Tired front suspension bushes...
- Thread starter Toed64
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My first question is why do you want to fit offset bushes?
They will increase the caster angle. This only improves steering if your car has a tendency to roll (caster creates dynamic camber). But for the rest extra caster gives less turn in (generally less steering force before the apex of a corner).
The difference in steering 'feel' comes from that with more caster you slightly lift the car when steering. So there is more force involved. The weight of the car will also give the tendency to push the steeringwheel back to it's center position. So more caster gives a more stable straightline feel.
I think if you want more steering force/grip, stiffer rollbars are a better way to go imho, correcting the problem instead of adjusting according to the problem. Also alignment can be more of a gamble; the z4m front suspension is different, so which alignment do you choose?
Most cars that steer really well (cars like lotusses) don't run that much casterangle.
As far as poly busings: I would never fit poly bushings that cant act as a bearing. So always a 2 piece bushing on moving parts.
Polyurethane is good for pressure, not for stretching.
As for the FCAB's I think it's important to have a bushing that has on the inner sleeve (so the part that goes on the hex part of the control arm) a metal insert. The powerflex bushings have those. I say this because bushings from strongflex are known to fail at this point; the inner part can work itself slowly out of the outer bushing (creeping up the control arm pin so to speak). On the german z4 forum there are examples of this failure.
They will increase the caster angle. This only improves steering if your car has a tendency to roll (caster creates dynamic camber). But for the rest extra caster gives less turn in (generally less steering force before the apex of a corner).
The difference in steering 'feel' comes from that with more caster you slightly lift the car when steering. So there is more force involved. The weight of the car will also give the tendency to push the steeringwheel back to it's center position. So more caster gives a more stable straightline feel.
I think if you want more steering force/grip, stiffer rollbars are a better way to go imho, correcting the problem instead of adjusting according to the problem. Also alignment can be more of a gamble; the z4m front suspension is different, so which alignment do you choose?
Most cars that steer really well (cars like lotusses) don't run that much casterangle.
As far as poly busings: I would never fit poly bushings that cant act as a bearing. So always a 2 piece bushing on moving parts.
Polyurethane is good for pressure, not for stretching.
As for the FCAB's I think it's important to have a bushing that has on the inner sleeve (so the part that goes on the hex part of the control arm) a metal insert. The powerflex bushings have those. I say this because bushings from strongflex are known to fail at this point; the inner part can work itself slowly out of the outer bushing (creeping up the control arm pin so to speak). On the german z4 forum there are examples of this failure.
On the offset bushings the black outer part is aluminium, on the centered bushings, the black part can be either chosen aluminium or shore95a PUZulu4 said:
Thank you very much for the detailed reply GuidoK. Much appreciated. 
My reasoning for wanting to try an offset bush is exactly as in your second sentence "They will increase the caster angle. This only improves steering if your car has a tendency to roll".
The roads in my area are very poor. Not only do they have a lot of potholes, but they have regular uneven patches, sloping left or right. This causes the car to want to suddenly pull left or right. This is most disconcerting when at speed on a single track road and I have to concentrate quite hard on just going straight.
I will eventually go for slightly less low profile tyres, but as I'm currently refreshing the suspension, I thought I'd try altering the lollipops while I'm at it.
As for which alignment to choose, I thought I'd go for Z4M but I suppose it may be a bit of trial and error.
You've definitely given me food for thought, so perhaps I'll go for a two-part SuperPro poly bush or a standard Z4M lollipop. (I don't really want to harden the ride any more).
My reasoning for wanting to try an offset bush is exactly as in your second sentence "They will increase the caster angle. This only improves steering if your car has a tendency to roll".
The roads in my area are very poor. Not only do they have a lot of potholes, but they have regular uneven patches, sloping left or right. This causes the car to want to suddenly pull left or right. This is most disconcerting when at speed on a single track road and I have to concentrate quite hard on just going straight.
I will eventually go for slightly less low profile tyres, but as I'm currently refreshing the suspension, I thought I'd try altering the lollipops while I'm at it.
As for which alignment to choose, I thought I'd go for Z4M but I suppose it may be a bit of trial and error.
You've definitely given me food for thought, so perhaps I'll go for a two-part SuperPro poly bush or a standard Z4M lollipop. (I don't really want to harden the ride any more).
In a sloping road situation more caster will result in more steering pull I think.
I think it works as follows: the tyre is (seen from left hand side to right hand side) a flat piece of rubber; the contact patch.
When the road slopes to left or right, that tyrepatch will want to follow that slope.
If you have no caster, so that the lower and upper kingpinjoint are in a straight vertical line (the lower is the pillowball in the suspension arm, the upper is the top bearing on the shock), the tyrepatch cant put a force on the steering rack. The upper and lower kingpinjoint take all the force.
When you add caster, a part of that force is transponed on the steering rack. Say if you had 90 degrees caster, so that the kingpin would be 100% horizontal, all the force resulted from that slope would be transponed onto the steering rack.
So more caster induces more tramlining. That is that extra steeringwheel 'feel' people are talking about. More force sent through the steering rack. The offset bushing give I believe 0,5deg extra caster. The standard suspension has 5,3 - 5,5 deg caster (depends on whether you have lowered suspension or not)
I think it works as follows: the tyre is (seen from left hand side to right hand side) a flat piece of rubber; the contact patch.
When the road slopes to left or right, that tyrepatch will want to follow that slope.
If you have no caster, so that the lower and upper kingpinjoint are in a straight vertical line (the lower is the pillowball in the suspension arm, the upper is the top bearing on the shock), the tyrepatch cant put a force on the steering rack. The upper and lower kingpinjoint take all the force.
When you add caster, a part of that force is transponed on the steering rack. Say if you had 90 degrees caster, so that the kingpin would be 100% horizontal, all the force resulted from that slope would be transponed onto the steering rack.
So more caster induces more tramlining. That is that extra steeringwheel 'feel' people are talking about. More force sent through the steering rack. The offset bushing give I believe 0,5deg extra caster. The standard suspension has 5,3 - 5,5 deg caster (depends on whether you have lowered suspension or not)
Oh, now I'm a bit confused because I thought people had previously said that the offset bush improved tramlining effects :?
So are you saying that with an offset bush on a smooth, flat road the car would be more stable, yet on a sloping road it will be less stable ?
So are you saying that with an offset bush on a smooth, flat road the car would be more stable, yet on a sloping road it will be less stable ?
Zulu4 said:
Yes.
More caster means the steering rack tends to follow the road with more force.
So on a straight flat road that means straigt ahead, and when there's ridges or a sloping bank, it'll try to follow that. And with patches and potholes, the steering wheel will jerk/bounce more from left to right. It basically amplifies the forces on the steering wheel.
Here's a quote from the book "the automotive chassis" (Reimpell/Stoll) that also describes that (I'll spare you all the formula's, it's a very heavy read)
There's also more sensitivity to sidewinds, but that is hardly an issue on a low slung car like this.
And when you want to steer, initial steering input is less direct with more caster. I don't know if you ride a motorcycle, or a bicycle, but compare a chopper (lots of caster) to a sportbike (less caster), or a normal bicycle (more caster) to a mountainbike (less caster).
Less caster has more direct steering input.
The advantages of more caster are that straightline on a good road it's more stable, and when you have lots of body roll, caster gives dynamic camber, so on the outer wheel you can get a larger contact patch. But of course it's better to reduce that body roll (but that means stiffer suspension or stiffer anti rollbars)
