Backspace and Offset, How do they relate?

Discount Tire

Discount Tire

Active Member
Offset and backspace are essentially two different ways of looking at the same thing. They determine the location of the wheel and tire assembly when bolted to the vehicles hub.

Offset is the measured distance between the hub mounting surface and the wheels center line.

With X being the determined offset, the hub mounting surface on positive offset wheels is X amount forward from the wheel center line. The hub mounting surface on negative offset wheels is X amount backwards from the wheel center line. The hub mounting surface on 0 offset wheels is the wheel center line.​

Backspacing is the measurement from the hub mounting surface to the back edge of the wheel.


backspace_offset.jpg

***Typically speaking, the higher the offset/backspace, the more the wheel will tuck inwards towards the suspension or away from the fender. The lower the offset/backspace, the more the wheel will push out away from the suspension or towards the fender.

For Example:

*If the offset on a 9" wide wheel is +12mm, the hub mounting surface will be 12mm forward from the wheels center line. Measuring from the hub mounting surface to the back edge of the wheel, the backspacing is 5.5"

*If the offset on a 9" wide wheel is -12mm, the hub mounting surface would be 12mm towards the back of the wheel from the wheels center line. Measuring from the hub mounting surface to the back edge of the wheel, the backspacing is 4.5"​


Here's how to calculate the backspacing using the rim width and offset:

First - add 1" to the rim width and then divide by 2 to find the wheels center line (you have to account for the wheel flange which is why you add 1" to the rim width) . Second - convert the offset which is in millimeters into inches. There is 25.4mm in 1 inch so divide the offset by 25.4. Lastly - add the converted offset to the wheels center line if the offset is positive for the correct backspacing. Subtract the converted offset to the wheels center line if the offset is negative for the correct backspacing.

For Example:

17x9 +12mm offset

*9" rim width + 1" = 10"
*10"/2 = 5" (wheels center line)
*+12mm offset/25.4 = 0.47" (0.50" rounded up)
*0.50" + 5" = 5.50" Backspace


17x9 -12mm offset

*9" rim width + 1" = 10"
*10"/2 = 5" (wheels center line)
*-12mm offset/25.4 = 0.47" (0.50" rounded up)
*0.50" - 5" = 4.50" Backspace​


***Please note that the wheels center line is the backspacing for 0 offset wheels.

Last, here is a simple chart to help convert offset and backspace.
Offset_backspace_Chart_zps4469bc93.jpg


I hope this helps.


-Matt
 
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Wow, Nobody chimed in on this? We appreciate it Discount. This should be a sticky. Its a very common question with a quick solid answer.


Paratroopers stay up longer!
 
This is the best explanation and illustration I have seen so far. Thanks for posting this up! :thumb:
 
Hmm what's going on here ? Thanks for the write up. Welcome to Wayalife. You and Mel haven't gone threw proper procedure of giving a wave though. It's good to have some more pros on here.:thumbup:

Sent from my PC36100 using WAYALIFE mobile app
 
Tackerdown said:
Hmm what's going on here ? Thanks for the write up. Welcome to Wayalife. You and Mel haven't gone threw proper procedure of giving a wave though. It's good to have some more pros on here.:thumbup:

Sent from my PC36100 using WAYALIFE mobile app
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Shame on us. DONE!
 
Matt, this has been a huge help.

As a newb in the Jeep world I have been looking at the Black Rock Type D series 909S wheels at discount.

They have 4.5 inches of back space and the lift requires 4.5 or less, so I should be good correct? By the way it reads the wheel will be tucked under the fender as far as it could with that much back space correct or is it the other way around?
 
Jethro71- 4.5" backspace and skinny ~275x?x? tires should be flush with the flares. Your variables are the rim width and your chosen tires width. So if you go with a backspace of say 4" - then the tires will stick out further from the frame/ flare. Hope this helps.
 
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BlackRockBurner said:
Jethro71- 4.5" backspace and skinny ~275x?x? tires should be flush with the flares. Your variables are the rim width and your chosen tires width. So if you go with a backspace of say 4" - then the tires will stick out further from the frame/ flare. Hope this helps.
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Planning on running a Black Rock wheel with 4.5 back space, 17x8 with a 35x12.50x17 tire. Thanks for the info.
 
Jethro71 said:
Matt, this has been a huge help.

As a newb in the Jeep world I have been looking at the Black Rock Type D series 909S wheels at discount.

