CMiller95

I feared a 450lb front spring was going to be too stiff since it was more than GM used. I'm glad to hear that the 600 lb still rides nice! If I really get into Autocross or something I will look into investing in new springs. I currently have my single adjustables on their lowest setting, I figured once I get used to how it handles again I will play with the settings and find the best spot!

Thanks!
Colton

VikingTrad3r

Cmiller, have you read about this being done somewhere?

From your photos im trying to visualize what you mean.

like take the whole coil and strut assembly and flip it 180deg?

that would fit in the early c4 shock towers ?

STEVEN13

I am considering doing this to my 92 as I have coilovers in the rear (solid axle). My current front spring is VBP adjustable which I installed for track events (road course).

Do you have any issues with it bottoming out? How does the car sit in the front. Any photos?

Nice job!
Steve

MatthewMiller

I realize this is an almost 2-year-old post, but I somehow missed it the first time around. So...
I have some questions about the wheel rates the OP calculated for the leaf spring up front. I ahven't found an accurate published figure for the motion ration of a C4 monoleaf and LCA. Just eyeballing a C4 lower control arm, it looks like the MR is closer to 0.66, because the spring pad contact area is about 2/3 of the way from the inner control arm pivot to the ball joint. It depends on where you measure the pad contact point.

The Hib Halverson chart where you got that 30.8 number is wrong, I think. For that Challenge spring rate, he's figuring a MR of 0.51. OTOH, for some other springs on that same chart, he has a MR of 0.59, others are 0.57, and so on. They're all over the place, and I'm not convinced any of them are accurate. The Challenge front spring had a rate of 659lb (115.5N), but if the MR is 0.66 then it gives a wheel rate of 287lb. Even with Halverson's range of MRs, it's wheel rate is anywhere from 171lb to 229. So I think your 450lb coilover spring is actually in a similar range to the Challenge front leaf spring. Also, I don't understand how he figured the rear wheel rates. It should be 1.0, since the leaf spring ends connect directly to the upright.

I do agree with your wheel rate calculation for your 450lb coilover spring, and I agree that the MR for the rear is 1.0. It is also 1.0 in the rear for the leaf spring, so the rates are directly comparable with coilover spring rates.

Aside from the different thoughts on wheel rates of stock leaf springs, I think it's important to understand that front spring rates have very little to do with perceived ride quality from the driver's seat, especially in a C4. Springs in general don't generate as much force as most dampers in high-speed bump situations - at high shaft speeds the shock has a higher rate than the spring, by a lot. So in general, shock rates have more to do with ride comfort than spring rates. Furthermore, in a C4, the occupants sit very close to the rear axle line, and far from the front axle line. Whatever bumps the front end encounters will barely be felt compared to bumps the rear wheels encounter. So it makes perfect sense to me that 600lb springs wouldn't make a noticeable difference in ride quality compared to 450lb springs. Also, you mention you have (had?) the dampers on their softest settings. That has a lot more to do with your improved ride quality with the coilovers than the form of the spring. The wheel rate is the wheel rate, regardless of whether it comes from a monoleaf or a coil or a torsion bar. Coil springs don't inherently ride smoother than monoleafs. Lots of people think they do, but there's no explanation for why that would be true.

Careful here. Again I think that Halverson chart has incorrect wheel rates. But also, don't forget that in cornering, swaybars add to the wheel rates also, and they also have motion ratios as well as lever arm ratios. C4 front swaybars have much shorter arms than their rear bars, but a roughly similar MR as the front monoleafs (maybe 0.6ish?), whereas the rear bars have long arms but a 1:1 MR. I haven't done the math to figure their wheel rates (I note that Halverson didn't either), but just be aware that for figuring wheel rates related to cornering balance, the swaybars need to be accounted for.

I don't understand this statement at all. How does efficiency relate to springs?

blackozvet

I don't understand this statement at all. How does efficiency relate to springs?[/QUOTE]

I will support the comment with my experience, as i have been racing with the transverse leaf and coilovers (and going between using both this year)

Running at my home hillclimb track (which is quite bumpy entering, during and exiting turn 2) and using the z51 front spring rated at 582 lbs the car was bottoming out quite badly.


