New Performance Suspension Bushings
11-16-2018, 04:57 PM
#61
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So regarding the rear LCA bush, the bracket dimensions are as follows and are approximate, the lower allloy half is 60 mm in diameter, the upper half is 27 mm deep. However I got 58 mm diameter. I know there is some variation there but maybe that is for compression of the factory bushings.
The Powerflex outside diameter is approximately 61.50 mm in diameter, that makes sense when you look at the 60 mm diameter in the lower cap, just enough compression to stop the rotation but not enough to cause distortion on the running surface on the inner part of this bush. I don't think the same can be said for the upper half. There will be an uneven amount of compression and distortion there.
There is a relatively easy fix, please ignore my previous suggestion that a piece should be inserted to build it out in height and capture the lugs on the rear of the bush. This is not a good idea. I was thinking on my feet. The best solution is for the bush to be milled just on the upper half by about 2 mm, the cut wouldn't be uniform but follow the upper crossmember's shape. This would also be the best anti rotation method as the diameter would be different from top to bottom as such there would be no chance of movement. The pressure throughout the bush would be uniform in regards to it's clamping.
Your thoughts?
The Powerflex outside diameter is approximately 61.50 mm in diameter, that makes sense when you look at the 60 mm diameter in the lower cap, just enough compression to stop the rotation but not enough to cause distortion on the running surface on the inner part of this bush. I don't think the same can be said for the upper half. There will be an uneven amount of compression and distortion there.
There is a relatively easy fix, please ignore my previous suggestion that a piece should be inserted to build it out in height and capture the lugs on the rear of the bush. This is not a good idea. I was thinking on my feet. The best solution is for the bush to be milled just on the upper half by about 2 mm, the cut wouldn't be uniform but follow the upper crossmember's shape. This would also be the best anti rotation method as the diameter would be different from top to bottom as such there would be no chance of movement. The pressure throughout the bush would be uniform in regards to it's clamping.
Your thoughts?
11-16-2018, 08:39 PM
#62
Rennlist Member
So regarding the rear LCA bush, the bracket dimensions are as follows and are approximate, the lower allloy half is 60 mm in diameter, the upper half is 27 mm deep. However I got 58 mm diameter. I know there is some variation there but maybe that is for compression of the factory bushings.
The Powerflex outside diameter is approximately 61.50 mm in diameter, that makes sense when you look at the 60 mm diameter in the lower cap, just enough compression to stop the rotation but not enough to cause distortion on the running surface on the inner part of this bush. I don't think the same can be said for the upper half. There will be an uneven amount of compression and distortion there.
There is a relatively easy fix, please ignore my previous suggestion that a piece should be inserted to build it out in height and capture the lugs on the rear of the bush. This is not a good idea. I was thinking on my feet. The best solution is for the bush to be milled just on the upper half by about 2 mm, the cut wouldn't be uniform but follow the upper crossmember's shape. This would also be the best anti rotation method as the diameter would be different from top to bottom as such there would be no chance of movement. The pressure throughout the bush would be uniform in regards to it's clamping.
Your thoughts?
The Powerflex outside diameter is approximately 61.50 mm in diameter, that makes sense when you look at the 60 mm diameter in the lower cap, just enough compression to stop the rotation but not enough to cause distortion on the running surface on the inner part of this bush. I don't think the same can be said for the upper half. There will be an uneven amount of compression and distortion there.
There is a relatively easy fix, please ignore my previous suggestion that a piece should be inserted to build it out in height and capture the lugs on the rear of the bush. This is not a good idea. I was thinking on my feet. The best solution is for the bush to be milled just on the upper half by about 2 mm, the cut wouldn't be uniform but follow the upper crossmember's shape. This would also be the best anti rotation method as the diameter would be different from top to bottom as such there would be no chance of movement. The pressure throughout the bush would be uniform in regards to it's clamping.
Your thoughts?
If both the rear bush and the front bush are free to rotate then the only thing axially restraining the car to the lower arm would be the flanges cast into the arm and they are not exactly a precision type of finish yet alone particularly beefy. The original design does not allow differential rotation- it only permits torsion of the bush material and I understand we are aligned in that regard. The clamping action not only ensures the torsion is controlled by the bush material it also stops axial movement and the shoulder therefore presumably would act as a secondary stop given there will probably be a small amount of axial deflection. The compressive action of the rear clamp on the bush material is so obvious Powerflex could not possibly have overlooked that or they would have to be right numpties.
In your note the other day you stated that both bushes are free to rotate when assembled and indeed I am quite happy to accept that. However the thought that struck me this evening [early morning now] is that considering the above maybe the rear bush of the Powerflex design, contrary to your current thinking is designed to stop rotation and rely on torsion as per the original concept whereas the front one is free to rotate. This begs the question of Scott whether he did a test with the rear clamp torqued up to see whether the lower arm still moved freely- he probably did not given the procedure is to torque when settled,
The urethane Powerflex use has higher torsional rigidity and I noted it seems they use the black urethane material for the outer bush which is stiffer than the blue material grade. Perhaps they reasoned that with the dimensions used, the geometry of the clamp and the 85 ft lbs of bolt torque pushed through the cap would stop the outer bush from moving relative to the stainless insert and the total torsional resistance from the two concentric bushes on the rear mount alone would provide similar torsional resistance to the stock arm and also restrain axial movement.
