Wheel design offsets and physics

[mzivtins]

Hi everybody,

When designing wheels for cars or race cars, is it stronger to have the wheel spokes meet the barrel as close the to centreline of the wheel as possible?

Is this an easy assumption to fall into?

In a lot of LMP cars we can usually see wheels where the spokes and face line up with the outer most edge of the barrel (like a high offset wheel) and this is true for practically all road cars:


I'm confused about this because i have the need for some negative offset wheels (-62mm) given the size of the wheel this will put the face and spoke closer to the centre line of the wheel... is this a good idea or not? or does it not matter and I should stop over-thinking?

(I didnt know where to post this question, mods please move is needed)

[Rodak]

Brake clearance is obviously a factor.....

[Hoffman900]

Brake clearance is obviously a factor.....

That and there might be some benefits in terms of what you can do with suspension packaging. Also aero.

[Zynerji]

Brake clearance is obviously a factor.....

That and there might be some benefits in terms of what you can do with suspension packaging. Also aero.

I thought offset was mostly for kinematic response like camber curves and scrub radius?

[PhillipM]

Ideally you would, but then aero, brakes and kinematics get in involved and you just don't have space to do that if you want to maximise the others.

[Zynerji]

Ideally you would, but then aero, brakes and kinematics get in involved and you just don't have space to do that if you want to maximise the others.

So, the inevitable compromise rears it head, yet again!!

[Greg Locock]

Offset is entirely controlled by packaging. if you want a reasonably small scrub radius and a reasonably small KPI then you have to tuck the lower ball joint in towards the centre line of the wheel. The brakes will obviously be outboard of that, thereby pushing the face of the wheel out towards the outside of the car.

In principle you can have very large scrub radii, 101.6 mm was not unusual way back when, but I don't think anybody is a huge fan of big KPIs.

Strengthwise, yes, if you could get the face of the wheel to line up with the vertical edge of the tire fitting well it would be good, and that's a typical design.

[mzivtins]

Strengthwise, yes, if you could get the face of the wheel to line up with the vertical edge of the tire fitting well it would be good, and that's a typical design.

Does this mean that it has less strength if the spoke and face meet the barrel close to the centre line rather than the outer edge?

[mzivtins]

I thought offset was mostly for kinematic response like camber curves and scrub radius?

Could you simply this for me in terms of:

what differences in feel and performance would you expect to experience in: for example, the different between a 50mm offset wheel vs a 50mm inset wheel?

This can be as over-simplified as possible, think "road car"

[mzivtins]

Brake clearance is obviously a factor.....

I guess the great thing about negative offset (why do we call it that? surely inset is better) is that you have clearance for days on pretty much all face designs out there

[Greg Locock]

No, the inner wall nearer the centreline would be better, but given that most designs need a lot of offset then using the 'outer' wall of the tire fitting well is a good use of material.

[J.A.W.]

As I recall, back in the 1990's wheel aerodynamics considerations led the likes of Mercedes-Benz to go
for light wheel rims with a flush disc-type outer face (albeit with carefully designed brake dust, ah sorry,
I mean, heat-rejection slots), whereas BMW seemed to prefer a narrower track with a 'deep dish' rim look,
& far more open 'spoked' design, presumably to actively 'show off' their big brakes & 'race heritage' maybe?