C6 Rear Suspension Geometry lesson sought
10-25-2007, 08:02 PM
#21
Le Mans Master
Thread Starter
Well put, glass slipper. The stock C2/C3 rear suspension most definitely does not have anti-squat. ...and, don't get me started about rear toe-steer. 
The thing that got me motivated to look more deeply into the C6 suspension in the first place is that it obviously does so much right. It can't be directly related, but understanding the C6 better may help me decide on how to incorporate future mods into my old ox cart. What can I say, I love it anyway.
The thing that got me motivated to look more deeply into the C6 suspension in the first place is that it obviously does so much right. It can't be directly related, but understanding the C6 better may help me decide on how to incorporate future mods into my old ox cart. What can I say, I love it anyway.
10-25-2007, 09:00 PM
#22
3rd Gear
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Good job, you solved the mystery!
As somebody already mentioned, the angle of the lower A-arm relative to the upper A-arm produces anti-squat. Anti-squat is the rear suspension's reaction to the torque imparted to the wheels during acceleration.
Here's a diagram to calculate anti-squat in a given rear suspension:
The red lines are the upper and lower trailing arms, if you extend those lines forward (the blue lines), they will intersect. The vertical green line (through the white circle) is the center of the front axle and the horizontal line (through the white circle with the cross) is at the height of the center of gravity. The orange line drawn from the bottom of the rear tire through the intersection of the green lines (we'll call that point A) is the 0% anti-squat line. Anything below that line and the rear suspension will multiply the torque reaction while anything above the line will reduce the torque reaction or provide anti-squat. The light tan line drawn from the bottom of the rear tire through the intersection of the blue lines and continuing on to intersect the vertical green line (we'll call that point B) gives the amount of anti-squat by the following calculation: (B-A)/A*100= C where C is anti-squat in %.
C2 and C3 Corvettes with the single trailing arm have no anti-squat capability and depend solely on the spring to counter the torque reaction on acceleration. If you've ever sat in a C2/3 and put it in gear or goosed the throttle (in an automatic), you've felt the rear end drop when the spring absorbed the torque. The big blocks have stiffer springs because of their higher torque and as a result, the ride suffered.
The C4s incorporated anti-squat geometry which allowed lower spring rates and a more comfortable ride. But because of the trailing arm arrangement, as you lower the car or as it squats on acceleration, it changes the angle of the arms in the direction which causes a reduction in anti-squat. Anti-squat is desired from a road racing perspective so the chassis doesn't get upset (suspension movements are minimized) when loaded or unloaded by the engine. Weight transfer is desired from a drag racing perspective...they are two different things.
The C5/6 Corvettes went to a double wishbone rear suspension. It still has anti-squat but you use the line drawn through the pivot points of the upper and lower control arms to establish the red lines in the drawing above. A couple of advantages to this arrangement are: 1) as you lower the car, anti-squat increases (the center of gravity, point A decreases while point B remains the same distance above A), 2) as the rear squats on acceleration, anti-squat increases (the intersection of the two blue lines is higher). With the stiffer chassis of the C5/6 and the increased anti-squat, spring rates are reduced for a better ride and each wheel is allowed to react to bumps instead of the chassis "reacting" to it. Go to an autocross and you'll see the difference between the C2/3/4/5/6 (I'm talking stock to stock). The C2/3s are wallowing all over the place with the front and rear suspension jacking up/down on acceleration/braking and bumps upsetting the chassis. The C4s don't wallow and you don't get much jacking of the suspension, but bumps will upset the chassis. Now watch the C5/6s, no wallowing, not much jacking, and you'll see the wheels go over the bumps without upsetting the chassis (unless they're big bumps)...very refined.
As somebody already mentioned, the angle of the lower A-arm relative to the upper A-arm produces anti-squat. Anti-squat is the rear suspension's reaction to the torque imparted to the wheels during acceleration.
Here's a diagram to calculate anti-squat in a given rear suspension:
The red lines are the upper and lower trailing arms, if you extend those lines forward (the blue lines), they will intersect. The vertical green line (through the white circle) is the center of the front axle and the horizontal line (through the white circle with the cross) is at the height of the center of gravity. The orange line drawn from the bottom of the rear tire through the intersection of the green lines (we'll call that point A) is the 0% anti-squat line. Anything below that line and the rear suspension will multiply the torque reaction while anything above the line will reduce the torque reaction or provide anti-squat. The light tan line drawn from the bottom of the rear tire through the intersection of the blue lines and continuing on to intersect the vertical green line (we'll call that point B) gives the amount of anti-squat by the following calculation: (B-A)/A*100= C where C is anti-squat in %.
C2 and C3 Corvettes with the single trailing arm have no anti-squat capability and depend solely on the spring to counter the torque reaction on acceleration. If you've ever sat in a C2/3 and put it in gear or goosed the throttle (in an automatic), you've felt the rear end drop when the spring absorbed the torque. The big blocks have stiffer springs because of their higher torque and as a result, the ride suffered.
The C4s incorporated anti-squat geometry which allowed lower spring rates and a more comfortable ride. But because of the trailing arm arrangement, as you lower the car or as it squats on acceleration, it changes the angle of the arms in the direction which causes a reduction in anti-squat. Anti-squat is desired from a road racing perspective so the chassis doesn't get upset (suspension movements are minimized) when loaded or unloaded by the engine. Weight transfer is desired from a drag racing perspective...they are two different things.
