Alloy wheels Offset

[Niks]

Im rather confused about how the offset is calculated on alloy wheels so hoping someone could shed some light on this?

For example, if we take two 17" alloys for a W124 and both are 8" wide, one has ET34 and the other has ET47 (remember this is an example as I do not know what the real ET figures are!)

Does this mean the alloy with ET34 stick out more from the wheel arch and the ET47 sit inside it, or the other way round?

Sorry for my ingorance but ive been thinking about it quite a bit and ive just confused myself stupid!

 

[kjay]

You're correct.

A lower offset moves the wheel outwards

An increase moves it inwards.

 

[kth286]

A tip.

It is not like you have a choice often.

You have to go with a certain offset, depending on the width of wheel, to clear the suspension strut and brake caliper etc depending which 124 model you are fitting wheels to.

An example.

My original 124 wheels were 6.5 inches with offset of approx 44

Replacement wheels are 7.5 inches with offset of 35 and they just clear the 4pot brake calipers in the design I have chosen.

 

[Niks]

Thanks for clearing that up. I thought I was right but just needed reassurance.

So an offset of ET34, does the 34 figure calibrate in cm or inches? or does that have nothing to do with it?

 

[Mudster]

It's actually Millimetres....

These images help a little.......so ET34 is a positive offset of 34mm

 

[bolide]

The offset (ET), in mm, is the horizontal distance between the plane of the hub and the plane of the centre of the wheel width

An ET of zero has a wheel where the two planes are coincident. A 7" rim, therefore, would have 3.5" either side of that plane. Imagine a 9" rim on the same hub. A zero offset would see the inner edge of the rim 1" closer to the suspension upright than before. In most situations that won't work so the offset is changed to move the rim outwards and maintain the tyre-to-suspension clearance

Ideally the centre of the rim (and hence the centre of the tyre contact patch) will remain in the same place, regardless of rim width, so the handling will remain predictable

Nick Froome
www.w124.co.uk

 

[Niks]

Ok that makes sense now, so the ET is actually there to compensate for the width of the actual wheel so it clears the calipers and brake disc/suspension?

So if im looking at alloys which have a width of 8", realistically I need a ET47 or higher so it sits inside the wheel arch?

 

[kth286]

Actually NO.

It needs to be less than 47 - much less. Probably 32/34.

The best way is to go into a wheel seller and ask to have a wheel fitted to the hub to see the actual clearance.

That is what I did and they did not mind as they thought they would be ultimately selling me some wheels.

I did in fact order some wheels from a wholesaler and put them through that dealer to have the tyres fitted etc.

 

[Mudster]

Ok that makes sense now, so the ET is actually there to compensate for the width of the actual wheel so it clears the calipers and brake disc/suspension?

So if im looking at alloys which have a width of 8", realistically I need a ET47 or higher so it sits inside the wheel arch?

Judging by your avatar you have a W124.

The AMG alloys that were produced for this car were 17" x 8" (Aero I's)

They were ET28.....as long as you're a few millimeters within that area you'll not have any problems.

 

[Niks]

Brilliant, thanks for all the help guys!