Coil-overs or Leaf Springs–That be the Question.

[Hib Halverson]

I have some C4 suspension pieces listed in the ZR-1s and Parts for sale forum and the listing started a conversation which sort of progressed from being a F/S thread into a discussion of the FE7 suspension, aftermarket coil overs and SRC shocks. Because that's really hijacking a thread in the for sale forum, I wanted to move the conversation into the Technical posts area.

Spoiler alert!
This post discredits some "conventional wisdom" about coil-over-shock-absorber conversions on Corvettes with transverse composite leaf springs. My discussion of this issue is based on my experience in researching and reporting on Corvette ride and handling issues. It's also based on interviews I've done over many years with suspension engineers at GM, such as Scott Allman, who was the Lead Engineer for Ride and Handling during the late C4 era, along with other R/H engineering team members who worked on C5, -6 and -7.

First, let's look at where the idea to convert leaf-spring Corvettes to coils originated. It came from road race teams who ran C4-based race cars back in the 90s in classes which allowed modified suspensions and which wanted to have more choices in spring rates and ride heights which be used to tailor handing to different tracks. Since coil-over-shock assemblies allow one to easily adjust trim height, spring rate and, ultimately corner weights, racers liked to use them where legal.

Some who were building these road racing coil-over conversions, the resourceful capitalists that they were, decided to sell the conversions to C4 owners who were show car owners, street drivers or street/track users. The obvious reason for doing this was the business of selling racing-inspired parts for street use was far more lucrative than just selling the parts to racers.

Now, here's a key fact about suspensions: for a given wheel rate, the tire contact patch has "no idea" what spring/damper system is there. The ride rate curves with a leaf or any coil system on cars which GM engineers have measured are reasonably linear and similar in nature.

The pro’s of a coil spring conversions are:
1) Ability to vary rate, but there is a caveat: unless the damping is optimized, as well as the spring rate, ride and/or handling performance likely will not improve and may even be degraded.
2) Much easier way to adjust ride height and corner weights.
3) Ability to use a variable rate spring.
4) They look cool, especially on the C4 front end as, when the hood is open, they compliment the eye candy that is the LT5.

The con’s of coil overs are:
1) Because of the restricted packaging, a loss in ride travel. That is a big loss because, if you want a suspension to handle well and ride comfortably, as one of my suspension engineer friends says, "...ride travel is sacred."
2) A mass penalty. In many cases, coil-over-shock assemblies weigh more than a pair of composite leaves.
3) With coils you loose roll stiffness, so for a given ride rate, stabilizer bars must be larger. Bigger stab. bars weigh more along with sometimes creating new problems.
4) A lot of work to set-up properly.
5) They are a poor value for the street because of high cost to benefit ratio.

Relative to customers who believe the ride and/or handling would be better, even with coils: if they want a stiffer ride, they have to add stiffer springs. If they want a softer ride, they must add softer springs.*With both these conditions, without damper adjustments, neither will be optimized.*Indeed, there would be a perceived increase in vehicle response with higher rates, but the phasing of the steering, harsher ride, and tire saturation at the limit could be the down sides.

The aftermarket makes a killing selling coil-over packages. It's hard to believe the companies selling them have the resources, both analytical and subjective, to optimize their products in a manner which preserves the delicate balance between track performance and daily driving. What a lot of these aftermarket suspension vendors do have is good marketing. People who spend a ton of money on these packages, in their own mind, are going to think the car is better, but, in many cases objective testing shows they*are not "as better" as they might think and may not be any better at all.

Let's talk about "crosstalk". This is the characteristic of a transverse leaf that, when one side of the spring moves, the movement affects the other side of the spring. Crosstalk has similar effect on handling as does a stabilizer bar, it creates roll stiffness i.e.: it resists body roll. While crosstalk does occur, it does not occur to the extent the vendors of coil-over conversions would like us to believe. Nevertheless, a big selling point of coil-over-shock conversions is that they eliminate "crosstalk" but, on the other hand–and especially if the coil-over maker also sells stabilizer bars– everyone loves roll stiffness. So, is this a case of there being "good" roll stiffness (from stabilizers) and "bad" roll stiffness (from springs)? Perish the thought.

In fact, the elimination of leaf spring crosstalk is why, when you convert to coil-overs, you, also, have to add bigger stabilizer bars. If you don't go to bigger bars, then, so as not give away handing performance, you have to add a lot of spring rate and that tends to make the car's ride more stiff–which is the situation you are trying to eliminate in the first place...right?

Ok. Here's another myth propagated by those selling coil-overs: the rates of transverse leaf springs are progressive rather then linear. Take a look at a leaf off a C4/5/6/7. Ever wonder why they are shaped oddly, i.e.: their widths and thicknesses are not consistent? Well, that's done to make their rates linear. GM has been testing the composite leaves out of Corvettes since 1980. They may have started as somewhat non-linear 36 years ago, but by the C4 development, their linearity was enough that what progression in rate was left was so minor as not to be important.

