Before going anywhere, the vast majority of the research work was done for me by this fine chap on E90 Post;
https://www.e90post.com/forums/showthread.php?t=230973
When looking underneath a Z4, the suspension is actually a bit of a mash up of it's own components, E46/E85 bits and E90 parts. The rear borrows heavily from the E85 with some upgraded bushings a heavier duty differential and bigger brakes, the front on the other hand is almost entirely lifted from an E90 3 series with the same aluminium subframe and suspension control arms which are the main focus of this post.
Numerous 3 series owners have been able to swap these control arms with items from the E90 M3 to improve the overall handling of the car without necessarily having to change springs/dampers and impacting the ride comfort. The main difference between the standard components and the M3 items (to the best of my knowledge) being ball joints in place of rubber bushings in the case of the lower control arms and much stiffer bushings on the front tension struts. These give much less deflex under cornering load resulting in a car that feels tighter, more predictable and offering better feedback.
The last item that the 3 series guys can swap is the M3 anti roll bar, I tried this on the Z4 however there is a plug on the electric power steering unit that fouls the M3 bar, the Z4 item is shaped differently in the middle to take this into account. The M3 roll bar will just about fit with a push however will rub this plug, therefore the only upgrade option here is a roll bar from H&R and the likes which is a shame because the M3 bar is significantly lighter and having made a short test run with both, there's little to no difference in the feel of either. (both are a massive improvement over the standard roll bar)
Options on the rear are slightly more limited, in terms of bushings there's only the lower bushing on the trailing arm where the wishbone connects that can be swapped for the ball joint from a Z4M or E46 M3, the upper bushing is already a ball joint on E89 Z4's (not so on E85's) and the main trailing arm bush that the E46 guys get trouble with and fit limiters from Turner to is also a different item on E89 Z4's. Visibly it's a wider item so presumably gives less deflex than the old ones.
That just leaves the rear anti roll bar for which any upgrade roll bar for the E85 will fit. M3/Z4M items will not fit here, as with the front H&R etc are the only options.
So which M3 parts can be used?
On the front suspension:
Left Tension Strut With Rubber Mounting - 31102283575
Right Tension Strut With Rubber Mounting - 31102283576
Wishbone Left - 31102283577
Wishbone Right - 31102283578
On the rear suspension:
Ball Joint (X2) - 33306852895
Thus far I've tackled the front suspension and fitted the control arms and a H&R anti roll bar, we took a copy of the cars alignment settings beforehand as the M3 wishbones are longer than the standard items meaning the toe will need to be readjusted afterwards. It will also add a small amount of camber however in my case it was still within BMW's recommendations (-0.1 to -1.1 on the front), my camber went from -0.25 to -0.7.
It is also worth noting that for reasons unknown to me, BMW decided to mount the headlight levelling sensor on the right hand side wishbone on the Z4 instead of the left hand side as on the 3 Series or M3. With the car on the floor I took a measurement of the distance of the sensor arm to the floor them made a small bracket to fit onto the M3 wishbone that'd maintain the same height.
The two front tension struts, visibly these are quite similar to the standard items albeit with a different bushing.
Comparison of the M3 wishbone and standard item
All fitted together on the left hand side
I'll be having a go at the rear in the next few months but the result of all of this at the front is enormous. I'd changed the roll bar over a few weeks before fitting the M3 parts so was familiar with the improvement in front end response and reduction in body roll, the M3 components finally turn the Z4 into a proper sports car.
There is actual steering feel for a start, the initial turn in is slightly lighter than previously but the steering weights up as its loaded and give you little nibbles of information through the wheel about what the front tyres are doing at the same time, I'm finding this very addictive
Going through corners the whole car feels much much tighter with movements being far better controlled, instead of understeer and ploughing into the traction control it now just stays dead flat and outright grip is vastly improved. Along a twisting A road, the standard 35i wouldn't see which way this went :rofl:
One of the best parts of all of this is that there's been virtually no impact on ride comfort at all, being picky I'd say its a little more jiggly over really bad surfaces but the rest of the time it's still just as good a cruiser as before.
https://www.e90post.com/forums/showthread.php?t=230973
When looking underneath a Z4, the suspension is actually a bit of a mash up of it's own components, E46/E85 bits and E90 parts. The rear borrows heavily from the E85 with some upgraded bushings a heavier duty differential and bigger brakes, the front on the other hand is almost entirely lifted from an E90 3 series with the same aluminium subframe and suspension control arms which are the main focus of this post.
