Ordering Kodiak rims 18 inch or 17 inch ?

[mturo]

I can't make up my mind...for the track should I use 18" or 17" rims all around. I know SCCA mandates 17" but is 18" better? Why can you only use 17" in SCCA. In my region no one cares what rim you use. I am ready to order....but which size makes most sense for max performance? Any real life experiences would be appreciated.

[Silver Bullet]

If racing T1 you legally have no choice but 17x10,11's or stock size rims's if I recall.

18's will handle better in the front and allow a big brake package if ever upgrading.

[Arnoldir]

If planning on future big brake upgrade, discuss it with the wheel manufacturer before buying.

I've got 18x10.5 CCW classics up front, fitted for stock brakes and they will need either new centers or to be taken apart and machined if I go big brakes.

I love the 18" rims all around (nitto 555R2's), only reason I see to go with 17" rims would be to satisfy SCCA. I believe the SCCA picked that rim size to allow a standard 315 width race tire to be used on all 4 corners.

Just my 2 cents.

[GaryK]

I haven't priced r tires lately, but usually 17" are much cheaper than 18". If rules and/or competiveness aren't a concern, I would just go with 17" for that reason alone.

[mturo]

Good points thank you. I am inclined to go 17" based on tire costs. I find the z06 brakes to work very well.

[DJWorm]

If you are not bound by SCCA T1 rules I recommend

305/30x18 on 10.5x18 with +0.25" offset Front
315/35x17 on 11x17 with +0.25" offset Rear

[tlaselva]

Originally Posted by DJWorm

If you are not bound by SCCA T1 rules I recommend

305/30x18 on 10.5x18 with +0.25" offset Front
315/35x17 on 11x17 with +0.25" offset Rear

Best recommendation yet....

[speedpup]

18 x 12 CCW's front and rear here. What is the purpose of the tires or car? Street, track days, or combo?

[Z06-Z16]

If you are not bound by SCCA T1 rules I recommend

305/30x18 on 10.5x18 with +0.25" offset Front
315/35x17 on 11x17 with +0.25" offset Rear


Why would you go with the 17's in the rear?
With the 0.25 offset would these clear brake ducts on the front?

[DJWorm]

Concerning:
305/30x18 on 10.5x18 with +0.25" offset Front
315/35x17 on 11x17 with +0.25" offset

This set up cures a lot of handling deficiencies and allows the tires to be run under the body work with no tubbing and no brake duct or other clearance problems.

Let's look at the race tire selection process:

1. Working up from the OEM sizes one would naturally run
275/40x17F and the 305/30x18R on stock sized rims to get more rubber on the road and more grip. However this combination still does not address the understeer problem and may even make it worse.

2. In addition SCCA allows a +0.25 offset in Solo II and we have found this enables a slightly wider track on the road course which aids high speed stability and handling and allows inside clearance without degrading outside clearance.

If the rules are open on rim and tire size but we still want to fit them without tubbing and without clearance problems we will use the largest tire and wheel combination.

We have found that by moving the rear 305/30x18 to the front has some distinct advantages over other tires.
- First of all it fits and you can turn lock to lock.
- The larger tire contact patch area increases front grip, increases turn in and reducs braking distances
- It equalizes the front/rear tire contant patch, reducing understeer and brings the car to a more neutral state.
- The shorter side wall again aids turn in but also increase stability, increases steering response and increases modulation and feed back
- This all results more time in a straight line, quicker turn in, higher corner entrance speeds, higher mid corner speeds with less understeer on a tighter arc and a later apex.

We have thus solved most of the front handling problems with this larger tire with a shorter sidewall. Can we then improve the rear tire? The answer is yes.

We need to increase traction. To do this we use the 315/35x17. The 315 is just a touch wider than the 305 but the side wall is more flexible. This aids traction by increasing rear grip slightly when we need it and the more flexible sidewall allows the car to squat under hard acceleration and cornering. This allows higher corner exit speeds and also aids in quicker transition maneauvers such as rotating the car with power and a quicker slalom type transition.

We could go bigger in the rear but then understeer again becomes a problem and we might have to discard the rear brake duct for clearance (no big problem), we might have to tub but the biggest problem is contact of the inner rim shoulder with the rear spring perch bracket. This is not readily apparant in a static condition but under hard acceleration or cornering contact will be made as the sprin perch moves lower. If you start seeing small spiral Aluminum shavings on the track you are filing the inside of you wheels away. If you see these you will need to add a wheel spacer.

I am contemplating that the C6 Z06 will need 335 or 345 in the rear with 305's still in the front to handle the increased power.

Using 315's in the front can be tricky with the front brake duct and contact with the rear portion of the lower A Arm with the inside of the rim shoulder. The T1 boys and SCCA (as a limiting factor) will only allow them on a 10" front rim which in fact is sub optimal for the 315 and in fact turns it into a "300".
Whats more you now have a larger flexible sidewall up front which destroys turn in, modulation, braking, response and stability.

It should be noted that the C5R does run 18's all the way around but the tires are much wider as are the wheels and the suspension has a much wider track (6") and the suspension is completely different.

It should also be noted that the 305/30x18 and the 315/35x17 are almost equal in tire width and are equal in overall diameter. This does not cause any AH or TC problems as the delta is not greater. (I run with them off anyway, except in the wet of course)

[mturo]

Just when I thought I had it figured out DJ jumps in. Makes lots of sense.