Z4M front subframe-- why is it a different part? Roll center correction?

[Obioban]

terra pointed out the other day that the Z4M front subframe is a different PN than the M3 part, despite bolting onto same location and looking similar. He speculated that perhaps it was to correct the roll centers change from the lower ride height on the Z4M.

Has anyone ever measured, to see what the actual difference in that part is?



Edit, from later in the thread:

And just to clarify, the Z4 non-M shares the front subframe with the E46 (both non-M and M)

[Casa de Mesa]

Intrigued by this and sub'd. I didn't realize this.

[Sharocks]

I was gonna say because it's for a Z4M lol

[Tbonem3]

Part weights are provided by realOEM, is it lighter?

[R60BBA]

I recall reading somewhere that the front end suspension components of the Z4M is based on the E36 M3, where as the rear end is the same as the E46 M3.

[heinzboehmer]

Part weights are provided by realOEM, is it lighter?

Says it's heavier :/

Edit: Looks like the N52 E85/6 Z4 uses the same front subframe as the E46 M3. According to TIS, both the N52 and S54 Z4s have the same ride height. Roll center correction theory is looking less likely to me, unless TIS is wrong/missing information. Maybe just an updated part number?

Double edit: For the Z4s, TIS lists both the regular ride height and the "Low-slung sports suspension" ride height. The sports suspension is ~15 mm lower than the regular one.

I don't know too much about the Zs, so I assumed the sports suspension was some sort of package you could get on any car. But it's possible that the sports suspension is just the M car. If that's the case, then the roll center correction theory is back on the table.

[terra]

And just to clarify, the Z4 non-M shares the front subframe with the E46 (both non-M and M)

[Obioban]

I do have a Z4M front subframe-- if anyone has the gear to accurately measure it, and has a stock M3 subframe to compare it to, I'd be happy to mail it to them (obviously I'd need it back).

[terra]

I do have a Z4M front subframe-- if anyone has the gear to accurately measure it, and has a stock M3 subframe to compare it to, I'd be happy to mail it to them (obviously I'd need it back).

Volke ?

[thegenius46m]

I recall reading somewhere that the front end suspension components of the Z4M is based on the E36 M3, where as the rear end is the same as the E46 M3.

Front struts are the same as E36 where the rear end is similar to the E46 M3. Now if the regular Z4 uses the same subframe as all E46s then the Z4M subframe should be a direct bolt in. E46 330i ZHP uses Z4M front control arms from the factory and the offset fcabs are plug and play so the z4m subframe should fit no problem. Curious how z4m subframe will affect track width with E46 M3 control arms as I have not measured them on both my E46s.

[Obioban]

Now if the regular Z4 uses the same subframe as all E46s then the Z4M subframe should be a direct bolt in.

They do.

https://www.realoem.com/bmw/enUS/part?id=BD33-USA-10-2002-E46-BMW-325Ci&mg=31&sg=05&diagId=31_0553&q=31111096902

[George Hill]

Obioban sent me his Z4M subframe to compare to my gaggle of E46 subframes.

Visually they look identical, but as pointed out above the Z4M is about 1# heavier. Looking into the internal cavities of the subframe I found the Z4M has an added piece of metal, it must be some kind of additional support but doesn't really "tie" into anything significant so your guess is as good as mine as to why it's there.

I also took some rough measurements with my dial calipers and the control arm ball joint boss is basically in the same place laterally and vertically, the variance I was seeing could simply be from how I was measuring which was not 100% accurately repeatable between subframes.

Lastly, here are some pics of the bracing as best as I could photograph them.

How I was lighting up the internal area and showing which hole we are "looking" into.



M3


Z4M

[bigjae46]

I would guess that BMW needed to make the subframe stiffer since it was a convertible? I just bought my daughter a Z3 and that is the opposite of stiff. I'd have some faith in old school BMW that they would fix that in the subsequent model.

[terra]

I would guess that BMW needed to make the subframe stiffer since it was a convertible? I just bought my daughter a Z3 and that is the opposite of stiff. I'd have some faith in old school BMW that they would fix that in the subsequent model.

Well it’s just the Z4M that got this subframe with the reinforcement. Standard Z4 (roadster or coupe) got the same subframe as us.

[George Hill]

I would guess that BMW needed to make the subframe stiffer since it was a convertible? I just bought my daughter a Z3 and that is the opposite of stiff. I'd have some faith in old school BMW that they would fix that in the subsequent model.

I don't think the added "brace" would make a difference between the chassis. I'll see if I can "draw" out the brace on the outside of the subframe to show how it fits in there.