 ET vs Backspacing, What the... |
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| Krieger |
 Oct 8 2024, 06:14 PM
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#1
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Advanced Member  Group: Members Posts: 4,766 Joined: 24-May 04 From: Santa Rosa CA Member No.: 2,104 Region Association: None 
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I am going to order some new wheels for my track car. 15 x 9. I understand backspacing. I think all 911 and 914/6 backspacing stock is 125mm. That is what I'd like. The manufacturer is listing three different ETs: 3mm, 10.6, 23.3. Which ET gives me the stock backspacing?
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| JamesM |
Oct 8 2024, 09:59 PM
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#2
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Advanced Member Group: Members Posts: 2,019 Joined: 6-April 06 From: Kearns, UT Member No.: 5,834 Region Association: Intermountain Region |
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| SirAndy |
Oct 8 2024, 10:05 PM
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#3
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Resident German Group: Admin Posts: 41,937 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
QUOTE(Krieger @ Oct 8 2024, 05:14 PM)  I am going to order some new wheels for my track car. 15 x 9. I understand backspacing. I think all 911 and 914/6 backspacing stock is 125mm. That is what I'd like. The manufacturer is listing three different ETs: 3mm, 10.6, 23.3. Which ET gives me the stock backspacing? ET (Einpresstiefe) is the distance from the center-line of the wheel to the mounting surface. It's normally measured in millimeters and is negative if the mounting surface is closer to the inside of the wheel and positive if closer to the outside of the wheel. (IMG:style_emoticons/default/driving.gif) PS: To get backspacing use simple math: (wheel width / 2) + ET PPS: Of course, make sure you convert all variables to the same system first, either imperial or metric. PPPS: Don't forget that ET can be negative! |
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| SirAndy |
Oct 8 2024, 10:24 PM
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#4
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Resident German Group: Admin Posts: 41,937 Joined: 21-January 03 From: Oakland, Kalifornia Member No.: 179 Region Association: Northern California |
Found this in the info listed on an online calculator.
Not sure how accurate it is but it assumes that a 9" wheel is actually wider than 9" if you account for the wheel "lip". (IMG:style_emoticons/default/idea.gif) QUOTE Once you have your wheel width, it’s common practice to add an inch to it to account for the wheel’s outer lips before dividing it by two. |
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| sixnotfour |
Oct 9 2024, 08:38 AM
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#5
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914 Wizard Group: Members Posts: 10,671 Joined: 12-September 04 From: Life Elevated..planet UT. Member No.: 2,744 Region Association: Rocky Mountains 
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A factory 15x9 is 3mm offset,and have a 117mm backspace they have the are the worst offset... 9x16s have 15mm offset and a 125mm backspace and fit standard flairs better..
Aftermarket suppliers may sell 15x9s with other offsets , I don't know.. |
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| Krieger |
Oct 9 2024, 08:39 AM
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#6
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Advanced Member Group: Members Posts: 4,766 Joined: 24-May 04 From: Santa Rosa CA Member No.: 2,104 Region Association: None
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Thank you! So a 9" rim =228.6. 228.6/2= 114.3. 114.3 + 3(ET) = 117.3 That is close to the factory backspacing of 125mm I'm looking for. I have an unmounted 8" Fuch. I will measure the od of the rim later and play this game again and report back.
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| sixnotfour |
Oct 9 2024, 08:50 AM
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#7
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914 Wizard Group: Members Posts: 10,671 Joined: 12-September 04 From: Life Elevated..planet UT. Member No.: 2,744 Region Association: Rocky Mountains
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Backing the day you could buy a actual 245/50 -15 ,, that tire on 9s was tough fit with factory GT flairs, Erics GT can barely run 15x8s ,,,
So results canvary a bunch, my GT has has 15x9s and 245/50 -15s with lip rolled up.. Attached image(s) 
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| technicalninja |
Oct 9 2024, 09:18 AM
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#8
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Advanced Member Group: Members Posts: 2,085 Joined: 31-January 23 From: Granbury Texas Member No.: 27,135 Region Association: Southwest Region
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QUOTE(JamesM @ Oct 8 2024, 10:59 PM) https://tiresize.com/wheel-offset-calculator/ https://tireswheelsdirect.com/wheel-offset-...ace-calculator/ QUOTE(SirAndy @ Oct 8 2024, 11:24 PM) Found this in the info listed on an online calculator. Not sure how accurate it is but it assumes that a 9" wheel is actually wider than 9" if you account for the wheel "lip". (IMG:style_emoticons/default/idea.gif) QUOTE Once you have your wheel width, it’s common practice to add an inch to it to account for the wheel’s outer lips before dividing it by two. Wheel width is supposed to be measured tire bead to tire bead. If you've got a 9" wheel it should be 9" between the beads, not total width of the wheel. JamesM's wheel calculators state the same thing. So, if you're close you should actually measure what you've got and NOT trust published widths. Some wheels have skinny lips, and some are fat. Adding an inch overall should be "safe" in most cases. |
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| BillJ |
Oct 9 2024, 10:16 AM
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#9
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Senior Member Group: Members Posts: 1,216 Joined: 4-March 13 From: charlotte, NC Member No.: 15,610 Region Association: None
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One of my combos is 15x9 and I used minilites from AircooledRacing.net. Steve has departed us but I think his shop lives on and may still be the US distributor for them. It is also period correct as the factory used them due to a fuch not being made that large in 1970. Mine fit great under my flares but it doesn't mean it will work for yours. All seem to.vary a bit.
Attached thumbnail(s) 
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| Olympic 914 |
Oct 10 2024, 08:03 AM
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#10
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Group: Members Posts: 1,709 Joined: 7-July 11 From: Pittsburgh PA Member No.: 13,287 Region Association: North East States
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This chart may be some help.
It is for 911 Fuchs offsets  |
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