameter will give you better acceleration. I run stock EG brakes so I run 13x6 with 185/60/13 RA/1's and I love them, just like a 4.9 final drive.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by cibola &raquo;</TD></TR><TR><TD CLASS="quote">Smaller tire + wheel diameter will give you better acceleration.</TD></TR></TABLE>

True, within any given gear. On an autocross or road course, that may help or hurt, depending on the course. (If it forces you to use a higher gear for much of the course, then your acceleration can actually be worse.)

But I think the original question was more about cornering than about acceleration or braking...

 

damn this place has went to **** over the past year...

to answer your finally question the smallest sidewall possiable will the best ie the larger rims. since we are speaking about the same overall diameter here. the guys telling about 13x8 etc yes that works on those old cars, but on your car probably now not the best choice.

Now with all of that said in your said car you have very short gearing already, so down sizing the overall diameter shouldnt be a goal of yours.

BTW the fastest Street Modified civic in the country use 15s not 13s, and also the fastest Sm2 miata use 15s that should tell you something.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Mattamotor &raquo;</TD></TR><TR><TD CLASS="quote"> the guys telling about 13x8 etc yes that works on those old cars, but on your car probably now not the best choice.</TD></TR></TABLE>

Can you explain how the age of the car matters here? I've always been curous about this subject because there always seems to be opposing opinions.

I've been told the reason Touring Cars run huge wheels is because they car run huge brakes and that the smallest wheel that can fit over your brakes is best. Something about the size and shape of the footprint when the wheel width and diameter are the same. But then there is the sidewall flex issue of a smaller diameter wheel. Do modern tires really flex all that much to where a sidewall difference of 1" will negate the advantage of a better contact patch?

 

just a note, F1 cars run a 13" wheel with a big tire. but the technology is much different than what we have available

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nonsense &raquo;</TD></TR><TR><TD CLASS="quote">I've been told the reason Touring Cars run huge wheels is because they car run huge brakes and that the smallest wheel that can fit over your brakes is best.</TD></TR></TABLE>

That's indeed another practical factor to consider, in addition to the "theoretical" aspects.

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nonsense &raquo;</TD></TR><TR><TD CLASS="quote">Something about the size and shape of the footprint when the wheel width and diameter are the same.</TD></TR></TABLE>

You mean, like a 15x15 wheel?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxtasy &raquo;</TD></TR><TR><TD CLASS="quote">
You mean, like a 15x15 wheel?
</TD></TR></TABLE>

no, no. I'm talking about tire diameter, tire width and wheel width being the same between the two comparison setups, the wheel diameter being the only difference. Like for example, take Hoosier DOT-R tires in these sizes:

14x7 with 225/50-14 (23" tire diameter, about 4.43" sidewall)
15x7 with 225/45-15 (23" tire diameter, about 3.99" sidewall)

They have the same diameter and the same section width mounted on 7" rims. The only difference is the sidewall. Which will have the advantage if they both fit over the brakes?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by drewbie &raquo;</TD></TR><TR><TD CLASS="quote">just a note, F1 cars run a 13" wheel with a big tire. but the technology is much different than what we have available</TD></TR></TABLE>

It is also the rule. Not like they have a choice. This is one of the way they limit the brake size.

 

the 15s

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BarracksSi &raquo;</TD></TR><TR><TD CLASS="quote">Again, given the same tires and widths, is it better to have a shorter sidewall, or a smaller rim diameter?</TD></TR></TABLE>
Both. Smallest wheel that will fit over your brakes with the shortest sidewall available.

It's obviously a bit more complicated than that though. There really can't be a rule of thumb because there are so many factors to take into account. Sometimes the shorter gearing isn't desireable, and often times not available anyway.

What class are you shooting for and what other modifications does your car have? Making recommendations without this information is just pissing into the wind.

Sounds like you're looking at SP since you mentioned DOT-R tires. If I were running an EP3 in SP, I'd try out the 225/45-15 A6 first (but I can't make any guarantees that it is the best setup). An LSD and a Hondata ECU with raised rev limit are crucial, otherwise you're just going to get wheelspin and be constantly bouncing off the fuel cut.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nonsense &raquo;</TD></TR><TR><TD CLASS="quote">14x7 with 225/50-14 (23" tire diameter, about 4.43" sidewall)
15x7 with 225/45-15 (23" tire diameter, about 3.99" sidewall)

They have the same diameter and the same section width mounted on 7" rims. The only difference is the sidewall. Which will have the advantage if they both fit over the brakes?</TD></TR></TABLE>
The 15s will have a more precise steering feel and the spring rate of the tire will be lower (though this is less of a problem with super-stiff hoosiers than it is with, say, a Kumho MX).

 

How competitive you looking to be?

If you're just doing it for fun, then get what you like and/or what is cheapest.

You seriously want to do well in a class, follow this advice:

STS: 17x7.5's with 225/45/17's
STX: 17x8's with 235/40/17's
DSP: 18x9's with 285/30/18's front, rear use 245/16's.

