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914World.com _ 914World Garage _ Back space # please

Posted by: cal44 Feb 10 2018, 02:47 PM

Can someone chime in what the correct backspacing is on a factory 5.5 rim? I measure 4 7/8 or 4 5/8" (damn old eyes). I thought it was supposed to be 4.5".

Best

Posted by: Tom_T Feb 10 2018, 02:51 PM

QUOTE(cal44 @ Feb 10 2018, 01:47 PM)

Can someone chime in what the correct backspacing is on a factory 5.5 rim? I measure 4 7/8 or 4 5/8" (damn old eyes). I thought it was supposed to be 4.5".

Best

ET40 which is 40 mm, so just use a dual measurement tape measure or ruler to get mm's.

Tom
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Posted by: Dave_Darling Feb 10 2018, 05:35 PM

ET is the offset, which is measured from the center of the rim. Backspacing is measured from the inside-most lip of the rim.

Offset is typically more useful to suspension engineers, who are worrying about stuff like scrub radius. Backspacing is typically more important to us, who are trying to figure out if something fits in the wheel well.

A 5.5" wide wheel is generally about 6" total width. (The nominal width is across the inside where the tire fits; the lip for the tire is about 1/4" thick and there is a lip on each side.) That's about 150mm. Half of that is 75mm. That gets you the center of the rim. Add 40mm for the ET40, and you get 115mm. Which is about 4.5".

So a 5.5" wide ET40 rim will have something close to 4.5" backspacing.

These numbers are all very approximate, so somewhere between 4.5" and 5" of backspacing seems reasonable for a 5.5 ET40 rim.

--DD

Posted by: cal44 Feb 10 2018, 07:51 PM

[quote name='Dave_Darling' date='Feb 10 2018, 03:35 PM' post='2577570']
ET is the offset, which is measured from the center of the rim. Backspacing is measured from the inside-most lip of the rim.

Offset is typically more useful to suspension engineers, who are worrying about stuff like scrub radius. Backspacing is typically more important to us, who are trying to figure out if something fits in the wheel well.

A 5.5" wide wheel is generally about 6" total width. (The nominal width is across the inside where the tire fits; the lip for the tire is about 1/4" thick and there is a lip on each side.) That's about 150mm. Half of that is 75mm. That gets you the center of the rim. Add 40mm for the ET40, and you get 115mm. Which is about 4.5".

So a 5.5" wide ET40 rim will have something close to 4.5" backspacing.

These numbers are all very approximate, so somewhere between 4.5" and 5" of backspacing seems reasonable for a 5.5 ET40 rim.

--DD

Thank you.
Mike