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-   -   Offset & Backspace - How Do They Relate? (https://www.thedieselstop.com/forums/f122/offset-backspace-how-do-they-relate-260418/)

Discount Tire 06-29-2010 02:25 PM

Offset & Backspace - How Do They Relate?
 
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.
http://i248.photobucket.com/albums/g...ace_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.

I hope this helps.
-Matt

SmokeyWren 06-30-2010 12:18 PM

Matt -

Although I should be able to figure this stuff out, it still escapes me. So work this example on '99-'04 SuperDuty pickups.

Stock wheels are 16x7 with 0.25 inch offset. I want to mount new wheels that are 16x8. What offset/inset should I get to keep the inside edge of the rim the same distance from the suspension as the stock wheels?

And here's a frequently asked question:

I want to put 2005-up Ford stock 18" wheels (take-offs available from my local Discount Tire store) on my '99-'04 SRW. The stock rims are 16x7 with 0.25 inch offset. The 2005-up rims are 18x8 with 1.57 inch inset. I know there's a 1big difference in inset/offset, but what does that mean when my Discount Tire store tries to mount those wheels on my pickup?




Discount Tire 06-30-2010 01:34 PM

Quote:

Originally Posted by SmokeyWren (Post 1918813)
Matt -

Although I should be able to figure this stuff out, it still escapes me. So work this example on '99-'04 SuperDuty pickups.

Stock wheels are 16x7 with negative 0.25 inch offset (Ford now calls it 0.25" inset). I want to mount new wheels that are 16x8. What offset/inset should I get to keep the inside edge of the rim the same distance from the suspension as the stock wheels?

My system shows the stock 16" wheels for your SuperDuty '99-'04 was a 16x7 +6mm offset. (roughly +.25" offset) If someone can pull off their stock wheel and look at the back of it, it should have the offset stamped into the back of one of the spokes. Sometimes its next to the letters "ET".

In terms of offset: Assuming my guide is correct, to have the keep the inside edge of your new 16x8" at the same location as the inside edge of your stock 16x7 wheels you would want a 16x8 with a negative 6mm offset. This is usually listed as -6mm. Note, a 16x8 with a -6mm offset will stick out 1" further than your stock wheels. (not counting the tire.)

In terms of Backspace. A 16x7 +6mm offset has a 4.25" Backspace. A 16x8 -6mm offset also has a 4.25" Back Space.



Quote:

And here's a frequently asked question:

I want to put 2005-up Ford stock 18" wheels (take-offs available from my local Discount Tire store) on my '99-'04 SRW. The stock rims are 16x7 with 0.25 inch inset. The 2005-up rims are 18x8 with 1.57 inch inset. I know there's a 1.32 inch difference in inset, but what does that mean when my Discount Tire store tries to mount those wheels on my pickup?
With an 18x8 + 45mm offset (1.77") the inner wheel edge of the 18" 05+ Stock wheel would sit about 1.5" closer to the suspension/spindles/frame than the stock 16x7" wheel (not counting the tire). Your Discount Tire would need to test fit the wheel to see if there is enough space for it to clear these areas. If they don't clear your local store would recommend another wheel option.

SmokeyWren 07-02-2010 06:34 AM

Quote:

Originally Posted by Discount Tire Direct (Post 1918843)
My system shows the stock 16" wheels for your SuperDuty '99-'04 was a 16x7 +6mm offset. (roughly +.25" offset)

Yep, you're right. The Ford specs confirm that it a quarter inch offset, not inset. For 2005-up, they changed to 1.57" inset.




niceshot 05-26-2011 07:31 AM

Newer stock wheels on 2004
 
Ok I have stock KR 16" wheels on my 2004 F250. I also have a newer set of stock 18's that I was thinking about putting on. After reading it looks like that's not possible. I have a set of Toyo OC MT's 35x12.50x18 and could you tell me what size aftermarket wheel I would need in order to ensure proper spacing? If this question has already been addressed, welcome to my ADD world:thumbsup:

SmokeyWren 05-26-2011 10:29 AM

Quote:

Originally Posted by niceshot (Post 2071820)
Ok I have stock KR 16" wheels on my 2004 F250. I also have a newer set of stock 18's that I was thinking about putting on. After reading it looks like that's not possible.