Numerous 3 series owners have been able to swap these control arms with items from the E90 M3 to improve the overall handling of the car without necessarily having to change springs/dampers and impacting the ride comfort. The main difference between the standard components and the M3 items (to the best of my knowledge) being ball joints in place of rubber bushings in the case of the lower control arms and much stiffer bushings on the front tension struts. These give much less deflex under cornering load resulting in a car that feels tighter, more predictable and offering better feedback.
The last item that the 3 series guys can swap is the M3 anti roll bar, I tried this on the Z4 however there is a plug on the electric power steering unit that fouls the M3 bar, the Z4 item is shaped differently in the middle to take this into account. The M3 roll bar will just about fit with a push however will rub this plug, therefore the only upgrade option here is a roll bar from H&R and the likes which is a shame because the M3 bar is significantly lighter and having made a short test run with both, there's little to no difference in the feel of either. (both are a massive improvement over the standard roll bar)
Options on the rear are slightly more limited, in terms of bushings there's only the lower bushing on the trailing arm where the wishbone connects that can be swapped for the ball joint from a Z4M or E46 M3, the upper bushing is already a ball joint on E89 Z4's (not so on E85's) and the main trailing arm bush that the E46 guys get trouble with and fit limiters from Turner to is also a different item on E89 Z4's. Visibly it's a wider item so presumably gives less deflex than the old ones.
That just leaves the rear anti roll bar for which any upgrade roll bar for the E85 will fit. M3/Z4M items will not fit here, as with the front H&R etc are the only options.
So which M3 parts can be used?
On the front suspension:
Left Tension Strut With Rubber Mounting - 31102283575
Right Tension Strut With Rubber Mounting - 31102283576
Wishbone Left - 31102283577
Wishbone Right - 31102283578
On the rear suspension:
Ball Joint (X2) - 33306852895
Thus far I've tackled the front suspension and fitted the control arms and a H&R anti roll bar, we took a copy of the cars alignment settings beforehand as the M3 wishbones are longer than the standard items meaning the toe will need to be readjusted afterwards. It will also add a small amount of camber however in my case it was still within BMW's recommendations (-0.1 to -1.1 on the front), my camber went from -0.25 to -0.7.
It is also worth noting that for reasons unknown to me, BMW decided to mount the headlight levelling sensor on the right hand side wishbone on the Z4 instead of the left hand side as on the 3 Series or M3. With the car on the floor I took a measurement of the distance of the sensor arm to the floor them made a small bracket to fit onto the M3 wishbone that'd maintain the same height.
The two front tension struts, visibly these are quite similar to the standard items albeit with a different bushing.
Comparison of the M3 wishbone and standard item
All fitted together on the left hand side
I'll be having a go at the rear in the next few months but the result of all of this at the front is enormous. I'd changed the roll bar over a few weeks before fitting the M3 parts so was familiar with the improvement in front end response and reduction in body roll, the M3 components finally turn the Z4 into a proper sports car.
There is actual steering feel for a start, the initial turn in is slightly lighter than previously but the steering weights up as its loaded and give you little nibbles of information through the wheel about what the front tyres are doing at the same time, I'm finding this very addictive
Going through corners the whole car feels much much tighter with movements being far better controlled, instead of understeer and ploughing into the traction control it now just stays dead flat and outright grip is vastly improved. Along a twisting A road, the standard 35i wouldn't see which way this went :rofl:
One of the best parts of all of this is that there's been virtually no impact on ride comfort at all, being picky I'd say its a little more jiggly over really bad surfaces but the rest of the time it's still just as good a cruiser as before.
