An Engineer Weighs in on Corvette Leaf Springs: Deep Talk on a Seemingly Simple Subject

Story by Lane Borg

A straightforward question came up as we worked on our Corvette Z06 project car: What are the rates for leaf springs–both OEM and some popular replacements?

So we asked Lane Borg, a mechanical engineer, Formula SAE instructor, Goodyear test driver and owner of Borg Motorsports, manufacturer of bespoke high-performance chassis bits for Corvettes. As it turns out, this question was waaaaay too difficult for our primitive lizard brains to tackle. His response required more than 1200 words.

Producing relevant data for Corvette transverse leaf springs can be immensely complicated because there are lots of variables and dynamic reactions affecting the action. There’s more going on than a simple energy absorption and release.

We’ve already mentioned the load transfer during roll that can occur with transverse leafs. But when it comes to the inside, unloaded part of the spring, we have to consider that it’s not producing a negative force of its own. It may be deflecting slightly due to tension, but it’s not pushing down the unloaded suspension since it’s mounted in such a way that it only gathers energy when compressed. Once it’s fully released, there’s no more force being applied at that corner besides gravity.

We also have to address possibly the most overlooked factor in transverse spring behavior: the material and stiffness of the mounting pads. These pads act essentially as another spring, and they compress and deflect enough to make significant dynamic changes in the way the spring behaves during deflection.

It’s my opinion that the mount stiffness is why people don’t like leaf springs. When you do a coil-over conversion, you have metal-to-metal contact from the A-arm through the coil spring to the chassis. Even with aftermarket leaf springs, you don’t. It would be like installing coil-overs with the stock rubber bushings on both ends of the coil assembly; of course it feels less direct. I think finding a way to eliminate the mount deflection would be a huge step forward for those of us who have to use leaf springs in our classes.

The easiest way to test transverse leaf spring rates is in ride, by compressing and releasing both sides of a front or rear axle at once. This eliminates complexity introduced by multiple bending moments, bushings, etc.

To do this we simply built a chart of movement versus load by slowly lowering the car onto scales and measuring wheel deflection versus load on the scales. Weight is just force of gravity, so you end up with a force versus a displacement: a spring rate. Making a table for each spring looks something like Diagram 1.


Diagram 1. Click to open in a new window.

We can then chart out the various brands of leaf springs we tested and come out with Diagram 2.


Diagram 2. Click to open in a new window.

There’s a couple of things to note on the data. The most important, to me anyway, is that the springs don’t provide a linear rate below 0.5 inch of initial displacement. This is very likely the mounting pads being loaded and the bushings being compressed.

The measured rates are all at the wheel, however. To get true spring rates, you need to run everything through the motion ratios to account for the mechanical forces being multiplied or lessened through the leverage and angles of the suspension arms.

Motion ratios for leaf springs work the same as they do for coils: A certain amount of wheel movement equates to a certain amount (usually less) of spring movement. But leaf springs are actually easier on motion ratios because the force they apply is 90 degrees to the A-arm, unlike a coil spring that is likely at some non-90-degree angle. The motion ratio for the leaf spring is simply the distance from the chassis mounts to the spring adjuster divided by the distance from the chassis mounts to the outboard ball joint. For the C5, this is 0.625 (10 inches divided by 16 inches) front and 0.611 (11 inches divided by 18 inches) rear.

One theory I’ve heard is that the leaf moves in an arc, so the motion ratio changes. While I understand where that idea is coming from, the change is significantly less on leaf springs than on coils. Due to the spring’s mounting arrangement to the chassis, it stays very parallel to the control arm during movement. This is unlike a coil setup, where the angle of the coil to the control arm is constantly changing. This behavior is one of the advantages of a transverse leaf spring setup. It’s also one of the reasons real race cars have pushrod suspensions: The pushrod (or pullrod) design can combat some of the kinematic issues of this angle change as the suspension articulates.

One thing to remember is that the leaf spring and damper motion ratios are different for a leaf spring setup because the spring and damper don’t connect at the same point. This can change what you need with your damper tuning. It’s simple math to figure out, but some people miss it. When they put the designed spring rate with the designed damping on the same axis and therefore motion ratio (as you do in a coil-over), the mismatched damping and spring rate issues go away and everything works better.

