Z4M getting some recognition

[PDJ]

I personally don’t agree with the comments about polly bushes making the car harsher.
My take on it is. The OEM, polly bush or even fully rose jointed (I did all 3 types on my Westfield) is the bush or bearing, it is just a pivot point for the suspension to move around and a small amount of movement at that.
The harsh ride will come from the shock absorbers and the springs.
If you take it to extremes and say the spring is that stiff it does not compress, it is not the bearing making the car harsh, now if you go the other way and make the spring very soft the car is soft and floaty.
The shock absorber takes the shock out of the movement and controls the rebound the spring has trying to return the energy it gained in compression
If anyone want a ride in my fully polly bushed Z4M to compare just get in touch car is still used as I have no garage at the moment but as soon as the salt is on the roads it will be staying put just outside where I am renting while I have a house built.

 

[plenty]

The harsh ride will come from the shock absorbers and the springs.

There are two types of ride: primary and secondary.

Primary ride is the car's ability to cope with undulating terrain, and is indeed a function of compression and rebound, affected by spring rates, damping and geometry but not by bushes. In fact stiffer bushes can improve primary ride in some circumstances, e.g. worn rubber RTABs flexing on compression and causing unwanted lateral movement of the rear axle leading to the feeling of a "loose" rear end especially over bumpy ground.

Secondary ride is the car's transmission of NVH and this is indeed affected by using harder bushes. So using poly bushes or rose joints won't affect primary ride, and in fact will make the car more precise and responsive to driver inputs, but it will degrade secondary ride by adding NVH.

 

[maupineda]

My car came fitted with H & R coil-overs that were only a few months old so I've kept them but they are really firm and only adjust for ride height. If/when they need changing I'd probably go for OE or if I was feeling flush KWs with adjustable compression and rebound!

When I needed new engine mounts Ross at RBM (bowser134 on here) recommended OE rather than Vibra-Technics for a road car - same with poly-bushes.

After raising the ride height, removing spacers and going back to 18" wheels from 19s I had a 4 wheel alignment last year to E46 M3 CSL settings and my car was transformed.

The H&R coils are seriously flawed from the get go, I had to modify springs and bumps stop to make a decent kit out of them

The main issues are:

- The front bump stops are toooo long, 70mm. I cut them down to 45mm
- their springs are too low, this makes the ride absolutely crap, you ride on the stops all the time, so with stiffer springs, plus riding on stops, you end with a very crashy ride

The above could be fixed, however, the dampers are too stiff and have close to 60lb of gas force, all these give you have a rock hard suspension (not fun).

To fix the first two issues I did the following

FRONT
- change springs to swift springs, this required new threaded perches from Öhlins. I chose 225 lb/in (all this is cheap if you DIY)
- cuts stops to 45mm

REAR
- used B8 shocks from non M, they are still valved very stiff, also have much gas pressure, but overall allow for shortening the rear stop and reduce droop which is needed with shorter springs, they are shorter than H&R coils shocks and OE.
- used barrel type springs from TCK, I chose 600 lb/in

With the above changes made the car ride much better and can now can round sharp bumps well to some extend, and the car is much less crashy and body control is superb.

The one thing I cannot fix without spending too much money is gas pressure in the dampers, but maybe 30-40 lb of gas force would improve things much further. I think 30lb of gas would be best for road.

Mexican roads are crap, so all the above was needed to coupe with our awful roads.

The issue was not much spring stiffness, but damping, plus ride height, plus bump stop length.

My setup above gives me 10mm more of suspension travel before touching the stops, so I have more effective suspension travel vs OEM, and the same overall travel until bump stops are solid blocks.

The above plus CSL alignment makes the car go on as in on rails up to 7-8 10ths, though the one thing the CSL alignment augmented is tram lining, so for poor and unleveled roads it needs some toe.

 

[Mr Tidy]

Many thanks for your detailed response.

You're right about the ride on the H & Rs, even with some added sidewall on the 18s mine is still very firm. I think I need to try one with a different suspension set-up to get an on-road comparison.