Wheel offsets, spacers and stuff.

[Rob M]

Can somebody explain, in simple terms ( because I'm simple) how to calculate wheel ETs and spacer size and the like?
To put everything into a context and make it all understandable, I'm looking around for some wheels for my Merc W126.
Now, for arguments sake, Ive seen some 18in monoblocks with the same ET as the standard set up of 25.
I have been told that the larger the diameter the wheel the less offset is required?
Moreover, the standard wheels are 5x112 so, in theory, a whole load of German rims are within reach BUT some of the ETs of said German wheels are, say 40.
Bloody hell, I'm not making sense am I.....
Ok, If I see a set of 17, 18 inch alloys with an offset of 40, what size spacers would I need to get them on my car?
If anybody can pick through my utter cobblers, Id appreciate some advice.
Thanks

[markbognor]

Use www.willtheyfit.com to compare wheels with known stats.

Calculate the ET of a wheel by measuring the back space from the mounting surface to the edge of the rim, then the overall width to the outside edges of the rim (wheels are actually dimentioned to the bead seat (where the tyre sits)but for this it's easier to get an accurate measurement to the outside edges).

Divide the width by 2. Take that away from the back space measurement.

Back space - 1/2 wheel width = Et

Et describes how far the mounting surface is offset in (-et) or out (+et) from the centre line of the wheel.

Spacers make a +tve increase to the offset so a 25mm spacer on a -15et wheel will put the wheel in the same position as a +10 wheel without the spacer.

Increase the et/offset (make it more positive or less negative), for wheels of the same width, moves the wheel into the arch, closer to the suspension.
Decrease the et/offset (make it less positive or more negative), for wheels to move out of the arch away from the suspension.

[Rob M]

Feb 24, 2015 20:11:38 GMT markbognor said:

Use www.willtheyfit.com to compare wheels with known stats.

Calculate the ET of a wheel by measuring the back space from the mounting surface to the edge of the rim, then the overall width to the outside edges of the rim (wheels are actually dimentioned to the bead seat (where the tyre sits)but for this it's easier to get an accurate measurement to the outside edges).

Divide the width by 2. Take that away from the back space measurement.

Back space - 1/2 wheel width = Et

Et describes how far the mounting surface is offset in (-et) or out (+et) from the centre line of the wheel.

Spacers make a +tve increase to the offset so a 25mm spacer on a -15et wheel will put the wheel in the same position as a +10 wheel without the spacer.

Increase the et/offset (make it more positive or less negative), for wheels of the same width, moves the wheel into the arch, closer to the suspension.
Decrease the et/offset (make it less positive or more negative), for wheels to move out of the arch away from the suspension.

I now need to unscramble my brain..
Thanks for that, I now understand the semantics.
Its going to make looking for wheels easier.
Edit: That site is brilliant fella, cheers!!

[stealthstylz]

Feb 24, 2015 20:11:38 GMT markbognor said:

Spacers make a +tve increase to the offset so a 25mm spacer on a -15et wheel will put the wheel in the same position as a +10 wheel without the spacer.

.

You've got that the wrong way round dude. Spacers effectively reduce the backspace and therefore lower the ET, so a 25mm spacer on a ET10 wheel will be in the same place as a ET-15 wheel without the spacer.

[adam73bgt]

Feb 24, 2015 19:36:12 GMT Rob M said:

Now, for arguments sake, Ive seen some 18in monoblocks with the same ET as the standard set up of 25.

Ok, If I see a set of 17, 18 inch alloys with an offset of 40, what size spacers would I need to get them on my car?

Thanks

If your standard offset is 25 and you want to fit et40 wheels (assuming wheels the same width) then you would need a 15mm spacer to have the wheel sit in the same place as your original wheel

[markbognor]

Of course, thanks for noticing. Confused myself with positive and negatime. I'll edit the original post when I'm on a more compliant device.

[MrSpeedy]

Wheel diameter has absolutely no effect on offset.

God knows where your 'expert in the pub' got that from?!

[Rob M]

Feb 25, 2015 9:58:49 GMT adam73bgt said:

Feb 24, 2015 19:36:12 GMT Rob M said:

Now, for arguments sake, Ive seen some 18in monoblocks with the same ET as the standard set up of 25.

Ok, If I see a set of 17, 18 inch alloys with an offset of 40, what size spacers would I need to get them on my car?

Thanks

If your standard offset is 25 and you want to fit et40 wheels (assuming wheels the same width) then you would need a 15mm spacer to have the wheel sit in the same place as your original wheel

That's what I assumed to be the case.
I think the diameter of the wheel v offset comment threw me somewhat, hence Ive asked on here as its THE place to ask stupid questions and get intelligent answers.

[Rob M]

Feb 25, 2015 10:47:54 GMT MrSpeedy said:

Wheel diameter has absolutely no effect on offset.

God knows where your 'expert in the pub' got that from?!

I'm going to hazard a guess that they confused width with diameter.
I'm being generous to a fault here.