I’m trying to calculate offset for a change of wheels and wondered if someone can please have a look at my maths / logic?
I have 2 dimensions I'm sure of… 7” rims and 106mm backspace… so…
7” = 177.8mm
177.8mm / 2 = 88.9mm
The backspace is 106mm
106mm – 88.9 = 17.1mm
ET = 17
Is that right?
I only ask as when I look at the ‘backspace to offset’ conversion charts (see pic below), they come up with something else. Their logic apparently
being 106mm backspace = 4.17”. The convergence point of the 7” rim and 4.17” backspace on the chart = 3 (approx).
Standard wheel width is measured to a different place from backspace, hence the discrepancy:
http://i198.photobucket.com/albums/aa48/afmomto2/WheelBackspacing.jpg
[Edited on 14/12/15 by matt_claydon]
Just a thought.......I am sure you are aware of this anyway , 7" rims are bead to bead ....so the actual rim is possibly about 8". Usually if I am doing a quick check to see if a wheel might be ok , I would measure the back depth by placing a straight edge over the rim and measuring to the mounting face. Sure you would have done it that way too. If you measure your rim from outside edge to outside edge....then half that , you might get an answer that fits in with the chart???
Cheers guys.
I'm taking the 7" from the stamp on the wheels and the backspace I've measured is indeed to the outer lip.
I think you need to measure the wheel from lip to lip........this will be greater than 7", then half that measurement and calculate as you did before. The offset figure , either positive or negative refers to the relationship between the centreline of the wheel and the mounting face of the hub. A wheel with 0 offset would have exactly equal amount of rim to each side of the hub mounting face . Positive offset would put the wheel further out in the arch....negative further in.....
It doesn't matter where you measure width from as offset is taken from cl of rim.
dont forget the consider your tyre size if changing wheel width etc
Easy way to measure is put steel rule across rim and measure distance to mounting flange. Subtract the distance from front rim to mounting surface
measured same way and divide the difference by 2.
As others say the discrepancy is due to different reference points
Thanks all.
Complete Kit Car mag shows how and has pictures for those of use that find reading difficult
quote:
Originally posted by snapper
Complete Kit Car mag shows how and has pictures for those of use that find reading difficult