How is wheel offset determined?
Discussion
OK, I understand what the offset value is, what I'm interested in is how the designers came up with the figure. I have heard that it's to do with how the wheel bearings are arranged which would mean that if the wheel width was increased the offset should remain the same otherwise (at least in my mind) the forces exerted on the hub by the wheel would act differently on the bearings. I've also heard, somewhat contradictory, that you should increase (or decrease depending which way around you look at it) the offest as you increase wheel width.
All this rambling is to do with me reworking the rear suspension on my 240Z. The original set up is a Chapman strut with stub axle and the wheel offset is 0. My design changes things so that I use the rear hub (and it's nice bug brakes) of a Z32 300ZX and it's CV shafts. The Z32 wheels have something like a 35-40mm offset. My design shifts the wheel mounting surface outboard from the original location by 19mm. So, if the offset is determined by the hub bearing assembly, I should still use the standard Z32 offset wheel. If it's not then should I use a 19mm offset wheel which will position the wheel in the same overall location as a 0 offset wheel on the original hub, and so presumably keep the general points of the suspension geometry in the same place?
Or does it just not matter that much?!
Cheers,
Rob
All this rambling is to do with me reworking the rear suspension on my 240Z. The original set up is a Chapman strut with stub axle and the wheel offset is 0. My design changes things so that I use the rear hub (and it's nice bug brakes) of a Z32 300ZX and it's CV shafts. The Z32 wheels have something like a 35-40mm offset. My design shifts the wheel mounting surface outboard from the original location by 19mm. So, if the offset is determined by the hub bearing assembly, I should still use the standard Z32 offset wheel. If it's not then should I use a 19mm offset wheel which will position the wheel in the same overall location as a 0 offset wheel on the original hub, and so presumably keep the general points of the suspension geometry in the same place?
Or does it just not matter that much?!
Cheers,
Rob
If you increase the width of the rim/tyre combination you should idealy correct the offset so that the centre of the tyre still passes through the same plane of the hub as it did before i.e. the way the bearing designer intended it. The use of wheel spacers years ago put strain on the bearings by moveing that plane outboard. You can move it outboard a small amount without to much of a problem.
This only applies to the rear.
The front is a different matter where the effect on the bearings is the very least of the problem. In the front any change will effect the whole steering geometry. I know only enough about this to advise that you go find good reference material on steering geometry design before you start experimenting.
Steve
This only applies to the rear.
The front is a different matter where the effect on the bearings is the very least of the problem. In the front any change will effect the whole steering geometry. I know only enough about this to advise that you go find good reference material on steering geometry design before you start experimenting.
Steve
Suspension design is a huge subject so you aren't going to get a complete answer.
At the simplest level, the suspension designers decides what angle they want the steering axis to be, and then position the tyre so that the steering axis intersects the ground at the right position relative to the contact patch. The right position is typically close to the centerline of the contact patch, but not necessarily right on the centerline. The suspension designer can trade off the offset of the wheel versus the dimensions of the hub and upright to put the tyre in the right place. For example with front wheel drive the need to put the outer CV joint close to the steering axis means the brakes and hub need to go well outboard so the wheels typically have a huge inset to bring the tyre back to the right position.
If you change the wheel width you would aim to keep the centerline of the tyre in roughly the same place, but suspension and body clearance issues might mean you have to move it slightly.
At the simplest level, the suspension designers decides what angle they want the steering axis to be, and then position the tyre so that the steering axis intersects the ground at the right position relative to the contact patch. The right position is typically close to the centerline of the contact patch, but not necessarily right on the centerline. The suspension designer can trade off the offset of the wheel versus the dimensions of the hub and upright to put the tyre in the right place. For example with front wheel drive the need to put the outer CV joint close to the steering axis means the brakes and hub need to go well outboard so the wheels typically have a huge inset to bring the tyre back to the right position.
If you change the wheel width you would aim to keep the centerline of the tyre in roughly the same place, but suspension and body clearance issues might mean you have to move it slightly.
Well changing the offset will affect the Kingpin offset as the centre on the contact patch would be moved. If the offset is changed by a considerable amount then the KPI could change from +ve to -ve offset of vice versa (in relation to the centre of the contact patch). You would need to look up to advantages of both as I can't remember of the top of my head, but I know +ve offset is desirable for perfermance vehicles.
And then you have the effect of the wider/narrower track width to consider...
Sorry if this makes no sense but I'm just trying to remember it all, I'll double check with my notes in the morning.
D'oh beaten to it.
>> Edited by NightDriver on Thursday 2nd February 23:13
And then you have the effect of the wider/narrower track width to consider...
Sorry if this makes no sense but I'm just trying to remember it all, I'll double check with my notes in the morning.
D'oh beaten to it.
>> Edited by NightDriver on Thursday 2nd February 23:13
Cool, all roughly makes sense. One of these days I'll attempt to learn more about suspension design but really haven't got time at the moment. So I'll stick with making the wheel stay in the same location. Handy that the Z has a 0 offset to start with as it makes the maths easier So for my rears which will be an extra 19mm wide on each side I'll just use a 19mm offset wheel, job done, thanks!
Cheers,
Rob
>> Edited by B19GRR on Friday 3rd February 12:46
So for my rears which will be an extra 19mm wide on each side I'll just use a 19mm offset wheel, job done, thanks! Cheers,
Rob
>> Edited by B19GRR on Friday 3rd February 12:46
Problem with having the KPI intersection different at the ground, on a fwd at least, will effect torque steer quite alot.
I guess you could remove torque steer by getting the contact patch offset right, but on most cars that would probably require a wheel 2 inches further out or something silly like that.
I always get confused with offset.
Alot of people actually widen the track when fitting wider wheels, as the strut scrubs the tyre otherwise. Basically go wider by 10mm, increase the offset by half that, so 5mm, so as the outer edge goes out 10mm, you bring the whole wheel back 5mm, so the effect is 5mm wider out, and 5mm wider in.
When you hit the strut, then you can only move out, and that is when problems happen.
Best to leave as the manufacturer inteneded. Anyone who thinks they can alter a finely setup system (for the road at least) without bringing in further compromises must be a setup god, so why are they not doing this as a job anyway
Dave
I guess you could remove torque steer by getting the contact patch offset right, but on most cars that would probably require a wheel 2 inches further out or something silly like that.
I always get confused with offset.
Alot of people actually widen the track when fitting wider wheels, as the strut scrubs the tyre otherwise. Basically go wider by 10mm, increase the offset by half that, so 5mm, so as the outer edge goes out 10mm, you bring the whole wheel back 5mm, so the effect is 5mm wider out, and 5mm wider in.
When you hit the strut, then you can only move out, and that is when problems happen.
Best to leave as the manufacturer inteneded. Anyone who thinks they can alter a finely setup system (for the road at least) without bringing in further compromises must be a setup god, so why are they not doing this as a job anyway
Dave
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