I am looking at a 90 C2 coupe which has 16" 993 wheels (see pic). The fronts say 7", 55mm offset but the rears are 9" and 70mm and there is a bolt on spacer behnd the wheel. I know that I would want to change that out ASAP but what does the 70mm offset wheel come from?

 

Rick,

The 9J ET70 16" wheel shows up on PET as an M398 option for the 993 which I think is just "Cup wheels". I'm curious why you would want to remove the spacer, since the 964 would REQUIRE a spacer of around 15mm to 18mm to position the wheel correctly. The standard offset for the 964 is 53.2mm so the 70mm wheel would sit too far into the bodywork without a spacer.

Hope that helps.

Regards

Dave

 

Rick, Computamedic is correct. I had a set of these wheels on my '91 Cab for a while. They required spacers on the rear to keep from rubbing on the oil line. Don't run without them. Roy

 

I have them as well - and a 20mm spacer.

 

I don't want to remove the spacer it is just that many people subscribe to the theory that bolt on spacers like that are not as strong as having the proper wheel to begin with. Since I want to autox that car I do not want to create a weak link. I wonder why the factory would have shipped that wheel with the big spacer required.

 

Simple - they didn't. The wheel is designed for a 993 which has a completely different rear suspension and axle compared to the 964 and REQUIRES a wheel with that offset. The spacer is required on the 964 to adjust the offset to suit the suspension/axle arrangement. If you don't want to use a spacer then you cannot use those wheels.

Regards

Dave

 

Dave,
I believe that 964, 993, 996 narrow body cars all share the same "late" offset of about 55mm for the rear. That is why you can run Boxter or 996 wheels on 964, 993, late 944 turbos, etc. I know the factory used a spacer on 993 Targas but that is a totally different wheel, a 17" 3 peice wheel I think.

 

Rick,

I don't think there is a simple answer. PET shows a lot of wheel options for the 993 and the wheel offsets range from ET40 to ET70 for the rear wheels. Logic says that you'd expect the stub axle to be a constant - at least within the same narrow bodied cars - but why the wheel offset would be varied so much beats me. You might expect the wider wheels to have a smaller offset to move more of the rim outwards of the centre line.

I can see no evidence of the factory fitting spacers although others may know different. But the ET70 wheel will definitely be a problem on the 964 because of the inboard oil line in the right rear wheel well.

I'm sorry that I can't be any more specific.

Regards

Dave

 

Thanks Dave. I as baffled as you. I know I need to run the spacers or replace the wheels.

 

Rick,

I've just posted a query on the PCGB 993 forum asking if someone can enlighten us. I'll get back to you if/when I get any more info.

Regards

Dave

 

The standard 964 (Turbo and Turbolooks not included) was designed to accept a 52.3mm offset rear 8JX16 wheel to set the correct track width. In the 16 inch wheels this was never changed, but 55mm rear offset 17 inch wheels were offered as an option from model year 1992 onwards.

The 993 was designed to accept a 70mm offset 8J or 9J X 16 rear wheel to set the correct track width. This was NEVER changed and the standard (basic) 16 inch wheels for every 993 from start to finish was 70mm. From model year 1996 onwards these wheels became known as winter wheels. 17 inch steel winter wheels were also offered with 70mm offset for the rear.
In model year 1996 Porsche decided to widen the rear track width for summer conditions using 17 inch wheels with a 55mm offset. This was to improve the rear end traction with the slightly more powerful varioram engine.
16 and 17 inch winter wheels for all 993s remained with a 70mm offset to narrow the track again for driving in snow and ice.
Ask me how I know?

The 996 rear suspension design was changed to use 55mm rear offset wheels to obtain the correct track width from the outset.

The original Boxsters used a 40mm offset rear wheels. Later models were changed to use a 55mm offset wheel. I have not researched the Boxster to see the reasoning and what was done if anything. I suspect nothing was done except to use the higher offset to reduce the rear track width.

Wheel offsets used for the various models and versions does not reflect a design commonality between types.

The wheel shown in the picture leading this thread is the standard (basic) model year 1996, 16 inch 993 wheels. There were two slightly different designs. One for model year 1994 and 1995 (Design 93) and the other for model years 1996 to 1998 inclusive (Design 96). These wheels were sold in the USA because I purchased mine from the USA.
They are not cup nor cup design wheels either. By 1993 18 inch Speedline or 18 inch BBS racing wheels were used as the standard fit on the 964 Carrera RS 3.8, the 993-Cup and 993 Carrera Cup cars depending upon the series rules.
Ciao,
Adrian.

 

Thanks Adrian for sorting that out.

Regards

Dave

 

Thanks Adrian. That really clears things up and makes sense.