Suspension geometry question...
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Suspension geometry question... I understand the purpose of all the parts of the F-body rear suspension. The torque arm prevents the axle from rotating... My question is why is the torque arm wishbone shaped? The lower control arms should keep the bottom of the axle from pushing/pulling and it seems to me that a single upper rod would be sufficient. Anybody want to educate me on the subject?
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This looks like a diagram from the top view, but not for sure. To me the simpliest reason for doing this is to eliminate a pan hard bar tradition located on a 4-link setup, when the top bars angle like they are this prevents any sideways movement of the axle. Under accleration or hard accleration most of the load from the torque is applied to lower arms.
It adds rigidity to the torque arm. Otherwise, it could experience torsion (twisting), which would push 1 side of the axle higher than the other (this still happens on hard launches, but it would be far worse), or it may even fail. In fact, this twisting is minimized also by the LCAs. Furthermore, the connection is strengthened by having both sides. You run less risk of the ta failing at the contacts or having the bolts ripped out of the rear end due to the torque being transmitted. Can you design it differently? Sure, but I wouldn't recommend changing the triangular shape.
Mind you, I haven't analyzed the forces or moments associated with the design, so this is speculation, though I do have some background in design.
Mind you, I haven't analyzed the forces or moments associated with the design, so this is speculation, though I do have some background in design.
Last edited by 2002BlackSS; Jul 31, 2005 at 09:55 PM.
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it's an over simplified view from the side..
Makes sense BlackSS... I was just pondering bolting a bracket and rod end to the top of the 3rd member and affixing a beefy rod from it to the trans crossmember. One because I can't find a suitable traditional torque arm setup that does what I want. Also, it would free up space for a sweet true dual exhaust
Makes sense BlackSS... I was just pondering bolting a bracket and rod end to the top of the 3rd member and affixing a beefy rod from it to the trans crossmember. One because I can't find a suitable traditional torque arm setup that does what I want. Also, it would free up space for a sweet true dual exhaust
Originally Posted by OldeSkool
it's an over simplified view from the side..
Makes sense BlackSS... I was just pondering bolting a bracket and rod end to the top of the 3rd member and affixing a beefy rod from it to the trans crossmember. One because I can't find a suitable traditional torque arm setup that does what I want. Also, it would free up space for a sweet true dual exhaust
Makes sense BlackSS... I was just pondering bolting a bracket and rod end to the top of the 3rd member and affixing a beefy rod from it to the trans crossmember. One because I can't find a suitable traditional torque arm setup that does what I want. Also, it would free up space for a sweet true dual exhaust
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After looking on the web the picture makes sense, but it does look a lot like the rear suspension of a chevelle (A body). I don't own a ls1 camaro just a 68 camaro (project car), and 99 silverado.
Last edited by cbrich; Aug 1, 2005 at 12:53 AM.
This triangular shape is done as much to provide rigid mounting to the axle as it is to strengthen the torque arm itself. If there was only a single mounting point to the axle, the axle would still be able to rotate. Two spaced mounting points does a far better job of restricting the axles desire to rotate.
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The torque arm is designed to stop the rotational forces of the rear end. It is not designed to withstand any twisting/torsion forces.
The triangular shape of it is for strength in stopping these rotational forces and to attach at a single one-point mount in the front.
That being said... The best "simple" design of a torque arm is to use a mount like Spohn does on the rear end and to run 2 straight parallel tubes up to a similar mount near the transmission. When you accellerate, the rotational forces of the rear end are going to pull on the upper bar and push on the lower bar.
The problem lies in the ability to strengthen the front mount to take all the rotational forces without breaking. Spohn and others take this into consideration by attaching the two bars together to mount at a single point. The attachment of these two bars equalizes the pushing of one bar and pulling of the other and attaches them at a single front mount.
Ignore everything said here. The torque arm always sees twisting. Every time you go through a corner and experience body roll, or hit a bump with one side and not the other, the car sees one side go up and the other side stay the same or drop. The rubber mount is designed to allow this movement.
Also, LCAs are supposed to allow the rear end to move that way. This is why rubber is okay, rod ends are best, and poly is sub-standard for cornering. Suspension travel is GOOD, not bad.
If you have problems with one-side squat coming off the line, fix it correctly with a sway bar. The poor man's route is to use an air bag.
The triangular shape of it is for strength in stopping these rotational forces and to attach at a single one-point mount in the front.
That being said... The best "simple" design of a torque arm is to use a mount like Spohn does on the rear end and to run 2 straight parallel tubes up to a similar mount near the transmission. When you accellerate, the rotational forces of the rear end are going to pull on the upper bar and push on the lower bar.
