|
1. The M coupe and Z coupe have the same rear axle length
2. The M coupe has longer axles than the Z
3. Tire-Rack is full of S***
4. I don't own an M coupe
5. That, because the M coupes rear end assembly is different, it must be wider than the Z
I've proposed that the axles are at least the same length, based on wheel offsets and doing a little math. It appeared the Z offset being larger (41mm with 17" wheels, 43mm with 16" wheels)vs 8mm for the M coupe, meant that the axle could not be longer in the M coupe. Intuitively, I then postulated that the axles in both cars are the same length (also using an assumption that BMW would share some components from the parts bin to hold down costs).
Of course this caused a great deal of B.S. to get batted about. Then it expanded to assume that, based on the axle length, that the two cars used the same rear end assembly, which just couldn't be right (how does two components sharing a dimension mean they are the same part?)! Sharing dimensions does not nessesitate the sharing of component assemblies(and assuming BMW does anything to save money is a wrong headed move).
Anyway... after all this, and after some further imperical (tape measure and ruler in hand) investigation, based on the fact that tire/wheel centers show such different offsets, and that I have never seen a Z car with M wheels (making me believe that something is amiss), and noting that the wheel fitment guides by wheel manufacturers show two different offset dimensions for the two cars, even for exactly the same wheel width.... you gotta get the message...
And here it is:
Measured from the wheel mounting face to the opposite side mounting face, using a crude measuring system, to avoid dismanteling two cars, the following came to light:
We are all full of crap! The M coupe rear end is actually narrower than the Z coupe! The difference between the two is just about 3", with the 2.8 coupe being the wider of the two!
To verify, let's do some more math. If you figure the Zcoupe has a rear track (from BMW lit) of 58.8" (distance between the centers of the rear tires), with a wheel offset of 43mm (1.69"), then the calculated distance from mounting surface to mounting surface would be +/-62.18" for the Z coupe. On the M coupe, the track is 58.7", with an offset of 8mm, indicating a mounting to mounting distance of 59.33". This calculates to a difference of 2.85" (62.18" on the Z vs. 59.33" on the M), very close to the +/- 3" we were able to measure on the cars. In other words, the wheel offsets backs up the imperical measurements.
This also explains why you won't see any Z coupes sporting the M's take-off wheels. The M wheels would stick out of the Z cars fender wells, by about 1.5"! Also, it explains why the rear Roadstars have so much obvious posiitve dish from the outside, while the Z's look flat. This is why I doubted the M's axles were longer in the first place, it just didn't look right.
The M coupe's rear assembly is subtly different all over the place, from the swing arms to the axles themselves, and the differential housing. The narrower dimension puts the axle bearing and suspension mounting points closer together, and closer to the wheel centerline. This is a good thing. This means that the rear end camber change acts on the center of the wheel, instead of being offset. Overall, the M geometry certainly looks more refined.
While we were looking, it was noted that the reason the cooling fins on the M coupe are different, appears to be about making room for the battery box. The Z coupes fins are on the rear face, which would interfere with the battery on the M coupe, so the fins were relocated to the underside of the diff housing...
Man, now I can get some sleep.
I hate conjecture, even my own (especially my own). In the information age, we should all spend more time being accurate, and less time conjecturing B.S.!
I'm going to go get a life now. This, by the way, includes driving the h*** out of my 2000 Imola Red M coupe!
© 1997 -
2009
Jump Internet Inc. All rights reserved.
