backspace vs. offset
Thread Starter
Le Mans Master
Joined: Feb 2011
Posts: 5,943
Likes: 61
From: Southern NH
backspace vs. offset
we all know we need 4" backspace on an 8" wheel for our cars, but how does this translate into offset? I was looking at a set of wheels and they are 17"x8" but the offset is listed as -12mm. Is this correct for our cars?
(Coys won't have ANY 4" Backspace wheels until July so I need to find an alternative)
here's the wheels:
http://www.rev-wheels.com/rev-wheel....=5838&cat=1270
(Coys won't have ANY 4" Backspace wheels until July so I need to find an alternative)
here's the wheels:
http://www.rev-wheels.com/rev-wheel....=5838&cat=1270
Last edited by qwank; Apr 8, 2013 at 10:05 PM.
Thread Starter
Le Mans Master
Joined: Feb 2011
Posts: 5,943
Likes: 61
From: Southern NH
actually, I found this really cool wheel calculator and it answered my question:
http://www.rimsntires.com/specs.jsp
http://www.rimsntires.com/specs.jsp
Melting Slicks
Joined: Apr 2002
Posts: 2,337
Likes: 14
From: Norfolk VA
Okay. When purchasing wheels most deal with backspace b/c offset is not really a factor. Im sure you know a wheel width of 8" wheel is actually a 9" wheel. The wheel of your choice, 17x8 wheel is acutally a 9" wheel. It is a good idea to buy a set of wheels with a 4" backspace. However if you wish to run a wider tire (greater than 255mm/wheel(10" or greater) you will most likely need wheel adapters. Im sure you already know this. Good luck.
Melting Slicks
Joined: Apr 2002
Posts: 2,337
Likes: 14
From: Norfolk VA
Thread Starter
Le Mans Master
Joined: Feb 2011
Posts: 5,943
Likes: 61
From: Southern NH
the wheels are $13 less on the vendor's website. they probably raise the price a little to make up for Ebay fee's.
anyways, I'm going to give them a call on Monday and hopefully they have the wheels in stock. Doing research online, it looks like these wheels come in American Racing boxes.
anyways, I'm going to give them a call on Monday and hopefully they have the wheels in stock. Doing research online, it looks like these wheels come in American Racing boxes.
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
Oldguard. Can you explain this to me? I looked at the website referenced above and it seems that backspace is the distance from the edge of the rim to the mounting pad and offset is the distance from the mounting pad to the centerline of the wheel. If you get a wheel with the proper backspace then it seems to me that you wouldn't need an adapter because all the extra width is on the outside of the wheel. Adding an adapter would only push the wheel out farther. Are you saying that wider wheels generally have a bigger backspace so you need adapters to prevent the tires or wheels from rubbing on the inside?
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Most wheels have an overall rim flange width of about 1" greater than the given width at the bead. Using that 1" difference as the default, the offset for an 8" rim with 4" BS is -0.50", or -12.7mm, with negative values equating to the mounting surface being further from the outside of the wheel than is its CL...
Offset = BS" - ((bead width" + 1") / 2)
mm = inches x 0.0393700787
That said, I'd stick with BS and not worry about offsets unless made absolutely necessary.
Offset = BS" - ((bead width" + 1") / 2)
mm = inches x 0.0393700787
That said, I'd stick with BS and not worry about offsets unless made absolutely necessary.
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Melting Slicks
Joined: Apr 2002
Posts: 2,337
Likes: 14
From: Norfolk VA
Oldguard. Can you explain this to me? I looked at the website referenced above and it seems that backspace is the distance from the edge of the rim to the mounting pad and offset is the distance from the mounting pad to the centerline of the wheel. If you get a wheel with the proper backspace then it seems to me that you wouldn't need an adapter because all the extra width is on the outside of the wheel. Adding an adapter would only push the wheel out farther. Are you saying that wider wheels generally have a bigger backspace so you need adapters to prevent the tires or wheels from rubbing on the inside?
Last edited by Oldguard 7; Apr 6, 2013 at 11:37 PM.
Race Director
Joined: Jan 2007
Posts: 14,856
Likes: 3,308
From: NYC NY
IMCO (and only IMCO 
) I find most "off the shelf" wheel manufacturers use the term offset. The custom order wheel manufacturers (who build wheels to specific specs) use the term backspace. On top of that the "off the shelf" manufacturers state the offset in mm which requires another couple of calculations to determine backspace. It just adds to the confusion.
Melting Slicks
Joined: Mar 2007
Posts: 2,520
Likes: 11
From: Hinckley OH
You are right if you get a wheel with the proper bs you will n.ot need a wheel adapter. The bs and width of the wheel can vary greatly. In my case my replica c6 wheels (in c5 size) measure 17x8.5" (actually 9.5") front, 18x9.5 (10.5") rear. The bs for my rear wheels ia 7.5" I cannot put the wheel on the lugs b/c the edge of the wheel/tire hit the shock absorber and the wheels were nowhwre on the lugss I had to buy 2" apapters to decrease the bs from 7.5" to 5.5" so that means 5.5" of the wheel/tire (on a 10.5" wheel) is inside the fender and 5" is on the outside but the wheel/tire does not stick outside the fender or rubbing against the fframesuspenframe/suspension. For the front wheels a 1.5" adapters with a 7" bs decreased the bs to 5.5" (on a 9.5" wheel) which means 5.5" of the wheel/tire is on the inside of the fender and 4" is outside but does not stick outside the fender or rub against the suspension issues with turning. My tires are front 245/4517, rear 275/4018. With all that said, ometimes you mustuse wheeo adapters
Thread Starter
Le Mans Master
Joined: Feb 2011
Posts: 5,943
Likes: 61
From: Southern NH
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
On the 275/60's, tho the TRA standard wheel width may be only 7.5", an optimum width would be more like 9" to 9.5". I'll accept that you could at least defend the 8" rim here on the TRA technicality, but that standard was pretty much only devised to better facilitate comparisons between tire sizes, not to indicate optimum fitments. However, on the 205/75's, with the TRA standard wheel width being 5.5" and the optimum ~6.5", I'm afraid your installing them on 4" rims puts you well into balloon territory.
Bottom line: just because you can force a tire onto a rim doesn't make it a good fitment. Then again, I guess handling isn't everyone's priority, either. JMHO
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Well, excuse me if this comes off as my flaming you, but you apparently gave little regard to sidewall stability or distortion. From a technical perspective...
On the 275/60's, tho the TRA standard wheel width may be only 7.5", an optimum width would be more like 9" to 9.5". I'll accept that you could at least defend the 8" rim here on the TRA technicality, but that standard was pretty much only devised to better facilitate comparisons between tire sizes, not to indicate optimum fitments. However, on the 205/75's, with the TRA standard wheel width being 5.5" and the optimum ~6.5", I'm afraid your installing them on 4" rims puts you well into balloon territory.
Bottom line: just because you can force a tire onto a rim doesn't make it a good fitment. Then again, I guess handling isn't everyone's priority, either. JMHO
On the 275/60's, tho the TRA standard wheel width may be only 7.5", an optimum width would be more like 9" to 9.5". I'll accept that you could at least defend the 8" rim here on the TRA technicality, but that standard was pretty much only devised to better facilitate comparisons between tire sizes, not to indicate optimum fitments. However, on the 205/75's, with the TRA standard wheel width being 5.5" and the optimum ~6.5", I'm afraid your installing them on 4" rims puts you well into balloon territory.
Bottom line: just because you can force a tire onto a rim doesn't make it a good fitment. Then again, I guess handling isn't everyone's priority, either. JMHO
