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The ideal offset for racing wheel front is 55. It is the offset that clear the offset at the wheel (ie : the offset between center plan of the wheel and the pivot axle of the wheel) But, if you want to run big foot as it is a way to gain mechanical grip on track, yon can't choose that offset without rolling the fender. So you have to compromise.
I run 16" cs design wheel 8" large ET60 with special rubber from kumho in 245. It is competitive. Essentially because the car is very simple to run. I have to work again on it because the mechanical grip is not optimal at the limit and the car tend to under steer a little. I don't like under steer's cars.
The estate runs 18x10x65mm all around.....BUT it rubs on the inside in U turns and requires a bit of fender rolling up front to work..... The 265/35-18's we ran in lemons fit pretty easy....the Pirelli Slicks in 275/645-18 are TIGHT up front....
The front wheels on the widow were 18x10x 9" backspacing...with 295/30-18 tires it was perfect....barely rubbed at full lock on the inside...fine on the outside....but those are custom rims that are pricey....the common 18x10x65mm were $99 each....
This car has the rear spacers removed, 18x8.5 front 18x11 rear. Please call or email if I can help you with a set of Fikse wheels built to fit your particular needs.
This is what I run on my GT:
9x18 et52 with 255/35 fronts
11x18 et65 (with 15mm spacer effectively making them et50) with 295/30 rears.
Stock'ish ride height, it doesn't rub anywhere with street driving - but it's a GT so fenders are rolled already. Sorry for low-quality cellphone pic, but that's the only one I have currently.
Edit: I would not recommend such an extreme offset on fronts if you're going for custom set of wheels, rather get ET70-75ish, as recommended.
But for rear 11" wide rims, it looks like ET50-55 is close to optimal.
Edit2: From the first pic, you could imagine that there's still quite a lot of space in rear wheel well - but that's not the case, check the second super-high quality pic. Tire fills the fender very nicely with little space to spare.
And here we go.
The spec were on is what fits a non rolled fender S4.
This car has the rear spacers removed, 18x8.5 front 18x11 rear. Please call or email if I can help you with a set of Fikse wheels built to fit your particular needs.
The spec were on is what fits a non rolled fender S4.
Well, sorry, I can't provide more info than I have, we don't have that much 928s running around here. So, you will have to extrapolate from that. Or, well, just ignore my post completely.
All I can add to my post, that the setup should also work on an S4 with stock riding height. Also that you don't really want to run 11" on rear with more than 55 offset (rephrasing, you don't want to move wheel inwards from that).
11x18"ET64 on the rear, very close to control arm: Sorry for trying to be helpful.
For front wheels, and stock 25" tire diameter, the steering axis will be centered laterally on the tire patch with an offset of 55mm (ET55), i.e. neutral steering forces on each tire. With a larger offset the tire patch moves inboard, this positive offset is considered "safe" because the car tends to steer away from any soft stuff that you may get into on one side. For the S4 the factory fitted ET65 wheels, and ET60 on the Club Sports.
Lots of folks have fit other wheels with different offsets, but I think 55-65mm is a good target. Above 70mm, you're getting into a pretty big offset between steering axis and contact patch. And much below 55, the negative steering-axis offset becomes an issue, and the outer rim gets outside the fenderline depending on width.
The other issue is handling. Lots of folks, especially with big horsepower, fit wider tires in the rear. But that promotes understeer, which I personally dislike-- unless you can bump up the front width, also.
For the record. Anyone looking for the max wheel and tire fitment on an unmodified 928 and have no rubbing, this is what it takes:
Front: 18x9.5 et75 with a 275/35R18.
Rear: 18x11.5 et68 with a 295/30R18
You can go bigger tires and you can mess with the offsets, but you will not be able to run full wheel articulation without rubbing. The clearance to the control arm in the rear is about 4mm with a 3-pc wheel, which means that some cast wheels might rub. You can also use a 40 series front, but the tire would be significantly taller than the rear, which ends up looking odd and can also contact the front sway bar at full lock depending on the shoulder profile.
This is what the above combo looks like on my S4 under hard cornering, so you can see that the tire tucks right in to the fender. Hope this helps the next person trying to optimize his fitment.
Last edited by GreatWhite87; 02-27-2019 at 11:09 PM.
Reason: Adding pic
For the record. Anyone looking for the max wheel and tire fitment on an unmodified 928 and have no rubbing, this is what it takes:
Front: 18x9.5 et75 with a 275/35R18.
Rear: 18x11.5 et68 with a 295/30R18 (non XL rated, those would be too wide)
You can go bigger tires and you can mess with the offsets, but you will not be able to run full wheel articulation without rubbing. The clearance to the control arm in the rear is about 4mm with a 3-pc wheel, which means that some cast wheels might rub. You can also use a 40 series front, but the tire would be significantly taller than the rear, which ends up looking odd and can also contact the front sway bar at full lock depending on the shoulder profile.
