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Old Mon, May-10-2010, 03:24:27 AM   #4
Obioban
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Offsets:

First, let's start out with a definition of what wheel offsets are, so people can follow.

From tirerack.com:

Quote:
Originally Posted by tirerack.com
The offset of a wheel is the distance from its hub mounting surface to the centerline of the wheel. The offset can be one of three types (measured in millimeters).*

Zero Offset

The hub mounting surface is even with the centerline of the wheel.

Positive

The hub mounting surface is toward the front or wheel side of the wheel. Positive offset wheels are generally found on front wheel drive cars and newer rear drive cars.

Negative

The hub mounting surface is toward the back or brake side of the wheels centerline. "Deep dish" wheels are typically a negative offset.
If the offset of the wheel is not correct for the car, the handling can be adversely affected. When the width of the wheel changes, the offset also changes numerically. If the offset were to stay the same while you added width, the additional width would be split evenly between the inside and outside. For most cars, this won't work correctly. We have test fitted thousands of different vehicles for proper fitment. Our extensive database allows our sales staff to offer you the perfect fit for your vehicle.

On the M3, and all modern BMWs for that matter, the offsets are negative. Therefore, the smaller the offset, the more aggressive (or pushed out) the wheels are. Moreover, adding a spacers will decrease the effective offset. For example, adding a 10mm spacer to a 35mm offset wheel will make the offset 25mm.


Spacers: Whenever possible, spacers should be avoided. Neon01 explained why very well:

Quote:
Originally Posted by Neon01 View Post
1. You add to the weight of the wheel, effectively, which is never good for bearings

2. Assuming the "face" of the wheel is relatively flat, you're adding a moment arm through which the wheel will torque on the hub (bearings). For example, if I have a wheel that is an ET22, the wheel face will be located pretty close to the center of the wheel, and the compression forces of the car acting through the hub are in the same vertical plane as the wheel face. The same is true in the case of the ET45 wheel, but when I add a 20mm spacer, I've now added a torque to the face of the hub in addition to the simple shear force I had before. With the spacer, the force is vectored down in the plane of the hub, which is transmitted to the wheel through the spacer. The wheel is directing its force on the spacer vectored upward, so the spacer moves the planes apart by the 20mm, which creates a torque on the wheel and the hub from the upward force of the wheel and the downward force of the car. Maybe this wasn't that clear, but it is true.
Spacers create more stress, add weight, and create more possible sources of vibration. Whenever possible, buy wheels in the correct size/offset and skip the spacers.

If you do run spacers, and sometimes they are necessary (I need them for my snow wheels to clear the BBK, for example, Whenever possible use hubcentric spacers. They're much safer, and stress the car much less than non hubcentric. It's also an extremely good idea to get a stud kit. They make a stronger construct, they're less likely to loosen themselves, and they make mounting/dismounting tires much easier.


M3 Offsets:

Okay, so, now that you know what they are, how do you find the right one for the car? Well, originally with a measuring tape, shims, spacers, etc… but we're past all of that at this point. What works well on the e46 M3 is pretty well establish at this point.

So, here's how I'm going to do this: I'm going to list the best offset for a given size wheel (best meaning biggest tire fitting. You bling guys can make your own thread about stance, flush, etc), and the tire size you can fit on that. From there, if you wan to run a more aggressive wheel, you just decrease the tire size from there (aka, for every 5mm more aggressive you go, reduce one tire width (tire widths go in increments of 10mm, half goes on the inside, half on the outside, so every 10mm decrease in width brings it in 5mm on the outside)).

And I'm just going to focus on wider wheels with bigger tires, as smaller wheels/tires fit fine over a wide range of offsets.

Assume stock fenders, no rolling, no fender liner removal.


Front:

9": ET38. All tires 265 and smaller will fit, some 275s will fit.
9.5": ET35. All tires 265/35/18 and smilers will fit. All 275s will fit with some extra camber in the front. SOME 285s will fit-- this will vary from one tire model to the next.
10": There's no reason to go with a 10" front

Rear:

9.5": ET27 will fit all 275s and most 285s with the proper alignment
10": ET27 will fit all 285s and some 295s with the proper alignment

Special Case, track setup:

For the track, it's beneficial to run a square setup so you can rotate tires (extends tire life) and dial out understeer (matching front and rear tire sizes). The Optimal setup for this is 18 x 9.5 ET35, with a 10mm hubcentric spacer in the rear. This setup allows you to easily run 265 tires square, 275 square with some alignment tweaking, and 285 square if you work at it.

Take away: Clearly the offsets listed above are not the only offsets. What they are is the ideal offsets. Generally speaking, for every 5mm of variance of the above offsets, you need to decrease the width of the tire by 10mm.


NOTE WELL: There are lots of factors involved with tire fitment. Alignment, the specific fenders on your car, the model of tire you go with, etc. The above is generally true for the e46 M3, but there are exceptions. If you want to be guaranteed to have no fitment issues, don't push the tire sizes. The smaller you stay, the less issues you can run into!
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Current Cars: 2005 IR/IR M3, 2001 LMB/blk M5, 03 530i, 04 M3 wagon, and some boring stuff
Past cars: 04 M3, 96 M3, S50B32 e36 M3 CM race car

Last edited by Obioban; Tue, Sep-28-2010 at 11:53:29 PM.
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