Trying to understand rim backspacing and offset
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Joined: Jan 2005
Posts: 29
From: Dracut, MA
Trying to understand rim backspacing and offset
I currently have a set of stock 16x8 rims on my 91 Z28. The fronts are 16x8x0 and the rears are 16x8x16.
My assumption is the fronts are offset by 0mm and the rears are offset by 16mm.
With that in mind I am looking into a set of American Racing Torq Thrust II 17x8 rims. However, I am going to get them all with the same offset so I can rotate them if I choose too. So I am using the calculations from the rear rim as my base.
Using a real crude way of measuring the required areas of the rim this is what I came up with. However, I my intention was to see if I am able to get in the ball park of what is on the rim. It’s a bit of a pain to measure it accurately since the tires are on the rim. Here is what I came up with:
Overall width of the rim = 8.3565
Centerline = overall width / 2
= 8.3565 / 2
= 4.17825”
Backspacing = 4.9”
Offset = backspacing – centerline
= 4.9 – 4.17825
= .72175”
Offset in mm = (offset in inches)(25.4)
= (.72175)(25.4)
= 18.33mm
Now knowing this I went to American Racing’s website. They offer the Torq Thrust II in three styles for the 17x8. According to their calculator on the website the following rims will mount up as follows.
Backside spacing = 4.5” / 114mm
Offset = 1.27mm
The clearance from stut housing to the inside of the wheel will be 15mm MORE
The outside edge of the wheel will EXTEND an extra 15mm
Backside spacing = 4” / 102mm
Offset = -11.43mm
The clearance from stut housing to the inside of the wheel will be 27mm MORE
The outside edge of the wheel will EXTEND an extra 27mm
Backside spacing = 4.75” / 121mm
Offset = 7.62mm
The clearance from stut housing to the inside of the wheel will be 9mm MORE
The outside edge of the wheel will EXTEND an extra 9mm
What I do not understand is the calculator online does not account for the backside spacing. It also does not tell me what the backside spacing is used for the calculation.
Out of the three options it seems the last one of the three with the 7.62mm offset will get me closest to where I need to be. My goal is to get the as close to stock appearing as possible without going with a custom wheel.
Any ideas? If my theory is incorrect please let me know. I appreciate everyones input.
My assumption is the fronts are offset by 0mm and the rears are offset by 16mm.
With that in mind I am looking into a set of American Racing Torq Thrust II 17x8 rims. However, I am going to get them all with the same offset so I can rotate them if I choose too. So I am using the calculations from the rear rim as my base.
Using a real crude way of measuring the required areas of the rim this is what I came up with. However, I my intention was to see if I am able to get in the ball park of what is on the rim. It’s a bit of a pain to measure it accurately since the tires are on the rim. Here is what I came up with:
Overall width of the rim = 8.3565
Centerline = overall width / 2
= 8.3565 / 2
= 4.17825”
Backspacing = 4.9”
Offset = backspacing – centerline
= 4.9 – 4.17825
= .72175”
Offset in mm = (offset in inches)(25.4)
= (.72175)(25.4)
= 18.33mm
Now knowing this I went to American Racing’s website. They offer the Torq Thrust II in three styles for the 17x8. According to their calculator on the website the following rims will mount up as follows.
Backside spacing = 4.5” / 114mm
Offset = 1.27mm
The clearance from stut housing to the inside of the wheel will be 15mm MORE
The outside edge of the wheel will EXTEND an extra 15mm
Backside spacing = 4” / 102mm
Offset = -11.43mm
The clearance from stut housing to the inside of the wheel will be 27mm MORE
The outside edge of the wheel will EXTEND an extra 27mm
Backside spacing = 4.75” / 121mm
Offset = 7.62mm
The clearance from stut housing to the inside of the wheel will be 9mm MORE
The outside edge of the wheel will EXTEND an extra 9mm
What I do not understand is the calculator online does not account for the backside spacing. It also does not tell me what the backside spacing is used for the calculation.
Out of the three options it seems the last one of the three with the 7.62mm offset will get me closest to where I need to be. My goal is to get the as close to stock appearing as possible without going with a custom wheel.
Any ideas? If my theory is incorrect please let me know. I appreciate everyones input.
Last edited by Fast_25th; Mar 18, 2006 at 10:02 PM.
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