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Old 03-31-2005, 07:43 PM
culver culver is offline
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Corvette leaf springs

I saw this on a Corvette Forum, thought I would share.

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Corvette and leaf springs. Many people are surprised to hear that the C6 Corvette uses leaf springs. This seams to conger up images of ox carts and old Ford pickups. I thought I would post my understanding of the technology with the hopes that other will post their insight.


Since 1984 the Corvette has used a transverse fiberglass composite leaf spring as part of the suspension. The C5 and C6 both have very similar double A-arm suspensions that wouldn't look out of place on any high end sports car. The only significant difference being instead of a coil over spring the Vette is using a single leaf spring. The suspension geometry and motion would be exactly the same if GM chose to use coil springs rather than the leaf. For those who might think the Corvetteís leaf spring is ďoutdatedĒ technology, keep in mind that the composite leaf spring was introduced as an option in 1981 and in itís current layout (acting as a partial anti-role bar) in 1984. Itís safe to say the coil spring is much older.


This is a picture of the C5's rear suspension.

http://www.corvettecavalry.com/exhaust/no-mufflers.jpg

The leaf is the black thing that runs from one side to the other just under the lower A arms.



A Brief History of Leaf Spring Suspension.

Excluding the Corvette I'm aware of 4 general types of leaf spring suspensions.

1. Model T style transverse leaf.

http://www.trainweb.org/toenailridge/vanradiator.jpg

This model shows the transverse leaf used on a Ford Model-T. The suspension has two lateral arms that keep the front axle perpendicular with the chassis. Lateral axle movement is controlled by the spring. This system suffers from poor control of the axleís movements among other flaws. Iím not aware of any production car that uses this suspension type.



2. Conventional truck type, longitudinal leaf springs:

http://www.chris-longhurst.com/carbi...ion_bible.html (scan down)

This is the one we all love to hate. Itís also about the only type of leaf spring suspension still in use. Itís cheep, durable and handles badly. It suffers from friction between the leaves and from poor control of the axleís location.



3. Golf cart style transverse leaf spring:

I couldnít find any pictures of this but it basically looks like a double A-arm where the leaf spring is one of the A-arms. The geometry is probably OK under vertical loads but lateral loads would defect the spring and cause camber changes. Not an issue for golf carts but bad for sports cars.



4. Leaf with links. There are lots of variations on this suspension

Miller Indy Roadster

http://www.scaleautoworks.com/millerphoto.jpg

http://www.scaleautoworks.com/metalMillerFQ3.jpg (the black things on top of the front axle

Jaguar MkII rear suspension (canít find a picture)

Like #3, these suspensions uses a combination of links and the leaf spring to support the axle. The Jaguar set up looks similar to a 4 bar solid axle rear suspension except the lower link is the end of an inverted leaf spring. The other end for the leaf is attached to the chassis under the passenger compartment. The middle of the upside down (frown rather than smile) spring presses against a rubber block. The end connects to the bottom of the axle. This system offers better handling and axle control than #2 but is still suffers from friction between the leaves of the springs and compared to multi-link live axles, poor control of the axleís location.



What makes these all the same

All of the above have several things in common. First, multi-leaf springs that suffer from friction between the leaves as the leaf flexes. Second, the inherently flexible leaf spring is being asked to work as a spring AND a suspension arm. Springs (leaf, coil, torsion etc) are good at being springs. They are bad at being other things like rigid links. In those suspension designs the spring is being asked to hold the axle and be a spring. To itís credit, the leaf spring does this much better than a coil spring. How well would a coil spring do that job? Think of a bobble head doll.



Why is the Vette different?

First, the Vette actually has double A-arm suspension like many other high end sports cars. The A-arms are used to fully control the movement of the wheels. The only difference between the Vette and other cars with A-arms is the Vette uses a leaf to pull the lower arm down rather than a coil spring to push it down. In both cases the spring is doing what it does best, being a spring ONLY.

