 Back space # please |
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| cal44 |
 Feb 10 2018, 02:47 PM
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#1
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Member  Group: Members Posts: 165 Joined: 8-October 09 From: Encinitas, Ca. Member No.: 10,910 Region Association: Southwest Region 
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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".
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| Tom_T |
Feb 10 2018, 02:51 PM
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#2
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TMI.... Group: Members Posts: 8,318 Joined: 19-March 09 From: Orange, CA Member No.: 10,181 Region Association: Southern California
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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. (IMG:style_emoticons/default/beerchug.gif) Tom /////// |
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| Dave_Darling |
Feb 10 2018, 05:35 PM
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#3
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914 Idiot Group: Members Posts: 14,986 Joined: 9-January 03 From: Silicon Valley / Kailua-Kona Member No.: 121 Region Association: Northern California
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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 |
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| cal44 |
Feb 10 2018, 07:51 PM
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#4
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Member Group: Members Posts: 165 Joined: 8-October 09 From: Encinitas, Ca. Member No.: 10,910 Region Association: Southwest Region
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[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 |
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