Hi guys,

I have a bit an issue with a set of Cup 1's for my 928. It seems that I didn't do my homework and check the correct offset for the stock 928...stupid...so now the mags that I have stick out by about 30mm (front set only, rear set flush with the body without spacers). Can't machine off the the back of the mag, there's not enough material to make a difference. Wondering now if someone has ever had this issue and solved it by fitting a small wheel hub from another model? 944 if it fits at all? or perhaps an aftermarket one? No idea if this is a common issue and whether there is a simple fix. Unfortunately for me, the rims weren't cheap and now it seems I'm stuck with a beautiful set of cup 1's that don't fit my 928 Ill post the offsets of the mags that I have and repost the stock offsets just as an FYI. Open to all ideas and suggestions aside from selling them which is a last resort for me at the moment.

 

Borbet Cups 1's that I have - Front offset: 23 Rear offset: 17 ....

 

I would suggest you sell the rims to a 911 owner and buy the correct size,
the original ET for the 928 is ET 65,
the lowest you can fit on the front is an ET of 50,
and the rear without spacers is ET 45.

The lower the ET the further the rim center line will be from the hub surface with an ET of 65 being the stock fitment for the 928

 

No simple fix...sorry.

 

I think you have this one bass-ackwards, Stan. A zero offset puts the wheel mounting (hub) surface exactly at the wheel centerline.

I think the only way the OP's problem could be solved, if at all, is if the wheels happen to be two or three piece, in which case one might fabricate a "negative" spacer for between the wheel rim and wheel center, and even then only if the wheel center is mounted to the rim from the front. I don't think his wheels are two or three piece.

 

To round out the discussion, the brake discs mount onto the hub before the wheel, so trying to make a 'shorter' hub would demand a different brake rotor, and would demandsome pretty interesting offset caliper mounting bolts to get it in a bit away from the wheel.

 

I think the OP just set a record for the wildest offset to be attempted on a 928- sad to say those are not even close and the only thing they have in common for your need are the 5x130 bolt holes.

The 928 steering/suspension design was very advanced for its time and has completely different requirements to the likes of the 911 which most Porsche wheel sets seem to be based on.

Besides the need to actually fit in the wheel wells and look sensible, the steering is designed to have a NSR [neutral scrub radius] of some 13mm [if my memory serves me correctly] and to achieve this you need an offset of 65mm. To achieve this offset and avoid clouting the brake caliper you need a wheel design wherein the spokes are close to line of the tire flange- those things you have will have the spokes well inboard of the tyre flange- correct?

As a 928 owner you have a set of very nice "garden ornaments" - cut your losses and try to flog them to a 911 punter. One size most certainly does not fit all when it comes to 928's at least.

Rgds

Fred

 

//

 

Stan,
Your original statement " the lower the ET number the further away from the hub surface the rim centerline will be" is correct. The HIGHER ET will make the wheel more INWARD. The LOWER offset will push the wheel more OUTWARD. Wasn't that what you said and meant?
So for the OP, the offset is not POSITIVE enough to allow the wheel to tug in. Selling the wheels will be the only option.
Steve

 

Exactly, the ET value is the distance from centerline of the wheel to mounting surface of the wheel. So if this value is low, the mounting surface is close to the centerline, the position of the wheel will be more outward. If this value is high, the position of the wheel will be more inward.

http://www.tirerack.com/wheels/tech/...jsp?techid=101

 

So I stand corrected by my original statement.

The lower the ET the further the rim center line will be from the hub surface with an ET of 65 being the stock fitment for the 928

 

No, Stan. You are wrong again, and this time it is not a typo. Talking about where the whole wheel goes, in or out in respect to the offset, does not make your original, and now your reaffirmed statement, correct, if that's what you mean by your post just above. As the offset number gets smaller, the closer the wheel centerline gets to the wheel mounting surface. And as it does so, of course, the whole rim is moving with the centerline in an outward direction. Movement of the whole wheel in or out cannot make your statement correct.

The smallest offset is zero, or no offset at all, and that means that the centerline of the wheel is exactly on the same plane as the mounting surface. If it were the other way, as you now suggest and as the others are trying to help with, where do you and they think the offset will be as it gets smaller and supposedly moves farther away from the wheel centerline? Where will it be when it reaches zero? If it reaches zero somewhere farther away from the wheel centerline, what number is it when it is at the wheel centerline?

 

The bold part is correct as long as you bear in mind that we're talking about a negative offset, which is what the stock 928 wheels have and what the 928 suspension is designed for.

If the wheels were a positive offset, then the first part would hold, but asthe number fell, the wheels would move inwards.

 

Either positive or negative, as the offset number gets smaller the mounting surface and wheel centerline get closer until they are are on the same plane at zero. Offset is simply the distance from the wheel centerline to the mounting surface, whichever side it is on, either positive or negative. When the number is getting smaller how can they be getting farther apart?????

Hilton, the link posted above suggests that the Porsche wheels we use are positive offset. What's up with that, from your perspective? Is it different down under?

 

[edit:] The wheels we use are positive offset. Correctly specified, we are looking for a 67mm offset with 25.5" inch tall tires to maintain the correct negative scrub radius.