BS/Offset?
Thread Starter
On The Tree
Joined: Nov 2005
Posts: 187
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From: OKC, OK
BS/Offset? Given that the popular Grand Sport (rear) wheels spec at:
17" x 11" (+50mm offset/7.93" backspacing)
Now, here's my (incorrect) math:
rim width...11 / 2 = 5.5 wheel centerline
backspacing - centerline = offset in inches (convert to mm)
7.93 - 5.5 = 2.43
convert (2.43) inches to mm should be (2.43x25.4) is 61.72mm offset
but the offset s/b 50mm
What am I doing wrong?
Thanks
17" x 11" (+50mm offset/7.93" backspacing)
Now, here's my (incorrect) math:
rim width...11 / 2 = 5.5 wheel centerline
backspacing - centerline = offset in inches (convert to mm)
7.93 - 5.5 = 2.43
convert (2.43) inches to mm should be (2.43x25.4) is 61.72mm offset
but the offset s/b 50mm
What am I doing wrong?
Thanks 
Offets does not include the bead thickens of a wheel . . . where the BS does . .
Thus BS and Offset are RELATED but you cannot really 'guess' one from the other . . .
Average bead thickness of a wheel is anywhere between 3/8" to 1/2"
The offset is calculated from the advertised rim width, which is the distance BETWEEN the beads.
Backspacing is calculated from the full overall width of the wheel, where an advertised wheel is 11" it will have an overall width of up to 12", including the mounting beads.
Think of a wheel as a channel like this " [ " where the advertised width is the inside distance between the two little legs and the overall is to the outside of the little legs.
Offset = 50 mm which equals 1 31/32" or 1.96"
11" divide by 2 to get center line of wheel is 5.5"
5.5" + 1.96" = 7.46"
Advertised Backspace of 7.93" - 7.46" gives a bead thickness of .47" or 15/32" . . .
Make sense?
Thus BS and Offset are RELATED but you cannot really 'guess' one from the other . . .
Average bead thickness of a wheel is anywhere between 3/8" to 1/2"
The offset is calculated from the advertised rim width, which is the distance BETWEEN the beads.
Backspacing is calculated from the full overall width of the wheel, where an advertised wheel is 11" it will have an overall width of up to 12", including the mounting beads.
Think of a wheel as a channel like this " [ " where the advertised width is the inside distance between the two little legs and the overall is to the outside of the little legs.
Offset = 50 mm which equals 1 31/32" or 1.96"
11" divide by 2 to get center line of wheel is 5.5"
5.5" + 1.96" = 7.46"
Advertised Backspace of 7.93" - 7.46" gives a bead thickness of .47" or 15/32" . . .
Make sense?
Last edited by Xsta Z 28; Oct 14, 2008 at 09:13 AM. Reason: Typo . . .
Thread Starter
On The Tree
Joined: Nov 2005
Posts: 187
Likes: 0
From: OKC, OK
Offets does not include the bead thickens of a wheel . . . where the BS does . .
Thus BS and Offset are RELATED but you cannot really 'guess' one from the other . . .
Average bead thickness of a wheel is anywhere between 3/8" to 1/2"
The offset is calculated from the advertised rim width, which is the distance BETWEEN the beads.
Backspacing is calculated from the full overall width of the wheel, where an advertised wheel is 11" it will have an overall width of up to 12", including the mounting beads.
Think of a wheel as a channel like this " [ " where the advertised width is the inside distance between the two little legs and the overall is to the outside of the little legs.
Offset = 50 mm which equals 1 31/32" or 1.96"
11" divide by 2 to get center line of wheel is 5.5"
5.5" + 1.96" = 7.46"
Advertised Backspace of 7.93" - 7.46" gives a bead thickness of .47" or 15/32" . . .
Make sense?
Thus BS and Offset are RELATED but you cannot really 'guess' one from the other . . .
Average bead thickness of a wheel is anywhere between 3/8" to 1/2"
The offset is calculated from the advertised rim width, which is the distance BETWEEN the beads.
Backspacing is calculated from the full overall width of the wheel, where an advertised wheel is 11" it will have an overall width of up to 12", including the mounting beads.
Think of a wheel as a channel like this " [ " where the advertised width is the inside distance between the two little legs and the overall is to the outside of the little legs.
Offset = 50 mm which equals 1 31/32" or 1.96"
11" divide by 2 to get center line of wheel is 5.5"
5.5" + 1.96" = 7.46"
Advertised Backspace of 7.93" - 7.46" gives a bead thickness of .47" or 15/32" . . .
Make sense?
sure would be nice, if they (offset & bs) were calculated from the same point!
Last edited by Xsta Z 28; Oct 14, 2008 at 09:14 AM. Reason: Fixed my typo!
