Wheel Offsets

[VladD]

I'm trying to get my head around wheel offsets. I know how it's measured, but I don't understand why you have a variation. What are the actual benefits of positive and negative offset? Is the idea to keep 50% of the weight of the wheel each side of the mounting surface?

Edited by VladD on Thursday 15th July 14:25

[Mr2Mike]

The primary function is to set the correct scrub radius i.e. the distance between the center of the contact patch and the (virtual) center of the steering axis. This greatly affects how a car feels to drive, especialy stability under braking and acceleration.

[Sam_68]

What he said ^^^

A very much secondary consideration is the loads on the wheel bearings.

But you need to remember that the steering axis offset (or scrub radius, whichever you prefer to call it) is dictated not just by the wheel offset, but also by the relationship between the hub face and the steering axis; thus different cars with different hub designs might need quite different wheel offsets to achieve the same scrub radius.

It's all to do with the overall 'package' of the hub design and steering geometry, which can include fitting in the brakes and (on FWD cars) the CV joints within the available space.

Edited by Sam_68 on Thursday 15th July 18:18

[VladD]

Thanks for that.

The reason I ask is that I'm trying to work out what offset I'd need on new wheels if I were to change them on my car. The original wheels were 14x6J with an ET of 45. I want to know what offset to use if I go to 15x7J, but I'm now not sure that it can be worked out without knowing more. Surely I'd need to know the current scrub radius, and how far across the tyre the contact patch is. Is the contact patch half way across or does camber alter that?

[cen]

Increasing the size of the wheels does not alter the offset. If your ET is 45 then it remains with the new wheel purchase. There is a tolerance that can be worked to allowing a small variation dependant on your vehicle. Many wheel websites will supply this info

[Mr2Mike]

cen said:

Increasing the size of the wheels does not alter the offset.

This is true if the overall rolling radius does not change, i.e. larger diameter wheels compensated for by lower profile tyres. If the overall radius does increase the the scrub radius will also change since the steering axis is normally inclined. The change is likely to be quite small unless conversion to monster truck is being attempted.

[VladD]

Mr2Mike said:

cen said:

Increasing the size of the wheels does not alter the offset.

This is true if the overall rolling radius does not change, i.e. larger diameter wheels compensated for by lower profile tyres. If the overall radius does increase the the scrub radius will also change since the steering axis is normally inclined. The change is likely to be quite small unless conversion to monster truck is being attempted.

But if the rolling radius stays the same, but the tyre is wider, surely the centre of the contact patch moves and thus alters the required offset.

[chevy-stu]

VladD said:

Mr2Mike said:

cen said:

Increasing the size of the wheels does not alter the offset.

This is true if the overall rolling radius does not change, i.e. larger diameter wheels compensated for by lower profile tyres. If the overall radius does increase the the scrub radius will also change since the steering axis is normally inclined. The change is likely to be quite small unless conversion to monster truck is being attempted.

But if the rolling radius stays the same, but the tyre is wider, surely the centre of the contact patch moves and thus alters the required offset.

The offset is measured from the centre of the wheel to the mounting face of the hub so whatever the width of the wheel it should stay roughly the same, extra width will be added to both inner and outer of the new wheels so scrub will stay the same.

[VladD]

Excellent. Thanks for the replies everyone.