This is what the above combo looks like on my S4 under hard cornering, so you can see that the tire tucks right in to the fender. Hope this helps the next person trying to optimize his fitment.
Nice pics, nice looking wheels [Fikse?] and a nice looking 928.
I see you only have a few posts on this list- I see you are relatively new to the list, are you new to the 928? Did you set the car up with the wheels or did you purchase it that way as a matter of interest?
First of all for your safety, may I politely suggest you check with your tyre manufacturer whether your 295's have any business on 11.5 inch rims. I typically use either Michelin, Pirelli or Continental and non of those marques would allow a 295 to be fitted on such a wide rim. If you had the optimal rear offset [60mm] you could easily run a 305 in there but not with ET68. A 10mm wheel spacer would solve that issue but at that thickness I suspect you would have to change the wheel studs and that is messy. There are bolt on spacers but whether such can be made at 10mm thickness I do not know- marginal at best..
With the rear offset you have I suspect you cannot run anything because as you correctly advise, you have next to no clearance because the rear offset is too big. A look at your nice photo taken from the rear suggests that the tyre wall can sit 20mm [?] further outboard so with ET60 and a 305 you would be laughing all the way. You also state something about XL tyres could not be used as they are wider. Last time I read the conventions for tyre sizing the 295 dimension is the maximum width of the sidewall with the tire at rated pressure and rated full load and as I am aware the XL rating makes no difference to that dimension but if you are aware of anything that does kindly steer me in the direction that explains what such change might be.
For the front wheels you should be able to get a 275 section in there but anyone I know who has achieved that advises that there will be a rub on the roll bar. If the wheels were fitted to the car when you acquired it, I suspect you may find that the steering rack has at least two spacers in it to restrict the lock. Even then with ET75 and a 275 section I would be pleasantly surprised if it did not rub. FYI plenty of folks run with a slight rub and have no issues but it does not appeal to me..
As a matter of interest, what measures do you have to keep roll in check? Do you have a stiffer front roll bar by any chance? I find that even with a 265 front section my Devek bar at full tension only just keeps the rear in check. I would imagine that if you have stock anti roll bars the rear might be rather exciting but then many track junkies like their cars "loose".
The beauty of three piece wheels is that by changing the rim sections the offsets and widths can easily be reconfigured if and when they are less than optimal and easily repaired if they dinked by a pothole!.
Trust the above helpful and accurate. I have no plans to change my 18 inch ET68 9.5 inch fronts or my ET60 10 inch rears any time soon but do feel free to try and convince me why I should..
Nice pics, nice looking wheels [Fikse?] and a nice looking 928.
I see you only have a few posts on this list- I see you are relatively new to the list, are you new to the 928? Did you set the car up with the wheels or did you purchase it that way as a matter of interest?
First of all for your safety, may I politely suggest you check with your tyre manufacturer whether your 295's have any business on 11.5 inch rims. I typically use either Michelin, Pirelli or Continental and non of those marques would allow a 295 to be fitted on such a wide rim. If you had the optimal rear offset [60mm] you could easily run a 305 in there but not with ET68. A 10mm wheel spacer would solve that issue but at that thickness I suspect you would have to change the wheel studs and that is messy. There are bolt on spacers but whether such can be made at 10mm thickness I do not know- marginal at best..
With the rear offset you have I suspect you cannot run anything because as you correctly advise, you have next to no clearance because the rear offset is too big. A look at your nice photo taken from the rear suggests that the tyre wall can sit 20mm [?] further outboard so with ET60 and a 305 you would be laughing all the way. You also state something about XL tyres could not be used as they are wider. Last time I read the conventions for tyre sizing the 295 dimension is the maximum width of the sidewall with the tire at rated pressure and rated full load and as I am aware the XL rating makes no difference to that dimension but if you are aware of anything that does kindly steer me in the direction that explains what such change might be.
For the front wheels you should be able to get a 275 section in there but anyone I know who has achieved that advises that there will be a rub on the roll bar. If the wheels were fitted to the car when you acquired it, I suspect you may find that the steering rack has at least two spacers in it to restrict the lock. Even then with ET75 and a 275 section I would be pleasantly surprised if it did not rub. FYI plenty of folks run with a slight rub and have no issues but it does not appeal to me..
As a matter of interest, what measures do you have to keep roll in check? Do you have a stiffer front roll bar by any chance? I find that even with a 265 front section my Devek bar at full tension only just keeps the rear in check. I would imagine that if you have stock anti roll bars the rear might be rather exciting but then many track junkies like their cars "loose".
The beauty of three piece wheels is that by changing the rim sections the offsets and widths can easily be reconfigured if and when they are less than optimal and easily repaired if they dinked by a pothole!.