I fitted 600 lb coilovers - keeping in mind that the mounting point is above the leaf mounting point so motion ratios and wheel rates should be a moot point - and the bottoming out was gone. Chassis heights the same. Car was quicker.
I talk to people (who know more about the subject than me) and this comparison is given to me, imagine the coil spring upright - it operates at 100% of its potential efficiency, lay it over 10 degrees and it loses spring rate, lay it over 20 degrees and it loses even more spring rate. Now imagine a lateral leaf spring laying on its side - how much spring rate is needed to operate in a performance environment, double, 1000 to 1200 kg spring rate, double what you would need if you were running a coil spring. you could argue all day long about coil v leaf but I guess that tells a story.
Having said that Im not all pro coilover and anti leaf, my car is slightly quicker with a leaf in the rear than the coilovers !

Nokones

Be very careful with the coil overs on a C4. Coil overs are a great idea and since Vette Brakes and Products no longer exist along with the custom spring rates, coil overs now are the best option. However, the front upper shock mounts were not designed to carry the load. Also, the lower rear shock mounts were not designed to carry that load. I have seen pictures where as the rear lower shock mounts have separated from the spindle. The front shock mount will need to be reengineered and fabricated to carry the new loading. The lower control arm shouldn't be a problem because it is already carrying that load pretty much in the same spot.

A friend of mine that autocrosses a C4 is using 1,000 pound coil overs. I thought that may a tad too high of a spring rate but, he seems to be happy with that rate. I'm still happy with my Vette Brakes & Products adjustable 1470 pound monospring. If that breaks then I will have to go coil overs.

MatthewMiller

Motion ratios and wheel rates always matter. As noted above, wheel rates are really all that matter. You went from a wheel rate of 200-250lb with the Z51 spring (depending on the exact MR of the stock spring pad) to a wheel rate of 347lb: somewhere between a 38-73% increase in front wheel rate! I would imagine that makes a big difference in bottoming out and handling balance!

I don't understand this at all. A spring is a spring is a spring. They are all rated in the same way: how much force does it take to compress or extend the end a certain distance? A monoleaf rated at 582lbs/in requires 582lbs of force to move its end 1". A coil spring rated at 582lb/in also requires 582lbs to compress or extend its length 1". Same with a torsion spring (like a sway bar): it takes a rated amount of force to move it's arm a certain distance.

Control arms and their spring attachment points travel in arcs, not straight lines. A coil spring is only directly perpendicular to the direction of control arm travel at one instantaneous point in the arc of the control arm. But if you look at any coilover setup, the shock and spring are always canted inward quite a bit in order to clear wheels and to reach the frame mounting points. They never have enough bump travel to reach that perpendicular (i.e. 1:1 motion) point. OTOH, a leaf spring is installed pretty close to parallel with the control arms, and actually follows a similar (if not identical) arc as the control arm moves. Therefore, their load paths do a better job of staying close to perpendicular to the control arm's arc than a coil spring ever does. This isn't really a discussion of "efficiency," as much as it is about geometry. But in your context, the leaf spring is actually more "efficient." Add in the fact that the front monoleaf on a C4 actually does some anti-roll duty (allowing the use of a smaller/light front sway bar), and it actually is more efficient.

I have no personal experience with this. I worry a lot more about the rear lower mount than the front, since the former is a cantilevered chunk of aluminum. Frankly, I worry about it even when just using shocks with really high high-speed damping rates and/or that hit the bump stops a lot, let alone spring loads. Like you, I've seen pics where these actually broke. A friend of mine, Chris Ramey, recently built his C4 with coilovers for Optima events. He fabricated some steel braces to go around that part and reinforce it.

Yep, I find that most people tend to underestimate the spring rates they should be using in competition, especially for modern high-grip tires. My avatar pic is my car with its 1125lb VBP spring on BFG Rival S tires, and you can see it's still leaning a good bit in pure cornering loads. Obviously bumps and such confuse the situation, but we generally don't want to approach full-range suspension movement just from cornering and braking loads.

PS - I'm in the same boat as you regarding the demise of VBP, if my springs ever fail. Supposedly another company is working to fill their void, but who knows? Otherwise, we need someone to market braces for the rear shock mounts, at the very least. And folks with the narrow (early) front shock towers might have to do serious hacking to use high spring rates on their cars.

