Will these wheels fit correctly?
02-14-2010, 08:12 PM
#16
Rennlist Member
It seems to me that there are two questions going in this thread. One has to do with the wheel width and offset and the other has to do with proper tire size for these wheels.
I went through this once or twice before recently, here, but it turned out that no one was interested in "crunching the numbers!" At the risk of repeating myself with a similar result, and on the expectation that you might actually be interested, I'll do it again.
You need to start with what you have on the car now. That might mean taking a couple of wheels off and seeing what the offset actually is. I would do it this way, but I already know what I have on my car. I have 7.5 by 17's on the front with 52mm offset. The wheels you are looking at are 8.5 by 18, so they are an inch wider. That means that there is half an inch more on each side of the center-line of the wheel. If the offset were the same as what you have that alone will put the outer rim half an inch closer to the inside of the fender. That may be all the room you have--I have slightly less with my wheels. But, the offset you are looking at is less by about 6mm, and that is about a quarter of an inch, so you will be about 3/4 inch farther out than mine are and that is too much. You can probably adjust the ride height up some and get by with it, but it is likely that you are going to rub.
The same approach applies to the rears. I have 9 by 17's and they allow about a half inch of fender clearance. They have 47mm offset. With your new wheels having 54 offset that is 7mm more offset than mine. That means that the hub face of the wheel is 7mm farther out from the centerline of the wheel. Therefore, the extra inch of width, giving half inch on the outside, is reduced some by the 7mm greater offset which pulls them back in. Overall they will be only about a quarter inch closer to the frear fender than mine, so they will fit.
Now, the tire size question. The numbers you are mentioning are the widths of the tire profile crosssection. You will want the have a tire width that is about an inch wider than the wheel width. So, with an 8.5 inch wheel you will want a tire with about a nine and a half inch width. 9.5 inches times 25.4mm is 241.3mm. That means that you should be looking for a 245 front tire and not a 265 or wider. Then the other number in the tire size in the height expressed in a percentage.
Since you should be looking to have about 25 to 25.5 inches overall height, take that number, 25.5, and subtract the wheel diameter of 18. That leaves 7.5inches, and then divide that by 2 for 3.75 cross section height. Convert that to mm's and you get 92.25, then divide that by 245 for the percentage which is 38.8. Therefore I would round down for the front and look for tires that are 245 by 35 by 18. I would round up for the rear unless it is really close.
Take this tire size and work it the other way and you will come up with the resulting tire height. 245 times .35 is 85.75, times 2 is 171.5 then divide by 25.4 is 6.75 inches. So, you end up with 18 plus 6.75 or overall tire height of24.75 inches. Just a hair under 25, which I think would be just right for the front or just a little short. If youthink you might like a slightly taller tire, go with a 40 profile.
For the rears I come up with 285 by 35 by 18. That give a tire height of 25.85 which will be just about right for the rears, I think.
I hope this helps.
Jerry Feather, FBA Certified
S4's (6)
KGJT
P.S. I just bought a set of similar wheels for one of my car projects, but in 19 inch. They have very similar offsets to what you are looking at. They are two- piece wheels however, and I am going to make a ring for the fronts that reduces the offset by 3/8 inch and I may machine about an eighth of an inch off the hub face of the fronts to bring then back a little more. That way they will only be about a quater inch wider than what I have now, and I think there is room for that. The rings will fit between the rim and the wheel center which is bolted in. JF
I went through this once or twice before recently, here, but it turned out that no one was interested in "crunching the numbers!" At the risk of repeating myself with a similar result, and on the expectation that you might actually be interested, I'll do it again.
You need to start with what you have on the car now. That might mean taking a couple of wheels off and seeing what the offset actually is. I would do it this way, but I already know what I have on my car. I have 7.5 by 17's on the front with 52mm offset. The wheels you are looking at are 8.5 by 18, so they are an inch wider. That means that there is half an inch more on each side of the center-line of the wheel. If the offset were the same as what you have that alone will put the outer rim half an inch closer to the inside of the fender. That may be all the room you have--I have slightly less with my wheels. But, the offset you are looking at is less by about 6mm, and that is about a quarter of an inch, so you will be about 3/4 inch farther out than mine are and that is too much. You can probably adjust the ride height up some and get by with it, but it is likely that you are going to rub.
The same approach applies to the rears. I have 9 by 17's and they allow about a half inch of fender clearance. They have 47mm offset. With your new wheels having 54 offset that is 7mm more offset than mine. That means that the hub face of the wheel is 7mm farther out from the centerline of the wheel. Therefore, the extra inch of width, giving half inch on the outside, is reduced some by the 7mm greater offset which pulls them back in. Overall they will be only about a quarter inch closer to the frear fender than mine, so they will fit.
