Off-Sets?
01-30-2002, 02:07 PM
#1
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Off-Sets?
I've asked around here and elsewhere and I've been told it's possible to run tyres as wide as 255/45 on 17x9.5" in front and 315/40 on 17x11.5" on the rear of a shark.
...if you're prepared to roll the rear fenders slightly. No problem.
...and you have to have the right off-sets.
Can any one please tell me the right off-sets?
...if you're prepared to roll the rear fenders slightly. No problem.
...and you have to have the right off-sets.
Can any one please tell me the right off-sets?
01-30-2002, 04:51 PM
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Hi 928 Maniac
It's a hell of a job trying to find out actual numbers isn't it! I'm going through the same process and have gathered some info - haven't got my wheels yet but have zeroed in on a few likley offsets.
For 9.5" fronts I have read in a few places that a 68mm offset is a good number.
For the rears i'm a bit more vague. For 10.5" I don't have any numbers but for 10" I have info that people have used values from 51-60mm. So it looks as if there is enough tolerance in that band to fit your extra 0.5". I don't know whether this includes spacers or what size.
I am going to play safe and get my ruler out come the time!
(how's it going with the light covers?)
Cheers
DaveW
It's a hell of a job trying to find out actual numbers isn't it! I'm going through the same process and have gathered some info - haven't got my wheels yet but have zeroed in on a few likley offsets.
For 9.5" fronts I have read in a few places that a 68mm offset is a good number.
For the rears i'm a bit more vague. For 10.5" I don't have any numbers but for 10" I have info that people have used values from 51-60mm. So it looks as if there is enough tolerance in that band to fit your extra 0.5". I don't know whether this includes spacers or what size.
I am going to play safe and get my ruler out come the time!
(how's it going with the light covers?)
Cheers
DaveW
01-30-2002, 06:21 PM
#4
Burning Brakes
If you plan on using 315/40-17 tires then be prepared to have more sluggish acceleration and a higher CG than the 255/40-17 tires normally used. If you do the math you should be able to figure out that you are making the gearing 7% taller, and raising the rear of the car by nearly an inch.
With that warning, based on Porsche's offset recommendations for their wheels, you should be able to deduce what offsets will work when you go to wider wheels.
For 928s with S4 or GTS brakes, the front and rear offsets are about 62mm and 52mm respectively on Porsche's 7.5x17"" and 9"x17" wheels. Therefore, with a 9.5"x17" wheels up front (with a 62mm offset) you'll have tires that may extend one inch further inward and outward than the OEM setup. If so, the outward part of the tire will rub or get cut by the fender lip. To avoid this, you'll have to get a wheels with more offset. A wheel with a 75mm offset may cause the tire to extend 1/2 inch further outward, and 1.5" further inward than the OEM setup. This might work, but then there may be rubbing of the tire on the inner fender, especially during turns.
I'll leave it to you to figure out the "right" offset for the rear. (It's actually easier since you need not worry about turns.) Good luck.
With that warning, based on Porsche's offset recommendations for their wheels, you should be able to deduce what offsets will work when you go to wider wheels.
For 928s with S4 or GTS brakes, the front and rear offsets are about 62mm and 52mm respectively on Porsche's 7.5x17"" and 9"x17" wheels. Therefore, with a 9.5"x17" wheels up front (with a 62mm offset) you'll have tires that may extend one inch further inward and outward than the OEM setup. If so, the outward part of the tire will rub or get cut by the fender lip. To avoid this, you'll have to get a wheels with more offset. A wheel with a 75mm offset may cause the tire to extend 1/2 inch further outward, and 1.5" further inward than the OEM setup. This might work, but then there may be rubbing of the tire on the inner fender, especially during turns.
I'll leave it to you to figure out the "right" offset for the rear. (It's actually easier since you need not worry about turns.) Good luck.