The other problem was friction between the leaves of a leaf spring. Well the Vette uses a single piece leaf so there is no internal friction, just like a coil spring.

So what we have is double A-arm geometry just using a different type of spring.



So why does the Vette use it

To be honest, I have no idea how GM got started with the transverse leaf spring. The used to use coils in front but in 1984 they switched too leafs front and rear. I suspect itís a tradition they maintain for the same reason Porsche keeps their engine out back even though the platform mate Boxster moved it to the middle.



What are the advantages for the Vette?

This is an article written around the time the C4 was released. It covers a lot of the reasons why GM retained the leaf suspension

http://web.telia.com/~u60113742/misc...ass_spring.gif

The big advantages are:

-It weights A LOT LESS than coil springs. One leaf replaces two coils. The two coil springs weigh 3 times as much as the one leaf. Additionally the leaf is placed at the bottom of the car. In addition to removing weight you lower the CG.

-It acts as an anti role bar. The article above explains how this works so I wonít. The advantage is you can run lighter anti-role bars because the springs are taking care of part of the job for you.

-The leaf springs never wear out. The vendor of these springs has never had to replace one due to fatigue failure. Coil springs to were out but you typically donít notice on smaller, lighter cars. You do see it more on old, heavy Caddies and such. The improved fatigue life was really evident compared to the C3ís steel leaf spring. Thus this is an advantage over coils but not a big one.



What are the drawbacks for the Vette?

-They are expensive. We normally donít think of leaves as the expensive suspension but in the case of the Corvette, coils would be cheaper. The Vette already has all the parts a coil sprung double A arm suspension would use. Pull the leaf off, replace the shock with a coil over and youíve converted the Vette. Since the rest of the system is the same, the cost comes down to the price of 2 coils or one spring. Well if it was a steal leaf spring it might be cheaper (remember truck suspension is cheaper because the leaves also act as links).
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Old 03-31-2005, 07:44 PM
culver culver is offline
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Part 2
**********************************************

If itís so good why donít other people use it?

Itís legitimate to ask, does GM know something that Ferrari, Porsche etc donít know or are the people at GM just being pig headed and sticking with ďoutdatedĒ technology.



Street cars:

-You must design them into the car in the first place. This seams obvious but consider these springs span across the bottom of the car. In the front they have to clear the engine oil pan and in the back they have to stay out of the way of the differential. Basically, you can retro fit coils on the Vette because the mounts can be shared with the shock mounts. For the most part you canít retrofit Corvette style leaves onto other cars because you would have to add mounts that donít exist on the regular car.

-GM and their supplier spent a lot of time and money developing the Vetteís composite spring. Currently they are the only manufacture with the knowledge and understanding to make the springs work. On the other hand, coil springs are common and well understood. Lots of vendors can make them in a wide variety of configurations. Itís easier for the other manufactures to stick with what they know. Other manufactures would have to study the design and manufacture of composite leaf springs before they could pop them on the next Supra-NSX-Type-GT. GM did that work years ago. Toyota could certainly afford to develop their own composite springs if they wanted. The same may not be true for smaller companies like Ferrari and Porsche.

-Engineers like to stick with what they know. Lots of suspension engineers are familiar with using coil springs. They could experiment with leaves if they wanted or they could stick with coils and get the job done. See the point about undertaking a research project.

-Coils are cheaper. This automatically keeps them off lower cost cars (Miata, Civic) and cars that share platforms with lower cost siblings (Audi TT). Porsche isnít worried about saving every last dollar but there suspension and chassis design may not allow packaging a Corvette type leaf. The same is probably true of Ferrari. Even if packaging isnít a problem they still have to pay for tooling to make the springs. Unlike the GM who spreads that cost over 30,000 Vettes a year, Ferrari would spread that over maybe 2000 cars a year. Porsche would be somewhere in between. Conversely I can get coils made with relatively low setup cost and a cheaper per part cost. So not only would they have to spend more per car, they have to spend a lot more up front.