The problem lies in the ability to strengthen the front mount to take all the rotational forces without breaking. Spohn and others take this into consideration by attaching the two bars together to mount at a single point. The attachment of these two bars equalizes the pushing of one bar and pulling of the other and attaches them at a single front mount.
Originally Posted by 2002BlackSS
It adds rigidity to the torque arm. Otherwise, it could experience torsion (twisting), which would push 1 side of the axle higher than the other (this still happens on hard launches, but it would be far worse), or it may even fail. In fact, this twisting is minimized also by the LCAs.
Also, LCAs are supposed to allow the rear end to move that way. This is why rubber is okay, rod ends are best, and poly is sub-standard for cornering. Suspension travel is GOOD, not bad.
If you have problems with one-side squat coming off the line, fix it correctly with a sway bar. The poor man's route is to use an air bag.
Last edited by Roadie; Aug 1, 2005 at 10:10 AM.
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If it helps, take a look at a set of bridge trusses. They're triangulated for strength instead of straight up and down... and they only see load in one direction (ok... you structural engineers... lay off... 
).
).
Originally Posted by Roadie
The torque arm is designed to stop the rotational forces of the rear end. It is not designed to withstand any twisting/torsion forces.
The triangular shape of it is for strength in stopping these rotational forces and to attach at a single one-point mount in the front.
...
Ignore everything said here. The torque arm always sees twisting. Every time you go through a corner and experience body roll, or hit a bump with one side and not the other, the car sees one side go up and the other side stay the same or drop. The rubber mount is designed to allow this movement.
Also, LCAs are supposed to allow the rear end to move that way. This is why rubber is okay, rod ends are best, and poly is sub-standard for cornering. Suspension travel is GOOD, not bad.
If you have problems with one-side squat coming off the line, fix it correctly with a sway bar. The poor man's route is to use an air bag.
The triangular shape of it is for strength in stopping these rotational forces and to attach at a single one-point mount in the front.
...
Ignore everything said here. The torque arm always sees twisting. Every time you go through a corner and experience body roll, or hit a bump with one side and not the other, the car sees one side go up and the other side stay the same or drop. The rubber mount is designed to allow this movement.
Also, LCAs are supposed to allow the rear end to move that way. This is why rubber is okay, rod ends are best, and poly is sub-standard for cornering. Suspension travel is GOOD, not bad.
If you have problems with one-side squat coming off the line, fix it correctly with a sway bar. The poor man's route is to use an air bag.
As I stated before, though, my whole line of reasoning was conjecture based on the geometry of the suspension and not actual numbers. I don't know how significant any lateral forces or moments are on the torque arm, though they do exist, especially if you're not going in a straight line. Thank you for your clarification though.
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I understand about keeping it from rotating and the triangular element. But I see the lower control arms as keeping the "bottom" of the axle from pushing or pulling... and a single rod fixed to the "top" of the axle and to the chassis to keep the top from pushing or pulling, this will prevent rotation. If the lca's weren't there, then i could see why the triangular shape was needed.
These are quick simple sketches, hope they make sense. But you should be able to see why axle rotation is solved in both cases. For simplicity lets say that there are rod ends at the end of each red line. This would prevent any push/pull and thus rotation and also would allow for pinion angle adjustment in either design. Also, about the second design, the location points on the chassis normally wouldn't be strong enough... but assume welded bmr subframe connectors to reinforce.
I guess what i'm saying is, I understand why the factory design is the way it is... All the parts are made out of sheetmetal and everything has rubber bushings which give and flex and wouldn't be suitable for either of these two alternative designs. I just see the factory design as the LCA's and torque arm "fighting" eachother (because of the differing radii) once the slop (rubber bushings) is taken away...
These are quick simple sketches, hope they make sense. But you should be able to see why axle rotation is solved in both cases. For simplicity lets say that there are rod ends at the end of each red line. This would prevent any push/pull and thus rotation and also would allow for pinion angle adjustment in either design. Also, about the second design, the location points on the chassis normally wouldn't be strong enough... but assume welded bmr subframe connectors to reinforce.
I guess what i'm saying is, I understand why the factory design is the way it is... All the parts are made out of sheetmetal and everything has rubber bushings which give and flex and wouldn't be suitable for either of these two alternative designs. I just see the factory design as the LCA's and torque arm "fighting" eachother (because of the differing radii) once the slop (rubber bushings) is taken away...
