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 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.
In reply to Coupefan :
You know, maybe the vacuum tube analogy is a good one. (Says the guy sitting beside two tube amps.)
I wonder if you could go old school and stack leafs to change the rates?
In reply to Appleseed :
You could conceivably make multileaf packs, but then you end up with all of the downsides of a multileaf pack.
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.
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.
This is verrrrry interesting to me, but so far only talks about the spring rate in ride. Whatb happens in roll as the weight transfers into and thru the corner is what I'm most fascinated by, and this article is only a teaser of that. Argh! I'll be panting for the next one. I have never been a fan of the C7 chassis, and I suspect these variable wheel rates under load may be the reason why the car feels unpredictable to me. I am also curious about the choice of the transverse leaf in the first place, given its foibles.
A GM engineer I met while testing press cars for a magazine told me (as I recall...) that in roll, the unloaded side of the transverse leaf is still affecting the loaded side, and that the wheel rate varies with travel. It is not linear. I believe I feel this when driving the C7 hard.
Thank you GRM staff and Lane Borg, great stuff and cant wait for the spring rates in roll story!
i would think like on a live axle you would get energy transfer from one wheel to the other when a wheel hits a bump, being the spring is directly connected to each other. there would also be a fair amount of deflection on the mounts to the chassis because they are not directly acting on the chassis but are cantilevered away from the wheel area, so the forces loaded into the chassis is far higher than a coil sprung car.
often wondered if composite springs are any different to leaf springs, being you can tailor fiber orientation compared to steel leafs.
Just in general lief springs are not linear at all plus they have inherit friction between the leaves of the springs (steel leafs springs) .They are very inefficient as far as energy stored /lb
the non linear spring rate isn't really bad as long as you have that engineered into the system.
For all those commenting on a roll or single sided bump scenario transferring forces to the other side: have you forgotten about sway bars? Because they do the exact same thing.
As stated before I was always tought the transverse leaf also acts like a sway bar. Isn't conventional knowledge that you could run less spring rate and more bar? Could that be why the coil guys are having to run so much more rate, because they lose roll stiffness moving to a coil?
I think he's onto something with the mounting, when you take a leaf out and see how mangled and beat up the mounts and pads are its obvious those thing deflect a bunch.
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.
It's a matter of the right tool for the job. Microwave ovens still use vacuum tubes, with a few very rare exceptions. Solid state amps still aren't as effective at that particular combination of frequency and power. Actually, the Corvette's use of lead springs is a bit more ideosyncratic than the microwave.
Leaf springs on a live axle are even more complicated when it comes to calculations, even if you have fewer moving parts. Many of these have different spring rates on the front and back of the leaf and different rates side to side. Which is one reason the leaf spring love axle has fallen out of favor - not that they are too simple, but that getting good results with so few parts is too complicated.
By my estimation, the transverse leaf springs do not act like a stabilizer bar, but the opposite effect, pushing one side up pushes the other side down a small amount, like a Z bar but not as extreme.
Why does everyone seem to think that monoleafs on Corvettes are only mounted at one single point in the middle? Are these the same people who think that cutting coil springs raises your car?
In reply to rustomatic :
Of course they are supported across the middle and not laid out like a walking spring. Even so, I am fairly sure that, even supported across a couple feet in the middle, pushing one side up would still affect the other side.
It would be a neat experiment to test, at least.
A thoughtful analysis, but all done from a Corvette-centric point of view.
Another high volume production series featured rear transverse multi-leaf springs:
Triumph 'small chassis' cars: Herald, Vitesse, Spitfire, GT6.
There were 3 rear suspension variants, each with their own design influences on spring rates.
Early Herald, Spitfires and some Vitesse and GT6 use a swing axle arrangement where the spring is rigidly clamped to the diff. A pivoting Vertical Link between the swing axle bearing hub and the spring tip, along with the swing axle, the UJ pivot center, and the edge of the rigid diff spring mount formed a parallelogram that served to keep the VL nearly vertical as the suspension articulated through it's range of movement, while transferring wheel loads to the spring.
Later models of Vitesse and GT6 featured an SLA geometry where the spring serves as the longer upper lateral link, with a reversed wishbone shorter lower lateral link with single training arms.
This variant is called 'rotoflex' due to the use of a large rubber donut coupling in the drive axle, instead of the CV axles that are so commonly used today.
Later models of Spitfire and GT6 swing axle cars employed the so-called 'swing spring' variant, in order to mitigate the 'jacking' behavior of swing axles. The lower most spring leaf is still rigidly attached to the diff, but the upper leaves feature a rounded 'bump' in the center that is supported by a cage structure that permits them to rock or 'swing' when the left & right wheels are at different levels, such as in a turn. Thus, only the lowermost leaf contributes any roll stiffness while all the spring leaves support the full rear weight load. This behavior is the opposite of an Anti-Sway Bar, but same as a Z-bar, or blade style Camber Compensator, to prevent 'jacking'.
Although the article dismisses effect of length changes in a Corvette deflected leaf spring, ie measured distance between the center and the leaf tip, for the Triumphs, particularly the 'rotoflex' cars, it is important.
Ever wonder why leaf springs are sometimes called 'quarter-elliptic' or 'semi-elliptic'?
It's because as they deflect under load, the tip end traces a path in the shape of an ellipse segment, not a circle.
So the effective length of the upper link of a 'rotoflex' suspension changes as it deflects under load. This has an important impact on the rear camber, and due to the geometry, affects the spring rate as measured at the tire contact patch.
I thought quarter elliptical, semi elliptical springs were named because a quarter elliptical spring is a quarter of an ellipse (has only one "end"), a semi elliptical is only half of an ellipse (the kind commonly used on cars)
Old buggies used full elliptical springs - one big loop.
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