-Perception. Just like pushrods, the leaf spring as a stigma attached to it. The reasons for the stigma are legit (key component to heavy and typically poor handling suspension). However the reality is the sum of the older parts was the problem, not a specific part of it.



What about race cars? (this section is almost verbatim from another post of mine.

To start off, not all race cars use coil springs. Some F1 cars (Ferrari and others) use torsion springs instead. Years ago Indy and F1 cars DID use leaf springs but those days are long past.



The current design of open wheel racecars places great restrictions on suspension packaging. The Corvetteís transverse leaf spring must span from one side of the car to the other. Also, to be most effective the links between the spring and suspension arms should be under tension. This makes a bottom mount spring most effective. This packaging doesnít work well on an open wheel car because the spring would have to pass though the gear box around the dif (or the gear box would have to be raised and hurt the carís CG). At the front the driverís legs would get in the way. Additionally the spring is wide and would have to extend past the body work where it would hurt the carís aero package.



NASCAR rules dictate coil springs on the rear axle. They probably originally used leaves but given the option any car designer (modifier back in the day?) would have replaced the leaves with a multi-link set up. As I said before the multi-link offers better control of the rear suspension.



Another good reason is only a few companies understand the technology necessary to make the springs. Hypercoil is currently the top race spring manufacture. They can make very precise, matched spring pairs. The level of precise spring rate control and matching may not exist in the composite bow springs.



Coil race springs are not car specific. You select rates, diameters, length etc but you donít have a specific spring for a specific car. If you want to order a custom spring Hypercoil will wind it to your specifications on the same machine they use for the next custom spring. A custom Porsche, Formula Ford and LMP car spring can all be made on the same machine. By the time the C6 evolves into a C6-R (they donít start off with a production Corvette) the suspension geometry is so different that they couldnít just mount a C6 leaf spring. Itís far too expensive to have a few custom leaf springs tooled up (you would have to buy the tooling as well as the springs) so they use readily available coil springs.



This type of universal tooling isnít availible for the composite leaf spring. Only the Vette currently uses the spring so you are making a Vette only part. This seriously reduces the market for aftermarket composite leaf springs (still there are after market leaf springs available for the Vette). The business case for custom equipment to make Vette springs is harder to justify since itís a smaller market.



Why donít other cars retrofit leaf springs? Well they also donít retrofit torsion springs despite the fact that F1 cars use them. Put simply it would be VERY difficult. The Vette was designed to have them. It has mount points under the car where the springs fit to the suspension sub frames. Itís not easy to just add that to a car that was designed to use a coil spring. All of the cars you mentioned would have to be re-engineered to add leaf springs. Replacing the factory spring with a racing is easy by comparison.



The other VERY significant reason is racers will use what they know. They will put effort into learning about new technology (torsion springs in F1) but ultimately it is too there advantage to stick with what they know.



Would the Vette be better with coil springs?

Well that depends. As I said before, there are a lot more options available in coil springs. If I want to substantially change the Vetteís spring rates then I will need to go to coils. Also, if I want to totally get rid of the Vetteís anti roll I need to dump the leaves because they provided some roll resistance.



BUTÖ

If for some reason I just lost my leaf spring (maybe someone stole it to make a very strong bow and arrow) and had to replace it with coils. I want the same ride quality, the same spring and roll rates etc. Basically I want the car to be the same as before but with coil springs. Assuming you didnít change anything but the springs (same tires, shocks, ride height, same spring rate and effective roll rate, etc) the Vette would unquestionably be SLOWER with coils instead of the leaf setup. Basically if all else is equal, the coils are heavier and raise the CG of the car. One other small advantage is the shocks on the leaf sprung car will move more freely than the car with coilovers. When used as a coil over, coils impart a bending load on the shocks that cause them to bind a bit. On a street car you will never notice but on a race car it can cost a tenth of a second or so. (Hypercoil markets a pivoting spring perch to reduce the effects of side loading in coil over shocks).