Just a hypothesis at this stage but surely that would make more sense than a complete **** up that needs to be corrected not to mention we have one happy camper using it.
Feel free to kick the crap out of the above mentioned theory whilst I try to get some shut eye.
Rgds
Fred
11-16-2018, 09:08 PM
#63
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Well I was doing some thinking and then something in the grey matter flashed some warning lights!
If both the rear bush and the front bush are free to rotate then the only thing axially restraining the car to the lower arm would be the flanges cast into the arm and they are not exactly a precision type of finish yet alone particularly beefy. The original design does not allow differential rotation- it only permits torsion of the bush material and I understand we are aligned in that regard. The clamping action not only ensures the torsion is controlled by the bush material it also stops axial movement and the shoulder therefore presumably would act as a secondary stop given there will probably be a small amount of axial deflection. The compressive action of the rear clamp on the bush material is so obvious Powerflex could not possibly have overlooked that or they would have to be right numpties.
In your note the other day you stated that both bushes are free to rotate when assembled and indeed I am quite happy to accept that. However the thought that struck me this evening [early morning now] is that considering the above maybe the rear bush of the Powerflex design, contrary to your current thinking is designed to stop rotation and rely on torsion as per the original concept whereas the front one is free to rotate. This begs the question of Scott whether he did a test with the rear clamp torqued up to see whether the lower arm still moved freely- he probably did not given the procedure is to torque when settled,
The urethane Powerflex use has higher torsional rigidity and I noted it seems they use the black urethane material for the outer bush which is stiffer than the blue material grade. Perhaps they reasoned that with the dimensions used, the geometry of the clamp and the 85 ft lbs of bolt torque pushed through the cap would stop the outer bush from moving relative to the stainless insert and the total torsional resistance from the two concentric bushes on the rear mount alone would provide similar torsional resistance to the stock arm and also restrain axial movement.
Just a hypothesis at this stage but surely that would make more sense than a complete **** up that needs to be corrected not to mention we have one happy camper using it.
Feel free to kick the crap out of the above mentioned theory whilst I try to get some shut eye.
Rgds
Fred
If both the rear bush and the front bush are free to rotate then the only thing axially restraining the car to the lower arm would be the flanges cast into the arm and they are not exactly a precision type of finish yet alone particularly beefy. The original design does not allow differential rotation- it only permits torsion of the bush material and I understand we are aligned in that regard. The clamping action not only ensures the torsion is controlled by the bush material it also stops axial movement and the shoulder therefore presumably would act as a secondary stop given there will probably be a small amount of axial deflection. The compressive action of the rear clamp on the bush material is so obvious Powerflex could not possibly have overlooked that or they would have to be right numpties.
In your note the other day you stated that both bushes are free to rotate when assembled and indeed I am quite happy to accept that. However the thought that struck me this evening [early morning now] is that considering the above maybe the rear bush of the Powerflex design, contrary to your current thinking is designed to stop rotation and rely on torsion as per the original concept whereas the front one is free to rotate. This begs the question of Scott whether he did a test with the rear clamp torqued up to see whether the lower arm still moved freely- he probably did not given the procedure is to torque when settled,
The urethane Powerflex use has higher torsional rigidity and I noted it seems they use the black urethane material for the outer bush which is stiffer than the blue material grade. Perhaps they reasoned that with the dimensions used, the geometry of the clamp and the 85 ft lbs of bolt torque pushed through the cap would stop the outer bush from moving relative to the stainless insert and the total torsional resistance from the two concentric bushes on the rear mount alone would provide similar torsional resistance to the stock arm and also restrain axial movement.
Just a hypothesis at this stage but surely that would make more sense than a complete **** up that needs to be corrected not to mention we have one happy camper using it.
Feel free to kick the crap out of the above mentioned theory whilst I try to get some shut eye.
Rgds
Fred
11-16-2018, 09:49 PM
#64
Three Wheelin'
Well I was doing some thinking and then something in the grey matter flashed some warning lights!
If both the rear bush and the front bush are free to rotate then the only thing axially restraining the car to the lower arm would be the flanges cast into the arm and they are not exactly a precision type of finish yet alone particularly beefy. The original design does not allow differential rotation- it only permits torsion of the bush material and I understand we are aligned in that regard. The clamping action not only ensures the torsion is controlled by the bush material it also stops axial movement and the shoulder therefore presumably would act as a secondary stop given there will probably be a small amount of axial deflection. The compressive action of the rear clamp on the bush material is so obvious Powerflex could not possibly have overlooked that or they would have to be right numpties.
