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06-24-2019, 12:56 AM | #1 |
Blue 67
Join Date: Apr 2004
Location: Western WA
Posts: 1,179
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Wheel Mathematical question
Hi:
I am confused about understanding wheels back spacing and offset, even after all these years! First thing first: Back spacing is the distance between wheel mounting surface to the lip of the wheel on the back side, Offset is the position of the wheel mounting surface in regards to the center of the wheel based on its width. Correct? Now, this seems to be a simple calculation unless my understanding of the backspacing and offset above is wrong. For example, If a wheel is 8" wide with 4" back spacing, that means the wheel mounting surface is 4" away from wheel rear lip which means the mounting surface is at the mid center of wheel which is 4" which means the offset has to be zero! So why and how an 8" wide wheel with 4" back spacing can have positive or negative offset? What am I missing? Thanks
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1953 GMC 3/4 ton: current project 1967 C20: 5.3 voretc with T5 conversion, 4 wheel 8 lug disc brake 1972 Nova: my 4 door sport sedan! 5.3 voretc + T5 conversion. drivable project, FUN!! 1979 Camaro Z28: 5.3 vortec conversion with build TH350: drivable project, Fun to drive 1992 Camaro RS convertible (Z28 clone): 5.3 conversion with build T5: on going project 2005 Silverado 2500, 4x4 Duramax, original owner |
06-24-2019, 06:37 AM | #2 |
Special Order
Join Date: Nov 2004
Location: Mt Airy, MD
Posts: 85,858
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Re: Wheel Mathematical question
I don't know, but I've never gotten into the whole modern wheel thing. Wheels used to all have a backspace that would allow any wheel with the same bolt pattern to fit a vehicle with the same bolt pattern. When buying wheels you just got the diameter and width you wanted and unless you measured for your own curiosity, you never knew the backspace. From that standpoint and through all those years, to me "offset" equates to what we referred to as "dish", the opposite of backspace, the depth to center on the front, which would be the balance of the backspace figure when subtracted from the width. You could do the math if you really needed to know what those figures equated to in what you refer to as "offset", but I don't know why that would matter. I've never run across any cases of wheel offset not being the balance of the width after subtracting the backspace. Do you have an example?
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"BUILDING A BETTER WAY TO SERVE THE USA"......67/72......"The New Breed" GMC '67 C1500 Wideside Super Custom SWB: 327/M22/3.42 posi.........."The '67" (project) GMC '72 K2500 Wideside Sierra Custom Camper: 350/TH350/4.10 Power-Lok..."The '72" (rolling) Tim "Don't call me a redneck. I'm a rough cut country gentleman" R.I.P. ~ East Side Low Life ~ El Jay ~ 72BLUZ ~ Fasteddie69 ~ Ron586 ~ 67ChevyRedneck ~ Grumpy Old Man ~ Last edited by special-K; 06-27-2019 at 10:11 PM. |
06-24-2019, 08:23 AM | #3 |
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Re: Wheel Mathematical question
This might help
https://mgaguru.com/mgtech/wheels/pi...eloffset_2.gif |
06-24-2019, 08:28 AM | #4 | |
67-72 parts collector,…
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Re: Wheel Mathematical question
Quote:
A 8" wheel actually measure 9" overall. So a zero offset 8" wheel will have a 4.5" backspace.
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06-24-2019, 08:29 AM | #5 |
67-72 parts collector,…
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Re: Wheel Mathematical question
Look at this thread,...
http://67-72chevytrucks.com/vboard/s...d.php?t=530806
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Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823 |
06-24-2019, 08:30 AM | #6 |
Registered User
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Re: Wheel Mathematical question
a high offset (Positive) usually means deep backspacing, a low offset (negative) means less back space and deeper dish.
think in the terms of a FWD car of a high performance car with the track width pushed out wide...you need a deep offset to get under the car. recent heavy duty trucks are receiving the same treatment...look at the newer gm hd trucks, wheels have like 6" BS. an 8" wheel that has 50mm positive offset / or 6" bs. |
06-24-2019, 09:24 AM | #7 |
Blue 67
Join Date: Apr 2004
Location: Western WA
Posts: 1,179
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Re: Wheel Mathematical question
Thanks for the all replies. This subject has been bothering me since it is one of those things that you never learned and you always have taken somebody's else world for it without understanding it!
First, it doesn't sounds right to me that these two values have been measured by different measuring standards, one by inch and one by millimeter! This is where this old brain of mine hurts! If you have an 8" wide wheel ( which would be 9" wide from outside lip to lip approximately) with zero offset, which means the mounting surface is at the exact center of the wheel (4" from the either side of the inside of the lips and approximately 4.5" from the outside of the lips) this wheel has to have 4.5" backspacing. In another word an 8" wheel with 4.5" back spacing, mathematically, has to have zero offset since if you change the offset (wheel mounting surface) the backspacing will change automatically but then how come they are different offset ratings for such a wheel?! Does this make sense?
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1953 GMC 3/4 ton: current project 1967 C20: 5.3 voretc with T5 conversion, 4 wheel 8 lug disc brake 1972 Nova: my 4 door sport sedan! 5.3 voretc + T5 conversion. drivable project, FUN!! 1979 Camaro Z28: 5.3 vortec conversion with build TH350: drivable project, Fun to drive 1992 Camaro RS convertible (Z28 clone): 5.3 conversion with build T5: on going project 2005 Silverado 2500, 4x4 Duramax, original owner |
06-24-2019, 10:10 AM | #8 |
67-72 parts collector,…
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Re: Wheel Mathematical question
Not everyone wants a zero offset wheel.
