Artful-Bodger said:
The tightening torque according to online specs is 74ftlb, that torque has tightened the side mounting plates onto the steel tube that runs through the rubber, so how can it move, well the clue is in the fact you are supposed to tighten the bolt with the cars weight on the wheels, so the ride height is as it is on the road, because what happens is the arm moves up and down and the rubber in the bush twists, it's only a small amount due to the length of the arm and the limited suspension travel.
That is the point of pretention on the rtab's
for the lateral control arms you need to tighten them indeed in the normally loaded situation if you use the stock rubber bushings. For poly bushings this is not a issue.
Before I had poly bushings in my rear axle, I used to use a transmission jack to jack up the rear suspension (up to when the lateral arm would sit horizontal) and tighten the bushings.
If you have poly bushes or similar the bush cannot twist so it must move, for it to do this the fit must be very precise, any side load applied to the bush by the mounting plates will hinder it's movement so the bush must be able to move freely without being slack, in my experience most of these bushes need a little work on the lathe to get the fit right as the cars have different tolerances and wear and tear.
For strongflex&powerflex this was not an issue (from first hand experience). No slack present and no hinder of movement. For the lower outer bushing you can best use the same balljoint that is used on the upper lateral arm.
Also polyurethane bushings up to shore95a still have some flex so I cant imagine it being a problem up to that stiffness, unless its a really cheap crappy brand that has no eye for quality. But who buys that stuff for their p&j?
With what bushing (brand, model) on what place on the z4 precisely did you have this experiece? That would be good to know so people know what particular brand&model they should avoid.
A harder bush also imparts more vibration into the car, so you get more road noise and rattles developing, the ultimate for this was the aluminium bushes I made for an MX5, the ride was rather noisy, squeaky and harsh bordering on unbearable on the road, it worked ok on the track but driving back and fore to events was not amusing!
Of course, but so are stiffer arb's, stiffer lowering springs or coilovers. The trick is where to find the right adjustment to your needs. Thats why poly bushings are nice because you generally can choose hardness.
Although I went for shore 90A on all the control arm bushings and shore80A on the bushings that bolt directly to the chassis (to prevent high loads on chassis studs) and shore80A on the diff (reduce noise), all these stiffer than stock bushings, plus the stiffer arb's I use had less impact on how much stiffer the car got than compared to my KW V3 coiloverset. To put it all into perspective so to say. So people who can stand a kw v3 coilover set, certainly can stand some shore95a bushings.
Of cours I didnt use 100% solid bushings, that is something I would never advise for a streetcar. Not only because of NVH but also cracks around studs in the chassis. ARB mounging points and subframe mounting points would need much reinforcement.
But if you think the stock suspension is already on the harsh side and you dont crave for more handling, poly bushings is not the way to go. A merc SLK is
