Leaf Springs - Pros/Cons
Discussion
Specifically in relation to the Corvette C6.
By all accounts this seems to handle pretty well, but it still uses leaf springs. Something that is often brought up in the media over here as a bad thing. My question though is - is it?
I mean is it that leaf springs are older technology, but still a decent solution/alternative to a coil spring or is the coil spring superior in all respects.
By all accounts this seems to handle pretty well, but it still uses leaf springs. Something that is often brought up in the media over here as a bad thing. My question though is - is it?
I mean is it that leaf springs are older technology, but still a decent solution/alternative to a coil spring or is the coil spring superior in all respects.
It is a very complicated area, so apologies if this goes on a bit.
The traditional leaf layout (the cart spring layout - two longitudinal springs on a beam axle, as on the Mustang, IIRC) is archaic, introduces several problems, and should be bettered by several alternative layouts.
For ultimate (race) performance, I would always tend towards double wishbones, probably with coil overs, or with some variation on that theme (push- or pull-rods, torsion bars or disc springs, depending on the requirements). However road cars generally do not require ultimate performance, have other priorities, and other options are worth considering.
The 'Vette uses a transverse leaf spring. The spring acts only as a springing medium, with the suspension arms (double wishbone I think, but perhaps MacPherson struts) handling wheel location. The transverse leaf idea has some interesting possibilities (though how well Chevrolet have exploited them is another matter) - it did win two World Championships after all - and when people dismiss it they are only showing their ignorance.
Benefits:
- Roll stiffness: A quirk of the transverse leaf is twice as stiff in roll as it is in bump. That suggests you could eliminate an anti-roll bar or at least reduce its thickness - saving weight, cost, and reducing the problems an arb introduces (an arb reduces overall grip, after all).
- Wheel arch width: Adding coilovers to a damper strut increases the radius by about an inch each side. So removing the coilovers could give you an exta 2" between the wheelarches - rather useful for vans, estate bootspace, or if you want to drop a wide V8 in there.
- Floor height: It may also be possible (though not certain) to lower the floor height - again good for bootspace.
- Load distribution. The weight of the car has to be transferred to the wheel hub carriers. In a coilover design this generally means down into the floor structure, then back up the wheelarches, and into the top of the spring. That means some significant structure up there. With a transverse leaf, you should be able to take it straight from the floor into the spring. Loads at the top of the damper (without coil) are much lower. So you should be able to simplify the structure considerably, saving weight.
- Hub mounting: If you have a driveshaft running straight through the centre of the hub, it's rather in the way for that damper/spring unit. If the coil spring is playing a part in defining the damper strut length, this could be quite a problem.
You could mount the strut to the hub above the driveshaft, but centrally. But this would result in a very high wheelarch (packaging, load distribution risks)
You could mount it lower, but off-centre (fore or aft). But because of the extra diameter of the coilover, it would be considerably more off-centre than just a damper unit. That introduces a load trying to rotate the hub in bump - loading up bushes and trying to move the wheel position. That's unnecessary loads that need to be countered (weight) and unnecessary wheel movement (loss of grip/handling).
However, a leaf could be mounted directly above or below the driveshafts - no off-centre loads, no artificially induced wheel movements.
- Subframes: It's quite common to construct the complete suspension onto a subframe. It can be easier to install, easier to set up, and easier to control Noise, Vibration & Harshness (NVH) with dedicated bushes between subframe and chassis. In some cases, a transverse leaf is far better suited to this. Load paths and packaging can be well-suited to a compact, simple sub-structure.
- Aero: If, as on a Se7en, you have those bloody great coilovers waving in the wind, you can imagine that a leaf could present a much smaller obstacle to the air, especially if you hid it inside the Vee of the wishbone.
So, if some of these are relevant to your car, a transverse leaf might be worth considering.
Downsides Of a Leaf Spring
- Weight; There's a lot more mass in just one leaf of spring steel than two comparable coils (the Corvette uses a plastic composite spring).
- Inertia: Part of that weight is unsprung mass. If the wheel goes over a bump, you've got more mass to move. You've also got more complexity in the spring-mass chain (if you did a decent A-level Physics course, you would have done the thing with springs and masses in series). Net result is problems with handling and NVH.
- Wheel location: Unsurprisingly, springs are, well, springy. But not just in an uppy-downy way. In the traditional cart spring layout, the leaves locate the axle, and not very well. The axle can move left-right, fore-aft, and the spring can wind up. The way the bushes work only makes this worse - do you use small bushes for location, but risk NVH problems, or larhe pliant bushes tha tlet the wheels move all over the place? You can partly solve this with exta bits (and weight) like a Panhard rod, anti-tramp arms, etc. But really that's making the best of a bad job. If you want to use a leaf, you should consider using it only as a springing medium, not for location.
