Demon Z28
08-08-2001, 06:07 PM
First of all, I'd like to setup a frame of reference for where the stock setup is..
The Stock setup for 3rd/4th-gen cars is actually not so bad (bind-wise) as some other solid axle designs. Fox+ Mustangs and older A/G bodies represent an awfully good example of how NOT to do it.. The Stock F-body setup gets around any bind issues through the use of soft bushings, and semi-flexible parts. The front A-arms are stamped steel too, but you don't see them flexing that much....
Now, you're probably wondering WHAT bind is... Suppose the springs/shocks weren't there, and you had the ability to move the suspension around on it's mounts without their resistance. The only resistance you would feel, would be any created by bushing friction, and any bind in the system. Bind is caused when the arcs of travel that the links in the suspension act within apply forces away, or into, each other.
F-bodies have what's referred to as a 3-link suspension design. Those 3 links are the two LCAs, and the torque arm. It's actually one of the simpler designs, and is really hard to screw it up. However, economics has a really good ability to make factories do just that. If you think of the 3 links separately, you'll see that each link has an arc that it travels in as the axle moves. For now, just imagine the axle is moving up and down while level. Since the LCA's are shorter than the torque arm, their arc is sharper than that of the torque arm. So, what happens is that, as the LCAs travel further from center, the arcs get further apart. This would cause bind, but GM saw that one, and made the nose of the torque arm slide forward/rearward in it's rubber mount.
If you got this far, you're better than most. There are several aftermarket torque arms that mount solidly to a crossmember with no fore/aft movement possible. (yuk)
Stay with me, 'cuz I'm gonna get 3D on ya...
Now, what happens when the axle move up and down, and it's NOT level? All hell breaks loose that's what... It's when the axle is tilted compared to the body that most of the problems start. Those same 3 links become major sources of bind, and the net effect of all three is the same as suddenly increasing the rear spring rate on whichever side the body is tilting toward.
When the axle tilts, it tries to rotate (twist) the LCA's and the torque arm to accomodate. As long as the stock parts and bushings are there, there's enough "give" to do it. But, if you put Polys at both ends of tubular LCA's and torque arms, it can no longer do this. The polys don't give much at all, and the tubular parts give even less. If it can't twist the parts, that load gets transferred to the body. The counter-reaction is applied to the axle, and goes more towards the side in the same direction as the tilt. This adds so much resistance to movement, that it acts like you just doubled, or tripled, the rear spring rate.
What happens when you put stiffer springs in the back? Right, you get oversteer. In this case, it's not constant, and it's combined with the rather nasty tendency to lift the inside rear wheel. The big problem with it, is that, as a driver, you don't know WHEN it's coming, nor can you do much about it when it does happen, because it's so sudden, and such a large change. What you CAN do, is make the car understeer more under normal conditions in a vain attempt to make this change more controllable, but that will simply make you slower.
Surprisingly, the panhard bar introduces very little to this. It's long enough such that it's arc of travel doesn't have much time to do anything since F-bodies don't have that much suspension travel in the first place. The important part it plays is in it's placement relative to the ground. Alas, we're getting into roll centers there, and that's gonna have to be a topic of another discussion.
Spohn's torque arm actually doesn't "require" the front rod-end since he uses the telescopic tubes under the rubber boot. Not only does it accomodate the fore/aft movement needed during level axle movement, it accomodates the "twist" needed when the axle tilts. I got the rod-ended one because it's a race car, and race cars have rod-ends dammit....
The LCA's should have at least one end of the arm rod-ended, and that end should be the body end. You can use polys at the axle end since one rod-end has enough movement in it to allow the "twist" that the LCA needs when the axle tilts.
All of this is assuming you want the car to "handle". Mine is an autocross car, so handling is most important, even over power. If you're drag racing, you might even WANT the bind that happens when the axle tilts because the bind resists body movement, attempting to keep the body level, kinda like a rear swaybar does. Just BE AWARE that, unlike a rear swaybar, bind loads the spring that bears that load. That will, by extension, load that side's tire accordingly.
Along with that, comes the torque-arm's length. The stock arm is long, not because that is the optimal length, but because it's cheaper to mount it to the tranny, instead of it's own mount somewhere in the middle.
For drag racing, the optimal length places the front torque arm mount wherever a line drawn from the CG to the center of the rear tire's contact patch crosses the floorpan. On most F-bodies, the CG is about camshaft height, and at the firewall. This makes the optimal length for drag racing to be about 30-32 inches.
For handling, there a BIG gotcha in there... It's called brakes. 31-33 inches for a torque arm is too short, and because the rear brakes have the opposite effect the gas pedal does on a torque arm, the rear brakes induce wheelhop on braking. So the idea is to go somewhere in between.
The stock mount type aftermarket torque arms are also stock length, so, other than strength, there's not much to gain there.
BMR (the track pak design) and Spohn's torque arms are positioned more in between stock, and drag racing optimal. The BMR unit has no provision for a "twisting" motion in it's design, and it's mounted with poly, so bind will be inherent with it if it's used on a handling car. So, Spohn's arm got the nod....
For drag racing optimal, Jeg's makes a 31.5" long torque arm that looks like a good setup for maximizing force on the rear tires. Just don't use killer rear brakes with it.
Herb Adams used to make what was called a de-coupled torque arm that had TWO arms. One was short and the other was pretty long. The short one worked when accelerating, and the long one took over when braking. I don't know whatever happened to it, though it looked rather expensive to make.
WHEW!