Then we have the typical response of aftermarket companies selling coil-over conversions that a C4 with coils will ride nicer than a one with stock FE7 suspension. Of course it will–but only as long as the coils have lower rates. So, now, we're back to an aforementioned problem with coil-over conversions: since you give up roll stiffness and then you use soft spring rates to get the ride where you can back up your claim that cars with coil-overs ride nicely, you end up with wheel rates the same or less then even base C4s. Reality is that, if the goal is better handing, even with coils, you have to give up ride for handing, so in that respect, coil-over-conversions are no different than leaf springs.

To convert to coils and maintain roll stiffness, their rates have to be quite high and that would degrade ride quality with a net gain of zero compared to the factory suspension. You could go down in the rates of the coils and get the roll stiffness back by significantly increasing stabilizer bar sizes, but then you end up with the "soft springs/big bars" problem: lots of "head toss" when driving on roads which are uneven or potholed. Drive a car with really big bars and soft springs on rough roads and the head toss gets old really quick. Finally, when you go the soft springs/big bars route on a C4, you need to be careful because, due to the car's weak structure, you can get to a point with bar rate increases where you start bending the car's structure. Once you start doing that, the car's handling gets a bit spooky at the limit.

As for my own tastes? I have not cared much for the spring and bar rates on my '95. I get why GM used soft–and then, later, even softer–wheel rates on ZR-1s. In fact, ZR-1s had wheel rates about the same as did base cars for their entire six-year production. The soft ride was all about marketing the car to an older demographic, but, when I bought the car in 1995, I wasn't old enough to care about a "cushy ride" and, even now, in my 60s, I still like cars that are more stiff. Oh...I don't want a ride that's annoying or uncomfortable, but I like stiffness. Maybe that's why, these days, my two favorite cars to drive really hard are my '12 Z06/Z07 and my Wife's ATS-V, both of which have Magnaride–one of the greatest suspension developments of the last ten years or so. Fun fact, some of the software algorithms used by MR today, trace their origins back to the ZR-1 "active ride" program of the late 1980s, but...I digress.

Back in the mid/late-90s, I remember talking with Scott Allman about putting the Z07 suspension parts on my ZR-1. I remember him telling me to use the later FE7 front spring (90-93-N/mm), the 30-mm front bar, the FE7 rear spring and either the 24- or the 26-mm rear bar, depending on my driving style. He also said a set of the Z51 front lower arms would be good because of their higher bushing rates but, initially, that was a non-starter because GM never serviced them as replacement parts. Luckily, a couple years later, a GM friend of mine had a set pulled out of stock at the plant and shipped to me.

I took the first step, by sending my SRC shocks to Bilstein to be revalved to better match the anticipated higher wheel rates, but...like some projects on which we ZR1 DIYs embark, my Z07 upgrade never was completed. All those parts are still sitting in my parts storage. At this point, my ZR-1 is for sale, but if I was going to keep it, I'd eventually, use those suspension parts rather than converting to coil-over-shocks. On a street car or a street/track, I would never give up ride travel or the leaf springs' roll stiffness for coil over shock assemblies.

Again, the biggest practical advantage of coils comes with a track car with a cage in it, running slicks on a relatively smooth surface. In that case,*a well-developed, coil-over-shock package–with a significant rate increase to make up for the loss in roll stiffness when you scrap the leaf springs–is better, as long as the user knows how to "tune" the coil-over system once it's installed.

I also have to, once again, mention the other big advantage of coil-over conversions and that is they look so darn cool when you go to Cars-and-Coffee on weekend mornings and open your hood. There is room for everyone in the Corvette hobby and there is no doubt that the LT5 engine is automotive artwork. Add a colorful coil-over conversion and you've "framed" that LT5 and have an underhood area that is great eye candy at car shows.

[Paul Workman]

Thanks for the interesting read, Hib!

[RussMcB]

Yes, thanks, Hib, a good read. Thanks for sharing.

I'm still a relatively new Corvette owner, and still coming to terms with its suspension. The one obstacle in my mind is the "independence" of the spring at each corner.

Recently I needed to replace a rear leaf spring because it showed a bit of splintering. At the same time I wanted to address a couple of issues: 1) visually too high ride height in the rear (compared to front), and 2) a larger tire-to-fender gap on the right rear side of the car compared to the left rear.

I bought longer lowering bolts, and with the rear ARB disconnected, I kept making adjustments (trying to lessen the RR fender gap) until I ended up with the right lowering bolt longer than the left, but it did not make much difference in the tire-to-fender gaps side to side. My uneducated conclusion was the two center mounting points of the rear leaf spring are so close to each other, and mounted with some rubber shims, it almost acts like a single center mounting point.

