Wheel Backspacing with Offsets
Thread Starter
JK Enthusiast
Joined: Nov 2006
Posts: 232
Likes: 0
From: San Antonio TX
Wheel Backspacing with Offsets
Colleagues,
Just wanted a quick check on my math wrt wheel backspacing and offsets (yes I read the FAQ like a good little jeeper should):
Backspacing + offset = "true overall backspace from hub mount"
4.75" + 0mm = 4.75"
4.75" + -6mm = 4.5" (as -6mm = ~-.25")
4.75" + 6mm = 5.0" (as 6mm = ~.25")
So basically given the examples above, 4.75" backspacing + -6mm offset provides the same clearance for tires as a pure 4.5" backspace with 0mm offset.
Am I tracking the right way?
Cheers...
Just wanted a quick check on my math wrt wheel backspacing and offsets (yes I read the FAQ like a good little jeeper should):
Backspacing + offset = "true overall backspace from hub mount"
4.75" + 0mm = 4.75"
4.75" + -6mm = 4.5" (as -6mm = ~-.25")
4.75" + 6mm = 5.0" (as 6mm = ~.25")
So basically given the examples above, 4.75" backspacing + -6mm offset provides the same clearance for tires as a pure 4.5" backspace with 0mm offset.
Am I tracking the right way?
Cheers...
JK Freak
Joined: Sep 2009
Posts: 553
Likes: 1
From: Houston, TX
Backspacing is more of an absolute measurement (same wheel to wheel) whereas offset is relative to wheel width.
Usually people tend to talk backspacing more with trucks and offset more with cars. With trucks, the clearance issues are usually more with the swaybar and wheel well (with turning). With cars, the clearance issues are usually more with A-arms, struts, and spindles (static clearance).
Thread Starter
JK Enthusiast
Joined: Nov 2006
Posts: 232
Likes: 0
From: San Antonio TX
I've continued searching the internet far and wide--because everything you read on the internet is true right?
The website eHow had a very good explanation of offset...here it is (I'm not intentionally violating any forum rules I hope..apologies if I am) as this is meant to augment WOL's FAQ write-up that is a great resource for all:
Offset Explained
The offset of a wheel is measured in millimeters and is determined by the distance from where the wheel mounts to the hub to the centerline of the wheel. The centerline of the wheel is the center of the wheel if it is being viewed from the rim side of the wheel (the part the tire stretches across), not the face of the wheel.
Zero Offset
If a wheel has a zero offset, then the hub mounting surface is even with the wheel's centerline.
In English translation [jeeperaf] the centerline of your tire is theoretically centered over the vertical axis of your hub's mounting surface. Therefore, a rim with 4.75" backspacing and 0mm offset would ride with a tire's centerline that is also 4.75" from the rim's inner lip. Positive and negative offset have the effect of providing a given backspacing while then moving the centerline to a position that mimics pure x" backspacing with 0mm offset as shown below.
Positive Offset
In a positive offset wheel, the hub mounting surface is toward the outside of the wheel. In effect, this makes the wheel fit closer under the wheel arch, rather than stick out. Positive offset wheels are commonly found on front-drive cars, though recent rear-drive cars have begun using them.
In English translation [jeeperaf] the centerline of your tire is theoretically centered inside of the vertical axis of your hub's mounting surface, and this is where it gets a little counter intuitive. If you had a rim with 4.75" backspacing and +6mm (~.25") offset, the tire's centerline is riding .25" closer to the brake drum and sitting closer inside the wheel arch. Or in other words, the above measurement equals the exact same place relative to your vehicles brake drum and wheel arch as if you had a rim that with 5" backspacing and 0mm offset.
Negative Offset
If a wheel has a negative offset, then the mounting surface is closer to the inside of the wheel and closer to the brakes and suspension. This has the affect of pushing the wheel away from the car. Deep dish wheels, which feature a deep lip, are usually negative offset.
In English translation [jeeperaf] the centerline of your tire is theoretically centered outside of the vertical axis of your hub's mounting surface, and this continues to be counter intuitive. If you had a rim with 4.75" backspacing and -6mm (~.25") offset, the tire's centerline is riding .25" farther from the brake drum and sitting farther outside the wheel arch. Or in other words, the above measurement equals the exact same place relative to your vehicles brake drum and wheel arch as if you had a rim that with 4.5" backspacing and 0mm offset.
How Does Wheel Offset a Car?
A car's wheel offset is specifically designed by the engineers who design the car to work with the type of car it is, the kind of performance it delivers and the way it is intended to be driven. If the wrong offset wheel is used, a car's handling can be adversely affected. The wrong offset wheel can also put undue stress on wheel bearings. Suspension components will also experience more stress, occasionally causing them to break. This is most likely to happen when a positive offset wheel is replaced by a wheel with a strong offset that moves the wheel further outward from the suspension than it should be. It should be noted that this is rare occurrence.
In English translation [jeeperaf] let's take an extreme example to show you the "lengthening of the beam effect" of your entire axle relative to your hub mounting surface and tire centerline and the role they play as weight supporting points (like columns and I-beams in a building).
Same 4.75" backspacing with 0mm offset. Now take a 2" spacer and put that between your hub and wheel. This would create an effect whereby your hub mounting surface moves inside the centerline by 2" and creates a negative offset of a substantial -48mm. By doing this, you've extended the centerline of your wheel out fairly far (and the weight bearing effects of gravity on the axle "beam"). In other words, you had a given length of a support beam between two columns, and now you've just lengthened that beam without thickening it...at some point it will sag in the middle under the stress.
