Wheel backspacing and offset
Thread Starter
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
Wheel backspacing and offset
Hi, everybody!
I thought I had a pretty good grip on wheel backspacing and offset, but apparently I don't. I am hoping somebody can straighten me out. First, a little backstory.........
I am trying to figure out what new wheels to buy for my 1980 'vette. I have read that the backspacing and offset for my stock, 8" wide aluminum wheels is 4" of backspace, 0 offset. Made sense to me, 8" wheel, 4" backspace, 4+4=8, 0 offset, right? Well, what I'm trying to do is find a 17" diameter wheel that has the same backspacing and offset as the stock wheel does, and it's proving to be somewhat of a challenge. I keep looking at wheels and the measurements just don't add up. For instance, look at this wheel:
http://www.summitracing.com/parts/ARE-5157861/
How can an 8" wide wheel have 4" of backspace, yet also have -12mm of offset? Where is the 12mm coming from? Is there something basic here that I am just not aware of?
Keep the shiny side up!
Scott
I thought I had a pretty good grip on wheel backspacing and offset, but apparently I don't. I am hoping somebody can straighten me out. First, a little backstory.........
I am trying to figure out what new wheels to buy for my 1980 'vette. I have read that the backspacing and offset for my stock, 8" wide aluminum wheels is 4" of backspace, 0 offset. Made sense to me, 8" wheel, 4" backspace, 4+4=8, 0 offset, right? Well, what I'm trying to do is find a 17" diameter wheel that has the same backspacing and offset as the stock wheel does, and it's proving to be somewhat of a challenge. I keep looking at wheels and the measurements just don't add up. For instance, look at this wheel:
http://www.summitracing.com/parts/ARE-5157861/
How can an 8" wide wheel have 4" of backspace, yet also have -12mm of offset? Where is the 12mm coming from? Is there something basic here that I am just not aware of?
Keep the shiny side up!
Scott
Safety Car
Joined: Jan 2001
Posts: 3,637
Likes: 27
From: Norwalk Ct
It’s simple....backspace and offset are measured differently, by using two different points of reference.
Lets start of by choosing a wheel, say 17x8. The 8 part is the "bead" width so to speak, the inner distance between the front and rear wheel lips, where the tire would mount.
Now if you put that same 8" wide wheel flat on the ground and measured from the floor, to the outer edge, it would = 9", the 8" bead width, plus 1/2" front and back for the wheel lip.
Ok backspace measurement - this is the measurement from the hub mounting face, to the outer edge of the lip at the back of the wheel (the 9” width)
Offset, is the distance (in MM’s) the hub mounting face is moved from the center line of the wheel, forwards or backwards, using the bead width as the total width (the 8” width) .
So an 8” wide wheel, with 4” backspace (so would have 5” dish), but it will have -11 offset (the mounting face is moved ½” rearward of the wheel center line)
Nick
Lets start of by choosing a wheel, say 17x8. The 8 part is the "bead" width so to speak, the inner distance between the front and rear wheel lips, where the tire would mount.
Now if you put that same 8" wide wheel flat on the ground and measured from the floor, to the outer edge, it would = 9", the 8" bead width, plus 1/2" front and back for the wheel lip.
Ok backspace measurement - this is the measurement from the hub mounting face, to the outer edge of the lip at the back of the wheel (the 9” width)
Offset, is the distance (in MM’s) the hub mounting face is moved from the center line of the wheel, forwards or backwards, using the bead width as the total width (the 8” width) .
So an 8” wide wheel, with 4” backspace (so would have 5” dish), but it will have -11 offset (the mounting face is moved ½” rearward of the wheel center line)
Nick
Race Director
Joined: Jan 2007
Posts: 14,919
Likes: 3,333
From: NYC NY
with what Nick said. Good explanation. I'll add that it is important to remember that wheel width is measured from bead to bead as Nick said so the wheel is an inch wider than it's stated width. That is important when you are working with wheel specs based on offset. For what it's worth most of the custom built wheels made in the USA are built to a backspace spec. So yes if you order 8" wide wheels you want 4" backspace. The wheel you linked to IS the correct backspace wheel for your car.
Drifting
Joined: Mar 2011
Posts: 1,347
Likes: 121
From: Kernersville NC
That wheel is $70 less at wheelsforless.com and includes lugs, valve stems, and free shipping. The TT2.1 comes with a 4.07' backspace and fits fine.
http://www.wheelsforless.com/detail.aspx?ID=7935
http://www.wheelsforless.com/detail.aspx?ID=7935
Thread Starter
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
It’s simple....backspace and offset are measured differently, by using two different points of reference.
Lets start of by choosing a wheel, say 17x8. The 8 part is the "bead" width so to speak, the inner distance between the front and rear wheel lips, where the tire would mount.
Now if you put that same 8" wide wheel flat on the ground and measured from the floor, to the outer edge, it would = 9", the 8" bead width, plus 1/2" front and back for the wheel lip.
Ok backspace measurement - this is the measurement from the hub mounting face, to the outer edge of the lip at the back of the wheel (the 9” width)
Offset, is the distance (in MM’s) the hub mounting face is moved from the center line of the wheel, forwards or backwards, using the bead width as the total width (the 8” width) .
So an 8” wide wheel, with 4” backspace (so would have 5” dish), but it will have -11 offset (the mounting face is moved ½” rearward of the wheel center line)
Nick
Lets start of by choosing a wheel, say 17x8. The 8 part is the "bead" width so to speak, the inner distance between the front and rear wheel lips, where the tire would mount.
Now if you put that same 8" wide wheel flat on the ground and measured from the floor, to the outer edge, it would = 9", the 8" bead width, plus 1/2" front and back for the wheel lip.
Ok backspace measurement - this is the measurement from the hub mounting face, to the outer edge of the lip at the back of the wheel (the 9” width)
Offset, is the distance (in MM’s) the hub mounting face is moved from the center line of the wheel, forwards or backwards, using the bead width as the total width (the 8” width) .
So an 8” wide wheel, with 4” backspace (so would have 5” dish), but it will have -11 offset (the mounting face is moved ½” rearward of the wheel center line)
Nick
Keep the shiny side up!
Scott
Thread Starter
Le Mans Master
Joined: Mar 2008
Posts: 5,948
Likes: 72
From: Oxford MA-----You just lost the game!!!!
with what Nick said. Good explanation. I'll add that it is important to remember that wheel width is measured from bead to bead as Nick said so the wheel is an inch wider than it's stated width. That is important when you are working with wheel specs based on offset. For what it's worth most of the custom built wheels made in the USA are built to a backspace spec. So yes if you order 8" wide wheels you want 4" backspace. The wheel you linked to IS the correct backspace wheel for your car.
Keep the shiny side up!
Scott
