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914World.com _ 914World Garage _ Backspacing

Posted by: McMark Jun 25 2006, 06:30 PM

Somehow all of the "guides" for finding back spacing and offset don't seem to make sense between their examples and their formula.

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

In their chart they list a 5.5 wheel with 3.75 back spacing as having a 12 mm offset.

They say to find the offset that you subtract centerline (5.5 / 2 = 2.75) from back spacing (3.75). Which gives us (3.75 - 2.75) 1 inch. Which converts to 25.4 mm, NOT 12 mm.

What I'm I missing?

Posted by: Jeroen Jun 25 2006, 06:47 PM

you're missing nothing... their table is just wrong

Posted by: McMark Jun 25 2006, 06:51 PM

WTF, that's the SECOND site I've seen with BS info.

Posted by: trekkor Jun 25 2006, 07:07 PM

Welcome to the internet

Didn't someone say, "Just because it's on the internet doesn't make it true"?

KT

Posted by: pupwag Jun 25 2006, 07:26 PM

Might be that the 5.5 is bead to bead and backspacing is measured to the outside of the wheel rim.Offset is also from the outer edges at least on that diagram.That might account for most of the difference.

Posted by: Demick Jun 25 2006, 08:58 PM

pupwag has it right. A 5.5" wheel is 5.5" from bead to bead (rim width in their diagram). You cannot accurately determine backspace from offset by just knowing the bead-to-bead width. You need the overall wheel width. Without it, you can just approximate what the backspace is (or offset is). Which is exactly what that table does.

For the most part, I have seen the wheel flanges to be around 3/8" wide for an alloy wheel. Looks like their table assumes about 1/2" wide wheel flanges, which is where your missing ~13mm comes from.

Demick