AtlBlkZ06

Hello all,

MY friend was trying to convince me that I absolutely need coilovers, and somewhere in the argument he mentioned that the stock suspension transfers motion like a live axle, because the suspension is one big piece going from end to end.

I said his logic was flawed because the suspension has two fulcrum points at each end which means the two sides inflect independently, though there will be some slight interaction as spring rates may vary depending on the position of the other side (true?)

I think the stock springs have a stigma attached to em that makes people think they're junk just because they're a different version of old-skool.

Please post your thoughts.

z06vin

The Z06 suspension sounds archaic & "weak" but it WORKS GREAT IN REALITY. So unless you are a hardcore racer why bother? The Z06 has a great street ride as it is too.

6Speeder

If the fulcrum points were hard mounted, no "give" then there would be no transfer whatsoever, it would be completely independent. With a street suspension there could be some transfer, but to where? It would transfer a very small amount of energy to the center of the spring, not to the other side's suspension. For all intents one side of the leaf spring is as isolated from the other side as coilovers are. Any real transfer would be the same, coilovers or leafs, from one side's suspension to the chassis, and then from the chassis to the other side. Samo-samo

mstan

The transverse leaf spring is an elegant design that is totally acceptable for any application where you do not need to adjust quickly. eg street. The strenth of coilovers is easy adjustability to racetrack conditions.

wishihad1-2

Sorry, a bit late to this thread but...
Actually the leaf spring does transfer forces across the car. But before I go into that lets get some terminology straight. IRS refers to geometrically independent rear suspension. That moving the left wheel does not cause a change in the caster, camber or toe curves for the right wheel. If you think about it, on a car with a live axle when the left wheel moves up, the right wheel has to camber out because the two wheels have to stay in line.

The Corvette doesn't do that. The camber, caster, and toe for each wheel is solely dependent on how compressed the suspension is on that side of the car. That is what we SHOULD think of when we talk about independent suspension.

However, some people think "independent" means no forces transfer. Well that's just not the case. I can't think of a single late model performance car that doesn't transfer loads across the car's suspension. Really, how many cars do you know of without anti-roll bars? We actually want that communication between the left and right wheels. The anti-roll bar is what does that communicating. I

Without an anti-roll bar, the force required to compress the right wheel would be independent of the force required to compress the left wheel. However the anti-roll bar installed on all those cars in the world does exactly the opposite. When the left wheel is pushed up, it makes it easier to push the right wheel up. In short it causes communication across the two rear wheels. All of the Civic fan bois that bash the Corvette for "not having IRS" simply don't understand that their Civic's have the same "cross talk".

Now what makes the leaf spring as used on the Corvette special is the leaf spring can do part of the anti-roll bar's job. Not only does it "cross talk", it does it by design. Why? Well the leaf spring is very light. GM was hoping with the C4 to not need anti-roll bars at all. In the end they needed them but they could be smaller and lighter because they were helped by the leaf spring.

Basically, all the stuff about the leaf spring being a bad design and having cross talk is from people who don't know any better. I remember a lot of people saying "I told you so" when the Z06 suspension was considered skittish. How did GM fix the issue? Well a new set of shocks but no change to the springs.

Here's some good reading on how the leaf spring works in the Corvette.
http://temp.corvetteforum.net/c4/gcr...ass_spring.gif

John Shiels

My VBP leafs work great at the track. Haven't seen a coilover Vette do better.

glass slipper

Thank you, thank you, thank you!!! I get so sick of that "crosstalk" term when people refer to the Corvette transverse composite monoleaf spring. I always tell them if "crosstalk" is a bad thing, you better unhook your anti-roll bar because it does the exact same thing. Supposedly, DRM started the "crosstalk" BS term years ago as a way of marketing his racing coilovers for the street on the C4 platform. Of course since the rear of a C4 doesn't have the two wide mounts for the spring to allow it to bend into an "S" and provide roll resistance/"crosstalk", I wonder why DRM marketed the coilovers for the rear.

DRM has a post in another thread to "demonstrate" the "crosstalk" with pictures even.
All it accomplished was proving how the factory design provides roll resistance via a novel design but he still insisted on saying it was a bad thing. It was so embarrassing from a technical standpoint, I didn't bother to respond. I wish that "crosstalk" term would die out, it is the single worst misnomer I've ever seen.

I'm not a coilover hater as they have their place. They are a much cheaper way to get racetrack spring rates and be able to "tune" the car for a particular track with different/cheap coil springs. (Priced a monoleaf lately.) But be prepared for a much stiffer ride, there are no free lunches. Some people like it, some people are surprised (nobody told them it would ride like a buckboard) and don't like it. A coilover setup with the same spring rate as the stock monoleaf spring versus a stock car/spring with the same shock from the coilover setup will lose everytime.

wishihad1-2

I will not defend a company I know little about but…

The crosstalk term MIGHT be correct with respect to the C4 rear end. I’m really not that familiar with the C4 rear end, I’ve only ever looked at it in pictures. However, here is how the crosstalk MIGHT be applied to the rear of that car.

