atb

FBIII wrote:

Wide track for a wide motor means narrower tires upfront when maintaining the overall shape of the car. I seem to recall hearing also that the narrow front tires had to do with reducing drag at speed. Don't know how much truth there is in that though.

Rufus Sanders

Dave R.

Thanks for clearing this up. I was really hoping it was possible to have those Modas all around. What if you went with a 245 or 235 tire up front? But hey the idea of widening an existing wheel is great too. Maybe that's the way to go.

Also, Glen, is that your son's 944 in your avitaar photo? Where are those Boxster wheels I saw on it (or was I seeing things???) - Ruf

DR

Ruf,

>Dave R. Thanks for clearing this up. I was really hoping it was possible to have those Modas all >around.

No problem.

>What if you went with a 245

245/45-16 is doable, 245 17 or 18 is not made in the proper diameter(25") to my knowledge.

>or 235 tire up front?

Same deal as above (wrong diameter tire IMHO), and since it is taller it would only amplify rubbing issues from incorrect offset wheels on the front.

DR

> At least they seem OK.....

So did my second wife..... at first :-)

GlenL

Hey, Ruf.

Seein' things. In the Summer it gets S4 wheels that I got from Rick via eBay. Those in the pic are the "winter wheels" (old S wheels) with snow tires.

928ntslow

Hmmm....911/912's originally had the same sized tires all around. As the engines got larger, it made a somehat bad situation worse. To counter act the increased rear weight issue, Porsche added wider wheels and tires in the back for increased contact patch to keep the car on the road around curves. It worked....kinda.

Having narrower tires in the front of a 928 seems unneccessary if you ask me. 928's had the same sized tires all around until the late 80's. I think they put larger tires in the rear for a sportier look and quite possible for increased hp in putting the power to the ground. Keeping narrower tires up front allowed for , not so bad gas mileage and relatively easier steering for the common driver.

It just makes sense to me that a 50/50 car should have the same sized wheels and tires all the way around. I think the wider tires in the back end up pushing the car so much, it makes it tougher for the fronts to actually steer to the optimum. Uuuuuuuunnnnnnnfortunately, most all wheels out there for the 928 are designed with the 911 in mind. I wish I could have put the same sized wheels and tires all around my GTS...it would have been too costly for me to do.

The 82 will though...and they WILL be 9.5" all around!

MrLexse

Someone had mentioned to me that putting spacers in, lessens the offset. I'm confused. Is that the case and if so why?

heinrich

Lex, offset is the distance between the wheel's imaginary centre line and the mounting surface. Positive offset is when the mounting flat surface with bolt holes, is toward the outside of the wheel. Adding a spacer pushes the whole wheel outward, and thus the centre line of the wheel has been pushed outward, or put differently the spacer has become a new mount surface, which sits closer to the centre line of the wheel. Remember the spacer mounts to the wheel hub face. So if offset was originally 50mm and you add a 10mm spacer, the gap between centre line and mounting face now is smaller, and effective offset is now 40mm.

928ntslow

Question
Answer
Heinrich is correct, BUT...the offset is actually increased over all. It is a tricky question to answer really. Let's say you need a 65mm offset overall. Say you have a wheel that has a 40mm offset as Heiny says. To mount it to a 928 without a spacer, would put the inner lip of the rim in too close or right against the chassis/body/shocks. You will then need a 25mm spacer to push the wheel further away from the chassis/body/shocks to get it positioned where you need it out in the wheel well. In essence, you have increased the necessary offset to allow the wheel to fit.

HOWEVER, when looking at this from the angle of the person who told you this info, the spacer lessens the offset, based on the fitment of a particular wheel with a given offset less than the necessary 65mm....as in the wheel with a 40mm offset.

If you can't follow that, try this. You need an offset of 65mm. You have a wheel that is 40mm. Adding a 25mm spacer will lessen the needed offset to allow the 40mm offset wheel to fit properly.

HTH

John Veninger

This may help:
http://www.1010tire.com/WheelOffsetCalculator.asp

Currently I run 9.5X17 on the front of the track car with 275 Hoosiers and rolled fender. The have a 73mm offset. They make no contact except when at full lock parking the car.

MBMB

Would you say a spacer increases the offset of the wheel (from that point of view) or decreases the offset needed by the car (from that POV)?

John Veninger

You CANNOT "space" a 52mm offset wheel to a 65mm offset.

MBMB

Then I'm confused, because Keith says you can "space" a 40mm offset wheel to a 65mm offset. Why 40 but not 52?

Peter F

A spacer decreases the offset, thats why.
Higher offset number moves the wheel closer to inside of wheel well.
Lower offset puts the wheel further out, that is also why adding spacers lowers the offset the same number of mm as the spacer is thick.

Cheers/Peter

DR

>Then I'm confused, because Keith says you can "space" a 40mm offset wheel to a 65mm offset.

Keep in mind that our good friend Keith lives in California and I think the officials were burning some "bushes" nearby when he wrote that :-)

The confusion that causes such mistakes is that we do not generally refer to the offsets on our 928s correctly, we have NEGATIVE offsets, not positive offsets. You can't add "positive" (spacers) to a "negative" offset and increase the "negative offset"... what you do is decrease the "negative offset".

Add 20 mm spacers (positive) to a 65 negative offset and you get a 45mm negative offset ..

If you are talking positive offset then yes adding a 20 mm spacer to a 45 mm positive offset will give you a 65mm positive offset.

Here is hopefully a simple way to describe "offsets".

Imagine a line directly down the middle of the width of your wheels. Then measure the distance to the hub/rotor mounting surface (where the studs stick out), the distance to this surface from this "center line" is the "offset".

If the hub/rotor mounting surface is towards the outside of the center line (towards fender lip) then the offset is negative. If the same surface is towards the inside of the center line (body chassis), then it would be positive offset. Our 928s have and use negative offsets.