In your note the other day you stated that both bushes are free to rotate when assembled and indeed I am quite happy to accept that. However the thought that struck me this evening [early morning now] is that considering the above maybe the rear bush of the Powerflex design, contrary to your current thinking is designed to stop rotation and rely on torsion as per the original concept whereas the front one is free to rotate. This begs the question of Scott whether he did a test with the rear clamp torqued up to see whether the lower arm still moved freely- he probably did not given the procedure is to torque when settled,
The urethane Powerflex use has higher torsional rigidity and I noted it seems they use the black urethane material for the outer bush which is stiffer than the blue material grade. Perhaps they reasoned that with the dimensions used, the geometry of the clamp and the 85 ft lbs of bolt torque pushed through the cap would stop the outer bush from moving relative to the stainless insert and the total torsional resistance from the two concentric bushes on the rear mount alone would provide similar torsional resistance to the stock arm and also restrain axial movement.
Just a hypothesis at this stage but surely that would make more sense than a complete **** up that needs to be corrected not to mention we have one happy camper using it.
Feel free to kick the crap out of the above mentioned theory whilst I try to get some shut eye.
Rgds
Fred
If both the rear bush and the front bush are free to rotate then the only thing axially restraining the car to the lower arm would be the flanges cast into the arm and they are not exactly a precision type of finish yet alone particularly beefy. The original design does not allow differential rotation- it only permits torsion of the bush material and I understand we are aligned in that regard. The clamping action not only ensures the torsion is controlled by the bush material it also stops axial movement and the shoulder therefore presumably would act as a secondary stop given there will probably be a small amount of axial deflection. The compressive action of the rear clamp on the bush material is so obvious Powerflex could not possibly have overlooked that or they would have to be right numpties.
In your note the other day you stated that both bushes are free to rotate when assembled and indeed I am quite happy to accept that. However the thought that struck me this evening [early morning now] is that considering the above maybe the rear bush of the Powerflex design, contrary to your current thinking is designed to stop rotation and rely on torsion as per the original concept whereas the front one is free to rotate. This begs the question of Scott whether he did a test with the rear clamp torqued up to see whether the lower arm still moved freely- he probably did not given the procedure is to torque when settled,
The urethane Powerflex use has higher torsional rigidity and I noted it seems they use the black urethane material for the outer bush which is stiffer than the blue material grade. Perhaps they reasoned that with the dimensions used, the geometry of the clamp and the 85 ft lbs of bolt torque pushed through the cap would stop the outer bush from moving relative to the stainless insert and the total torsional resistance from the two concentric bushes on the rear mount alone would provide similar torsional resistance to the stock arm and also restrain axial movement.
Just a hypothesis at this stage but surely that would make more sense than a complete **** up that needs to be corrected not to mention we have one happy camper using it.
Feel free to kick the crap out of the above mentioned theory whilst I try to get some shut eye.
Rgds
Fred
11-17-2018, 05:16 AM
#65
Rennlist Member
I had that same thought so I torqued the LCA whilst in the air. I could move the control arm right through it's range of vertical movement with no binding or catching at all. There are plenty of cases in the Porsche world where very clever engineers have done away with the rubber and gone with monballs. They offer even less resistance than the urethane.
That is a good data point
So what that tells me is that the outer bush must be structurally of such resilience that the clamping force does not translate through to the steel sleeve and urethane sliding face.
Did you check for any axial movement as a matter of interest? I remain somewhat puzzled about how that is controlled.
11-17-2018, 05:23 AM
#66
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Fred, the axial movement is controlled by the slight pre-loading on the thrust faces of the bushings. I helped them get this bit to close tolerance from my accident free car. That arm will not move forward and backwards as much as the factory bushings would. As mentioned I think if I machined an arc into the top of the black urethane then this bush would be perfect.
11-17-2018, 08:35 AM
#67
Rennlist Member
Fred, the axial movement is controlled by the slight pre-loading on the thrust faces of the bushings. I helped them get this bit to close tolerance from my accident free car. That arm will not move forward and backwards as much as the factory bushings would. As mentioned I think if I machined an arc into the top of the black urethane then this bush would be perfect.
That mitigates my concern for the most part if the differences example to example are minimal. When I first studied the front suspension system I was puzzled as to why the rear bolt required 85 ft lbs and the front ones somewhat less at 50 ft lbs. I concluded that this was because the rear clamp required more torque to enable it to anchor the rear bush. I figured that if anything, the front bolts would take more load than the rear ones from the braking force component. Hopefully the thrust collars on the lower arm are up to the new task at hand- specifically the rear most one as that is going to be restraining the braking force which amounts to 1G or roughly 1 tonne each side on a fully loaded car.
I do not know how critical end float is on this item but from what you are saying it sounds as though the thrust faces on the arm have been promoted from long stop to wicket keeper!
Regarding the machining of the bush given Scotts experience I would think that is just not needed. The bush needs to be clamped something of the same radius to keep the load spread even albeit I remain a bit puzzled about how it works given there is something in the region of 20 [?] degrees of clamp face missing on the chassis side of the joint.
Now the interesting bit is how the missing chunk of torsional resistance impacts the springs and dampers which presumably are going to be more heavily loaded