They have 4.5 inches of back space and the lift requires 4.5 or less, so I should be good correct? By the way it reads the wheel will be tucked under the fender as far as it could with that much back space correct or is it the other way around?
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Yes, 17x8 with a 4.5" backspace should sit out past the fenders by about 1/4" (not counting the tire). With a 35x12.50R17 mounted to it the tire will stick out about 1.5" past your fenders. If you choose a wider wheel with the same backspace your wheels and tires will stick out further. That said, although a 35x12.50R17 can be mounted to this narrow of a wheel but it is not ideal. I would recommend choosing a wheel that is 8.5" or 9" wide if you want to run a 35x12.50R17 tire. These wider widths will stick out further but the wider wheels will also help you get more of our your tires in the long run.
 
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Discount Tire said:
Yes, 17x8 with a 4.5" backspace should sit out past the fenders by about 1/4" (not counting the tire). With a 35x12.50R17 mounted to it the tire will stick out about 1.5" past your fenders. If you choose a wider wheel with the same backspace your wheels and tires will stick out further. That said, although a 35x12.50R17 can be mounted to this narrow of a wheel it is not ideal. I would recommend choosing a wheel that is 8.5" or 9" wide if you want to run a 35x12.50R17 tire. These wider widths will stick out further but the wider wheels will also help you get more of our your tires in the long run.
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I hear what you are saying about a wider wheel but a large majority of the wheels I am looking at only come in a 17x8 option and the ones that are 17x9 have a back space greater than 4.5. Black Rock wheels is what I would like to run and none of the aluminum wheels I have looked at have a 17x9 option. So kind of stuck on the 17x8...
 
Discount Tire said:
Yes, 17x8 with a 4.5" backspace should sit out past the fenders by about 1/4" (not counting the tire). With a 35x12.50R17 mounted to it the tire will stick out about 1.5" past your fenders. If you choose a wider wheel with the same backspace your wheels and tires will stick out further. That said, although a 35x12.50R17 can be mounted to this narrow of a wheel it is not ideal. I would recommend choosing a wheel that is 8.5" or 9" wide if you want to run a 35x12.50R17 tire. These wider widths will stick out further but the wider wheels will also help you get more of our your tires in the long run.
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Matt would it be better to run a 315/70x17 tire like a DuraTrac on the 17x8 wheel with 4.5 of backspace? I see a lot of guys running the DuraTracs on stock wheels which are 17x7.5. The DuraTracs get good reviews and honestly are about the same size in diameter as a 35" tire.

Thanks for all your help.
 
Jethro71 said:
I hear what you are saying about a wider wheel but a large majority of the wheels I am looking at only come in a 17x8 option and the ones that are 17x9 have a back space greater than 4.5. Black Rock wheels is what I would like to run and none of the aluminum wheels I have looked at have a 17x9 option. So kind of stuck on the 17x8...
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I am running 315/70 17 Duratrac's on Mickey Thompson Classic III's - 17x9 @ 4.5BS. They fit and look great. I can't picture this tire on a narrower rim, I think it would be pretty pinched in though. As it is I'm not really using the outer 1" or so of the tread with proper inflation.
 
Rivertoys said:
I am running 315/70 17 Duratrac's on Mickey Thompson Classic III's - 17x9 @ 4.5BS. They fit and look great. I can't picture this tire on a narrower rim, I think it would be pretty pinched in though. As it is I'm not really using the outer 1" or so of the tread with proper inflation.
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I hear what you are saying about pinched but all through this forum there are folks running them on stock rims at 7.5 inches. I have found a Black Rock 997 (steel) that is 17x9. Goodyear shows the approved wheel width on the 315/70-17 as 8.5 - 10. The only problem is I wanted aluminum. Thanks for you info.
 
Jethro71 said:
I hear what you are saying about pinched but all through this forum there are folks running them on stock rims at 7.5 inches. I have found a Black Rock 997 (steel) that is 17x9. Goodyear shows the approved wheel width on the 315/70-17 as 8.5 - 10. The only problem is I wanted aluminum. Thanks for you info.
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No problem! My 2-door is very light with a soft-top, two doors, no winch... so I'm not pressing them down much.

As a side note, I really wanted steel wheels (for an oldschool look), and was close to pulling the triger but then found they are SO much heavier. My set up with 17x9 aluminum is 91lbs/wheel. The stock 32" MTs on stock rims were 74lbs. I didn't think that was too bad of an increase.
 
Jethro71 said:
Matt would it be better to run a 315/70x17 tire like a DuraTrac on the 17x8 wheel with 4.5 of backspace? I see a lot of guys running the DuraTracs on stock wheels which are 17x7.5. The DuraTracs get good reviews and honestly are about the same size in diameter as a 35" tire.

Thanks for all your help.
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You can run either size, the 315's are barely narrower. I think you are on the right track in looking at aluminum wheels. Steel wheels are quite a bit heavier and you're already adding a decent amount of weight just by moving up to a 35" tire. You can still run the 17x8 wheels that you want to run you'll just need to pay closer attention to your tire wear and tire pressure.
 
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