what is backspace?

**kugzs** · 14-09-2006, 10:32 AM

Ive been looking for some wheels and people keep mentioning back space.
what is it?

thanks in advance

**wilbo666** · 14-09-2006, 10:47 AM

The distance from the wheel to rim mounting face to the inboard side of the rim.

http://www.rsracing.com/tech-wheel.html

or even (who would have thought

)
http://www.google.com.au/search?hl=e...e+Search&meta=

Cheers
Wilbo

**kugzs** · 14-09-2006, 10:54 AM

so does it basically mean you need at least that distance from the hub to the guards?

**bobob** · 14-09-2006, 10:58 AM

Inboard side might mean towards the inside of the car, i.e. away from the gaurds

**takai** · 14-09-2006, 12:05 PM

Its basically offset, but measured differently. And yes, inboard side is away from the guards.
Its normally measured in inches, wheras offset is in mm and from the centerline. So to convert offset to backspace you add half of the rim width in mm and then convert back to inches.

I.e. a -9 offset 7" wide rim would have a backspace of

Code:

(-9 + (7/2)*25.6)/25.6 == 80.6mm or ~3 1/8"

**Joshstix** · 14-09-2006, 12:15 PM

Offset is also generally measured from the bead seating point rather than the edge of the rim which can make it a bit harder to compare backspace numbers.

**takai** · 14-09-2006, 12:18 PM

Originally Posted by Joshstix

Offset is also generally measured from the bead seating point rather than the edge of the rim which can make it a bit harder to compare backspace numbers.

Interesting, i didnt realise that. Ive always measured it form the rim edge.

**Joshstix** · 14-09-2006, 12:20 PM

I guess generally it's the same as long as the rim lips are the same both sides of the rim but for a lot of the top line billets and things they have different inner and outer lips.

I guess just make sure where the manufacturer is measuring from before putting money down based on your measurements is the moral of the story I'm telling.

**Joshstix** · 14-09-2006, 02:23 PM

Originally Posted by takai

I.e. a -9 offset 7" wide rim would have a backspace of

Code:

(-9 + (7/2)*25.6)/25.6 == 80.6mm or ~3 1/8"

Actually on the topic of measuring to the bead or the rim this formula can get you into trouble. A 7" rim is wider overall than 7" most are about 8".

Lets say you have a car that has 102mm between the plane of the wheel mounting face and the strut body. This means the absolute maximum backspace you can run is 100mm or so. If you decide you're going to run an 8 inch rim and do the maths from your origonal formula you'd get

(X + (8/2)*25.6)/25.6 = 100mm

You'd get a result of X = ~5mm offset.

If you put that wheel on it won't fit though as you have the inner lip from edge to bead on top of the desired backspace.