MM951

Okay, quick question about wheels and offset.

I have a car that has 0mm offset on the front wheel and -10mm on the rear wheel. Some wheels I would like to get someday are the same bolt pattern, but with a 24mm offset (5.95" backspace) What would be needed to do to make them fit..if I could do anything at all?

If it helps, my wheels now are 16X8 and 16X9 and the wheels I want are 17X9 all around

Thanks
mike

Geo

http://www.comp.co.uk/techinfo/techinfo1.htm

MM951

Intresting read, thanks Geo

What I dont understand since I have a lower offset and the wheels have a 24mm offset..how will they bolt up? I can't use spacers because the rim is already too far out, correct? Same reason early offset porsche wheels dont fit on late offset Porsches ,rgiht?

RMills944

sounds correct. I don't know if you are going to be able to get those wheels to fit without doing some major work to move the spindle inward instead or just have a custom set of wheels made.

Chris_924s

Pic stolen without authorization from "parting out a red 86" thread.
This is what late offset PD's look like on an early offset car

RMills944

LOL!!! thats hilarious! (no offense to the owner)

MM951

Haha, yeah I know what they look like..

Early offset is ~23mm right? Late is ~52.3mm?

So I WOULD need spacers to fit wheels with a 24mm offset to my car which normally has 0 and -10mm stock?

why am i even thinking about wheels when my car needs to run?! LOL...guess its always better just to know

Chris_924s

Correct grasshopper.

MM951

But the wheels should fit when I put the spacers on, no?

Danno

"What I dont understand since I have a lower offset and the wheels have a 24mm offset..how will they bolt up? I can't use spacers because the rim is already too far out, correct?"

"So I WOULD need spacers to fit wheels with a 24mm offset to my car which normally has 0 and -10mm stock?"

"But the wheels should fit when I put the spacers on, no?"

Mike, check out the "944 Wheels FAQ- Measurement Basics" section on my 951 RacerX website. Learn to draw your own wheel diagrams (like the one I have on there) for the specific wheels your're looking at:


Besides the offset, you want to measure the clearance you have on your existing wheels, especially the front. Since you're going 1" wider, do you have an additional 0.5" clearance on both the inside AND outside? If not, you'll need to juggle effective offset (with spacer) to place all of that extra 1" either on the outside or the inside.

So here's your before & after dimensions (assuming 0.75" thickness in rim-flanges):

16x8: 0mm offset = 4-3/8" on inside (backspacing) and 4-3/8" on outside
16x9: -10mm offset = 4-1/2" on inside (backspacing) and 5-1/4" on outside

17x9: 24mm offset = 5.8" on inside (backspaing) and 3.93" on outside

Ideally, you'd use a 24mm spacer in front and a 34mm in rear to place the centerline of your new wheels in exactly the same place as before to preserve the same suspension-geometry and steering scrub-radius as before. But depending upon the fender clerances in the front, you may want to go with smaller front spacer to place most of the extra width on the inside.

Finally at the end of all this, you can rotate your wheels for even tyre wear.

Geo

Don't ask me.

I've got 5 sets for my 944 and it doesn't run (yet).

Come to think of it, I have 1.5 + .75 sets for my G20 and it's totaled, waiting to be parted out still.

Hmmm, my SE-R isn't running right now and I have, uh, 3 sets for it.

Shoot, my Bimmer runs and I have the fewest wheels for it.

I need to run a used wheel store. Actually, I have someone working on a wheel finishing fixture for me and I'm assembling my bead blasting cabinet. Once those are done I'll be refinishing some of my 944 wheels and selling them (probably 2 sets). Need to sell my second set of Bimmer wheels too. Just been too distracted (or perhaps lazy) to get them up on eBay.

Geo

One thing to watch out for in dealing with offset is to be sure you're using the same convention for positive/negative. Sometime in the last 20 or so years the convention has flip-flopped. It used to be that backspacing larger than half the wheel width was considered negative. Now it's generally considered positive.

Some cars 15-20 years old (and wheels that old) may get you twisted around.

Danno

yeah, the equation used to be:

offset = (0.5 * width) - backspacing

but I think as more and more cars were built with larger backspacing to hold bigger brakes and ABS sensors, people got tired of saying "negative 10mm"or "negative 20mm" all the time. So they dropped the "negative" word, but implied in the measurement was the negative.

Nowadays, the equation for offset has been revised to have non-negative offset as the more common one where backspacing is larger than 1/2 the width of the wheel (mounting surface is outboard of the centerline):

offset = backspacing - (0.5 * width)

BTW- the "width" used in these equations are the outside-width or overall-width which is based upon measurements from the outside of the flanges. This typically is about 0.5-1.0" wider than the official bead-width which is used to refer to the size of the wheel. Bead width is where the tyre-bead sits and is impossible to measure with the tyre on the car.

So when plugging in numbers, use the actual measured outside width, like 7.75" for a 16x7" rim.

Geo

Exactly right Danno.

I think a lot of the reason also is the change in front suspensions in the domestic market. With the change from largely double A-arm suspension to McPherson struts, I think the move to more backspacing is to improve scrub radius and stress on things like bearings.