GlenL

Gosh. I wanted to answer this but Hilton completely nailed it. I'm verklempt.

Donald

Hilton-
Thanks very much, this is the first time I have seen scrub radius introduced as a factor in front offsets; and well explained.
Glen- Thank you for bringing up the handling issue! (&loosen up those shorts, being verklempt is painful)
Jerry & finally- thanks, I am planning on a tire closer to stock width.

Jerry Feather

Donald, I am not sure what you mean by "stock width," but if the wheels are not stock width the tires are not going to be stock width unless you would like the assenbly to look like those young latins or whatever they are referred to now who have really wide wheels on their 60's or 70's chevy or other car with a really outlandish offset that puts the wheels about half way outside the wheel wells, then putting tires on them that are the cheapest they can find that have about 4 inches of tread and the side walls are streched to barely get the beads in place.

That is an exageration, but with 9.5 inch wheels you are not going to be using stock width tires.

Jerry Feather

Vilhuer

CS, SE and '89 GT used 8" ET60. That was "wildest" wheel version factory was willing to use and is good starting point in trying to figure out what works at front end.

Jerry Feather

I guess that the question is "what wheel offset is it that gives the factory stock 10mm or 15mm scrub radius? According to the analysis we are paying attention to, ET60 will not do it.

Jerry Feather

Jerry Feather

OOPS! I forgot to close the quote. JF

Donald

Jerry:
I think you missed this modifier: "I am planning on a tire closer to stock width", when people assumed that because I stated that AO's calculator gives 295 as a maximum fit that I was going to use that width. The GT came with 9" factory rear rims.
Thanks,
Donald

Jerry Feather

Yeh, I guess I misread it. JF

Jerry Feather

Donald, Actually I did read and understand the "modifier" you pointed out. What troubles me mostly is that the original factory width has almost nothing to do with the proper size of tire for the rims you want to put on. You seem to have completely missed the point of the simple analysis for tire sizing that I took a few minutes to give you. The rim width and tire diameter are the two ONLY criteria for determining the correct tire size for a given rim width and diameter. The original tire size has essentially nothing to do with it. The offset of the new wheels and the resulting position in respect to the car are the other criteria that you need to understand in respect to a given set of later style/size wheels. You need to understand both; and again, the original tire size has nothing to do with it except to help you decide the overall diamater you need to stay within.

Jerry Feather

Donald

Jerry:
Thank you for your comments and the calculation. I do understand the issues.

JWise

Read the first paragraph of that Wikipedia article. It mentions a fact not often discussed in these conversations: camber setting

I suspect many of us run fairly aggressive camber settings, I know I do (~ -2.0 IIRC). This also affects the scrub radius - the more aggressive you get with negative camber, the more you diminish the amount of negative scrub radius. This means you need to be even more careful when going with a lower than stock offset up front.

The negative scrub radius design issue is one of the reasons I went with a high offset OEM wheel like the ET70 Cup II available in both 8" and 9" widths.

Hilton

Yes Jerry, it's based on the stock ET65 offset found on all 928's except those rare cars fitted with Clubsport Wheels in 1988 and 1989 (i.e. Clubsports, 928 SE, and 1989 GT's), which as Erkka points out, used ET60 8" wide front wheels. Every other 928 front wheel, from 15/16" phone dials to GTS Cup II's, had ET65 front offset.

I checked a handy aussie document - its -10.2mm for late cars, and -16mm for early ones (i.e. before the hubs changed for the big 4-piston Brembo calipers). So to retain some amount of negative scrub radius, the theoretical minimum offset is ET56 for late cars (retaining 1.2mm negative scrub radius), and ET50 for early ones (retaining 1mm negative scrub radius).

For more good reading, see this document:

http://www.landsharkoz.com/forms/928wheels.pdf
(scrub radius discussion is page 8)

My personal stance is that the above PDF is not conservative enough - based on driving my own cars with a variety of wheels. When I got my hands on a set of ex-GTS ET65 fronts it was a real eye opener as to how much difference it made compared to the ET55 Boxster twists I had on the car prior to that. It was enough of a difference that I'm retaining the stock manhole covers on one of my '87 S4's, and will be getting rid of the Boxster wheels.

dr bob

For those that have the WSM's, the suspension diagram on page 40-4 is a fabulous illustration of the relationships among the ball joint centers and the wheel center. The illustration shows how a line through the ball joints, which is the axis around which the wheel turns, passes the tread 10.2mm outside of the center of the tire. Pressure on the tire from braking, road forces, etc. all work to turn the wheel towards the center of the car about that axis. If the pressure is uneven, such as from a road hazard or a blowout, the car woul normally swerve towards the wheel that's dragging more. The slight offset shown lets the steering exactly counteract that uneven pressure. The target is to allow you to have a front tire go flat at speed (BIG speed) and have the car stop relatively straight with no lurching or pulling. That also means no tramlining or other bump-steering issues from uneven pressures on the wheels.

The WSM Page:

Donald

Dr. Bob:
Thanks for the reference and explanation. Personal experience: I cut the left front and had it "catastrophically" deflate a year ago, and barely noticed a difference in steering input w/ the D90s.

Fikse Wheels

Have you decided on wheels for your 928? Our wheels are built to order and we can fit your car properly. http://www.fikse.com