Originally Posted by OldeSkool
I just see the factory design as the LCA's and torque arm "fighting" eachother (because of the differing radii) once the slop (rubber bushings) is taken away...
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Originally Posted by jRaskell
I don't believe either of those designs would maintain the proper pinion angle of the axle, another job the torque arm performs.
Originally Posted by OldeSkool
For simplicity lets say that there are rod ends at the end of each red line. This would prevent any push/pull and thus rotation and also would allow for pinion angle adjustment in either design.
Im just trying to kick around ideas for alternative setups. I look at a solid rear axle and I know that all that needs to happen is for it to be able to move up and down in one plane (may be a mild arc) There are dozens of different ways to do this. 3-link, 4-link, 5-link, Z-link, etc.. I have no real intention of doing anything different to my car, just enjoying the tech discussion.
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Simple answer:
The TA can't be rigidly mounted on both ends (using the LCAs as a twist limter as you suggest) because that would result in a suspension that has 2 different lever lengths mounted rigidly...result is suspension bind. You wouldn't be able to articulate the rear suspension up and down.
What you want to do could be done if you mounted this new-fangled TA on the same bolt line as the LCAs (in effect adding a 3rd LCA to the system, but it would mount at the top of the diff case). This would remove any binding issue during suspension travel, but would result in wildly changing pinion angles as the axle articulated.
There's nothing extraneous in the OEM design. They'd love to save material, cost, and weight if they could.
The TA can't be rigidly mounted on both ends (using the LCAs as a twist limter as you suggest) because that would result in a suspension that has 2 different lever lengths mounted rigidly...result is suspension bind. You wouldn't be able to articulate the rear suspension up and down.
What you want to do could be done if you mounted this new-fangled TA on the same bolt line as the LCAs (in effect adding a 3rd LCA to the system, but it would mount at the top of the diff case). This would remove any binding issue during suspension travel, but would result in wildly changing pinion angles as the axle articulated.
There's nothing extraneous in the OEM design. They'd love to save material, cost, and weight if they could.
The way I read OldeSkools idea was that he wasn't rigidly mounting the TA at all. He said rod ends at each end of all those mounting points. He's essentially turning the TA into a single upper control arm. This would allow the suspension to articulate, but it would cause the axle to rotate as the suspension moved up and down, and as I said, I don't think this would maintain desired pinion angle.
The first setup you diagrammed OldeSkool, the axle would rotate counter-clockwise as the suspension compressed. In the second, the axle would rotate clockwise as the suspension compressed. I'm not sure how these would affect pinion angle or handling, but I am sure they both would affect both of those things in different ways. You don't have any rotation at steady state, but suspension motion induces rotation of the axle.
The first setup you diagrammed OldeSkool, the axle would rotate counter-clockwise as the suspension compressed. In the second, the axle would rotate clockwise as the suspension compressed. I'm not sure how these would affect pinion angle or handling, but I am sure they both would affect both of those things in different ways. You don't have any rotation at steady state, but suspension motion induces rotation of the axle.
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Originally Posted by jRaskell
The way I read OldeSkools idea was that he wasn't rigidly mounting the TA at all. He said rod ends at each end of all those mounting points. He's essentially turning the TA into a single upper control arm. This would allow the suspension to articulate, but it would cause the axle to rotate as the suspension moved up and down, and as I said, I don't think this would maintain desired pinion angle.
The first setup you diagrammed OldeSkool, the axle would rotate counter-clockwise as the suspension compressed. In the second, the axle would rotate clockwise as the suspension compressed. I'm not sure how these would affect pinion angle or handling, but I am sure they both would affect both of those things in different ways. You don't have any rotation at steady state, but suspension motion induces rotation of the axle.
The first setup you diagrammed OldeSkool, the axle would rotate counter-clockwise as the suspension compressed. In the second, the axle would rotate clockwise as the suspension compressed. I'm not sure how these would affect pinion angle or handling, but I am sure they both would affect both of those things in different ways. You don't have any rotation at steady state, but suspension motion induces rotation of the axle.
- This does result in a single upper control arm, but doesn't have to result in pinion angle changes, it just depends on where you fix the front of the new upper control arm. Consider the front suspension, you've got an upper and lower and by choosing the right length of the 2 arms and the right mounting location you can design a suspension travel that has minimal camber change...this is analogous to a ladder bar suspension, but with only one upper arm which would probably be a strength issue.
- I stand corrected @ there being suspension bind, I don't think that was right, but both ends of this new upper control arm (revised torque arm) has to be able to articulate.