Again, if I decide I really want to race I will likely dump the leaves because I have more options with coils. For a reasonable (in racing terms) price I can get custom coils made. The same isnít true of the composite leaf spring.
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Old 03-31-2005, 07:59 PM
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Matra et Alpine Matra et Alpine is offline
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Excellent post and thanks for the pic.

We debated this before and nobody coudl confirm for me where the leaf runs.

The BIG issue with leaf springs is the limited movement and control. So variable rate and LONG travel is very difficult.

Also using the leaf as sprign AND anti-roll bar is not the best idea as a spring of any form is a compormise in a suspension anyway and to have it do 2 jobs and meet TWO sets of compromises is likely why others steer clear of it. BTW leafs are used in many modern small Eureopan cars, so the world does know how to do it :0

Also you chose to compare composite leaf and steel coil. You can get lightweight coils too. It's best NOT to justiofy anuthing by comparing appleas and oranges. And to suggest that the few poinds in a coil make the car slower is forgetting that the driver is present - you're talking about the weight of a good dump and decent meal

Can you please explain the bending the shock in a coilover , cos it doesn't UNLESS the coilover is operating non-perpendicular to the movement and in that case it's happening to the shock whether it has a spring on it or not

You recognsie the drawbacks as well and is a most informative post.

Thanks again.
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Old 03-31-2005, 07:59 PM
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interesting reading .. i know very little about cloverleafs, im more of a Mcphearson Strut man myself
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Old 03-31-2005, 08:07 PM
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Matra et Alpine Matra et Alpine is offline
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Actaully foudn out the 'vette sprign is only 8lbs, a little bit more than a dumpa nd a meal difference then
Still not a lot in a car weighing 3000+ lbs !!!!
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Old 03-31-2005, 08:28 PM
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Sweeney921 Sweeney921 is offline
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My truck has rear leaf springs, and with the combination of that and the fact that there's nearly no weight in the rear, it's easy to find yourself going sideways after hitting a bump in a corner . I guess that's a bad thing, but WAY more fun . I have no issue with the corvette having rear leafs; it shows that the engineers like to have some fun.
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Old 03-31-2005, 08:40 PM
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KnifeEdge_2K1 KnifeEdge_2K1 is offline
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the corvette has leafs at all 4 corners
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Old 03-31-2005, 08:53 PM
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Quote:
Originally Posted by KnifeEdge_2K1
the corvette has leafs at all 4 corners
really? haha that completely changes my mind
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Old 03-31-2005, 09:09 PM
culver culver is offline
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Quote:
Originally Posted by Matra et Alpine
Excellent post and thanks for the pic.

We debated this before and nobody coudl confirm for me where the leaf runs.

The BIG issue with leaf springs is the limited movement and control. So variable rate and LONG travel is very difficult.

Also using the leaf as sprign AND anti-roll bar is not the best idea as a spring of any form is a compormise in a suspension anyway and to have it do 2 jobs and meet TWO sets of compromises is likely why others steer clear of it. BTW leafs are used in many modern small Eureopan cars, so the world does know how to do it :0

Also you chose to compare composite leaf and steel coil. You can get lightweight coils too. It's best NOT to justiofy anuthing by comparing appleas and oranges. And to suggest that the few poinds in a coil make the car slower is forgetting that the driver is present - you're talking about the weight of a good dump and decent meal

Can you please explain the bending the shock in a coilover , cos it doesn't UNLESS the coilover is operating non-perpendicular to the movement and in that case it's happening to the shock whether it has a spring on it or not

You recognsie the drawbacks as well and is a most informative post.

Thanks again.
The coil comparison was based on the C4 when GM said they saved weight. Presumably it was a savings over production suitable coil springs.