CMiller95

Definitely been a while since I've relooked at this stuff! On the wheel rates, I did use Hib's chart which looking back on it may be incorrect. Therefore the challenge cars may have had a higher wheel rate than what the chart states. The rear should definitely have an MR of basically 1. You are dead on with spring rates and ride comfort, the shock generally has a much larger impact on ride quality depending upon the scenario. (I.e. large bump, shock has larger effect , small bump, shock has smaller effect, this is due to travel velocity which is what dampers function off of). I agree that it doesn't matter what type of spring you utilize, wheel rate is wheel rate whether it is coils, traverse, torsion, etc... The more I think about the chart the more I disagree with some values. I can say that the car ride well on the street and the track with this spring setup. I did make some new upper coilover mounts for the front as the QA1 mounts had some inherent issues. For the rear I am using Vansteel's lower and upper mounts. I dislike the moment the lower coilover mount presents on the knuckle but quite a few people have abused them much further than myself and haven't had issues. If I get motivated one of these days I can go through and model the suspension and accurately get a motion ratio, although it does change throughout suspension travel.

CMiller95

I lowered it approx 1" and didn't bottom it out. I have since brought it back up to return to the factory height but again, not bottoming out issues. How the car sits is completely dependent upon the set ride height.
Colton

STEVEN13

blackozvet

1000 lb coils ! sounds like he is over compensating for a lack of sway bar rate.

blackozvet

The mounting point of the spring pad is directly under the shock mounting point - so what Im saying is that the motion ratios of the leaf and the coilover will be very similiar - there will be very little difference in the motion ratio - I wasnt saying that motion ratios dont matter.

"a spring is a spring is a spring" yeah, until you start laying it over and it starts losing it spring rate, then its not quite the same spring anymore.
if you need 1200 lbs of leaf to do what 600 lbs of coil do, then there is something quite different going on. And you could be right in that the sway bar effect of the leaf could be a contributing factor, my recent experience has convinced me that you need much more sway bar with coilovers than you do with leaf' springs (front and rear).

I understand that your very theory driven, my comments on the subject are as a direct result of converting from leaf to coilover, and my pragmatic findings.

JoBy

Some reading on how GM engineered the suspension.
Up front the leaf speing is also acting as a anti sway bar.
Not so at the rear because the center mounts are so close together.

blackozvet

This piece of theory has been around and accepted by many for a while, but maybe its just me - in my experience putting coilovers in the rear necessitated going to a bigger sway bar in the rear. I have been backwards and forwards between coils and leaf in the rear, needs small swaybar for leaf, needs bigger swaybar for coils.
I suppose people can theorize the sh.t out of anything, but I know what works on my car, maybe its coz I'm in the southern hemisphere and the water spins in the opposite direction down the drain, who knows ?
.

MatthewMiller

The shock mount is a little further outboard, at least on the 88-96 C4s. In the pic below, the red line is the axis on which the shock mounts to the control arm, and the yellow area is the spring pad. The early C4s have a plate that bolts on top, and the shock and swaybar bolt to it. Maybe on that version the MRs for the shock mount and spring pad are similar?



I'm not trying to be an a-hole when I say this, but I don't understand what you're talking about here. The leaf spring is oriented across the front of the car, such that the ends are bent vertically from rest by the control arm. However much force in lbs it takes to bend the spring's end up 1" is the spring's rating in lb/in. Whatever the motion ratio is, you square that and that is the wheel rate the spring provides. It's very simple. If you put a Challenge spring in the clamps, but not connected to anything, and apply an upward force of 582lbs, it will compress 1". Likewise, when the spring pad area of the control arm moves upward 1", the spring applies an additional 582lb to the pad. A coilover is rated the same way: however much force it takes to compress the coil spring 1", and again you square the MR to get the wheel rate.

You don't. If the MRs were the same for the coilover and the leaf, then a 600lb coil would provide the same wheel rate as a 600lb leaf spring (except that you could use a smaller swaybar because of the front leaf's swaybar function). On a later C4, the MR for the coil spring is higher, but not that much higher. If we use a MR of 0.6 for the leaf spring, then the equivalent of a 600lb coil would be a 963lb-rated leaf. But again, that has nothing to do with "efficiency" - it's just a matter of the different MRs. If GM connected the ends of the leaf spring directly to the upright (like they did on the rear suspension), then the tables would turn and you'd only need a 347lb leaf spring to get the same wheel rate as the 600lb coil.