That’s why they make different offsets. Personally I only talk in “backspacing”. Those number match my tape measure. Sometimes people run different width rearends to gain a deeper lip wheel. As in a narrowed rearend. I like my wheels tucked inside the fenders more than most. That requires custom backspacing. Bagged vehicles for example, need more backspacing than a static dropped or stock truck.
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Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823 |
06-24-2019, 08:56 PM | #9 | |
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Re: Wheel Mathematical question
Quote:
That was my expensive education about backspacing. I suppose one could use spacers but I've heard various opinions about the safety of those and besides they cost as much as new wheels. For some reason the trend of nearly all "newer" vehicles is wider axles and high backspace wheels. The only reason I can see for this is maybe the extra width is needed to allow for the CV axles of IFS FWD and 4WD vehicles ???
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Current/past Chevy/GMC trucks: 1958 Chevy C-60; 1965 GMC C-50; 1965 Chevy C-10; 1971 Chevy K-10; 1973 Chevy K-20; 1976 GMC C-20; 1977 Chevy C-10 Suburban; 1980 Chevy K-10; 1989 Chevy K1500; 1991 GMC V1500 Suburban; 2016 Chevy K2500 HD Other vehicles: 1988 Jeep XJ; 2011 Toyota 4Runner |
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06-25-2019, 09:37 AM | #10 |
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Re: Wheel Mathematical question
you really should measure from inside the lip, verify with an overall width for clearance.
i prefer BS to offset, but if understand 25.4 mm is 1", you can figure it out...just remember Positive vs Negative he bad is NOT ALL wheel manufactures make all BS/Offsets, you can get close...but not always. i run a COYS WHEEL, currently a 7" wide front with 4.25 BS, i would love it to have 4.5... |
06-25-2019, 10:05 AM | #11 | |
67-72 parts collector,…
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Re: Wheel Mathematical question
Quote:
Billet Specialties decided to come up with an affordable wheel (aka Chinese made like all the other cheap wheels),.. that actually FIT most old cars and trucks. THEY GOT THE BACKSPACE NUMBERS CORRECT,... when all the other companies were just guessing. They come in gray centered wheels and full polished as shown below. Contact member Way2lo2 here on the board or check them out at http://www.billetspecialties.com/whe...ies/speedway-/ I think Summit Racing even sells them.
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Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823 |
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06-25-2019, 01:23 PM | #12 |
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Re: Wheel Mathematical question
Unless of course you have a step-side.
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Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats? |
06-25-2019, 03:29 PM | #13 |
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Re: Wheel Mathematical question
Why do you say that?
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Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823 |
06-25-2019, 05:21 PM | #14 |
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Re: Wheel Mathematical question
So that concerns me as I thought about an 8" wide 17" Dia. wheel in the back of my stepside. With the bulge of the sidewalls I'm not sure, what do you think?:
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Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats? |
06-25-2019, 08:03 PM | #15 | |
67-72 parts collector,…
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Re: Wheel Mathematical question
Quote:
If you have a 17x8 Wheel you need a 4.5 inch bs to center it in the fender. And that leaves 1.5” of wheel gap on either side of the wheel. If I’m looking at your spec sheet right. What size tire do you plan to run?
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Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823 |
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06-25-2019, 08:49 PM | #16 |
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Re: Wheel Mathematical question
Not sure something 27" Tall that will fit.
Why do I keep thinking a 4" bs will center it in there? Where is my thought process flawed?
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Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats? |
06-25-2019, 09:11 PM | #17 | |
67-72 parts collector,…
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Re: Wheel Mathematical question
Quote:
The overal width of most all wheels are 1” wider overall. A 17x8 Wheel will actually measure 9” wide over all.
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Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823 |
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06-26-2019, 07:50 AM | #18 |
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Re: Wheel Mathematical question
Oh then that's a good thing. Finding an 8" wheel with a four inch backspace was getting tiring. They were all 4.5" backspace. I was thinking custom wheel cash was in the future.
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Master Chief, US Navy, Retired 1964 C10 Step-side Short Bed - In Progress 2009 2500HD Duramax LTZ 1948 GMC Panel 1956 Big Window Stepside 1953 5 Window Chevy Stepside 1932 Coupe New Seat Foam for Bench Seats? |
06-26-2019, 09:57 AM | #19 | |
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Re: Wheel Mathematical question
Quote:
i am all for the tariffs, but some one should start casting again within the US.. |
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06-26-2019, 10:06 AM | #20 |
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Re: Wheel Mathematical question
Great thread, I am gonna bookmark it!
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Dave 1968 Custom Chevy with turbo charged 5.3 gen III 4l80e swap 1967-71 GMC 3/4 ton long step 4x4 (not sure what year exactly?) "A good friend will bail you out of jail...but a true friend will be sitting next to you saying "that was frekin awesome". "If it doesn't fit force it...If it breaks then it needed to be replaced anyway!" |
06-26-2019, 10:13 AM | #21 | |
67-72 parts collector,…
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Re: Wheel Mathematical question
Quote:
He was the one that worked with Billet Specialties to develop these wheel. (they are Chinese, like Coys and Ridler, etc) He's one of the biggest dealers of Billet Specialties wheels in the country. I follow him on Instagram and Facebook.
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Keith Convert to disc brakes. http://67-72chevytrucks.com/vboard/s...d.php?t=444823 Last edited by lolife99; 06-26-2019 at 10:21 AM. |
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