I hope I clarified SOMETHING in all that...
By ~ CPCamaro
The Stock setup for 3rd/4th-gen cars is actually not so bad (bind-wise) as some other solid axle designs. Fox+ Mustangs and older A/G bodies represent an awfully good example of how NOT to do it.. The Stock F-body setup gets around any bind issues through the use of soft bushings, and semi-flexible parts. The front A-arms are stamped steel too, but you don't see them flexing that much....
Now, you're probably wondering WHAT bind is... Suppose the springs/shocks weren't there, and you had the ability to move the suspension around on it's mounts without their resistance. The only resistance you would feel, would be any created by bushing friction, and any bind in the system. Bind is caused when the arcs of travel that the links in the suspension act within apply forces away, or into, each other.
F-bodies have what's referred to as a 3-link suspension design. Those 3 links are the two LCAs, and the torque arm. It's actually one of the simpler designs, and is really hard to screw it up. However, economics has a really good ability to make factories do just that. If you think of the 3 links separately, you'll see that each link has an arc that it travels in as the axle moves. For now, just imagine the axle is moving up and down while level. Since the LCA's are shorter than the torque arm, their arc is sharper than that of the torque arm. So, what happens is that, as the LCAs travel further from center, the arcs get further apart. This would cause bind, but GM saw that one, and made the nose of the torque arm slide forward/rearward in it's rubber mount.
If you got this far, you're better than most. There are several aftermarket torque arms that mount solidly to a crossmember with no fore/aft movement possible. (yuk)
Stay with me, 'cuz I'm gonna get 3D on ya...
Now, what happens when the axle move up and down, and it's NOT level? All hell breaks loose that's what... It's when the axle is tilted compared to the body that most of the problems start. Those same 3 links become major sources of bind, and the net effect of all three is the same as suddenly increasing the rear spring rate on whichever side the body is tilting toward.
When the axle tilts, it tries to rotate (twist) the LCA's and the torque arm to accomodate. As long as the stock parts and bushings are there, there's enough "give" to do it. But, if you put Polys at both ends of tubular LCA's and torque arms, it can no longer do this. The polys don't give much at all, and the tubular parts give even less. If it can't twist the parts, that load gets transferred to the body. The counter-reaction is applied to the axle, and goes more towards the side in the same direction as the tilt. This adds so much resistance to movement, that it acts like you just doubled, or tripled, the rear spring rate.
What happens when you put stiffer springs in the back? Right, you get oversteer. In this case, it's not constant, and it's combined with the rather nasty tendency to lift the inside rear wheel. The big problem with it, is that, as a driver, you don't know WHEN it's coming, nor can you do much about it when it does happen, because it's so sudden, and such a large change. What you CAN do, is make the car understeer more under normal conditions in a vain attempt to make this change more controllable, but that will simply make you slower.
Surprisingly, the panhard bar introduces very little to this. It's long enough such that it's arc of travel doesn't have much time to do anything since F-bodies don't have that much suspension travel in the first place. The important part it plays is in it's placement relative to the ground. Alas, we're getting into roll centers there, and that's gonna have to be a topic of another discussion.
Spohn's torque arm actually doesn't "require" the front rod-end since he uses the telescopic tubes under the rubber boot. Not only does it accomodate the fore/aft movement needed during level axle movement, it accomodates the "twist" needed when the axle tilts. I got the rod-ended one because it's a race car, and race cars have rod-ends dammit....
The LCA's should have at least one end of the arm rod-ended, and that end should be the body end. You can use polys at the axle end since one rod-end has enough movement in it to allow the "twist" that the LCA needs when the axle tilts.
All of this is assuming you want the car to "handle". Mine is an autocross car, so handling is most important, even over power. If you're drag racing, you might even WANT the bind that happens when the axle tilts because the bind resists body movement, attempting to keep the body level, kinda like a rear swaybar does. Just BE AWARE that, unlike a rear swaybar, bind loads the spring that bears that load. That will, by extension, load that side's tire accordingly.
Along with that, comes the torque-arm's length. The stock arm is long, not because that is the optimal length, but because it's cheaper to mount it to the tranny, instead of it's own mount somewhere in the middle.
For drag racing, the optimal length places the front torque arm mount wherever a line drawn from the CG to the center of the rear tire's contact patch crosses the floorpan. On most F-bodies, the CG is about camshaft height, and at the firewall. This makes the optimal length for drag racing to be about 30-32 inches.
For handling, there a BIG gotcha in there... It's called brakes. 31-33 inches for a torque arm is too short, and because the rear brakes have the opposite effect the gas pedal does on a torque arm, the rear brakes induce wheelhop on braking. So the idea is to go somewhere in between.
The stock mount type aftermarket torque arms are also stock length, so, other than strength, there's not much to gain there.
BMR (the track pak design) and Spohn's torque arms are positioned more in between stock, and drag racing optimal. The BMR unit has no provision for a "twisting" motion in it's design, and it's mounted with poly, so bind will be inherent with it if it's used on a handling car. So, Spohn's arm got the nod....
For drag racing optimal, Jeg's makes a 31.5" long torque arm that looks like a good setup for maximizing force on the rear tires. Just don't use killer rear brakes with it.
Herb Adams used to make what was called a de-coupled torque arm that had TWO arms. One was short and the other was pretty long. The short one worked when accelerating, and the long one took over when braking. I don't know whatever happened to it, though it looked rather expensive to make.
WHEW!
I hope I clarified SOMETHING in all that...
By ~ CPCamaro