This is a link to a document on Hyperco's website explaining the issues with coaxial spring loading on shocks:
http://www.hypercoils.com/DOC/Centra...-Load-Axis.doc
It's part of their information on their hydraulic spring perches.
http://www.hypercoils.com/Products/H...ing-Perch.aspx

Thanks for the good feedback!
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Old 03-31-2005, 09:27 PM
culver culver is offline
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You can kind of see the spring in this picture. This is under the motor of a C5. There is a dark plastic looking thing that looks like it extends though the V of the suspension arms. That's the leaf spring. Each half holds up one corner of the car. As was mentioned in one of the links above the spring actually provides some role resistance. This allows GM to achive their desired total role rate with a smaller anti-role bar.
http://www.topflightcorvette.com/Str-1.jpg

This one is a picture of the rear suspension
http://www.topflightcorvette.com/ima...suspension.jpg
The spring is the black plastic thing that runs just under and parallel to the lower A arms.
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Old 03-31-2005, 09:39 PM
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BTW if they were to use leaf on the C5-R/C6-R it'd be more difficult to swap spring....While at the same token this technology of transverse leaf might not be beneficial in a NASCAR type of world of asymmetrical car setup as a single transverse leaf spring for 2 sides guarantees symmetrical rates....
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Old 04-01-2005, 04:37 AM
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Matra et Alpine Matra et Alpine is offline
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Great stuff Culver.

I'd never heard of that as an issue. Mind you a movement of 15 thous is well within the tolerances of the operation and I don't think is that relevant to a damper that is operating non-parallel as the forces initially try to bend the shock anyway.

Not sure if they're comparing like with like as competition shocks usually have a shaped cup ( most ordinary production have flat ) which supports most of the spring for it's first coil. By supporting the spring along the complete first coil and not relying I'm not sure if ti fully removes the need for there hydralic cup or not. Good stuff.
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Old 04-01-2005, 05:29 AM
culver culver is offline
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Quote:
Originally Posted by Matra et Alpine
Great stuff Culver.

I'd never heard of that as an issue. Mind you a movement of 15 thous is well within the tolerances of the operation and I don't think is that relevant to a damper that is operating non-parallel as the forces initially try to bend the shock anyway.

Not sure if they're comparing like with like as competition shocks usually have a shaped cup ( most ordinary production have flat ) which supports most of the spring for it's first coil. By supporting the spring along the complete first coil and not relying I'm not sure if ti fully removes the need for there hydralic cup or not. Good stuff.
It is admittedly a small difference. The difference between 1st and 2nd on the track is pretty much a non-issue with street cars.
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Old 05-09-2005, 06:28 AM
culver culver is offline
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I know this is an old thread but the link below has some great pictures of the new Corvette Z06 including some very good pictures of the leaf springs.

http://www.z06vette.com/forums/showthread.php?t=80265
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Old 05-09-2005, 06:53 AM
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Quote:
Originally Posted by culver
I know this is an old thread but the link below has some great pictures of the new Corvette Z06 including some very good pictures of the leaf springs.

http://www.z06vette.com/forums/showthread.php?t=80265
Great find Culver. Those are excellent pics.
Really clear how that rear works.

I was surprised to see how narrow the two clamps are mounting the leaf.
With it so narrow, there will be a natural frequency of suspension movement which will setup a harmonic oscillation sweet-spot of wavelength equal to double the distance between the clamps. There will also be harmonics at 2, 4 and 8 times that frequency of movement.

on SOME surfaces those harmonic points will AMPLIFY the movements and worsen the handling. Does anyone have rates for the leaf ? We could then estimate those harmonics and hence KNOW when the rear will get "jiggly". I hope it doesn't' match the frequency when driving over cobblestones or we'll see lots of Vettes smashed on Edinburgh streets { If only we saw ANY Vettes in Edinburgh }

Could be an opportunity for an additional damper on the centre section of the spring. Wouldn't need to be big, I think even a bike steering damper might introduce enough. Otherwise when setting the suspension ratings the mechanics going to HAVE to compromise to use the main dampers to control it
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