How-To (?) on Offset Calclation - PeachParts Mercedes-Benz Forum

ScotzBenz · #1 01-27-2008, 03:20 PM

I’ve been looking for, and indeed seeing very nice sets of used Mercedes 16” wheels for my ’92 400E (W124.034). The more I read the postings however, the more confused I become, particularly relating to offset. Given proper offset (ET), the two other issues are brake clearance, and of course lug-bolt size, but I’ll get to those later.

I believe that my analysis, and your correcting it as needed, would help a lot of readers figure out for themselves, “will it fit my car?”. If the assumptions are correct, you can just plug in your numbers.

To establish the fact: the 400E, out of the factory, has 6-1/2” x 15 ET44 wheels:

Which is a larger offset than those of other w124’s, the 260 and 300E for example, in order to accommodate the larger caliper for a heavier brake setup on this vehicle, right or wrong?

Now, from the factory, those ET44 wheels are matched to a hub which is 44mm outboard (away from center of vehicle) of wheel center-line, right or wrong?.

So that by putting on a ET44 wheel, that centerline of the wheel and thus of the tire which is centered on the wheel, is exactly where it is designed to be, right or wrong?

Now, if all of the above are correct – allow me to use an extreme example to demonstrate my line of thinking:

Imagine that I find a really, really cool set of wheels, that must go on my 400E. Same 6-1/2” x 15 rim size, but with a 0-ET (no offset), and I’m gonna use my same factory tires. If I slap those wheels on, the centerline (along with both sidewalls of the tires) is going to lye 44mm (about 1-3/4”!) outboard of where they did with the stock ET44 rims), right or wrong?

If the above is correct, given the identical size wheel, but one with any lesser offset/ET than those stock hubs were designed for, the centerline of the wheel/tire will be moved to some degree or another outboard.

Imagine now that, I quickly found out that those ET0 rims resulted in severe damage to my outer fenders, and four flat tires – as they would if they could even be fitted, so I go on E-Bay and get another set of wheels – “Cool, same wheel size as my stock wheels”, but when they arrive they’ve got an ET of 88!

Now, if I am correct in my thinking, mounted on my ET44 hubs, the wheel centerline will be moved inboard (toward the center of the vehicle) 44mm (again 1-3/4”). Right or wrong?

Ideally, we want to have a wheel with an ET/offset that is as close as possible to the original that the car was designed for (in my case 44mm), so that the centerline of the wheel/tire are where the suspension/steering intended them to be - in order to maintain the handling characteristics – right or wrong?

Now, bear with me and help me figure out if these wheels will fit:
7.5 x 16 ET46. By the above reasoning, the wheel centerline will lye 2mm Inboard of that of the original rim (nominal difference), and that extra 1-1/2” (assume 38mm) of rim width (over the factory spec rims) will be evenly distributed on each side of the centerline. Right?

So, in terms of measurement, the change will be:
Inboard:
+2mm for movement inboard owing to difference in ET
+19mm for extra wheel width
= additional +21mm width inboard

Outboard:
-2mm for movement inboard owing to difference in ET
+19mm for extra wheel width
= additional +17mm width outboard

Given the right tire, it seems to me that this rim size is ideal in terms of improvement in handling (?)

I’ve read the following:
Certain Mercedes wheels have 12mm, and others 14mm lug bolts – Does anyone know if/how this size is identified on the wheels themselves?

Thanks if you’ve made it this far – your feedback on any of the above is greatly appreciated. Scott