Understanding Offset on Rims

[ItsTwest]

Rims i'm looking at have 3.70" backspacing with an offset of -19mm. How much does that add up to in inches for backspacing?

 

[rebelgregory]

19 mm is about 3/4". A negative offset means that the mounting surface is towards the brakes, giving it a deeper dish appearance.

It doesn't change the backspacing number. The inner lip is 3.70" from the mounting flange. If you add the backspacing to the offset, it should give the wheel centerline - so about 4.5 inches which makes it a 9 inch wheel.

Basically, the negative offset means that the rims will give you a wider stance, but it doesn't change the backspacing number - it's already figured in.

 

[blazer74]

Offset - Distance from the wheel center-line (CL) to the wheel mating surface - greater positive offset moves the tires inward (narrower track)

Backspace - Distance from the inside lip of the wheel to the wheel mating surface.

 

[rampage]

Maybe these links will help.

http://www.discounttire.com/dtcs/infoWheelOffsets.dos

http://www.onlineconversion.com/length_common.htm

 

[shady]

backspace is all you need to know... offset doesn't mean anything to a truck. if you need 4" from the back of the rim to the mating surface..... get 4" bs... offset just confuses people

 

[Fred_M1010]

backspace is all you need to know... offset doesn't mean anything to a truck. if you need 4" from the back of the rim to the mating surface..... get 4" bs... offset just confuses people

I'd say just the opposite.
Backspace only affects wheel to TRE clearance.
Offset is what determines where the tire actually ends up.
When measuring how big tires I can fit and/or how much to cut the fenders, that's the only thing I look at.

 

[76k5grrl]

Ohhhhhh this makes my head explode. I don't know why but I cannot understand this!!!!!

 

[shady]

I'd say just the opposite.
Backspace only affects wheel to TRE clearance.
Offset is what determines where the tire actually ends up.
When measuring how big tires I can fit and/or how much to cut the fenders, that's the only thing I look at.

if you know you have a 9" wide wheel.... and you have a 4" bs, then you have .5" offset. its simple math really. look at any 2 wheels with the same width and bs and they will have the same offset. its just 2 different ways of saying the same thing.

 

[zonacats]

Yeah, offset and backspace measure the same thing, just in different ways. They cannot move independently of each other if the width of the wheel remains the same. In other words, if you know the offset, you can figure out the backspace and vise versa (assuming you know the wheel width).

 

[Fred_M1010]

if you know you have a 9" wide wheel.... and you have a 4" bs, then you have .5" offset. its simple math really. look at any 2 wheels with the same width and bs and they will have the same offset. its just 2 different ways of saying the same thing.

Yes of course, I just feel that offset is a much more relevant measurement.

When I try to figure out how big tires I can fit to a certain vehicle, I start with the wheels and tires that are mounted at the time, and measure clearances in all directions while cycling the suspension and steer left and right.
I think it's much easier to think offset in that situation, and how much in or out I need to move the center of the new tire in relation to the current wheel.
IE how much more negative or positive offset I need to fit the biggest possible tire with the least amount of lift.
The width of wheel or the backspacing is a later concern to me.
But maybe that's just me

Many people seems to think that since they had a 3.5" BS on their stock 6.5" wide steelies,
they can use a 12" wide wheel with a 3.5" BS without altering the steering characteristics.

BTW a 9" wheel with a 4" BS usually have -1" offset.
(Width is measured inside the lips, and backspacing to the inner edge of the inner lip)