So while nine times out of ten some arrangement of coilover units are likely to be better, there are situations where other options could be significantly better. The transverse leaf of the 'Vette offers some very appealing opportunities, and I've also seen designs based on rubber bands (remarkably effective) and spring steel wishbones that act as both springs and location (incredibly light if you are working on a very light car). I can envisage a system other than coilovers on some very surprising cars, such as the Se7en.
*My credentials are several years designing suspension systems, a considerable part of it on leaf-based designs.
The traditional leaf layout (the cart spring layout - two longitudinal springs on a beam axle, as on the Mustang, IIRC) is archaic, introduces several problems, and should be bettered by several alternative layouts.
For ultimate (race) performance, I would always tend towards double wishbones, probably with coil overs, or with some variation on that theme (push- or pull-rods, torsion bars or disc springs, depending on the requirements). However road cars generally do not require ultimate performance, have other priorities, and other options are worth considering.
The 'Vette uses a transverse leaf spring. The spring acts only as a springing medium, with the suspension arms (double wishbone I think, but perhaps MacPherson struts) handling wheel location. The transverse leaf idea has some interesting possibilities (though how well Chevrolet have exploited them is another matter) - it did win two World Championships after all - and when people dismiss it they are only showing their ignorance.
Benefits:
- Roll stiffness: A quirk of the transverse leaf is twice as stiff in roll as it is in bump. That suggests you could eliminate an anti-roll bar or at least reduce its thickness - saving weight, cost, and reducing the problems an arb introduces (an arb reduces overall grip, after all).
- Wheel arch width: Adding coilovers to a damper strut increases the radius by about an inch each side. So removing the coilovers could give you an exta 2" between the wheelarches - rather useful for vans, estate bootspace, or if you want to drop a wide V8 in there.
- Floor height: It may also be possible (though not certain) to lower the floor height - again good for bootspace.
- Load distribution. The weight of the car has to be transferred to the wheel hub carriers. In a coilover design this generally means down into the floor structure, then back up the wheelarches, and into the top of the spring. That means some significant structure up there. With a transverse leaf, you should be able to take it straight from the floor into the spring. Loads at the top of the damper (without coil) are much lower. So you should be able to simplify the structure considerably, saving weight.
- Hub mounting: If you have a driveshaft running straight through the centre of the hub, it's rather in the way for that damper/spring unit. If the coil spring is playing a part in defining the damper strut length, this could be quite a problem.
You could mount the strut to the hub above the driveshaft, but centrally. But this would result in a very high wheelarch (packaging, load distribution risks)
You could mount it lower, but off-centre (fore or aft). But because of the extra diameter of the coilover, it would be considerably more off-centre than just a damper unit. That introduces a load trying to rotate the hub in bump - loading up bushes and trying to move the wheel position. That's unnecessary loads that need to be countered (weight) and unnecessary wheel movement (loss of grip/handling).
However, a leaf could be mounted directly above or below the driveshafts - no off-centre loads, no artificially induced wheel movements.
- Subframes: It's quite common to construct the complete suspension onto a subframe. It can be easier to install, easier to set up, and easier to control Noise, Vibration & Harshness (NVH) with dedicated bushes between subframe and chassis. In some cases, a transverse leaf is far better suited to this. Load paths and packaging can be well-suited to a compact, simple sub-structure.
- Aero: If, as on a Se7en, you have those bloody great coilovers waving in the wind, you can imagine that a leaf could present a much smaller obstacle to the air, especially if you hid it inside the Vee of the wishbone.
So, if some of these are relevant to your car, a transverse leaf might be worth considering.
Downsides Of a Leaf Spring
- Weight; There's a lot more mass in just one leaf of spring steel than two comparable coils (the Corvette uses a plastic composite spring).
- Inertia: Part of that weight is unsprung mass. If the wheel goes over a bump, you've got more mass to move. You've also got more complexity in the spring-mass chain (if you did a decent A-level Physics course, you would have done the thing with springs and masses in series). Net result is problems with handling and NVH.
- Wheel location: Unsurprisingly, springs are, well, springy. But not just in an uppy-downy way. In the traditional cart spring layout, the leaves locate the axle, and not very well. The axle can move left-right, fore-aft, and the spring can wind up. The way the bushes work only makes this worse - do you use small bushes for location, but risk NVH problems, or larhe pliant bushes tha tlet the wheels move all over the place? You can partly solve this with exta bits (and weight) like a Panhard rod, anti-tramp arms, etc. But really that's making the best of a bad job. If you want to use a leaf, you should consider using it only as a springing medium, not for location.
So while nine times out of ten some arrangement of coilover units are likely to be better, there are situations where other options could be significantly better. The transverse leaf of the 'Vette offers some very appealing opportunities, and I've also seen designs based on rubber bands (remarkably effective) and spring steel wishbones that act as both springs and location (incredibly light if you are working on a very light car). I can envisage a system other than coilovers on some very surprising cars, such as the Se7en.
*My credentials are several years designing suspension systems, a considerable part of it on leaf-based designs.
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