Mar 28, 2019 update to the Chevrolet Corvette Z06 project car

Project Z06: Trading Our Leaf Springs for Coil Springs

If tires are the most important variable affecting a car’s handing, the mechanical links that manage those contact patches finish a close second. What good are great tires if they’re just flopping around?

We took a major step regarding chassis stability with the last installment of this project, when we replaced the stock rubber bushings with an AMT Motorsport spherical bearing kit. Now it’s time to look at the components that hold up those control arms and manage the suspension’s interface between the car and the road–namely, the shock absorbers and springs.

Trans-cendent

Let’s talk first about the springs, because the Corvette has used a fairly unusual arrangement since the 1960s. Instead of the traditional coil springs found on most cars, the Corvette uses a single transversely mounted leaf spring at each end of the car. This leaf laterally spans the chassis and provides spring action to each corner.

In the second- and third-generation Corvette, these springs feature traditional multi-leaf steel construction. Since the fourth-generation cars, however, these springs have been made from a glass-reinforced plastic composite.

The entire rationale Chevy used for leaf springs will probably never be known, but this setup does offer a few benefits. For one, it keeps the mass low in the chassis while delivering an inherently low profile for the suspension parts–which, in turn, allows a lower profile for the body, aiding aerodynamics and giving the stylists a bit more leeway for their designs.

Compared to steel springs, these composite pieces also tend to have better strain energy–the amount of energy stored by a system undergoing deformation. This means that they can produce the same amount of spring force as steel springs while using less material. End result? Less weight.

So why doesn’t every car use transverse leaf springs? Well, again, there’s no clear single reason, but we can make some educated observations. First, the same packaging advantages that allow for lower fender lines also create other challenges. That big spring spanning the underside of the car takes up space, and it’s space that is usually occupied by other stuff. Designing a chassis around a pair of transverse leaf springs requires the designers to address just about everything else found underneath that car.

Plus, there are economies of scale in effect. If 99.999 percent of all the other cars in existence use coil springs, developing a one-off solution for a single model line is hardly efficient; and when it comes to the aftermarket, this has an outsized impact. Producing a transverse leaf spring for a specific application isn’t such a big deal if you’re a multi-billion-dollar corporation, but it’s not quite the same for an independently owned speed shop. Now you’re looking at a lot of tooling and R&D just to develop one specific composite leaf spring.

If you’re dealing with traditional coil springs and looking to make changes from stock, the aftermarket already offers a nearly endless supply of lengths, diameters and rates at a fair price, all with the click of a mouse button. Easy.

Spring Rate vs. Wheel Rate

We should discuss wheel rate, too. This is the actual force being exerted at the wheel when the various mechanical and angular force components are taken into consideration. Leaf springs actuate at an almost perpendicular angle to the control arm, so their effect is very consistent throughout their range of motion. Coil springs, in the real world, aren’t always that consistent due to their placement in the system.

Most coil springs are placed around the dampers, meaning that they usually sit 25 to 30 degrees from vertical. This reduces the force of the springs upon the control arms because the direction of that force isn’t perpendicular to the arm. And when the suspension compresses, that angle only increases, serving to decrease the effective leverage. The result is effectively a dynamic wheel rate that actually decreases during travel.

Is that bad? Well, it’s neither good nor bad, it’s just another factor to consider when discussing spring types.

Does the Car Care?

Our real question is simpler: Does it really matter? Does the car ultimately care what sort of spring is supporting the weight of its chassis?

We asked this question of someone smarter than us, namely Chuck Edmonson, physics professor at the United States Naval Academy. He also wrote “Fast Car Physics,” a book that neatly combines his love of motorsport with his love of equations.

My racing friends complain that it’s too physics-heavy, and my physics friends complain that it’s too racing-centric,” he says. “So, I think I hit just the right balance.”

