'09 Ram: All-Coil Suspension

[565]

The issue I see isn't necessary one side affecting the other - I'm making the assumption that the transverse leaf is bolted stiff enough in the center such that there is a stress discontinuity (no "cross talk"), and hence it acts as two separate springs. If this is not the case, the Corvette suspension is a real mess.

The issue is that on the same side, half the spring is being asked simultaneously to counteract the affects from body motion (torsion) and wheel motion (bending). I will also add however the Corvettes still have sway bars, but they are smaller than you'd expect.

Two different stresses with two different consequences on suspension performance. It's too much compromise IMO.

You've completely misunderstood how the transverse leaf springs on the Corvette work. The springs only bend along one axis of motion, there is no seperate bending or torsion axis.  The problem you speak of doesn't exist.

There are no two seperate motion tracks under the control of the leaf spring.  The double A arm suspension only allows one track of motion for the assembly.  Put in a coil spring and it still follows that same track of motion as it articulates.  Remove any sort of spring all together and a double wishbone setup will still follow that same track of articulation.  All motion of the double wishbone suspension occurs along this single track of motion, it doesn't matter what kind of spring is attached.  The transverse leaf spring is arranged so that motion along this track carved by the double wishbone suspension is translated into a single bending axis in the leaf spring.  As there is only one motion axis in the double wishbone suspension, the leaf spring only bends in one axis under any circumstance.  To suggest that somehow the up and down motion of the double wishbone suspension due to cornering and the up and down motion of the suspension due to bumps results in a different track of motion carved by the double wishbone suspension setup is false.

The effect of transverse leaf springs acting as antiroll bars is a completely seperate effect.  For anything to act as an antiroll bar it obviously has to somehow link the two sides of the suspension together, it's obviously silly to suggest that it wouldn't.  What antiroll bar works by not linking the two seperate sides of the independent suspension together?  That's right, none.  The anti-roll bar inherently sacrfices suspension independence for roll stiffness, it's a balance that every manufacture must strike.

The single transverse leaf spring of the Corvette does indeed transfer motion between the two sides of the IRS.  If you look closely at the Corvette's transverse leaf spring, you'll see that it's not held flat but actually arched.  This is also why the Corvette uses a large single transverse leaf spring instead of two smaller seperate leaf springs.  The single transverse leaf spring transmits motion from one side of the suspension to the other via changes to degree of arch.  When the wheel goes up on one side, the arch is reduced and the wheel goes up on the other side as well.  When the wheel goes down on one side, the arch is increased and the wheel goes down on the other side as well.  Now I'm sure all you transverse leaf spring haters will gasp and point and say this is terrible.  But it is exactly the same case as with an antiroll bar.  When the wheel goes up on one side the antiroll bar transmitts that upward motion to the other side of the suspension. 

So which is better, using an antiroll bar or having the transverse leaf spring do the motion transfer?  Well lets think of it this way.  GM could have saved quite a lot of leaf spring material if it made two seperate leaf springs without the long linker in the middle.  And it's not like they saved money by eliminating a roll bar all together because they still had to include a softer roll bar.  And since the diameter of a stiffer roll bar only adds tiny amounts to the sum, I'm sure GM's setup wasn't to save money.  In fact it would have been trivial for GM to make a totally independent leaf spring setup and relied totally on the antiroll bar. 

But the antiroll bar isn't exactly sophisticated.  In fact the torsion bar is probably one of the oldest and most primitive forms of motion absorption ever imployed.  It's just a large rod of metal that twists.  There is no control or sophistication in it's motion, which is why few cars still use a torsion bar as the main spring of their suspension.  However the torsion bar remains in the antiroll bar setup because it's one of the few motion absorption setups that can be used in across such a long distance in such a tight space with low weight, and low cost (it's just a rod of metal).  It's evident that GM tried to take as much work away from the antiroll bar as possible onto a motion absorption device that was designed to give far more control (the composite leaf spring).  Only because getting enough roll stiffness would have required leaf springs that were too stiff did GM also add an anti-roll bar.

The take home messege is this.  If GM wanted to make the antiroll bar the sole source of antiroll stiffness, it would have been the simplest thing in the world, just mount seperate leaf springs and use thicker rods  Hell I could do it right now by going downstairs and sawing the leaf spring in half.  Obviously they spent extra money and effort to keep as much roll resistance off the antiroll bar as they could.



Edit:  So apparently older generations of Corvettes had their leaf springs mounted tight, which did not allow it to act as an antiroll bar.

http://en.wikipedia.org/wiki/Corvette_leaf_springs

"A single, perfectly tight center mount that held a small center section of the spring flat against the frame would isolate one side of the spring from the other. No roll or anti-roll effect would appear. The rear spring of the C2, C3, and C4 has this type of mount, which effectively divides the spring in two. It becomes a quarter-elliptic spring.

Since the C4, the Corvette has had widely-spaced double mounts on the front. The rear spring has had double mounts since the C5. The spring is allowed to pivot about these two points. When the suspension compresses and the end of the leaf is pulled up, the center of the leaf spring between the two mounts moves down. This in turn reduces the spring force on the wheel on the opposite side of the car. In this way, the leaf acts like an anti-roll bar."


So basically GM could easily have a totally independent leaf spring setup and relied completely on the antiroll bars, as they'd been doing it for decades on older cars.  GM especially developed mounts that allowed the leaf spring to pivot because they wanted to take as much antiroll duty off the antiroll bar as possible. I'm sure they didn't do this for shits and giggles.

[GoCougs]

Perhaps I am wrong as the torsion due to body sway, however A-arms (and I'd bet the spring) are not rigidly mounted in the vertical axis (bushings) - granted, movement isn't signficant, but there will be a bit of torsion.

Now as to data regarding side-to-side interaction - wow. At least with a more conventional setup, there are three different springs with signicant damping between each - here it's still one spring doing the work typically handled by three - and to me that still speaks of harmonics issues.

I still see massive compromises in an effort to hold onto a legacy.

[S204STi]

Edit:  So apparently older generations of Corvettes had their leaf springs mounted tight, which did not allow it to act as an antiroll bar.

http://en.wikipedia.org/wiki/Corvette_leaf_springs

"A single, perfectly tight center mount that held a small center section of the spring flat against the frame would isolate one side of the spring from the other. No roll or anti-roll effect would appear. The rear spring of the C2, C3, and C4 has this type of mount, which effectively divides the spring in two. It becomes a quarter-elliptic spring.

Since the C4, the Corvette has had widely-spaced double mounts on the front. The rear spring has had double mounts since the C5. The spring is allowed to pivot about these two points. When the suspension compresses and the end of the leaf is pulled up, the center of the leaf spring between the two mounts moves down. This in turn reduces the spring force on the wheel on the opposite side of the car. In this way, the leaf acts like an anti-roll bar."


So basically GM could easily have a totally independent leaf spring setup and relied completely on the antiroll bars, as they'd been doing it for decades on older cars.  GM especially developed mounts that allowed the leaf spring to pivot because they wanted to take as much antiroll duty off the antiroll bar as possible. I'm sure they didn't do this for shits and giggles.

First, excellent post.  I would add however that corvette suspension prior to the C4 used the rear CV axles effectively as the upper control arms.  You can imagine the traction issues created by that.  So while it might seem that the suspension took a step back with the revised spring mounting, on the whole it moved forward exponentially.

Would it still benefit from the original, isolated leaf spring mounting?  Probably.