When you are dealing with autox, sometimes tire width is more important than gearing. There is no 'firm' answer as every car has different things that it likes.

Testing will be the only way you'll find the fastest combo.

You also did not tell us what class you wanted to run in. Decide that first then go for the best wheel/tire choice that will work on your car.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Targa250R &raquo;</TD></TR><TR><TD CLASS="quote">
Both. Smallest wheel that will fit over your brakes with the shortest sidewall available.

It's obviously a bit more complicated than that though. There really can't be a rule of thumb because there are so many factors to take into account. Sometimes the shorter gearing isn't desireable, and often times not available anyway.

What class are you shooting for and what other modifications does your car have? Making recommendations without this information is just pissing into the wind.

Sounds like you're looking at SP since you mentioned DOT-R tires. If I were running an EP3 in SP, I'd try out the 225/45-15 A6 first (but I can't make any guarantees that it is the best setup). An LSD and a Hondata ECU with raised rev limit are crucial, otherwise you're just going to get wheelspin and be constantly bouncing off the fuel cut.</TD></TR></TABLE>

Gearing changes are a non-issue because the tire diameters are the same. I know it's not an exact science (the hoosiers in the example I gave the 14's are actually wider than the 15's even though they're both listed as 225's).

I guess I'm talking theory here. It's not really autox specific. My point was that there seem to be 2 theories when it comes to the question of wheel size. Some say the contact patch will be larger when using a smaller wheel (in theory it is). Some say that the larger wheel will have stiffer sidewalls so directional changes are going to be faster and more predictable, but it seems most just pick a side and argue it w/o understanding what's all involved (not that I understand it).

Just food for thought. If we had unlimited budgets we'd have the best of all worlds, optimal wheel/tire and gearing setups that are perfectly in tune with our race engines, but as it is everything's a compromise.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nonsense &raquo;</TD></TR><TR><TD CLASS="quote">Can you explain how the age of the car matters here?</TD></TR></TABLE>
Old cars (like the ED7) are lighter and require less tire (in theory, but you still want all the tire you can fit for autocross), and also have smaller brakes that fit under 13" wheels. The EP3 is a heavy beast that needs more rubber and requires a 15" wheel to fit over the brakes.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nonsense &raquo;</TD></TR><TR><TD CLASS="quote">Some say the contact patch will be larger when using a smaller wheel (in theory it is).</TD></TR></TABLE>

No, it isn't. As long as the tires are inflated and supporting the weight of the car, the size of the contact patches depend only on the air pressure and the weight of the car, and not on the width of the tires or the diameter of the wheels. For example, if you have a 2640-pound car with tires inflated to 33 pounds per square inch, the area of the contact patches is 80 square inches - regardless of the treadwidth or wheel diameter. The advantages of wider tires are primarily due to changing the shape, rather than the size, of the contact patch, as well as with better management of heat buildup, due to less deformity of the sidewalls and a lower percentage of the tread in contact with the ground. Read this article for a more detailed explanation.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Todd00 &raquo;</TD></TR><TR><TD CLASS="quote">You also did not tell us what class you wanted to run in. Decide that first then go for the best wheel/tire choice that will work on your car. </TD></TR></TABLE>
Yeah, that's something that I haven't decided on yet, either.

I've been thinking of swapping in a K24A2, which would drop me into SM and add the option of running R-comps. The way the car is now, though, places it in STX, which changes the tire choices a bit.

Gearing has depended on the course. I think I've touched the rev limiter in 2nd once so far, but most of the time it ends up shy of redline. I'll have to check again if I do the swap -- it might be possible to hit the top of second gear quicker, which would negate the need for shorter tires. BUT, the tight sections may still work better with shorter tires even though it would have a significant bump in torque.

So far, I'm doing the autocross thing for fun. I don't have enough free weekends to race for points, and I don't see myself making trips to national events until after I grow old & retire. I kinda doubt that the EP can be genuinely competitive, although a driver in a base RSX won the challenge portion of a local ProSolo a couple weeks ago. As I improve as a driver, then maybe I can use the EP better and find out just how far it can go.

Boy, it sure is easier to discuss dyno graphs & engine mods... lol

 

Thinking some more...

What's been holding me back on corner exit hasn't been a lack of torque, but a lack of traction. I have to squeeze into the throttle, unwind the steering, and avoid wheelspin that way. Without an LSD, shorter street-legal tires would just make that worse, but R-comps might be sticky enough to let me get on the gas a little sooner.

But again, with a swap (which would include an LSD; might as well go all the way), everything's gonna change.

I'm basically considering three scenarios -- keeping the EP as-is, adding an LSD, or doing the motor swap.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BarracksSi &raquo;</TD></TR><TR><TD CLASS="quote">So far, I'm doing the autocross thing for fun. I don't have enough free weekends to race for points, and I don't see myself making trips to national events until after I grow old & retire.</TD></TR></TABLE>
I wouldn't even bother worrying about high vs. low profile or any of that crap then. Pick up some 15s because the tires are cheaper.