I have an 83' 930 and have known for a long time now that 18" wheels are not approved yet alot of people use them anyhow. Does Porsche actually have a reason for saying this? From experience is there really an issue if you use 18" wheels? Then in what way are 18" wheels better than 17" ones or is it just looks?

Thanks,

Brent

 

I think Porsche's tech bulletin said something about the rear wheel bearings.

Not sure of anything beyond that. I read the bulletin once, that is all I remember.

Lots of people I know are using 18's on the track, without any issues...

Maybe the issues are things like the wheel bearings wearing out at a higher frequency?

Brian

 

Thanks Brian, it would seem as long as you get the right tire so your overall diameter is nearly the same as stock spec. that it wouldn't be an issue. I guess you could even go 19" but not sure if it would look good.

Brent

 

There are a ton of cars running around with 18s out there. Some with near 100K and still on the orginal bearings. I wouldn't worry to much. It is the spacers that bother me. They uncenter the load a lot more than a 18 will.

 

So basically from what I understand the ideal set up will be using the correct offset wheels and avoid the use of any spacers if all possible.

RC

 

I was talking with my mechanic today about it and he told me a little story. He said some rich guy (what else) put 18" wheels on and ended up breaking a window from the stress??? So he made sure Porsche knew about it for compensation reasons I'm sure. Then Porsche to cover there asses said 18" wheels are not approved.

By the way good points about the uncentered load with wheel spacers and getting the correct offset to start with. Looks like 18" wheels are not a problem and should handle a little better.

Thanks,
Brent

 

I have had some experience with the 18" wheel issue on my 95 993. Porsche does not approve 18" wheels on early production runs of the 993 due to the inability of the steering rack and other suspension components to handle the stress generated by the shorter sidewall of the low profile tires on 18" wheels. In fact, Porsche sells a retrofit brace (only $14 if you can believe it) that simply bolts on to offer the added support needed.

Common opinion with the 993 group however, is that is was a bunch of BS concocted by Porsche to get out of any warranty claims. Many people (including me) ran 18" wheels without the brace with no problems.

 

Stephen

I wonder if you could give me some clarification of what you mean about the spacers uncentering the load, I am sure that it is me missing somthing but I cannot work out what difference it would cause as I thought the spacers just corrected the offset? I have 17" with the correct offset i.e a big chunk of metal between the wheel and hub and 18" with the incorrect offset but spacers instead of the big lump of metal.

I have drawn the two configarations out and cannot for the life of me work out why the loading would be any different?

Like I say I am sure it is my inability to comprehend the problem in front of me!

Thanks

 

The way I understand it is, though I don't know who comes up with the offset measurement is the wheel spacer just extends the wheel out while the offset changes where the load is to the wheel. I was thinking the bigger the wheel spacer the more load at the hub? Am I think correctly? So what is the theory behind what offset to use?

 

I have attached a diagram that explains offset, I know you all know what it is! the reason why I am posting it is to show what I mean about it not making a difference if you have more metal in between the wheel and hub or a spacer.

As far as I can see you increase offset in manufactre by not putting as much metal at the fixing point of wheel to hub.

I just cant see how it moves the load?

 

Sorry, what I meant to say is I can see that changing the offset moves the load, but I cant see why it makes a difference if you do it with a spacer or by casting more metal onto the wheel in manufacture.

If I bolt my spacer to one set of wheels and put them next to the wheels with the correct offset the result is the same.

 

I think your correct when you say the back spacing doesn't matter if you use a spacer or you have a thicker flange. It still places the wheel in the same spot.

BUT, I think if you add a spacer to a wheel, it places a wheel meant for one position is now moved out and is placing a greater load on the wheel/bearing/etc.

An exageration would be a spacer of 1" vs. 12". There is a smaller amount of load/leverage with a 1" spacer than a 12" spacer. The 12" spacer places a larger amount of load/leverage on the wheel/bearing/etc. I don't know if it's 12x as much, but there is a greater load.

Ummm.....I hope this makes sense!

 

I think Rocket's got it.

Spacers can be a problem by them selves if not made right. They can fail, most do not have the hubcentric lip built into them to center the wheels as Porsched designed them to. They add weight, especialy if they bolt one, they can add a lot of expence if you need longer wheel studs installed.

However, a wheel that has spacers on it that is placed in the wheel well in the very same place as a different wheel that does not require a spacer, will 'not' add any load on the wheel bearings or other suspention componets.

Spacers that move a wheel 'out further' will add an increase in load on the wheel bearings and other componets.

So will wheels with no wheel spacers but that have 'less' back spacing so as to move the wheels further out. (stock 911 w turbo flairs & spacers)

I think of 18's as just like stiffening up the suspention through fitting stiffer springs or shocks.

Anything that makes for a stiffer ride is going to potentaly add some additional stress and shock to the stock componets.

Also, change any componet in the suspention system and it would have some potental to creat some inintended or unplanned for issues like darting or other unpredictible handeling issues that Porsche had not tested for nor designed for.

I'd run 18's on a 930.

As a note, I had a girlfrend w 18's on a C70 turbo Volvo that had to replace 3 perfict tires in severl months do the sidewall damage as they don ont offer as much protection from bumps and are more sensitive to air pressure.

Just my opinions...

Keith

 

Here's my 18" rear tire with a 12" wide Goodyear.

 

Now that's hot!