Trust the above helpful and accurate. I have no plans to change my 18 inch ET68 9.5 inch fronts or my ET60 10 inch rears any time soon but do feel free to try and convince me why I should..
Thanks for the compliment. The wheels are Kinesis K18's, which took me several years to find. I have a few sets of wheels for this car, but I feel like none matches the car better than these. I bought this S4 stock and have made all the changes myself, it's not my first 928 as I used to restore these cars 20 years ago and have daily driven several dozen as well as raced a few. Over the years I have continuously improved the suspensions and wheels for autox and road race use, all the while retaining daily driver like comfort. I am an automotive engineer by trade, so my approach is rarely to buy anything off the shelf as it typically leaves me somewhat dissatisfied with the engineering behind them. Therefore very little in my car is out of a catalog. As an example, my shocks are factory koni's that have been re-valved with my own piston design and tuned to my spring rates. It rides way smoother than the stock sport suspension despite my spring rates being nearly 3 times higher. I use the factory rear swaybar and a custom front bar that I had bent up for the car. It's much like what you can find in the aftermarket, but locates better laterally and has much more clearance to the starter. I do not run rack limiters and I don't have any rubbing issues at full lock. I presume many who have commented on that may have been running a taller 40 series 275 tire. My car is setup for mild and very predictable oversteer. I have tried a number of spring rates and larger swaybars in the rear, but feel like you loose the amazing turn in when you tame the oversteer. This year I will be trying something truly out of the box with the rear suspension, if it works I will post the results for others to see. As you noticed, I don't spend a lot of times on forums, mostly because I get a lot of questions and I feel like answering them to my satisfaction leads me to write lengthy responses. I believe this post will is pretty good evidence of that.
As for your comment about clearances and offset, I believe your experience must be with a much higher car or a fairly narrow tire. While tire manufactures do try to keep the section width of a tire somewhat close to the "width" number noted on the sizing, there is a significant amount of variation in the actual sidewall profile and tread width. Tire Rack is an excellent resource as they post all the dimensions for the tires they sell. I assure you that my offset will place your tire in exactly the middle of an unmodified fender well, with the same amount of minimum clearance on the inside and outside. The 928 is actually quite difficult to size properly in the rear because the rear suspension has a lot of toe change under load, particularly with these wider tires. Thankfully the front has a good amount of camber gain, which pulls the tire in and away from the fender as the suspension articulates.
I'm using Proxes R888R's and Advan Neovas, which have a tread width of 11.3 and 11.2 inches respectively, as an example the same size Pirelli PZero is only 10.5" at the tread. On my car the result is about 4mm at full bump on the inside and outside. Using a lower offset would definitely rub, which incidentally I have tried. In fact, I found these offsets by using an 18x10.5 (et80) and adding thin spacers one at a time until I cleared the control arm, then was able to lower the car, then drive it around without rubbing on the inside, then eventually rubbing on the outside. Once I had those numbers I divided the distance between the two bookends of possible offsets (rubbing on the inside/outside) and got what I posted above.
The wheel width being a bit wide is a old racing trick. By going a little wider than the manufacturer's suggested width ranges you gain a little more lateral tread stability at the expense of tire wear. Going too wide will eventually lead in a loss of traction, but my 295's fit extremely square on the rims. I only tried two 305's, but both were too wide, rubbing both on the inside and outside equally. Mind you, my car is a little lower than most with the rear fender height being just under 22.5" inches, so I need the tires to tuck inside. These days hardly anyone makes a 305/30R18, so I'm not sure which ones you have. The comment about XL ratings may be outdated as it seems the two most widest 295 XL's I tried have been taken off the market. One was almost 11.6" at the tread according to my notes.
A curious development while we are talking about 295/18's. I was recently in a meeting with some off the Continental engineers and they informed me that Conti has started sharing it's tire development with their lower priced division General Tires. They went on to explain that their G-Max tire is very similar in construction and compound to the ExtremeContact Sport tires, which means it is an incredible value at the current price point. I might have to order a set for my street wheels, maybe then I wouldn't feel that bad about engaging in gross wheel-spin antics.
The wheel width being a bit wide is a old racing trick. By going a little wider than the manufacturer's suggested width ranges you gain a little more lateral tread stability at the expense of tire wear.
This is becoming more popular in the drag racing world too. One of our local guru's up here in Green Bay with a 1,000+hp 5 liter twin turbo 928 dropped down from 275's to 255's on 10" rims and noted traction and handing improved.
He's running Hoosier DOT drag radials.
A couple threads on his car here if you're curious:
I personally ignore the 255, 295, 305 etc width rating because it doesn't directly reflect how wide the tread actually is, which can vary greatly between brands.
I generally go off of tread width and will try to match that as best as I can to a rim's width. It really helps with treadwear and clearance.
Plus you wont have the puffy look of bulging sidewalls.
02-29-2012, 12:34 PM
