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[Z06] Stock transverse leaf spring properties

Old 02-05-2008, 06:42 PM
  #41  
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Originally Posted by wishihad1-2 View Post
Randy,

As we both agree that leaf does"crosstalk", do you consider that a good thing or a bad thing. When you sell a suspension upgrade kit do you pull the leaf because you feel there is something inherently wrong with how it works, or is it simply because you have found it easier to get the job done with the coils, dampers and anti-roll bars you have at your disposal?

I ask because I've seen many people disparage the design GM has chosen. I personally think it's clever and see nothing inherently wrong with it. I’ve seen a number of people who claim that this or that handling trait of the Corvette is due to the use of the leaf springs vs coils. They say this instead of saying that this or that handling trait is due to the combination of damping, wheel and roll rates chosen by GM.

However, I can see that the upgrade options are somewhat limited. I suspect that as a tuner there are more spring rates (and suppliers) available to you if you go coil vs leaf. Of course I could be wrong so I would be interested to know your take on the subject. Why did you guys choose coil overs vs a new set of dampers and a new leaf spring?

Given the factory shocks, and factory chosen spring and roll rates, do you think the car would have been better, worse, or effectively the same had the factory gone with coils (assuming they could fit) vs the leaf spring?
Personally,
The suspension meets all the Cafe standards and that makes them sell more cars which is a good thing all around. I would jump in one and drive it across the world, even if I were 80 years old.

I hate the stock suspension, enough droop travel to run the Baja 1000. Cheap shock (37 miles and one was leaking). The leafs do crack and ends get crappy over age. The swaybars on the C6 Z06 are the only thing worth anything. A bit of a upgrade up front will help some cars out. The way the leaf changes spring rate is completely different then a coil spring. I was around back when we raced one coilover car and one leaf spring car, the only time the leaf cars had the advantage was on fast smooth sweepers. The coilover car rocked over the leaf car. I was pretty young, but remember when we had about 50 different leaf springs, with all different rates. Some people call that cheating, we called it factory spec. The differences between them were pretty small but they had some. The coils that we use from Hypercoil are within a couple pounds even after a year of racing. The stock leaf bushings are crap, and do move around too much, also, what the hell is it with the fiction pads on the lower control arm. That is a big joke to me. The stock bushings are crap, the car pushes, and the wheel track is off a little.

Why do we convert leaf cars to coilovers. Well we are changing up a lot of different things, shock lengh, spring rates, spring rate change, the overall handling of the car. Why not use the leafs, well I would make more money doing that. So maybe I should start selling leafs, like we had the option in 1990.

Coilovers make our job easier to R and D new things. Hence everyone wins!!!

Randy
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Old 02-05-2008, 06:52 PM
  #42  
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What generation Vette were you involved with racing?

It sounds like you don't see the concept as flawed but you feel do you do a better job as an aftermarket vendor with coils. That seems like a very prudent choice. Would I be going out on a limb in saying that you agree that stock vs stock there is nothing wrong with using the leaf spring? And further that the stock suspension, leaf or no has loads of upgrade opportunities? I’m basically asking if you think GM was wrong in using the leaf or simply that it neither helps nor hurts, it just is.

BTW, I appreciate getting your take on the subject.
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Old 02-05-2008, 07:22 PM
  #43  
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Originally Posted by wishihad1-2 View Post
What generation Vette were you involved with racing?

It sounds like you don't see the concept as flawed but you feel do you do a better job as an aftermarket vendor with coils. That seems like a very prudent choice. Would I be going out on a limb in saying that you agree that stock vs stock there is nothing wrong with using the leaf spring? And further that the stock suspension, leaf or no has loads of upgrade opportunities? I’m basically asking if you think GM was wrong in using the leaf or simply that it neither helps nor hurts, it just is.

BTW, I appreciate getting your take on the subject.
The car works very well for 85 percent of the owners. The car is fast, cheap, and puts down all the performance numbers. I may regret saying this but, Yes the factory leaf works very well for the street-light track. For all the requirements that GM has to meet, the leaf springs meet or exeed the goals and raise the bar for everyone else. Building a factory style car has many different PITA, that us aftermarket companies don't have.

Yes the platform has many opprotunities to make it better

What cars have we raced, C3-C4-C5-C6 and soon C7-C8-C9-C10-C11-C12

Randy
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Old 02-05-2008, 10:34 PM
  #44  
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Originally Posted by wishihad1-2 View Post
Check out the 4th paragraph after the picture of the spring.
http://temp.corvetteforum.net/c4/gcr...ass_spring.gif
BTW, that section was from Micheal Lamm's book on the '84 Corvette (I got it from Wiki)

Many of the first year reviews of the C6 Z06 said it was skittish at the limits. The problem was corrected the next year. GM changed the shocks, not the springs.

