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The perfect offset on 18s....zen...

Old 02-28-2019, 02:06 AM
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Originally Posted by GreatWhite87 View Post
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.

Excellent response and thanks for sharing. My initial reaction was your wheels might be Kinesis but given I understand they stopped making them what seems like ages ago I thought it must be my decrepit imagination. I have read of folks using wider rubber than the rim with suggests but not come across anyone who actually has done that to feed back on. It was difficult to believe anyone doing what you are doing would make such a basic error so it is a relief to know it was a well considered choice. Best fitment and performance I had on my rear rims was with the 285x30 Michelin DOT legal rubber but the downside was they last no time before going off which on a car that only used to see a little track time is a waste but wonderful while they were on song.

The problem I have is getting a set of rubber at all these days as the local distributors struggle to supply current model fitment rubber and they are going away from 18 inch. Trying to being in rubber is also difficult as the authorities quite reasonably try to ensure that what comes in is fit for purpose.

The sizing designation has no commitment to tread width as you correctly point out no correlation to tread width- it is meant to ensure that manufacturers can specify a tyre size knowing that it will fit spacially thus the designated number is a maximum for the side wall and the rubber I have tested holds to this convention.

On my setup I do not go "silly low" rather I run at the low end of the approved range and with stiffer suspenders have no issues on the road. I also like the turn in abilities and set my front toe at the very bottom of the approved range and have experimented with below stock range when doing track days and the like. The Devek bar limits my wheel movement but I experience on issues with an additional stop in the rack. Not sure I would like to use it in UK parking lots though!

Do not be a stranger on this list- it is good to hear from folks like yourself with plenty of interesting input to offer.


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Old 02-28-2019, 08:43 PM
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When I’m bought Casper it had 17” wheels...315/35-17 and 295/35-17 fender rolling and zero rubbing. I think the wheels are custom forgeline but not sure other than 17x9.5 and 17x11..... sean still owns the wheels if you want. To measure them
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Old 02-28-2019, 08:49 PM
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315 left. 275 right.
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Old 03-01-2019, 07:58 PM
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Originally Posted by dr bob View Post
A couple posters use the term "positive offset" in error. You want NEGATIVE offset for your 928, moving the rim IN relative to the mounting face.
positive offset is the correct term "ET 65" or sometimes "ET 65P"
a deep dish wheel would be "ET -65"

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