wheel weight vs. wheel size.

[StephenB]

What do you think? Is bigger better?
Does anyone have experience testing different sizes?

I am compairing ITR rims in 17 vs 18 and 16.5lbs vs. 18.3lbs respectively.

Thanks for your thoughts and ideas,
Stephen

[Greg Amy]

As long as you can get the widths, offsets, and tire sizes that you want/need, the only wheel diameter you *need* is enough to clear the brakes, suspension components, and body parts.

In other words, a larger wheel diameter, in and of itself, is not faster, and in may ways may even be slower.

GA

[Knestis]

This occurred to me yesterday, car shopping with Mrs. Dr. K. The smallest wheels we saw on among the compact SUVs (CRV-sized) were 16s. Options include 18s.

K

[benspeed]

I selected 16 x 8.5 inch Kodiak wheels for ITR - tires are cheaper, wheels are lighter and I can put down the same contact patch. I think they will be faster than 17s but others may disagree...

[JoshS]

I agree Ben. I'm using 17x8.5 only because the Kosei K1 is available in that size for my car, it's cheap and light. 16x8.5 would have to be custom and would be a lot more money to get anywhere near the weight.

[erlrich]

What do you think? Is bigger better?
Does anyone have experience testing different sizes?

I am compairing ITR rims in 17 vs 18 and 16.5lbs vs. 18.3lbs respectively.

Thanks for your thoughts and ideas,
Stephen

If you're talking ITR the class, then you're stuck with 17" wheels - unless you're building an RX8, in which case you can use the factory 18" wheels. Generally speaking however, I've always thought that lighter was always better when it comes to wheels, as long as the tire size you want to run are available for that size rim.

[Sandro]

think you need to compare weights with the tires on, because otherwise you are comparing apples to oranges.

[lateapex911]

Larger diameter tires can give you a larger contact patch fore and aft.
But they reduce torque, so you must gear for that. Now, you can often get tires that are lower profile that keep the outer diameter, but allow larger wheels. With street tires, this can be good, as the shorter sidewalls can be more responsive, but race tires are constructed differently, so that's not always a benefit.

But larger diameter wheels and tires can add rotating mass which is hard to spin up and down, so it can hurt accel and braking. Of course, the tires and wheels must be combined to know what you have.

If the overall package is heavier the extra weight is unsprung, so that's bad...worset han if it were equal amounts of ballast. So it's a tradeoff. If you have lots of power and lots of unlimited braking capability, it can be worth it, but generally it's not a 'given' that larger is bigger. In lower hp cars, it's usually not better.

[philstireservice]

I would go with the lightest wheel you can find in your price range. Of course tire price and availablity is a factor.

just another bit of info:

Full tread 205/50R15 RA-1 - 20lbs
4/32nd shaved RA-1 - 18lbs
2/32nd shaved RA-1 - 16lbs
205/50R15 Hoosier DOT Wet - 17lbs
225/45R15 Hoosier R6 - 19lbs
245/40R17 Hoosier R6 - 22lbs

for every 1 lb you save in wheel weight it is like taking 20lbs out of the car.....goes for tire weight also

example 1lb each wheel x 4 = 80lbs

[GKR_17]

Larger diameter tires can give you a larger contact patch fore and aft. But they reduce torque, so you must gear for that.

Larger diameter tires will change the gearing, but will have zero affect on torque.

[lateapex911]

yea, of course, I typed that too fast. But it hurts torque multiplication unless countered with gearing. (However, that can be useful at certain tracks with certain cars)

[Gary L]

Larger diameter tires will change the gearing, but will have zero affect on torque.
Well now, wait a minute... :)

If you don't compensate for the larger diameter tires by changing some other gearing component, you will shift the entire torque curve to a higher ground speed. Therefore, to say it there is zero effect is not entirely true, IMO.

[Flyinglizard]

Smaller OD, goes fastest with these little cars. Up to about 400hp. Do a search and you'l find back to back test. I have raced the same car with many diff OD wheels. Smaller has alway been faster. Polar moment, leverage,inertia, yada yada.. I'll put cash on it.
MM

[StephenB]

Thanks everyone. This is exactly the type of info I needed. It is what I had decided before i posted but it's always nice to have more opinions than just the one in your head!

Thanks again, Stephen

[dtmracing]

One other thing is the gyroscopic effect of the spinning mass...

[StephenB]

I selected 16 x 8.5 inch Kodiak wheels for ITR - tires are cheaper, wheels are lighter and I can put down the same contact patch. I think they will be faster than 17s but others may disagree...

Ben, or anyone else...

Are we allowed to run 16" rims in ITR if it only lists a 17 on the spec line?

Stephen

[Z3_GoCar]

Is the spec line wrong? Or did it come with only 17" rims in the US? Can you even fit 16" rims over the brakes? Looks like the only way to fit smaller than 17" rims is if the spec line specifies less than 17" rim sizes.

[StephenB]

The spec line does have 16s for Ben's car... my bad: )

Can you put smaller wheels on the car than what the spec line says? For example could an rx8 run 17s

[erlrich]

Can you put smaller wheels on the car than what the spec line says? For example could an rx8 run 17s

"Cars originally equipped with twelve (12) inch wheels may fit thirteen (13) inch wheels. Cars originally equipped with metric 365 wheels may fit fourteen (14) inch wheels, and cars originally equipped with metric 390 wheels may fit fifteen (15) inch wheels. The above-mentioned cars as well as those cars originally equipped with thirteen (13) inch or fourteen (14) inch wheels may fit up to a fifteen (15) inch wheel. Cars may not fit wheel diameters smaller than those listed on their spec line. All other cars shall retain the wheel diameter fitted as original equipment for their make, model, and type. Knockoff/quickchange type wheels are prohibited. Wheels must be made of metal. Cars classified in ITR may utilize any wheel diameter up to 17” or retain their stock diameter wheels if larger."

Kind of contradictory, but I read that as saying ITR cars can use any diameter wheel up to 17", or their stock wheels if larger.

[StephenB]

That is also the way I read it but before I start looking and certainly before I spend a significant amount of money I want to be sure... I know for sure the auto rx8 came with 17" and I think all the grand am cars run 17"

[madrabbit15]

I am not sure but I think I remember Steve Eckerich telling me a couple weeks back that 16s wont fit on an rx8. Hes building one now for ITR.

[wally2]

See page 34 of May 2010 Car and Driver. They test different size wheel/tire combos on a VW Golf (15x6 up to 19x8.5). Seems to have effects on braking, acceleration, skidpad and even mpg.