I believe at least one of the guys who used to sell coil over conversion kits has since changed to selling a kit that used helper springs on the dampers but retains the original leaf spring.
I've seen that write up before on the C4 composite leaf spring, nice little marketing. I'm skeptical of their claim of the 1.8 ratio. No way, no how. Not on the front and not on the rear the spring will move so much between the mount points that it could amount to 1.8. An antiroll bar can twist in its mount freely the leaf is mounted two places which will limit its movement. The write up also claims that in "future" developments the antiroll bar could be simply eliminated. If it is so, why do we still have them?

Coil springs rates are linear the leaf spring solution is not.

I believe GM changed the shocks on the Z06 in an attempt to control wheel hop and oversteer, not skittish handling over bumps in a turn. On the ZR1 they changed the halfshaft diameters to reduce wheel hop. GM has NOT fixed the skittish handling of the Z06 with a new shock.

I don't think much of the half n half solution offered by "Caraweight", they do have nice Dymag wheels though.

If you looking for "inherent" flaws with the leaf springs how about friction pads, non-linear spring rate for starters.

The leaf springs are pretty good, if you want better you need coils.
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Old 02-05-2008, 11:39 PM
  #45  
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GM put stiffer shocks on the Z06 because increased suspension control was needed due the heavier wheels / tires. Also, the car is lighter and the weight distribution has changed.
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Old 02-06-2008, 12:04 AM
  #46  
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Originally Posted by petefias View Post
I've seen that write up before on the C4 composite leaf spring, nice little marketing. I'm skeptical of their claim of the 1.8 ratio. No way, no how. Not on the front and not on the rear the spring will move so much between the mount points that it could amount to 1.8. An antiroll bar can twist in its mount freely the leaf is mounted two places which will limit its movement. The write up also claims that in "future" developments the antiroll bar could be simply eliminated. If it is so, why do we still have them?

Coil springs rates are linear the leaf spring solution is not.

I believe GM changed the shocks on the Z06 in an attempt to control wheel hop and oversteer, not skittish handling over bumps in a turn. On the ZR1 they changed the halfshaft diameters to reduce wheel hop. GM has NOT fixed the skittish handling of the Z06 with a new shock.

I don't think much of the half n half solution offered by "Caraweight", they do have nice Dymag wheels though.

If you looking for "inherent" flaws with the leaf springs how about friction pads, non-linear spring rate for starters.

The leaf springs are pretty good, if you want better you need coils.
The mount points allow the spring to pivot. Like a suspension bushing the points limit vertical travel but they allow relatively free rotation. The older centrally mounted leaf spring (C3, C4 rear) was designed to be a rigid mount. The wide spacing allows the movement described in the text as well as the "crosstalk" Randy illustrated in the pictures. From what I recall reading somewhere the original hope was to do away with the anti-roll bar. From what I understand that would have required spring mounting locations which were not possible within the constraints of the car's design (sorry, as I can't give a citation this is only worth what you paid to read it).

I certainly see no reason to doubt the 1.8 ratio claims. It is possible that the GM engineers were not thinking of the Corvette but the other cars on which they were thinking about using the same system. Consider that GM did use the same technology on the Lumina. It's possible that the spring w/o an additional anti-roll bar might provide enough roll stiffness for a family sedan.

Coil springs can be both linear and progressive. The same is true of the leaf spring. It comes down to implementation.

I have never driven a C6 Z06 so I can not say with authority however the reviews I've read suggest the changes to he dampers for year two did quite a bit to help with skittishness. Also, do we know what actually is causing the ill handling traits? It is the dampers, the springs, the roll stiffness, or even the geometry. Why should be believe that the spring is the culprit as opposed to the others?

I will admit I am not so intimately familiar with the Corvette's suspension as to be able to say where the "friction pads" are. Are you talking about the pads between the leaves on the C3 Corvette?

As for progressive vs linear, my understanding formed composite leaf spring in conjunction with it's mounts can be either or. However, you may be right that once you choose one you are not free to retrofit the other.

Again, I think Randy made a very good case for using coils for aftermarket work. That doesn't mean that GM was wrong for using leaves in the first place.
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Old 02-06-2008, 12:11 AM
  #47  
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One of the major weaknesses, in my opinion, are the Sachs shocks. They are junk, even the "nurburgring-improved" ones. The old C5 shocks were downright dangerous in their lack of rebound control. The $300-a-set Bilstein shocks far outperform the Sachs.
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Old 04-30-2008, 10:45 AM
  #48  
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Recently I created an FEA model to show the flex of the leaf spring under load. It demonstrates how the wide mount points cause an upward pull on the right side to also somewhat lift the left side. This is where that anti-roll bar affect comes from. It is also the "crosstalk" term described above. You can see that the wide spacing of the pivots allows the center of the spring to flex down while raising the end of the spring on the opposite side.