The C5/6 Corvettes went to a double wishbone rear suspension. It still has anti-squat but you use the line drawn through the pivot points of the upper and lower control arms to establish the red lines in the drawing above. A couple of advantages to this arrangement are: 1) as you lower the car, anti-squat increases (the center of gravity, point A decreases while point B remains the same distance above A), 2) as the rear squats on acceleration, anti-squat increases (the intersection of the two blue lines is higher). With the stiffer chassis of the C5/6 and the increased anti-squat, spring rates are reduced for a better ride and each wheel is allowed to react to bumps instead of the chassis "reacting" to it. Go to an autocross and you'll see the difference between the C2/3/4/5/6 (I'm talking stock to stock). The C2/3s are wallowing all over the place with the front and rear suspension jacking up/down on acceleration/braking and bumps upsetting the chassis. The C4s don't wallow and you don't get much jacking of the suspension, but bumps will upset the chassis. Now watch the C5/6s, no wallowing, not much jacking, and you'll see the wheels go over the bumps without upsetting the chassis (unless they're big bumps)...very refined.
10-25-2007, 09:54 PM
#23
Le Mans Master
Thread Starter
For one, apparently the transverse leaf can't completely isolate the left and right from each other. But, even if all else was equal C/O's make ride height adjustments a snap, and if you follow thru with double or 4-way adjustables, as IMHO should be the case for anyone spending such serious money, a finer degree of tuning can be accomplished than with the leaf and separate damper. That's my take, anyway. Gentlemen...
10-25-2007, 10:07 PM
#24
Well put, glass slipper. The stock C2/C3 rear suspension most definitely does not have anti-squat. ...and, don't get me started about rear toe-steer.
The thing that got me motivated to look more deeply into the C6 suspension in the first place is that it obviously does so much right. It can't be directly related, but understanding the C6 better may help me decide on how to incorporate future mods into my old ox cart. What can I say, I love it anyway.
The thing that got me motivated to look more deeply into the C6 suspension in the first place is that it obviously does so much right. It can't be directly related, but understanding the C6 better may help me decide on how to incorporate future mods into my old ox cart. What can I say, I love it anyway.
10-25-2007, 10:46 PM
#25
The age old debate continues...
For one, apparently the transverse leaf can't completely isolate the left and right from each other. But, even if all else was equal C/O's make ride height adjustments a snap, and if you follow thru with double or 4-way adjustables, as IMHO should be the case for anyone spending such serious money, a finer degree of tuning can be accomplished than with the leaf and separate damper. That's my take, anyway. Gentlemen...
For one, apparently the transverse leaf can't completely isolate the left and right from each other. But, even if all else was equal C/O's make ride height adjustments a snap, and if you follow thru with double or 4-way adjustables, as IMHO should be the case for anyone spending such serious money, a finer degree of tuning can be accomplished than with the leaf and separate damper. That's my take, anyway. Gentlemen...
The leaf spring is actually designed to allow one side to affect the other. By putting the spring in two mounts that allow pivoting, when you go around a corner and the outer wheel compresses that side of the spring, it bends the middle section into an "S" shape via the mount on that side. The mount on the other side resolves the forces from the "S" bend causing the spring on side of the inner wheel to go up. In other words, by virtue of the dual pivoting mounts, GM is able to incorporate roll resistance into the spring. Thus, they are able to use smaller (as in lighter) anti-roll bars by making the spring do double duty. Also, anti-roll bars tie the left and right sides together already so they are never isolated...if it's a bad thing that the spring doesn't isolate the two sides then we need to disconnect the anti-roll bars.
The C6 leaf springs are just as easy to adjust ride height on, maybe easier since you don't have to take off the wheel. You can use a C/O shock in a leaf spring application but without the C/O spring to get the same level of control and the single transverse leaf spring weighs substantially less than two coil springs. The weight of the lighter leaf spring is lower on the car having a small impact on CG.
When somebody comes up with a way to move the leaf spring frame mounts, spring rates will be able to be adjusted also.
The only advantage to coil springs is they are cheaper and readily available in any spring rate/free height you want. This allows the serious/professional racer to fine tune the car to specific tracks. If you had a coil spring and a leaf spring that had the exact same wheel rate, you wouldn't be able to tell a performance difference except the coil spring car would weigh more. The problem is, there aren't a lot of choices in spring rates when it comes to transverse leaf springs.
GM did a lot of research on transverse leaf spring before the C4 came out and came to the conclusion that transverse leaf springs gave better performance but cost more. You can bet GM would not use a more expensive approach if it gave less performance.
Next....
10-25-2007, 11:09 PM
#26
Le Mans Master
Thread Starter
FYI, I have no rear bar on my shark, which is heavily sprung. If/when I ever go coil-overs, I won't be surprised if adding one (of the correct rate, of course) becomes necessary. I can't speak to newer generations, as I never even bothered to tinker with the suspension on my '92, but changing from a 40# multi-leaf on a problematic suspension to C/O's with Guldstrand's 5-bar looks pretty sweet for a C2/C3. I'm a fish out of water on these newfangled marvels...
10-26-2007, 01:38 AM
#27
How do you do this? Put a bottle jack under the side of the spring you want to adjust, then jack up the spring and turn the ****? All this time I've been taking the wheels off.
