Deciphering wheels: backspace, offset, etc...

Danny_Ocean · 04-14-2007, 06:13 PM

In order to find new wheels, I need to know what fits. I'm not exactly sure, however, what size wheels (offset, backspacing) I have now. My car has wide flares and the current OZ wheels use giant spacers to fit:

I removed one front wheel today and found the following engraved on the inside of one of the spokes:

40 01 0 P06
8.5J x 18 H2E41

Tire is 245/40-18.

The wheels use a 27mm spacer and the backspacing (inner wheel lip to hub mounting surface) = 159mm. I'm assuming you add the two numbers to come up with the total backspacing = 186mm?

So...what will I need, as far as offset & backspacing, to replace these wheels with ones that do not require spacers?

Kevin Stewart · 04-14-2007, 07:33 PM

the easiest way for a rookie, if you like how the wheels look and sit then try this

take the wheels off, bolt spacer back on, lay a flat edge across the back one side of wheel to the other) measure the air gap, this is the space between the flat edge and where the wheel adapter bolts to the car

Danny_Ocean · 07:50 PM

Well...I think I can do that without bolting everything together..."actual" backspacing = 159mm - spacer (27mm) = 132mm.

Problem is...on my American cars, wheels are fitted using the backspacing measurment. All the ads I look at for Porsche wheels list "off-set" as the only measurment. So, I guess my question is, how do I convert "backspacing" measurment to offset measurment?

And, is the "40" engraved on my current wheels indicative of the offset?

Kevin Stewart · 04-14-2007, 07:51 PM

what is the wheel width

Danny_Ocean · 04-14-2007, 07:55 PM

8.5 in.

Kevin Stewart · 04-14-2007, 07:57 PM

yes you would have a 40mm offset, 40mm minus 27 = 13mm offset

Danny_Ocean · 04-14-2007, 08:01 PM

Quote:

Originally posted by Kevin Stewart
yes you would have a 40mm offset, 40mm minus 27 = 13mm offset

Now you lost me.

Where does the "13mm" come in to play?

Kevin Stewart · 04-14-2007, 08:15 PM

imagine a center line at the center of the wheel, with your wheel and spacer bolted together it will be positive 13mm from the center line of the wheel, positive means the wheels sits more in the car a negative is like an old deep dish cregar

boyt911sc · boyt911sc's Garage

Danny,

To calculate the offset of any given wheel, use this simple calculation: Backspace minus half of the over-all width equals offset (BS-OW/2 =OS). Hope this helps.

Tony

Danny_Ocean · 04-14-2007, 08:32 PM

So, in order to replace these wheels, I'll need to find a wheel with 13mm offset (or ET)? Most of the Porsche wheels I've been looking at list ET55 (+/-).

Man, this is confusing stuff...why can't they just use backspacing measurments and be done with it?

Danny_Ocean · 08:44 PM

Quote:

Originally posted by boyt911sc
Danny,

To calculate the offset of any given wheel, use this simple calculation: Backspace minus half of the over-all width equals offset (BS-OW/2 =OS). Hope this helps.

Tony

OK...here goes:

Backspace = 159mm
Overall width = 8.5" (216mm)
1/2 of 216mm = 108mm
So...159mm - 108mm = 51mm offset or ET.

That was easy.

Now, I subtract my spacer (27mm) and I end up with 24mm which is, technically, my "simulated" offset. No-one makes an off-the-shelf 24mm ET wheel for Porsche, so I'll have to find a 51mm wheel and keep my spacers...? (or order a custom 3-piece wheel with a 24mm ET)...?

Kevin Stewart · 04-14-2007, 09:05 PM

the 17x9 wheels are 15mm offset for the rear and the fronts are 17x7.5 et24mm for the stardard sc-carrera fitment, which looks like what you have an sc?, I would bet those wheels will fit give some of the locals a call im sure someone has an old cup replica with sc spacing you could try, the 50mm plus wheels are used on the 964 up cars, i dought any one will even let you time trial with the spacers, Kevin

Danny_Ocean · 04-15-2007, 05:23 AM

Quote:

Originally posted by Kevin Stewart
the 17x9 wheels are 15mm offset for the rear and the fronts are 17x7.5 et24mm for the stardard sc-carrera fitment, which looks like what you have an sc?, I would bet those wheels will fit give some of the locals a call im sure someone has an old cup replica with sc spacing you could try, the 50mm plus wheels are used on the 964 up cars, i dought any one will even let you time trial with the spacers, Kevin

Yes, the car is an SC. But the wide flares make it necessary to move the wheels out.

I have been looking at modern wheels to fit on the car (Turbo Twists, etc.) which I know will require spacers. I'm trying to determine what wheels out there might fit without spacers. If there is no such animal, I guess I'm stuck with spacers.

I do not track the car. It is only an occasionally driven beach cruiser.

Oh Haha · 04-15-2007, 06:28 AM

With your SC/Carrera chassis, you should be looking at wheels for those models if you want to run without spacers. Any modern factory wheels will need spacers. You may even need to add a small spacer to get the SC/Carrera offset wheels to "fill out" the wells. IMO, the Techart wheels look awesome on your car. You could probably sell them off and buy a set of custom made wheels for your exact setup.

boyt911sc · boyt911sc's Garage

Danny,

Fuch (911-7 inch), Fuch (951-7 inch), and Fuch (951- 8 inch) wheels have offset at 23.3 mm. So if you need OEM wheels with no spacers, try these wheels and see how you like it before buying them. Remember one thing, you may have the correct offset but the over-all width of the rim/wheel will determine a good fit.

As an example 951-7 inch and 951-8 inch Fuch have similar offset value but diferent backspace of 121 and 134 mm respectively. So the 8" rim will be 13 mm more outboard. Hope this help you find the wheels to want.

Tony

JohnKo · 04-15-2007, 09:17 AM

Quote:

Originally posted by Danny_Ocean
Man, this is confusing stuff...why can't they just use backspacing measurments and be done with it?

Because offset or "ET"--the difference between the wheel centerline and the wheel mounting surface--is the dimension that is most relevant in terms of suspension geometry, bearing loads, etc., regardless of rim width.

Danny_Ocean · 04-15-2007, 09:49 AM

Quote:

Originally posted by Oh Haha
IMO, the Techart wheels look awesome on your car. You could probably sell them off and buy a set of custom made wheels for your exact setup.

Yeah, you're not the first person who has said this. Personally, I think they look a little "dated" (80's). If I could find a good wheel re-finisher who could paint the centers black, I might keep them. Otherwise, I'd like to go with something more modern. If I do find that perfect wheel, I'll put these for sale here before they go to E-Bay.

Thanks all. Things are a little clearer now.

D.O.