By the way, you can see this in real life if you have a metal ruler or other flexible beam. Have someone hold the ruler about 1/4 of the length from each end. When you pull up on one side the center will drop and the opposite side will rise up. Again this is the same thing that would happen when you have an anti-roll bar on the car.

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Old 04-30-2008, 11:10 AM
  #49  
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Originally Posted by TTRotary View Post
One of the major weaknesses, in my opinion, are the Sachs shocks. They are junk, even the "nurburgring-improved" ones. The old C5 shocks were downright dangerous in their lack of rebound control. The $300-a-set Bilstein shocks far outperform the Sachs.
all OEM shocks are junk. I forgot how many years ago but they were paying 9 bucks for a standard car shock.
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Old 04-30-2008, 12:31 PM
  #50  
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Originally Posted by wishihad1-2 View Post
Recently I created an FEA model to show the flex of the leaf spring under load. It demonstrates how the wide mount points cause an upward pull on the right side to also somewhat lift the left side. This is where that anti-roll bar affect comes from. It is also the "crosstalk" term described above. You can see that the wide spacing of the pivots allows the center of the spring to flex down while raising the end of the spring on the opposite side.

By the way, you can see this in real life if you have a metal ruler or other flexible beam. Have someone hold the ruler about 1/4 of the length from each end. When you pull up on one side the center will drop and the opposite side will rise up. Again this is the same thing that would happen when you have an anti-roll bar on the car.
If someone were to be concerned about the crosstalk, couldn't that be easily eliminated with a couple of additional clamping points in the center?
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Old 04-30-2008, 02:48 PM
  #51  
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Not really. First, you wouldn't want to remove the "crosstalk". Remember the crosstalk is the anti-roll spring action. It is exactly the same action as an anti-roll bar. The tires don't care if the anti-roll spring rate comes from a twisting bar or a bowed spring.

Also, if you were to clamp the center of the spring you would in effect create a much shorter, much stiffer spring. In all likelihood this would cause the center of the spring to fail.

Really, there is no reason to not want what this spring does. Having done a patent search Ford, Honda as well as a number of non-auto makers have filled recent patents around variations of this spring design. It really is a great, if not always cost effective, design. The Corvette wouldn't have been any better had GM decided to use coils instead of this leaf design. Heck, remember that it costs GM MORE to use the leaf spring. You have to figure they didn't use it just to be different.
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Old 04-30-2008, 03:10 PM
  #52  
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Now change spring rates. What happens then???

Randy
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Old 04-30-2008, 03:27 PM
  #53  
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Clearly in the Corvette the roll rate and bump rates are more closely tied together than in a car with conventional coils and an anti-roll bar. However, even in a conventional coil spring setup the roll rate is affected by the coil spring rate. Almost all cars, Corvette included, will have some measure of roll resistance even without an anti-roll bar (unless you are dealing with a Formula Vee with a zero roll rate rear end). With conventional coils if you stiffen the springs you will need a softer anti-roll bar if you want to have the same effective roll rate.

Of course for that large percent of owners who never change the suspension components out it doesn’t mater if the step up is harder to tune so long as it works well once tuned by the manufacture.
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Old 04-30-2008, 05:21 PM
  #54  
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I think there are different reasons to go coil-over vs. leaf spring and each may work for that person in a particular situation or in a particular class. It comes down to a personal decision based on your situation and how much time/money you want to invest.

I had the option of going to a T1 package, VBP, or coil over package for my C6Z. The car was/is going to see HARD track duty with minimal to no street duty, so that made my decision easier from the start. I wanted a track focused package with pretty stiff spring rates for very sticky tires....PERIOD!

When all was said and done the T1 package was overpriced and provided very minimal change from the stock C6Z package (as far as spring rates and shock quality). The VBP leafs are just one component and the rest of the set up is completely custom and is up to you to properly maximize (VBP sells the springs, so you need to match the shocks and get them valved correctly and choose the correct sway bar). So this is still pretty expensive option (like T1), there is a lot of potential guess work, and it may or may NOT work the way you want when all is said and done...kind of like here are a dozen choices....good luck, tell us how it works out!

I was not going to do the installation myself, so time was money and I could not afford to patch something together, pay for the install and have it not work correctly day one! Someone with the time, money, and expertise might find this their best option...who knows.

This left me with the coil-over option...very proven, relaible, relatively simple (no need to custom valve shocks to spring rates), easy/cheap to install, tons of adjustability, and high quality parts available. Most importantly, I could get the entire kit (shocks, springs, sways) for about $1000 cheaper than say a T1 package or potentially a well thought out VBP package. Also, there are enough very proven kits on the market (LG, DRM, PDFAT) that work...no muss, no fuss, they work. So for me the choice became easy. The coil overs were cheaper, as good or maybe better than leafs, and they are proven to work delivered in the box.