It's possible, but not recommended. You can add a spacer between the wheels and the hubs to push the wheels out about 2". But cheap spacers are killers, and you could buy new custom wheels for less than the good spacers cost.

Quote:

I have a set of Toyo OC MT's 35x12.50x18 and could you tell me what size aftermarket wheel I would need in order to ensure proper spacing?
The tire makers all recommend rims at least 8.5" wide. However lots of folks ran that size tires on 18"x8" rims with no problems.

Ford made factory 18"x8" rims for the last half of the 2004 model year Harley Davidson and King Ranch SuperDuty SRW pickups. That is probably the only 18" rims currently available that fit right and have the correct offset and hole pattern for a '99-'04 SuperDuty SRW pickup. And those are hard to find. But here's one source:

Hubcap Haven: ALY3551 Ford F250/F350 Harley Wheel Polished #4C3Z1007AC

That link includes Ford part numbers, so if you take those numbers to a Ford parts department, they could probably tell you the exact offset specs for that rim. I suspect it's 0.25", but I'm not sure.

A few other wheel manufacturers made custom wheels for the '99-'04 SuperDuty pickups way back when, but they stopped production a few years ago. Apparently, demand for high-priced wheels for '99-'04 SuperDuty pickups is not high enough to warrant production and sales of those rims, and they won't fit on anything else. So almost nobody still makes them. Alcoa and Weld Racing both made them for years, but neither still make them. But here's an Eagle Alloy rim that may be the right one for your truck. It even has the correct width for your tires at 8.5". These should be available at Discount Tire, but you may have to order them.

http://aewheel.com/wheels_detail_specs.cfm?id=287

Part nbr.....size . finish. Pattern. offset. Pilot bore
0646-2887 20x8.5 Chrome 8 on 170mm -1 mm . 4.649

And since Eagle Alloy has that size, they may have other styles with the same specs.




niceshot 05-29-2011 01:17 AM

Thanks Smokey. I went up to Discount Tire just to get their opinion. Said that for a 35x12.50x18 tire that an 18x9 with a -12 os or 4.5" bs would work. I have a 4" lift so not worried about clearance. My 2 main concerns are 1-I don't want my tires to hit my leaf springs on full turn and 2-I don't want my tires to stick out more than is necessary. Not sure either of these can be avoided. Thanks

niceshot 06-23-2011 11:15 PM

Quote:

Originally Posted by SmokeyWren (Post 2071896)
It's possible, but not recommended. You can add a spacer between the wheels and the hubs to push the wheels out about 2". But cheap spacers are killers, and you could buy new custom wheels for less than the good spacers cost.



The tire makers all recommend rims at least 8.5" wide. However lots of folks ran that size tires on 18"x8" tires with no problems.

Ford made factory 18"x8" rims for the last half of the 2004 model year Harley Davidson and King Ranch SuperDuty SRW pickups. That is probably the only 18" rims currently available that fit right and have the correct offset and hole pattern for a '99-'04 SuperDuty SRW pickup. And those are hard to find. But here's one source:

Hubcap Haven: ALY3551 Ford F250/F350 Harley Wheel Polished #4C3Z1007AC

That link includes Ford part numbers, so if you take those numbers to a Ford parts department, they could probably tell you the exact offset specs for that rim. I suspect it's 0.25", but I'm not sure.

A few other wheel manufacturers made custom wheels for the '99-'04 SuperDuty pickups way back when, but they stopped production a few years ago. Apparently, demand for high-priced wheels for '99-'04 SuperDuty pickups is not high enough to warrant production and sales of those rims, and they won't fit on anything else. So almost nobody still makes them. Alcoa and Weld Racing both made them for years, but neither still make them. But here's an Eagle Alloy rim that may be the right one for your truck. It even has the correct width for your tires at 8.5". These should be available at Discount Tire, but you may have to order them.

Eagle Alloy Wheels - Series 064

Part nbr.....size . finish. Pattern. offset. Pilot bore
0646-2887 20x8.5 Chrome 8 on 170mm -1 mm . 4.649

And since Eagle Alloy has that size, they may have other styles with the same specs.

Well I guess patience is a virtue! It's been almost a month since you told me about Ford making and 18" wheel in late '04 and believe it or not I was goofing around on Craig's list and found a set not an hour away for $500. They are perfect and just what I was looking for. Thanks so much for your info. I would have never known. Take care

blagislb 12-20-2011 11:19 AM

i have stock 16 inch wheels. im wanting to put 16.5 wheels on my truck, whats the backspacing for 16.5 inch wheels?

