Tire/Wheel size and gearing

GaryR · GaryR's Garage

I've been playing around with some numbers and not sure what (detrimental) effect I would get running different size wheels front/rear. I currently run 18" wheels in 8" + 9" widths with 245/35/ZR18 in front, 275/35/ZR18 in back (Hoosier R6's). I was told there are a couple guys running 15" wheels for the gearing advantage. Here is the tire data from Hoosier.

275/35/18 - 10.3" Wide, 25.5" Diameter, 80.2" circumference
275/35/15 - 10.1" Wide, 23.0" Diameter, 72.2" circumference

There are no 35 series tires in 15" so the only choice is:

245/50/15 - 9.2" Wide, 24.8" Diameter, 78.0" circumference

My current 18" fronts are:
245/35/18 - 9.1" Wide, 24.7" Diameter, 77.7" circumference, which are close to identical to the 15" size. I can see the gearing advantage to the rears, but what effect would the larger diameter fronts have on the overall setup? Doesn't seem to work to me. I'm going to look at other tires like the BFG for sizes also but methinks I need /35/15's all around..

Bill Verburg · Bill Verburg's Garage

When considering changing tire sizes there are several things to consider. You named one, the change in gearing due to different loaded rolling radius.

You can determine the absolute effect fairly simply in several ways, I like to look at the relationship between thrust(the push that goes toward forward acceleration) and speed.

here's a comparison I did for my C3 when I considered the effects of 2 possible rear tire choices, 275/40 vs 265/35, both x17

As you can see the absolute magnitude of the thrust changes as well as the speed where max thust occurs. The difference in magnitude is greatest in the lower gears and at lower speeds and thus becomes much less of an issue for track events than for street driving, unless it forces a shift at a place where one is undesireable(i.e you don't want to have to shift in a corner or too near a braking zone)

Other effects to consider are;
weight
sidewall & tread implications
tread compounds
availability

304065 · 02-06-2009, 12:06 PM

Gary, why not? Same price and the smaller package has a lower polar moment of inertia for spin up/down. Rules say any diameter you want and up to 1" wider than stock. For an SC with stock fenders I think 245/275 is about the max I've ever seen.

Too bad RSR Fuchs 15" are so expensive. When I bring my car back from a 5-year storage hiatus I'm going with a set of Jongbloeds. He will make whatever the hell you want and it will be STRONG and won't let you down.

GaryR · GaryR's Garage

Thanks Bill/John. Though I see on paper that my front 18" wheel/tire combo is almost identical to the 15" wheels with the 50 series tire I just can't get over the fact of running an 18" wheel in front with a 15" in the rear. Due to the lightness of my CCW wheels I doubt the weights would have an enormous difference. I would imaging the gearing change would be substantial as the difference in circumference is 10% (8")! There are a couple tracks I would love to have a lower gear to get through and out of corners better and that is why i'm thinking of this (and I believe it's legal in my class).. Part of my wanting to run my 245/35/18's in front with the /35 series 15's in the rear is economy (only need to buy two 15" CCW's), the other is it made sense to keep 35 series tires all around..

Bill Verburg · Bill Verburg's Garage

Quote:

Originally Posted by Gary

Thanks Bill/John. Though I see on paper that my front 18" wheel/tire combo is almost identical to the 15" wheels with the 50 series tire I just can't get over the fact of running an 18" wheel in front with a 15" in the rear. Due to the lightness of my CCW wheels I doubt the weights would have an enormous difference. I would imaging the gearing change would be substantial as the difference in circumference is 10% (8")! There are a couple tracks I would love to have a lower gear to get through and out of corners better and that is why i'm thinking of this (and I believe it's legal in my class).. Part of my wanting to run my 245/35/18's in front with the /35 series 15's in the rear is economy (only need to buy two 15" CCW's), the other is it made sense to keep 35 series tires all around..

concerns of running drastically disparate tire size, dimension or construction are;
* car trim wrt aero and corner weights
* differential slip angles and hence temperature profiles
* differential response curves to to side wall variance

In general you can get some pretty weird handling issues as a result

304065 · 02-06-2009, 01:51 PM

Quote:

Originally Posted by GaryR

Due to the lightness of my CCW wheels I doubt the weights would have an enormous difference.

What is the wheel weight? A look at the Complete Custom Wheel web site shows weights up around 20 pounds per wheel. A Factory Fuchs 8x15 weighs 14#. That is a HUGE difference to spin up and down.

GaryR · GaryR's Garage

I'm honestly not sure John but I know the stock 16" Fuchs on my SC with street tires felt like they weighed substantially more than my 18" CCW's with Hoosiers. Never weighed them and the CCW's were 8" and 9" vs the Fuchs at 7/8".

911st · 02-06-2009, 03:57 PM

Here is some good info on 275/15's: http://www.buckley-racing.com/products.html

GaryR · GaryR's Garage

Quote:

Originally Posted by 911st

Here is some good info on 275/15's: http://www.buckley-racing.com/products.html

He uses the 225/45/15 R6 on the front, which is a 9" width vs. the 245's 9.1", sure doesn't sound like I would lose anything going to the 225/15's. Is there really that little difference between a 225 and 245? The 225/15 has a 22.9" diameter and a 72" circumference, which is actually .2" smaller than the 275/15 rears... Now to find a proper set of 8x15" and 9x15" wheels!

911st · 02-07-2009, 08:49 AM

Order the Buckely-Racing rims. He had already worked out the fit and they are a good price for a custom 3 pice race wheel. Plus they look great.

I guess you could take a set of 7&8 fuchs, widen them and use spacers to get the back space right.

ninesixfour · 02-09-2009, 05:59 PM

Quote:

Originally Posted by 911st

Order the Buckely-Racing rims. He had already worked out the fit and they are a good price for a custom 3 pice race wheel. Plus they look great.

I guess you could take a set of 7&8 fuchs, widen them and use spacers to get the back space right.

I saw those wheels too. I'm already running 7&8 x 15" Fuchs but would love to run 8s and 9s at some point. These Buckley wheels seem like a good deal.