mark kibort

I have always been fine with the 275s up front. we even used the toyo 285x35x18 in wcgt for those two seasons where they used street tires. no problem.
My 10" rims, on which the 305s are fitted, dont fit because of the offsets. If I got a 1/2" further inside offset, it would have been fine, however, it would be a little too much tire on the inside, as the 10" with 8.5" backspacing is more than the 9.5" rim with my standard perfect fit, 8" backspaciing.
so, the real way to do this , and have the car as low as I have it, is to flare the fenders by .5". otherwise, as Brian has seen, it rubs on the fender, even with his greater offset, AND it limits range of steering dramatically. the 305x35 also is a very tall tire! now, with a low car you run into the tire hitting the top of the fender on the inside!

what i have done is a quick, cheap, effective fix at the cost of the some looks. the good news is that i can move one bolt and have it back to stock looking. the other good news is that in this set up, i can run 1/2" spacers with the older 275's and get a wider foot print for even better handling up front!

by the way, the tires due buldge a little on the 10" rims. but, its no different than when i used to use the 335s on the 11s. Here is a picture of the fit of the 275s on the front on either my 10 or 9.5" rim.

As a side note, that 285x30 has no sidewall as pictured in the 951 shots. that looks cool, but will be a bear for handling. race cars NEED sidewall on their tires!
mk

333pg333

I would still think that 305s on 10" are too big and that's what all their reletive website specs would have you believe, however just to contradict this I am told that the Hoosier R6 315 has a narrower track width than the 305 so go figure?

IcemanG17

Yes a 305 is slightly too wide for a 10" rim...this makes an aspect ratio of .83...vs the commonly used limit of .86....yes its close...

I do think a 305/30 would fit much better...I found it was the height of the tire 26.4" compared to the much shorter 295/30 at 25.0" that makes such the difference on rubbing....

Also I don't think a 10" rim with 60mm offset is enough....my 10" rims have 88.9mm offset (9" backspacing)....thats over an inch MORE on the outside than mine...take a look at the front wheel in this pic taking a fairly high speed left corner (say 70mph).....still plenty of clearance.....maybe 1/2" at that steering angle....now add 28mm (1.1") and it would be smashed into the fender...front ride height is around 110-120mm

mark kibort

I agree. Its not an optimal fit, but i remember this porsche 996 with 10" rims that had 305s on them. It worked fine and I even raced on those rims as loaners on the 'ole 79 at T-hill. the 335s on 11s looked the same too, but with less sidewall. one thing, the shape of the tire is slightly altered and makes the width a little more narrow. I remember the 335s on the 11s were only 1/4" wider than the 305s, but off the rim they were more like 1" wider!
Ive already driven on the 305s on the 11s and they feel fine. wear patterns look normal, its just the side wall is pinched in a bit (angled).

Im still trying to see if i can get the 295x30s from WCGT throwaways. that might make a better fit.

mk

IcemanG17

MK
I did some research on the tires WGT uses....it looks like it skips from R888's in 275 to 305 then 335......bummer...

mark kibort

edit: Your right brian.
http://www.world-challenge.com/files...x_A_Ver_10.pdf
However, it is the NEW viper comp coupes that get to use either the 305 or the 295 front tire. im sure noone will be using the 295 front, but you never know.

I have a phone number of someone that has access to some used 295 R888s

mk


mk

IcemanG17

MK
Thats true...the Viper ACR uses MPSC in 295 front 345 rear...so its possible they are used..... worth looking for anyway.....

333pg333

88.9 offset?? That way bigger than I've heard of before? Seems like the inside of the rim would be too unsupported but I'm no expert. Are those R888s in your pic? Nice pic by the way, car looks great.

mark kibort

The picture of my car above is an old one with the 275 on a 9.5" rim with the 8" back spacing. It is VERY common to use 8" backspacing for rims that will work with the big tires on a 928 front and rear. (11"s on the rear)

mk

mark kibort

The picture of my car above is an old one with the 275 on a 9.5" rim with the 8" back spacing. It is VERY common to use 8" backspacing for rims that will work with the big tires on a 928 front and rear. (11"s on the rear)

On the front, that 10" rim with the same fender position, goes to 8.5" backspacing. You think that is bad, look at Brians front rims. His backspacing is 9" on the 10" riim, and he still rubs with the 305. Thats why the fenders just had to be moved out. Ithink he has a little too much backspacing and it might be something to be concerned with as far as the canlevered forces on the rim


mk

333pg333

I just wouldn't have thought that anyone made a rim with Brian's offset of almost 89mm?? When you refer to backspacing are you guys including wheel spacers or something? I refer to the offset as what the wheel is manufactured with.
Are Brian's wheel arches made bigger? Looks like the rear fender is extended in the pic.

dprantl

Some measure backspacing including the wheel lip, others don't. Some just use a standard 0.5" for the wheel lip, even though wheel lip size varies greatly. If you really want an accurate offset measurement, measure backspace (wheel mounting to rear edge) and also measure wheel edge to where the tire edge mounts. Then use this formula: offset in mm = backspace in inches - wheel lip size in inches - (wheel width in inches / 2) * 25.4.

Dan
'91 928GT S/C 475hp/460lb.ft

IcemanG17

The wheels are custom made to order by http://www.ccwheel.com/files/home.php

Not cheap...but not too bad considering the quality......and a strong light forged wheel

My front fenders are slightly rolled, but not much....my rear fenders had GTS flares welded in, then rolled....I do have a spacer of about 1/4-1/2" in the rear...so I could easily run 345 tires or larger....the 335's clear easy

I do think running an 11" front rim with 315's using MK's fender mod might be the way to go, but I'm happy with how the car handles now.....we'll see if I get to a point where I need more front tire?

And yes they are R888's

mark kibort

flat edge across the rim edge and a distance to the mounting surface of the rim. 8" backspacing for 9.5 and 11" rims front and rear is as far as you can go and not do body work. (still rolling required to fold the inside lip inward).
I dont know what the offset is, but its pretty dramatic, and even more so with Brian's. any kind of other cars front wheel offsetwill be very hard pressed to work without rubbing. and if the car is lowered, forget about it. again, i think my offset works out to be around 82.5mm and rears in the 63mm range. If the standard offset system doesnt use the lip edge, then subtract that out.

mk

Benton

So Mark, I'm thinking about doing the exact same thing on my car. My eventual plan, when I get the time (not for another 10 months at least), is to widen my front wheels and flare my fenders. For now, I have two options: Put 275's on the front and 305's or 335's on the rear, or do what you did and put 305's on the front with 335's on the rear. My front wheels are identical in specs to yours (10" w/ 8.5" backspace).

Now that you have more time with this setup, would you recommend it? The cost difference is negligible, as the tires (used) are essentially the same price.