The only downside for the coil overs is the streetability, but that is more spring rate. The LG coil overs I installed are pretty darn stiff and not always comfortable on the street, but that is what I wanted.
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Old 04-30-2008, 05:43 PM
  #55  
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I think Randy's justification for using coilovers is absolutely solid. If he can do his job better and cheaper with coils then they are the way to go for his customers.

I'm also saying that from the factory, the car is absolutely no worse, and likely better because GM chose to use the leaf springs instead of coils. This assumes the same dampers and large scale production coil springs (which for what ever reason always seem bigger than aftermarket springs on just about every car I've ever looked at be it a Civic or Miata or Mustang).
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Old 04-30-2008, 08:33 PM
  #56  
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My newly installed Pfadt"s set between 1-16 at 6 front 6 back seem to have the exact same ride as stock on the street ,at 8/8 the ride is rough,however the spring rate for hard turns is "50" stiffer in either setting and the turns a very flat. That translated to more friction on the road and more control.
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Old 04-30-2008, 09:21 PM
  #57  
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Originally Posted by jan eric View Post
My newly installed Pfadt"s set between 1-16 at 6 front 6 back seem to have the exact same ride as stock on the street ,at 8/8 the ride is rough,however the spring rate for hard turns is "50" stiffer in either setting and the turns a very flat. That translated to more friction on the road and more control.
you're attributing more control to the wrong aspect of the setup. riding flatter in the corners does not (necessarily) equal more friction on the road. in fact it can mean less.

just because the body rolls does not mean the tires have less contact area.
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Old 05-02-2008, 03:11 PM
  #58  
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Originally Posted by mousecatcher View Post
you're attributing more control to the wrong aspect of the setup. riding flatter in the corners does not (necessarily) equal more friction on the road. in fact it can mean less.

just because the body rolls does not mean the tires have less contact area.
Thank you, I'm not the only one that understands this.

Randy
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Old 11-23-2008, 08:18 AM
  #59  
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I just became aware of this exciting discussion and I would like to add my personal experience and rise a question.

First, I have a C5 coupe (Z51 option) that I now drive in Germany. At the beginning I had tremendous problems driving fast on the autobahn because of severe side hopping in fast (120+ mph) curves on bridges (there the pavement is interrupted by some sort of connection pieces that compensate the flex of the bridge and that are somewhat bumpy and slippery, especially in the wet). To make a long story short: that problem was gone when I switched to Koni FSD dampers (FSD stands for "frequency selected damping" and that system is supposed to create higher damping forces for fast hits than for slow ones).

Now I frequently drive the car on the Nürburgring Northloop where I enjoy the very forgiving handling (I use non-EMT Michelin Pilot Sport PS2 tires), especially in the wet. Two dealers in my region offer coil over kits for the Corvettes together with (I believe) Bilstein dampers, but up to now I refused their offers for fear of actually deteriorating the handling of my car.

Now to my question: Somebody told me that the leaf springs in late Corvettes (that is, C5 and C6) actually have a degressive spring rate. That is, as the wheels travel due to the spring being compressed by the effect of a bump on the road, the force pushing the wheels against the road actually decreases as travel increases. Mathematically, the k in Hooke's law

F = -k x, k > 0

(where F is the force acting in opposite direction of the elongation x of the spring) then would be a function of x and actually decrease as x becomes larger. Of course one would rather prefer the opposite, that is, a progressive suspension to increase the resistance against bottoming out.

Indeed, on the Northloop there are a few instances where my car seems to bottom (e.g. after the "jump" before Schwedenkreuz and when I enter the Karussell) although not that hard that handling seems to be affected. Also in some twisty high-speed (120+ mph) sections (e.g. Fuchsroehre and Kesselchen) I feel that the suspension of my car works too much (that is, either the springs or the damping or both are too soft) so that I usually do not drive there as fast as the engine might allow.

So again my question is: are the suspension forces in late Corvettes linear, degressive or progressive with respect to wheel travel?

Thanks, Robert

Last edited by UncleRobb; 11-23-2008 at 09:42 AM. Reason: Grammar
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Old 11-23-2008, 12:43 PM
  #60  
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Originally Posted by wishihad1-2 View Post

The only problem with this picture is.

The one side has a ton of bump travel, which the car will never see because the stock shocks are bottomed out by then.

The other problem, the other side, to show this myth of a swaybar affect doesn't work either. GM has always used no attachment point for the way up. The pads on the front only act under bump and the rear has bolts. This theory would be ok if they attached the upper part of the spring.

Randy
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