Now, the tire size question. The numbers you are mentioning are the widths of the tire profile crosssection. You will want the have a tire width that is about an inch wider than the wheel width. So, with an 8.5 inch wheel you will want a tire with about a nine and a half inch width. 9.5 inches times 25.4mm is 241.3mm. That means that you should be looking for a 245 front tire and not a 265 or wider. Then the other number in the tire size in the height expressed in a percentage.
Since you should be looking to have about 25 to 25.5 inches overall height, take that number, 25.5, and subtract the wheel diameter of 18. That leaves 7.5inches, and then divide that by 2 for 3.75 cross section height. Convert that to mm's and you get 92.25, then divide that by 245 for the percentage which is 38.8. Therefore I would round down for the front and look for tires that are 245 by 35 by 18. I would round up for the rear unless it is really close.
Take this tire size and work it the other way and you will come up with the resulting tire height. 245 times .35 is 85.75, times 2 is 171.5 then divide by 25.4 is 6.75 inches. So, you end up with 18 plus 6.75 or overall tire height of24.75 inches. Just a hair under 25, which I think would be just right for the front or just a little short. If youthink you might like a slightly taller tire, go with a 40 profile.
For the rears I come up with 285 by 35 by 18. That give a tire height of 25.85 which will be just about right for the rears, I think.
I hope this helps.
Jerry Feather, FBA Certified
S4's (6)
KGJT
P.S. I just bought a set of similar wheels for one of my car projects, but in 19 inch. They have very similar offsets to what you are looking at. They are two- piece wheels however, and I am going to make a ring for the fronts that reduces the offset by 3/8 inch and I may machine about an eighth of an inch off the hub face of the fronts to bring then back a little more. That way they will only be about a quater inch wider than what I have now, and I think there is room for that. The rings will fit between the rim and the wheel center which is bolted in. JF
02-15-2010, 11:18 AM
#17
Team Owner
Jerry before you cut the wheel mount face down i would measure the thickness of the wheel mounting to where the lug nut face will be, if you cut the face of the wheel you may weaken the wheel so much that it peels off of the studs.
There are usually inserts in the lug area that the nuts seat against, and removing the rear side of their clamping area could weaken the wheel.
I dont know what the min thickness usually is but i would guess that about 20mm would be close
I will guess that you know this, as your other info seems so informative, however i am adding this so others dont get the same idea and cut away the wheel hub area so that their wheels will fit.
FWIW, its better to get the correct wheel offsets maybe you could swap out the wheel centers if your using 2 or 3 piece wheels
There are usually inserts in the lug area that the nuts seat against, and removing the rear side of their clamping area could weaken the wheel.
I dont know what the min thickness usually is but i would guess that about 20mm would be close
I will guess that you know this, as your other info seems so informative, however i am adding this so others dont get the same idea and cut away the wheel hub area so that their wheels will fit.
FWIW, its better to get the correct wheel offsets maybe you could swap out the wheel centers if your using 2 or 3 piece wheels
02-15-2010, 11:44 AM
#18
Rennlist Member
If you are serious about handling you need a 9 inch or better still a 9.5 inch front with a 68mm offset and a 265x35 section - 255 if you have a 9 inch.
On the rear you want a 10 inch ET60 with a 285 or 295 section- these will fit on either a S4 or GT with no mods or on a GTS with a thinner spacer. Some say they will fit on a GTS rear but on mine I found it was too close for comfort so fitted a 24mm spacer and open nuts.
If you want to fit turbo twist style wheels for cosmetics I suspect you will be stuck with 225's up front. Try to fit an offset of at least 60mm no matter what you do. A front offset of 52mm fits nicely but you will inevitably find it has a negative impact on the handling somewhere along the line due to the difference in NSR [negative scrub radius].
Regards
Fred
02-15-2010, 12:06 PM
#19
Rennlist Member
Jerry before you cut the wheel mount face down i would measure the thickness of the wheel mounting to where the lug nut face will be, if you cut the face of the wheel you may weaken the wheel so much that it peels off of the studs.
There are usually inserts in the lug area that the nuts seat against, and removing the rear side of their clamping area could weaken the wheel.
I dont know what the min thickness usually is but i would guess that about 20mm would be close
I will guess that you know this, as your other info seems so informative, however i am adding this so others dont get the same idea and cut away the wheel hub area so that their wheels will fit.
FWIW, its better to get the correct wheel offsets maybe you could swap out the wheel centers if your using 2 or 3 piece wheels
There are usually inserts in the lug area that the nuts seat against, and removing the rear side of their clamping area could weaken the wheel.
I dont know what the min thickness usually is but i would guess that about 20mm would be close
I will guess that you know this, as your other info seems so informative, however i am adding this so others dont get the same idea and cut away the wheel hub area so that their wheels will fit.
FWIW, its better to get the correct wheel offsets maybe you could swap out the wheel centers if your using 2 or 3 piece wheels
Jerry Feather
P.S. By the way, I wonder now what car this original poster is asking about the rim fitment? He doesn't seem to have a 928. JF
No spacers, and no rubbing
