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  #1  
Old March 10th 05, 06:53 PM
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Default leaf springs

I copied this off a corvette forum:
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
Corvette's leaf spring is "outdated" technology, keep in mind
that the composite leaf spring was introduced as an option in 1981 and
in it's current layout (acting as a partial anti-role bar) in 1984.
It's 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 axle's movements among
other flaws. I'm 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. It's also about the only type of
leaf spring suspension still in use. It's cheep, durable and handles
badly. It suffers from friction between the leaves and from poor
control of the axle's location.



3. Golf cart style transverse leaf spring:

I couldn't 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 (can't 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 axle's 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 it's 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 it's 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 a

-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 won't. 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 don't 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 C3's 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 don't 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 you've 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).





If it's so good why don't other people use it?

It's legitimate to ask, does GM know something that Ferrari, Porsche
etc don't know or are the people at GM just being pig headed and
sticking with "outdated" 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 can't retrofit Corvette style
leaves onto other cars because you would have to add mounts that
don't exist on the regular car.

-GM and their supplier spent a lot of time and money developing the
Vette's 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. It's
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 isn't 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
isn't 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 Corvette's 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
doesn't 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 car's CG). At the front the driver's 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 car's 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 don't 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 don't
start off with a production Corvette) the suspension geometry is so
different that they couldn't just mount a C6 leaf spring. It's 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 isn't 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 it's a smaller market.



Why don't other cars retrofit leaf springs? Well they also don't
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. It's 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
Vette's spring rates then I will need to go to coils. Also, if I want
to totally get rid of the Vette's anti roll I need to dump the leaves
because they provided some roll resistance.



BUT...

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 didn't 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 isn't true of the
composite leaf spring.

The End... almost

This is most everything I know about the Corvette's use of leaf
springs. I would love to hear from anyone who has additional insight or
questions.

Ads
  #2  
Old March 10th 05, 07:10 PM
Dad
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Posts: n/a
Default

When you get a chance, take a good look at how the springs are mounted on
the C6 versus the C5, neat huh?

--
Dad
05 C6 Silver/Red 6spd Z51
72 Shark Black/Black/4spd


  #3  
Old March 10th 05, 08:07 PM
Zorin Inc.
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Posts: n/a
Default

Funny you should bring up the leaf springs, my 95 was sagging in the rear
left... I was hoping it would be just an adjustment or maybe one of the
shocks lost it's precharge, etc... turns out my rear leaf was busted midway
between the diff and the wheel, splintered. Anyhow, an hour to swap it and
$299 for the new one later and I'm good to go again.

Zorin


  #4  
Old March 10th 05, 11:42 PM
RicSeyler
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Default

I have the lowering kit on my '94. In the rear I have
the hanger bolts with plenty of threads for adjustment
up or down.

Zorin Inc. wrote:

>Funny you should bring up the leaf springs, my 95 was sagging in the rear
>left... I was hoping it would be just an adjustment or maybe one of the
>shocks lost it's precharge, etc... turns out my rear leaf was busted midway
>between the diff and the wheel, splintered. Anyhow, an hour to swap it and
>$299 for the new one later and I'm good to go again.
>
>Zorin
>
>
>
>


--
Ric Seyler


 




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