Back to the data. Running the wheel rates through the motion ratios gets you the data in Diagram 3.


Diagram 3. Click to open in a new window.

That all looks well and good, but I like to validate my results wherever possible. In this case, GM actually published all of this information in SAE paper 970098, which can be purchased individually or as part of “PT-118: The Chevrolet Corvette: New Vehicle Engineering and Technical History,” a collection of technical papers that discuss the Corvette’s engineering from the first model through the current one. You can download these via the Publications section of the SAE’s website.

According to this paper, the Z51 (FE3) suspension has 1.20 hertz front ride rate and 1.45 hertz rear ride rate (without driver or luggage/load). On my C5, my measurements worked out to 1.18 hertz front and 1.42 hertz rear. That’s about a 2 percent error, which I’m very happy with, especially considering most scales are only good to 1 percent of their reading. (I did use calibrated scales for this testing.)

My big takeaway from this table is that the current crop of leaf springs doesn’t provide the necessary rate these cars want for competition, at least in ride. This is why I need to get my chassis rig built. I currently know how far off the leaf spring cars are in terms of ride rate, but I don’t have the test data to support how off leaf springs are from coils in roll.

As you can see in the table below, even with the stiffest leaf springs available, you’re still only in the range of 500 lbs./in. coil springs in ride. Most people I know run somewhere between 700 and 1200 lbs./in. rates depending on tires and aero. Overall, though, most successful cars I’ve looked at are competing on something in the 800/900 to 1100/900 front-to-rear range.

That basically means leaf spring cars are inherently half as stiff as coil cars (again, at least in ride). Combine that significantly lower rate with the less direct feel of the non-rigid mounts on both ends of the springs, and you have the story of why people like leaf springs less than coils. Plus, buying a bunch of leaf springs for tuning is insanely expensive compared to coil springs.

The other piece of info that table shows is how differently VBP and the GM supplier rate their springs. VBP may seem to be the stiffer spring by its quoted number, but when you actually measure it in ride, it’s less stiff than the more track-oriented Hyperco spring. That brings up another reason people don’t like leaf springs: tuning confusion. If you want to make a change to your car, switching from Brand X to Brand Y may not produce the desired result due to differences in how manufacturers rate their springs.

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Comments
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Coupefan
Coupefan Reader
5/22/20 2:48 p.m.

I'm still freaked out over their design choice. I know they work, like the old vacuum tubes we see in some audio gear, but still, I'm freaked out by the use. 

David S. Wallens
David S. Wallens Editorial Director
5/22/20 3:57 p.m.

In reply to Coupefan :

You know, maybe the vacuum tube analogy is a good one. (Says the guy sitting beside two tube amps.)

Appleseed
Appleseed MegaDork
5/23/20 10:44 p.m.

I wonder if you could go old school and stack leafs to change the rates?

Cooter
Cooter UberDork
5/24/20 9:04 a.m.

In reply to Appleseed :

You could conceivably make multileaf packs, but then you end up with all of the downsides of a multileaf pack.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
5/24/20 9:36 a.m.
Coupefan said:

I'm still freaked out over their design choice. I know they work, like the old vacuum tubes we see in some audio gear, but still, I'm freaked out by the use. 

There are a lot of dynamic as well as packaging benefits to their use.

wspohn
wspohn Dork
5/24/20 11:30 a.m.

Hey - as I type this I am sitting in front of a tube power amp fed by a tube preamp. Nothing wrong with tubes (or valves as the British call them), and certain military gear has long used tubes because they are much less problematic in an EMP (electromagnetic pulse) from a nuclear explosion.  The fact that Russian planes used tubes extensively for that purpose accounts for the continued popularity of Russian made (usually Sovtec) tubes in hi-fi.

(I also have a couple of systems that use 120 lb Class A mono solid state amps, so I go both ways). The irony is that my tube power amp puts out 70 W while my Class A amps put out 45 W (but they do it right down to 1 ohm).

 

BTW - very interesting spring article - thanks.

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