Long story short, the car, the shock absorbers, the tires and all of that other stuff doesn’t care what type of spring is supporting it. “If the material absorbs and releases energy at a specific rate, it doesn’t really matter what the material is,” Chuck says. “So, in this case, the type of spring doesn’t really matter to the action of the suspension. The shock doesn’t know whether it’s damping the motion of a leaf spring, a coil spring, a torsion bar or a ball of rubber. So long as that spring material has been designed and manufactured to absorb and release energy at a specific rate, it doesn’t matter what it’s made from.

But,” he continues (ever notice there’s always a “but” buried in these explanations?), “that doesn’t tell the whole story of Corvette transverse leafs, because there’s other physical actions involved.”

For one, Chuck explains, there’s the fact that Corvette leaf springs feature a single transverse element acting upon both wheels on the same axle. This creates some “coupling” effect in which the loads introduced to one side of the car are transferred to the other. And if that sounds like the anti-roll bar’s job, give yourself a cookie. Yes, on the Corvette, the action of the transverse leaf creates a small amount of roll couple as the spring itself acts a bit like an anti-roll bar.

Another but: Those transverse leaf springs also encounter some mechanical friction. How’s that? The Corvette’s springs rest on pads that aren’t connected to the control arms. These pads are allowed to slide back and forth as the spring and control arms describe their arcs. Friction at this interface can theoretically make for inconsistent actuation and cause a small amount of bind. However, independent testing has shown that the actual rates for the leaf springs are far more linear than the internet experts profess.

So while it’s possible to come up with a host of theoretical arguments against transverse leaf springs, in practice they basically appear to work as designed. And with the Corvette, they also provide one more advantage. Remember those pads found at the control arms? They can be height-adjusted on a corner-by-corner basis. This allows for some corner-weight adjustments with the parts delivered right from the factory, which is a huge advantage in setting up a car.

One last but: Thanks to that coupling tendency described earlier, corner weighting a stock C5 can be a frustrating process full of tiny adjustments, trials, errors and experimentation. Still, the end result can be a factory ride height along with a fully corner-weighted chassis. It beats a sharp stick in the eye.

Coils vs. Leafs

So, are coils or leafs the better choice? How about if we answer with a Yes for both?

We’ve implied that the car doesn’t care what kind of spring holds it up, and that’s basically true. The rest of the package, however, might care. Leaf springs may have a packaging advantage, but coils have an availability advantage. Leaf springs may offer more linear articulation, but coils don’t tie one side of the car to the other. Leaf springs may weigh less, but can involve some binding. Finally, consider the simple fact that more people know how to tune a coil spring suspension.

Once we weighed all the pros and cons, we decided to convert our Corvette to coil springs. Sorry, purists.

Tech for Our Ride

Our coil-over kit came from RideTech, a company that’s been at the high-performance muscle car game longer than many of us have been around. RideTech has been primarily known for their work with the older cars found in the pro touring scene, but as RideTech President Bret Voelkl puts it, “There’s damn little difference between a pickup truck and an F1 car. At the end of the day, these cars are a math problem. Give me the weight, wheelbase, roll centers, geometry, center of gravity, and tire size, and I will optimize the handling and ride quality.”

RideTech-equipped cars have won autocross championships in major muscle car and pro touring circuits, and we were eager to see what they had worked out for the C5. Our initial impressions of their kit were highly favorable: The shocks themselves are manufactured in partnership with Fox, who knows a thing or two about top-quality dampers.

The $4500 TQ-series RideTech setup that we installed consists of high-quality aluminum bodies featuring 4.1 inches of travel, remote reservoirs and independent adjustments for low-speed compression, low-speed rebound, and high-speed compression. Valving is done to RideTech’s specifications, but the units are rebuildable and re-valvable to the user’s requests.

Installation is straightforward and fairly simple. The factory leaf springs come out after a few bolts are removed, and the RideTech coil-overs bolt right into the holes vacated by the stock shocks.

If there’s any complication, it comes at the rear, where the chassis mounting holes must be ground back a bit to allow the installation of the upper shock mount. Chevrolet formed this part of the chassis from double-thickness structural steel, so you’re going to go through a lot of grinding bits. Just deal with it. Wear eye and ear protection and have someone hold your calls.

