Rear Pitman arms, What do they do?

[rob101]

Been getting back into suspension again, after not really reading anything about it for a while technically (used to do a bit for Formula SAE when i was at uni)
now i've been trying to work out what the purpose of the rear pitman arms. are they to reduce compliance steer (toe in/out due to deformation of the trailing arm bushes) during acceleration/deceleration? (which is a problem with trailing arm suspension) and that strange link (no.10 in this diagram http://www.realoem.com/bmw/showparts.do?model=HA11&mospid=47367&btnr=33_1099&hg=33&fg=30)that seems to only attach to one rear pitman arm on one side. Is that to ensure the subframe doesn't move sideways too much?

[e34.535i.sport]

I hope they do something as i've just forked out a few £ and a tuesday morning fixing them!!!!!

Nice job to do, and the new ones seem to have stopped clunking noises that were coming from the rear end. I'm gonna keep an eye on this thread... you saved me a job posting this. Cheers. :)

[M20Turbo]

Take a look at the video that Don made http://www.bimmer.info/forum/showthread.php?t=34598


Been getting back into suspension again, after not really reading anything about it for a while technically (used to do a bit for Formula SAE when i was at uni)
now i've been trying to work out what the purpose of the rear pitman arms. are they to reduce compliance steer (toe in/out due to deformation of the trailing arm bushes) during acceleration/deceleration? (which is a problem with trailing arm suspension) and that strange link (no.10 in this diagram http://www.realoem.com/bmw/showparts.do?model=HA11&mospid=47367&btnr=33_1099&hg=33&fg=30)that seems to only attach to one rear pitman arm on one side. Is that to ensure the subframe doesn't move sideways too much?

[rob101]

I hope they do something as i've just forked out a few £ and a tuesday morning fixing them!!!!!

Nice job to do, and the new ones seem to have stopped clunking noises that were coming from the rear end. I'm gonna keep an eye on this thread... you saved me a job posting this. Cheers. :)
Well I know they do something, of course in the context of the handling they brought them in the e28s which seemed to eliminate the turn in oversteer that was present in the e12 (or so i've heard). This seems to agree with what i was reading about semi-trailing arm suspension tending to toe out on braking/deceleration (because of bushing compliance) which causes oversteer (typical throttle lift off or underbraking before corner entry).
but that still doesn't tell me what the hell is that link that goes from one side of the dogbones to somewhere in the back as referred to in my original post.

[Rory535i]

but that still doesn't tell me what the hell is that link that goes from one side of the dogbones to somewhere in the back as referred to in my original post.

it's only on one side isn't it? maybe it's something to do with the direction of the rotation of the diff, to counteract it or something? I'm waiting on a set of them to come over from BMA. I'm in brisbane too, never seen you around though...

[BigKriss]

Who knows what item 10 does? I don't know. it just gets in the way. Bill R would know for sure, or Winfred.

http://www.bimmernut.com/%7Ebillr/images/Tracklink.jpg

[rob101]

it's only on one side isn't it? maybe it's something to do with the direction of the rotation of the diff, to counteract it or something? I'm waiting on a set of them to come over from BMA. I'm in brisbane too, never seen you around though...
yeah its only on one side, i was thinking along the lines of engine torque as you mentioned. my car doesn't get out much anymore haven't seen yours either looks very nice though!

[rob101]

Who knows what item 10 does? I don't know. it just gets in the way. Bill R would know for sure, or Winfred.

http://www.bimmernut.com/%7Ebillr/images/Tracklink.jpg
i am not so sure about that, tracklink article there. i can't really see how it reduces the pivot angle and thus the increases the Virtual swing axle length of the trailing arm. since it is so close to the trailing arm bush, it shouldn't affect the range of motion of the trailing arm, because nearest the bushes is the part of the arm that moves the least and also the arm is very short. If you wanted to decrease the compliance in the toe in/out axis of rotation of the suspension without affecting the original range of motion of the trailing arm you'd put a short link there. If you wanted to reduce the pivot angle, you would do so by moving the trailing arm bushes. (I am assuming the pivot angle is the angle that the trailing arm mounts make with the lateral axis.)
correct me if i am wrong, but the only reason i can see is to increase stiffness to toe in/out on acceleration and deceleration. I can't really see it as being anti-dive/squat either. where did that article come from?

I know there are some brilliant minds here, I just want to start a discussion, seems like most people here aren't all that interested so far.....

[Blitzkrieg Bob]

#10 is a torque strap between the front part of the sub frame and the left side of the diff carrier.

It's not on the trailing arm, but bolted on the sub frame side of the dogbone.

[BigKriss]

It's from Bill R.


i am not so sure about that, tracklink article there. i can't really see how it reduces the pivot angle and thus the increases the Virtual swing axle length of the trailing arm. since it is so close to the trailing arm bush, it shouldn't affect the range of motion of the trailing arm, because nearest the bushes is the part of the arm that moves the least and also the arm is very short. If you wanted to decrease the compliance in the toe in/out axis of rotation of the suspension without affecting the original range of motion of the trailing arm you'd put a short link there. If you wanted to reduce the pivot angle, you would do so by moving the trailing arm bushes. (I am assuming the pivot angle is the angle that the trailing arm mounts make with the lateral axis.)
correct me if i am wrong, but the only reason i can see is to increase stiffness to toe in/out on acceleration and deceleration. I can't really see it as being anti-dive/squat either. where did that article come from?

I know there are some brilliant minds here, I just want to start a discussion, seems like most people here aren't all that interested so far.....

[rob101]

It's from Bill R.
I meant more like what book it was from. I still stand by my statement that i don't think it has anything to do with constraining the path of movement of the trailing arm. for one thing the trailing arm in bump and droop the bolts tilts within the bushes of the pitman arm in a way in which is not constrained by the bushes

[BigKriss]

I thought it was BMW TIS.


I meant more like what book it was from. I still stand by my statement that i don't think it has anything to do with constraining the path of movement of the trailing arm. for one thing the trailing arm in bump and droop the bolts tilts within the bushes of the pitman arm in a way in which is not constrained by the bushes