Discount Tire 12-20-2011 02:27 PM

Quote:

Originally Posted by blagislb (Post 2142678)
i have stock 16 inch wheels. im wanting to put 16.5 wheels on my truck, whats the backspacing for 16.5 inch wheels?

It depends on the wheels width and offset.

For your 96, you'll want a wheel with a lower offset/backspace. Was there a particular wheel model you had in mind?

Maverick4x4 01-13-2012 12:46 AM

Hi Discount!

I won a set of MB rims from Discount, looking at the Gunner 8's in the 18X8.5 8-170 -6C

Using the math above, does that mean they will stick out 1.25" more than the factory 16x7's?

And in another 1/4"?

You said: " In terms of Backspace. A 16x7 +6mm offset has a 4.25" Backspace. A 16x8 -6mm offset also has a 4.25" Back Space." So an 18x8.5 -6mm would have 4.5" of backspace? (8.5/2 = 4.25, then subtract the 6mm =4, then add half of the 1", so 4.5")

This makes me nervous when it comes to running tire chains, there isn't a lot of room between the fenders and the inside of the tires...especially if I run the 325/65r18 size tires that I'm looking at. I'd better chain up the 305's that are currently on there and see how much room I have left.

Thanks,
Brett

Discount Tire 01-13-2012 05:56 PM

Quote:

Originally Posted by Maverick4x4 (Post 2151227)
Hi Discount!

I won a set of MB rims from Discount, looking at the Gunner 8's in the 18X8.5 8-170 -6C

Using the math above, does that mean they will stick out 1.25" more than the factory 16x7's?

And in another 1/4"?

You said: " In terms of Backspace. A 16x7 +6mm offset has a 4.25" Backspace. A 16x8 -6mm offset also has a 4.25" Back Space." So an 18x8.5 -6mm would have 4.5" of backspace? (8.5/2 = 4.25, then subtract the 6mm =4, then add half of the 1", so 4.5")

This makes me nervous when it comes to running tire chains, there isn't a lot of room between the fenders and the inside of the tires...especially if I run the 325/65r18 size tires that I'm looking at. I'd better chain up the 305's that are currently on there and see how much room I have left.

Thanks,
Brett

You're math is correct.

The 18x8.5 ET-6 Gunner wheels will stick out 1.25" further out when compared to the factory 16x7 +6 wheels and the inside clearance will be just over 1/4" more to the inside.

Maverick4x4 01-15-2012 10:19 AM

Quote:

Originally Posted by Discount Tire (Post 2151453)
You're math is correct.

The 18x8.5 ET-6 Gunner wheels will stick out 1.25" further out when compared to the factory 16x7 +6 wheels and the inside clearance will be just over 1/4" more to the inside.

Thank you! Maybe I'll go with the 17x8 Gunner instead...but then the tires aren't E rated in the diameter I want (35")

Discount Tire 01-16-2012 10:19 AM

Quote:

Originally Posted by Maverick4x4 (Post 2152053)
Thank you! Maybe I'll go with the 17x8 Gunner instead...but then the tires aren't E rated in the diameter I want (35")

I'm not sure which tire model you are after but Nitto offers their Trail Grappler and Terra Grappler in size 295/70-18. This size is just shy of 35" and both models are load range E with a maximum pressure of 80 psi :thumbsup:

Details: Tire Products - Discount Tire Direct

Maverick4x4 01-17-2012 01:32 AM

Quote:

Originally Posted by Discount Tire (Post 2152452)
I'm not sure which tire model you are after but Nitto offers their Trail Grappler and Terra Grappler in size 295/70-18. This size is just shy of 35" and both models are load range E with a maximum pressure of 80 psi :thumbsup:

Details: Tire Products - Discount Tire Direct

I think the Terra Grappler will fit the bill. The price is right!!! That's a neat size too. It will likely look pretty good on the 18x8.5" rims.

Getting to my cabin includes 5 miles of slick snowpacked roads followed by 1/4 mile of sometimes up to 12" of new snow (sometimes settled). The BFG AT has done a great job, but the Toyo AT's on my work truck do very well too. The Terra seems a lot like the Toyo AT's. Does it have the winter (mountain snowflake) rating?


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