If I recall, the C4 rear leaf is mounted directly to the dif housing. That housing is then mounted to the chassis. The two trailing links are mounted directly to the chassis. I think the toe link and the one lateral link are mounted to the dif housing. Of course the half shafts which make up the other suspension link are mounted to the housing.

Let’s just assume the spring is 100% securely attached to the housing. That means the left and right side act as if they were two completely separate springs. So looking at the car from the back we compress the driver’s side wheel. That causes the leaf to flex up. That in tern causes the dif housing to want to rotate clockwise as we look from the rear bumper. Well unless the dif housing is 100% rigidly attached to the body the housing will want to rotate at least a little bit clockwise.

That does two things. First, it slightly moves the inboard U-joints. Well those inboard U-joints are the pivot points for a set of lateral suspension arms. We would rather they not shift and move too much. Second, it will try to force the other side of the spring to move down. This is exactly the opposite of what we get with that whole “S-bend” in the front end (and both ends of the C5 and C6). Moving to coils would take that twist off the dif housing. In this way removing the loads of the leaf spring could (not saying it does) improve handling. However, it’s not “crosstalk” via the spring that’s to blame. It’s actually the fact that the leaf springs transfer their loads into the dif housing rather than the chassis. In a sense, it’s a problem due to a flexible spring mount, not the spring.

However, I can not say how real a problem this actually is with the C4. It is NOT an issue with the C5 and C6 as all the suspension loads are fed into the suspension subframes and chassis. The C5 and C6 most definitely have “crosstalk” because of the leaf. It’s just that it’s a good thing. If you think there is too much just get a softer rear anti-roll bar.

Regrettably, there is a lot of junk engineering used to sell automotive products to those who simply don’t understand the details.

GMuffley

Thank you for the informative post.

TTRotary

The other bad rap comes from the Leaf spring" term, a misnomer which evokes steel buggy springs, hopped-up '72 chevelles, and traction bars. Composite transverse monospring (CTM) is a better term.

In addition to what has been well said already, the CTM is also lighter in total than a comparable pair of steel springs, results in much less unsprung weight, and has better elasticity (the ability to return to the original shape). What it is not is cheaper.

LTC Z06

A very good write up.

wishihad1-2

Most certainly. To use the conventions we apply to other cars, the Corvette has double A arm suspension all around. I don't say a car has coil spring suspension. That term wouldn't tell me much. Coil spring suspension could include struts, double A arm, or even solid axles. The Tahoe is one of many vehicles with a multi-link, coil spring supported, live axle. We say it uses a live axle because that seems to be the most defining part of the suspension system. We don't call that "coil spring" suspension and people wouldn't confuse it with the coil spring suspension used on a 911.

However, for what ever reason people pick up on the leaf part of the Corvette's suspension and ignore the double A arm part.

petefias

Before anyone gets euphoric about the benefits or disadvantages of crosstalk think when do you want it or don't want crosstalk.

In a turn that is smooth you want it, to keep the car flat. In a turn that has a bump in it, probably not...

A coil over setup will work better by separating these functions, lets the anti-roll bar do its job and the springs do theirs. Anti-roll bars actually are a compromise between handling and comfort, they simply allow softer springs to be used for greater comfort.

I'm convinced DRM and others sell coilovers because it works. Do the C5R, C6R have leaf springs? It's been announced by GM engineers that the leafs on the C4 were used primarily for packaging reasons. End of story.

I remember people here were swearing up and down that crosstalk simply did not exist. Now that someone simply demonstrated that it exist now it is a good thing. Come on now... I never doubt it that it existed and don't believe it is a good thing. If it was a good thing all racers would have leaf springs.

I'm afraid this is turning into just another leaf vs. coil debate. If you're happy with the standard setup please keep it and enjoy it.

John Shiels

GM has a boat load of engineers and equipment to monitor everything. They can win a race by 6" so they look for the last 1". People go from a soft stock suspension and crummy and worn stock shocks to a stiffer coil-overs and think it was all the coil-over that increased the performance. My car has been faster than any cars I have seen over the rough or smooth portions of the track from low to high speeds.

Are coil-overs bad no. Can you swap spring rates cheaper yes. If you change the rates to much you need to change valving. How many here have the knowledge and equipment to analyize their shocks and springs?

wishihad1-2

As I never swore the crosstalk didn't exist that is irrelevant to my point. However, there is no reason to believe that the system is compromised by using the leaf spring to cover part of the job that would normally be handled by the anti-roll bar.

Consider it this way. When designing the suspension GM settled on a set of rates they wanted to achieve in both compression and roll. The wheels don’t care what combination of springs achieve those rates. If the roll rate is low enough, we will not need to add an anti-roll bar to increase the roll rate. Even without an anti-roll bar the Corvette, like virtually all cars still has some measure of roll rate in the rear end. It takes more force to compress the wheel 0.1” from static than it does to extend it 0.1” That is why the car doesn’t teeter-totter when you disconnect the roll bar.