01-30-2002, 07:03 PM
#5
Burning Brakes
Just for grins I decided to calculate the offsets that might work for you. Up front with 255mm tires, you shoud be able to get by with a 70mm offset. However, I also recommend you install much stiffer springs than OEM, and don't turn the wheel too sharpely as the tires WILL rub the inner fender. (BTDT with 235-45/17 tires on 8"x17" wheels with a 70mm offset).
In the rear with 315mm tires on 11.5"x17" wheels you will need at least 100mm offset. (You are looking at custom made, so be ready to pay for them.) You can get by with less offset, but you'll have to extend the fender. YMMV.
In the rear with 315mm tires on 11.5"x17" wheels you will need at least 100mm offset. (You are looking at custom made, so be ready to pay for them.) You can get by with less offset, but you'll have to extend the fender. YMMV.
01-30-2002, 07:29 PM
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Ed
Are your numbers taking into account the 25mm spacer that I think Fat Bully's model year will have fitted as standard, or does this assume it's removal (with new bolts)?
DaveW
Are your numbers taking into account the 25mm spacer that I think Fat Bully's model year will have fitted as standard, or does this assume it's removal (with new bolts)?
DaveW
01-31-2002, 01:29 PM
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Thanx Ed & Dave.
Sorry for being a bit pushy there, but when I'm looking at rims now at least I know what I'm looking for in the first place.
Long live Rennlist.
Sorry for being a bit pushy there, but when I'm looking at rims now at least I know what I'm looking for in the first place.
Long live Rennlist.
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01-31-2002, 05:15 PM
#8
I'm running 17X10 K-20s with 275-40-17 RE-730s on the front and 17X11 K-20s with 285-40-17 RE-730s on the back. I did need to do some rolling of the rear fenders since they just barely rubbed at first. The rims are off the car now and I will see if I can get some offset measurements for you.
I assume I need to measure from the center of the rim (inside part where the lugs are) to the inside outer rim (ie. lay a flat board across the inside of the rim and drop a ruler down to the flat spot at the lugs)?????
Later,
Tom
midlman@seanet.com 87S4 AT
I assume I need to measure from the center of the rim (inside part where the lugs are) to the inside outer rim (ie. lay a flat board across the inside of the rim and drop a ruler down to the flat spot at the lugs)?????
Later,
Tom
midlman@seanet.com 87S4 AT
01-31-2002, 05:27 PM
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Hi TMM
The offset number is actually the distance the flat mating surface (bound to have a proper name!) on the inside of the wheel is from the centreline of the wheel. Easy enough to figure out if you also take the actual wheel width measurement.
DaveW
The offset number is actually the distance the flat mating surface (bound to have a proper name!) on the inside of the wheel is from the centreline of the wheel. Easy enough to figure out if you also take the actual wheel width measurement.
DaveW
01-31-2002, 05:37 PM
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Hi TMM
The offset number is actually the distance the flat mating surface (bound to have a proper name!) on the inside of the wheel is from the centreline of the wheel. Easy enough to figure out if you also take the actual wheel width measurement.
DaveW
The offset number is actually the distance the flat mating surface (bound to have a proper name!) on the inside of the wheel is from the centreline of the wheel. Easy enough to figure out if you also take the actual wheel width measurement.
DaveW
01-31-2002, 05:50 PM
#11
Burning Brakes
I am not taking into consideration any spacers. If his 928 has the 21mm or 25mm spacer, then removing it will decrease the amount of offset needed by the width of the spacer. However, the lug bolts will then protrude that much further out beyond the wheel's lug nut holes. 
02-01-2002, 01:41 AM
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Dang! Ed you've just move the goalpost again!
I have spacers in the front and rear to clear my big reds. The thickness I can't remember now. I guess I'll have to check and the subtract the spacer width from the off-sets you've given me before to find the correct off-sets, Right?
I have spacers in the front and rear to clear my big reds. The thickness I can't remember now. I guess I'll have to check and the subtract the spacer width from the off-sets you've given me before to find the correct off-sets, Right?