When the car’s back on the ground, it will sit a bit lower than before: The RideTech shocks are slightly shorter than the OEM units, so they will lower the car a minimum of about an inch in the front and about an inch and a half in the rear. This moves the roll centers slightly downward, but not so far that they go underground or create an excessive amount of roll torque.

It’s possible to lower the car so much via the threaded adjusters that unfavorable suspension geometry will rear its ugly head, but you’ll first run out of fender clearance. So, it’s basically a problem that prevents itself from happening–or at least from getting completely out of hand.

On the Rack and Then the Track

Quality, technology and development time don’t always translate into performance. The real test is how this setup feels on track.

The answer, in a word, is good. The RideTech TQ shocks and coil-overs fairly transformed the manners of our C5. While the Corvette handled well from the factory, the limits of adhesion could be a little dicey to discover. It was a car that could easily be driven up to 95 percent of its limit, but unlike a Miata or Honda S2000, the Corvette’s ultimate edge of grip wasn’t totally predictable. The car now feels more comfortable and user-friendly when operating past 99 percent.

The kit’s supplied spring rates are interesting, since they’re flipped from the factory approach. The kit’s standard springs–the ones we installed–measure 700 lbs./in. at the front and 600 lbs./in. at rear, while the factory puts the stiffer rate at the rear. However, don’t forget that those factory springs also create some degree of roll resistance due to the fact that they’re formed from a single piece of material.

With the coils, all of the lateral load coupling is done through the anti-roll bars. In the front of our car, we already had a Strano Performance Parts 15/16-inch bar, which is basically the go-to part for a lot of top Corvette autocrossers. The additional spring rate plus the larger-than-stock front bar work together to give the C5 much better response than with the stock pieces, and the car now actually has some steering feel, something that was sorely lacking in the stock configuration.

Adjusting the RideTech shocks is simply a matter of turning the knobs: The rebound knobs are found on the shock bodies, while the combination high-speed/low-speed compression knobs are located on the remote reservoir canisters.

The actual layout of the C5 makes adjustments a little tricky. To access the rebound knobs, you have to get a hand way up into the rear wheel wells; up front, the coolant overflow tank and windshield washer reservoir need to be moved. Either way, it’s not the end of the world.

Adjusting compression is easy or hard, depending on where the canisters are mounted. We used the area vacated by the leaf springs, so our canisters are all but inaccessible unless the car is jacked up–not an ideal solution, but good enough for testing and good enough for now. Eventually we’ll make some access holes in the fiberglass so we can relocate the canisters into the trunk and engine compartment.

Setup-wise, we’re finding the car to be very reactive to rake changes, and those are easy to implement. Once we established a balanced corner-weight baseline, we can simply go in and raise or lower the front and change the balance of the car: A little lower in the front for some additional front-end bite and turn-in aggression, or a little higher for a less-agile but more stable feel.

On shock adjustments, we found a fairly happy median near the middle of the adjustment range. Our fine-tuning device then becomes adjustments made to the rear rebound. Overall, though, the shocks are truly exceptional, high-quality pieces that turn the C5 from a slightly edgy car at the limit to a very benign, drivable performance car.

Decisions, Decisions

So, why choose one system over the other? As mentioned more than once, from a pure standpoint of physics, there’s little difference between leafs and coils. They’re mechanical devices that store and release forces through tension at a known rate.

For us, and for many other Corvette drivers, the advantages of coils come down to availability and familiarity. Our decision was made easier by a rather liberal rule in our SCCA CAM autocross class allowing changes to spring type. And speaking of easy, coil springs are available from numerous suppliers at affordable prices and in a nearly unlimited number of rates. Also, corner-weighting with coils introduces none of the frustrating cross-loading that can crop up with leafs.

Then there’s the subjective feel: Our Corvette went from great to exceptional. Obviously much of the credit for this feel goes to the excellent RideTech shocks, but the valving and coils work together to create a package that feels so much better than the one Chevrolet installed.

And, of course, we still need to fully develop our new setup. We’ll dive into the data in the next installment.

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