This is a major reason why you need to strengthen your C-arms and axle housing as you move to bigger and wider tires (not to mention heavier) that require either less backspacing with 0 offset or deeper backspacing but compensate with negative offset in order to clear any rubbing issues full lock turn to full lock turn.
Measuring Wheel Offset
Calculating wheel offset requires the wheel backspace, wheel width and wheel width. If the distance from the hub mounting surface to the inner edge of the wheel is less than the wheel centerline, the wheel is a negative offset. If the distance is greater, the wheel is a positive offset.
I welcome any and all comments to make this explanation better for our community. If I have any points wrong above, I'm going to feel fairly stupid and a tad bit embarrassed, but I've always lived by "no one of us is smarter than all of us"
Cheers...
The website eHow had a very good explanation of offset...here it is (I'm not intentionally violating any forum rules I hope..apologies if I am) as this is meant to augment WOL's FAQ write-up that is a great resource for all:
Offset Explained
The offset of a wheel is measured in millimeters and is determined by the distance from where the wheel mounts to the hub to the centerline of the wheel. The centerline of the wheel is the center of the wheel if it is being viewed from the rim side of the wheel (the part the tire stretches across), not the face of the wheel.
Zero Offset
If a wheel has a zero offset, then the hub mounting surface is even with the wheel's centerline.
In English translation [jeeperaf] the centerline of your tire is theoretically centered over the vertical axis of your hub's mounting surface. Therefore, a rim with 4.75" backspacing and 0mm offset would ride with a tire's centerline that is also 4.75" from the rim's inner lip. Positive and negative offset have the effect of providing a given backspacing while then moving the centerline to a position that mimics pure x" backspacing with 0mm offset as shown below.
Positive Offset
In a positive offset wheel, the hub mounting surface is toward the outside of the wheel. In effect, this makes the wheel fit closer under the wheel arch, rather than stick out. Positive offset wheels are commonly found on front-drive cars, though recent rear-drive cars have begun using them.
In English translation [jeeperaf] the centerline of your tire is theoretically centered inside of the vertical axis of your hub's mounting surface, and this is where it gets a little counter intuitive. If you had a rim with 4.75" backspacing and +6mm (~.25") offset, the tire's centerline is riding .25" closer to the brake drum and sitting closer inside the wheel arch. Or in other words, the above measurement equals the exact same place relative to your vehicles brake drum and wheel arch as if you had a rim that with 5" backspacing and 0mm offset.
Negative Offset
If a wheel has a negative offset, then the mounting surface is closer to the inside of the wheel and closer to the brakes and suspension. This has the affect of pushing the wheel away from the car. Deep dish wheels, which feature a deep lip, are usually negative offset.
In English translation [jeeperaf] the centerline of your tire is theoretically centered outside of the vertical axis of your hub's mounting surface, and this continues to be counter intuitive. If you had a rim with 4.75" backspacing and -6mm (~.25") offset, the tire's centerline is riding .25" farther from the brake drum and sitting farther outside the wheel arch. Or in other words, the above measurement equals the exact same place relative to your vehicles brake drum and wheel arch as if you had a rim that with 4.5" backspacing and 0mm offset.
How Does Wheel Offset a Car?
A car's wheel offset is specifically designed by the engineers who design the car to work with the type of car it is, the kind of performance it delivers and the way it is intended to be driven. If the wrong offset wheel is used, a car's handling can be adversely affected. The wrong offset wheel can also put undue stress on wheel bearings. Suspension components will also experience more stress, occasionally causing them to break. This is most likely to happen when a positive offset wheel is replaced by a wheel with a strong offset that moves the wheel further outward from the suspension than it should be. It should be noted that this is rare occurrence.
In English translation [jeeperaf] let's take an extreme example to show you the "lengthening of the beam effect" of your entire axle relative to your hub mounting surface and tire centerline and the role they play as weight supporting points (like columns and I-beams in a building).
Same 4.75" backspacing with 0mm offset. Now take a 2" spacer and put that between your hub and wheel. This would create an effect whereby your hub mounting surface moves inside the centerline by 2" and creates a negative offset of a substantial -48mm. By doing this, you've extended the centerline of your wheel out fairly far (and the weight bearing effects of gravity on the axle "beam"). In other words, you had a given length of a support beam between two columns, and now you've just lengthened that beam without thickening it...at some point it will sag in the middle under the stress.
This is a major reason why you need to strengthen your C-arms and axle housing as you move to bigger and wider tires (not to mention heavier) that require either less backspacing with 0 offset or deeper backspacing but compensate with negative offset in order to clear any rubbing issues full lock turn to full lock turn.
Measuring Wheel Offset
Calculating wheel offset requires the wheel backspace, wheel width and wheel width. If the distance from the hub mounting surface to the inner edge of the wheel is less than the wheel centerline, the wheel is a negative offset. If the distance is greater, the wheel is a positive offset.
I welcome any and all comments to make this explanation better for our community. If I have any points wrong above, I'm going to feel fairly stupid and a tad bit embarrassed, but I've always lived by "no one of us is smarter than all of us"
Cheers...
Last edited by jeeperaf; Jul 25, 2010 at 09:39 PM.
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