It is also important to point out that there is some ratio of roll stiffness to bump stiffness. That is if you say increase the spring rate by 10%, the car will become say 5% stiffer in roll. In the case of the coil springs (ie no S bend cross over) I THINK (based on very veuge recollection from something I read a while back) the ratio is something like 1:0.7. That is increasing the stiffness in compression by 10% increases roll by something like 7%. This is not including the anti-roll bar. In the case of the leaf spring it was something like 1:1.4. That is for a 10% increase in bump we get a 14% increase in roll.

The purpose of an anti-roll bar is simply to increase the rate in roll beyond what we get from the springs without increasing the rate in bump.

Because the leaf spring ties the left and right side together more effectively the roll bar simply has less work to do.

Let’s say we have two Corvettes, one with coils, one with leaves. Next, let’s assume we want the same bump and roll rates. Let’s assume they have the same rates in bump already. That is it takes the same force to push the back end of the chassis down 0.1”. Well the wheel rate in bump is the same for both so the suspension sees no difference between the leaf and coils with respect to both wheels being compressed at the same time.

However, we also care about roll rate. This is where we get into some ratio type stuff and that whole S bend bit. In both the leaf and coil spring cases the suspension didn’t meet the roll rate target without adding an anti-roll bar. So in both cases we add the anti-roll bar. However, because the leaf spring does act in part as an anti-roll bar, the leaf equipped Vette is closer to the target roll rate than the coil model before we start adding the anti-roll bar. The leaf equipped car thus will use a smaller anti-roll bar to hit the target value.

Now a lot of people talk about how good the car handles when changing to coil overs. Are they wrong? Well probably not. But consider, was it the leaf that was the problem?
In changing to coil overs they have done several things. First, they have changed the dampers. That’s a big one. Odds are good that an expensive aftermarket damper can be better than the cost constrained factory parts.
In changing to coils you also are likely to have changed the spring rate. Certainly we can see that a change in spring rate will change the way the car feels. It will also likely change the roll rate. While the coil over kits often come with new anti-roll bars, do we know if in the end the new roll and bump rates are the same as before? If not how can we say the changes are comparable? In going to a coil over kit we have changed three critical suspension parameters, damping, rate in compression, rate in roll. How can we isolate which was the item which made the difference?

As for the racing question. I’ve seen this question before. It is a sensible question to ask and I would say two things in response. First, what is a good idea in racing is often not a good idea in road cars. Note that F1 cars are using torsion springs and flexure suspension joints rather than rod end/spherical bearings or bushings such as road cars use.

More to the point racecars are very much comprised of off the shelf parts. The racing shocks are standard parts that are configured for the C6R. Same is true for the springs. It would be quite expensive to make up sets of leafs for all the different rates the racing team might want. It is more practical and expedient to use what are essentially off the shelf items in a race car. There is nothing inherently wrong with the leaf spring for a race car but what do you do if you want 5% stiffer springs? Hypercoil can send you a new matched set in short order. Can the suppliers of composite leaf springs do that? Can they do it as quickly and cost effectively as the coil venders? Also, one of the stated advantages of the leaf is lower weight. Well if the C6R is able to get under the weight limit, then there is somewhat less incentive to try to save weight on the springs then have to add it back as ballast.

Finally, part of the reason GM chose the leaf was for packaging reasons. They felt that a set of coil overs would have forced higher shock towers which would have hurt trunk space and engine space. The book “All Corvettes are Red” stats this explicitly. We know you can get a set of coils into the stock damper location. We have all seen pictures that prove it. But we don’t know if GM would be able to source a set of reasonably priced, long lived, meet all the other spec, coil overs and put them in the same spot. It is likely GM felt they couldn’t. However, the C6R is a race car. The shocks don’t have to last 40,000+ miles. They don’t have to meet production cost targets. They don’t have to deal with a lot of the same restrictions. Heck, they probably move the upper shock mounts. What I’m ultimately getting at (in a long winded way) is the requirements, opportunities and constraints are different for the C6R vs the road car. What works for one should not be assumed to be acceptable for the other. The C6R doesn’t have locks or a stereo either. Would we choose to forgo those features on the road car?

TTRotary

...And corner weighting can make that difference. That is why race cars use coilovers. For a street car, the CTM is just fine.

LTC Z06

I've only heard bad things about the crosstalk from racers pushing the car at 10/10ths. I do believe it exist but only pros pushing the car at 10/10ths can notice it. The Corvette has been a world class handling car since 1984 and has only got better. Every sports car in the world has suspension modifications available so people can tune them to their taste and needs.

LTC Z06

...and it can be lowered and corner weighted also.

bbenavitz

and.............unless I missed something the ZR-1 will also have transverse leaf springs. It would be interesting to understand from GM engineers perspective why they chose to continue with this set up knowing full well that a coil over was very doable.

petefias

Just think how fast would you be with coilovers.