Tom400CFI

MM is right. It seems like you're confusing yourself by thinking about the leaf spring, in the context of how you think of a coil spring mounting; you're right that as you lay the coil over, it looses it's mechanical advantage on the arm, and you need more and more spring rate to maintain wheel rate. But the Leaf doesn't work in a telescoping fashion. It works like a coil that is perpendicular to the arm at all times. "laying it over" is misguided thinking, and you don't need a 1200 lb leaf to do what a 600 lb leaf can do....in spite of the leaf contacting the arm further inboard than the coil/shock does.

blackozvet

The red line is where the end of the leaf pad mounts on my 85 - directly under the coilover. Different to the later cars.



FACT - I fitted a 600 lb coil in place of a 600 lb leaf and it performed much better and stopped bottoming out.

FACT - If you want a 600 lb leaf to perform as well as a 600 lb coil you have to replace it with a 1200 lb leaf.
.
Being the pragmatic person I am I have no time for people splitting hairs over what I say and turning them into long winded technical jargon arguments.

MatthewMiller

Oh, maybe that's what blackozvet is thinking. I couldn't understand what he meant by "laying it over on its side." Okay, yeah. A leaf spring is rated with a force applied perpendicular to the length of the leaf, whereas a coil spring is rated with a force acting in line with the coil axis. Think of a monoleaf like a diving board. A diving board at a pool is nothing more than a big-*** monoleaf spring. If a 200lb guy stands on the end of it and it bends down 1", that diving board would be rated at 200lb/in. The leaf springs in Corvettes work exactly the same way, except the force is pushing up from underneath instead of down from on top. Whatever they're rated is the actual force they exert on the control arm, just as with a coil spring.

That's interesting, and I appreciate the info. I didn't realize the early C4s were that much different. So the MRs for the leaf vs coilover are at least very close. Good to know.

There are a number of possible explanations for your change in handling/performance that don't involve the form of the spring being used:

• You didn't just fit a 600lb coil in place of a 600lb leaf. Actually, you installed a 600lb coil plus completely different dampers (I assume the damper is different, at least). The dampers would presumably have a very different damping curve than what they replaced; and based on your pic they're also double-adjustable, so the overall rate could be a lot different.
• The new dampers may also have different bump stops that allow greater compression travel and/or have a more progressive "squish" to them so that you don't feel the bottoming out as with stock.
• Furthermore, there could obviously be a difference in front ride height with coilovers that have threaded collars for the spring seats, like yours. That could most definitely affect both handling and bottoming out.
• Your 600lb coil springs are actually starting out with a lower wheel rate than a 600lb leaf spring, because they are angled inward at 20*. If the control arm starts parallel to the ground, that angle means the coil is only applying 564lb of vertical force to the shock mount. As the arm moves upward into compression, it gets closer to perpendicular with the coilover, which means it's a mildly rising rate geometry from 564lb-600lb. The monoleaf is much more geometrically linear since it's load path to the control arms stays perpendicular to the arm throughout the suspension travel.
• Another possibility is that your leaf wasn't really a 600lb spring like you thought? Did you have an 84 Z51 spring in there? That would have been close at 582lb. No other OE spring for early C4s was even close to that stiff, and no C4 front spring of any year was rated at exactly 600lb. For that matter, how do you know for sure that your coils are actually 600lb? I've got friends in the industry who will tell you that when they actually test coil springs from aftermarket companies, they often turn out to have much different spring rates than their labeling says. Or owners cut them, which also increases their rates.

I'm sorry, but that simply isn't a fact. And I'm not trying to split hairs with you just to win an argument. It's really important that we not propagate misinformation on this forum, and the above quote and your statements about "efficiency" and laying a spring on its side are just flat wrong. As it is, too many Corvette owners spend money to switch to coilovers because they've been told that coil springs are inherently better than monoleafs, and that's simply not true.

blackozvet

ok - take your 1200 lb spring out and go racing with a 600 lb z51 spring and tell me how it goes.

grandspt

I know this doesn't have anything to do with coilovers but I can attest that these C4 suspensions are very sensitive to simple changes. A simple change to urethane bushings made the rear end of my car act pretty violently under power and in a corner. I swapped the urethane to the full Banski suspension and it got even worse.
What finally fixed this problem was adjustable Ridetech shocks. It appeared that I changed the friction/stiction of the suspension so much that the stock Z51 Bilstein shocks could not keep up.
But I can only imagine the mess that would become present by going to coilovers.
The GM engineers made the C4 suspension work well in stock form but any change seems to upset the hell out of these cars.
My theory is that the stock rubber suspension bushings were picked by GM very carefully. They wanted the rubber bushings to bind or have stiction.
I am sure the same kind of engineering went into the monoleaf springs.