ThetaTau87

I have 7x17 ET55 front and 9x17 ET55 rear Cup II wheels on my '89 GT. The rears have the original 17mm spacers installed making the effective offset in the rear 38mm.

I would like to get wider wheels for the front and go up to an 8" or even 9" wheel. There is an OEM 8x17 ET70 Cup II that was the rear from a 911 winter set that will work, but they are hard to find and expensive. The 70mm offset is also slightly larger than ideal. The OEM Clubsport wheels that my car came with originally were 8x16 ET60 in the front.

Will the 9" wheels I have on the rear (9x17 ET55) work well on the front? It may be easier and cheaper to find a pair of 9x17 wheels for the front if they will work than try to find a pair of the 8x17 Cup IIs.

My goal is to get more tire under the front for better balanced handling. The weight distribution is 50/50, but the widest tires I can fit up front on the 7" rims are 225/45/17. On the rear I have 245/40/17. This causes some understeer when pushed on the track. If I go up to 8" or 9" front rims I can use 245/40/17 all around or just on the front and go up to 255/40/17 in the rear for a slightly staggered look.

FredR

The 9 inch ET55 with a 245 section will be marginal. If the offset was 58mm with some fender rolling and perhaps a bit more camber it would definitely work. The increment of -3mm will make it very close indeed.

In your position I would want to know that the tyre/wheel combo I was about to purchase wil work.

Rgds

Fred

Vilhuer

There is also 9x17 ET70 version of Cup II or what factory calls Cup Design 93. It's 993 winter wheel for use with snow chains. Probably as difficult to find as 8x17 ET70.

http://www.pcarworkshop.com/images/3...1998_64028.pdf

EMan 928

Would this 9x17 ET 70 with a 255/40/17 be a good front tire for the 928?

Vilhuer

Probably better than stock setup but I would try 275/40/17 on all four corners. Rears with 9x17 ET55. Have to play with spacers to get them just right.

SMTCapeCod

Cue Gary & Jarrod in 5...4.....3...

Recent traffic: https://rennlist.com/forums/928-foru...-ii-wheel.html

ThetaTau87

There is one for sale on ebay. $575 BIN + shipping from the UK.

http://www.ebay.com/itm/191485273602?_trksid=p2055119.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

ThetaTau87

That's the post that got me thinking about this. $450 isn't a ridiculous price with free shipping, but if a 9x17 ET55 will work that would be cheaper and easier to find.

S4ordie

Erik, you have the best possible front tire wheel combo possible. It was well tested by the uber 928 gear heads (Greg, Rob, Gary, Jarrod, ....). Unless you were to fgure out how to squeeze a ten inch wide wheel under the front fenders, it's hard to figure out how to get more or better grip.

SMTCapeCod

Here.

https://rennlist.com/forums/928-foru...ml#post7500033

9 @ ET55, not likely. I'd anticipate mad tramlining, potential interference at fender. I bet Mark Kibort gave it a shot at some point though, as a test fit at least.

9 @ ET70, likely excellent.

8 @ ET70, I'm quite happy with, running 245.

ThetaTau87

Thanks for the link.

Why would you expect mad tramlining from a 9" ET55 wheel? The 7" wheels on the front of my car are ET55 without significant tramlining. Also, my GT came from the factory with 8" ET60 Clubsport wheels. 5mm makes that big of a difference? From what I've read a 55mm offset should be close to neutral where larger offsets than 55mm will give more self centering force.

It seems to me that clearance to the fender would be the main issue as the offset is very close to other factory wheel options for the front of the 928.

PHIL928

I have a set of 9" Cup 2 ET70 that I would like to sell, but getting them to you would be mad!

Gary Knox

In my opinion, the 8" et 70 front is perfect. That offset keeps the centerline of the wheel and tire within 1 mm of where it is in the OE GTS front (7.5" wide, et 65). Thus, there is no increase in the tendency to tramline etc.

Yes, the 9" et 70 gives the wider tire possibility, but it also sets the centerline out another 12.7 mm from where the engineers designed it to be for these cars. The 9" with the wider tires also results in a bit of rub when the wheel is at full lock, and some slippage of the wheels during sharp turns at low speeds (slip angle isn't correct vs the 7.5" et 65 or 8" et 70 wheels/tires). Offset of a 9" wheel would need to be ~et 83 to keep the centerline located correctly.

Yes, I KNOW, as I have sets of both the 8" et 70 cup II's and 9" et 70 Sport Design 2 piece wheels for the front of my '94. I like the looks of the 9" Sport Design 2 piece wheels enough to put up with a bit more tramlining and the slight wheel slippage at full lock. But, for best handling/driving, the 8" Cup II's are actually better.

There is a listing for ONE Cup II 8" et 70 on eBay currently (no relationship).

Gary-

ThetaTau87

Your calculations don't seem to add up in relation to the centerline of the wheel.

By definition the offset is the distance from the mounting face to the centerline of the wheel. That being so a wheel with a 70mm offset will have the same centerline location regardless of the width. If the wheel is 7", 8" or 9" wide and has a 70mm offset the centerline of the wheel is 70mm inboard of the mounting face. Only the location of the inner and outer lips of the wheel will change as the width increases.

For example if I removed the 7x17 ET55 wheels I have on the front of my car and put the 9x17 ET55 rears on the front the centerline of both wheels would be in exactly the same place. The inner and outer lips of the wheel would be 1" further away from the centerline. This could cause an interference with the fender from the added width, but shouldn't change the steering geometry at all.

I think you're confusing the change in location of the inner/outer lip of the wheel with the wheel centerline. Your calculations are correct to tell how far the lip of the wheel will move relative to the centerline. The only way to change the location of the centerline is with a different offset.

Gary Knox

ThetaTau,

One of us is wrong!! Probably me, as I had 3rd grade math a lot longer ago than you probably did - ha.