I think I'm probably wrong about that, but that's the way it seemed. I might try some type of thin shims in the center mounting points (slightly different on each side) to lower the right side a bit more. Fortunately, I have scales so I can make sure I'm not introducing any crazy cross-weights if I go too far.

[randy ransome]

Hey RussMcB let us know how you come out with the leveling of the rear when you get it finished. Mine leans to the left.

[Hib Halverson]

I would never adjust trim height my measuring the "fender to tire" distance. That's not accurate because of the inaccuracies of the body assembly process and inaccuracies in how the body is mounted to the frame.

If you're going to do trim height adjustment properly, you need to 1) put the car on a known level surface and 2) use the trim height measuring methods listed in the service manual which do not index off the body.

Once you get the trim height adjusted properly, then you can adjust your stabilizer bars to eliminate preload.

Also, in re: rear spring mount shims...in the rear of a C4 do not remove shims. You can move the shims from top to bottom or the other way but you can't remove them like you can in the front.

[batchman]

First, thanks for the writeup Hib. I am with you 100%, even though my race class restricts me to "stock suspension type and factory mounting locations". I have been well impressed with how light the springs and associated hardware end up on these cars.

I will say it took a couple of years to get the spring rates I was after, since the aftermarket is limited and production of these parts is not entirely predictable. It would be much easier to tune with standard coilovers but I am very grateful for what availability there is, and with the advent of C5 style adjusters on the front springs there is easy ride height/corner weight adjustment too.

One point I didn't see you mention though is load path. I hesitate to muck with the factory decision making process for how transient loads transfer to the structure. The front appears "worthy" but believe the addition of a "camber brace" would be highly helpful. I would really hesitate to put the full load of the car on the single-shear rear shock mount though - even though it is stout that is a pretty good lever to ask a chunk of aluminum to bear on a good pothole hit. Perhaps part of my trepidation is having needed to make a sleeve to space my bottom shock mount away from the knuckle (so it could articulate), making the "lever" even longer...

On the other hand, putting the spring and the shock co-axial has benefit in making the damping path directly aligned with the springs' forces.

Originally Posted by Hib Halverson

I would never adjust trim height my measuring the "fender to tire" distance. That's not accurate because of the inaccuracies of the body assembly process and inaccuracies in how the body is mounted to the frame.

Agreed. I too have a skew side-to-side in the rear, maybe as much as 3/8 inch. I don't think the car is bent, just the way it is and by looking at more than a few C4s it seems not unusual.

Originally Posted by Hib Halverson

Once you get the trim height adjusted properly, then you can adjust your stabilizer bars to eliminate preload.

Here's where I would love to find adjustable end links. I don't have a perfectly level garage (but I don't think it's really far out), but I do get a bit of preload on installing sway bars. Since Mr Guldstrand has left us, his fine (although spendy) adjustable links dried up just as I was ready to bite the bullet and order them.

Sigh,
- Jeff

[Mystic ZR-1]

C5 style adjusters on the front springs?
In place of messing with spring shims?
Please explain.
Thanks!

[RussMcB]

Originally Posted by batchman

Here's where I would love to find adjustable end links. I don't have a perfectly level garage (but I don't think it's really far out), but I do get a bit of preload on installing sway bars. Since Mr Guldstrand has left us, his fine (although spendy) adjustable links dried up just as I was ready to bite the bullet and order them.

Sigh,
- Jeff

I recently bought a pair of adjustable links for the rear of my car here:

http://emracingcorp.com/scripts/prod...?idproduct=319

They're nice. At first I thought they were a little over priced, but they have machined spacers to fit into the sway bars to replace the rubber parts.

If you think one adjustable link will do what you want, let me know. I would be content to use a stock link on one end and an adjustable link on the other. But you might want to install them on both ends to eliminate more rubber compliance.

[XfireZ51]

Hib,

Very informative write-up, as usual. My car came w coil overs and overall I am pleased w them. I won't convert back but I am interested in the comment of increasing the stab bar rate. Is there a guide to what this should be given the spring rates chosen, i.e. 425F/300R?

[Hib Halverson]

Originally Posted by batchman

(snip) and with the advent of C5 style adjusters on the front springs there is easy ride height/corner weight adjustment too.

I wasn't aware of any C4 front springs with C5-style adjustment screws. What company makes them?

One point I didn't see you mention though is load path. I hesitate to muck with the factory decision making process for how transient loads transfer to the structure. The front appears "worthy" but believe the addition of a "camber brace" would be highly helpful. I would really hesitate to put the full load of the car on the single-shear rear shock mount though - even though it is stout that is a pretty good lever to ask a chunk of aluminum to bear on a good pothole hit. Perhaps part of my trepidation is having needed to make a sleeve to space my bottom shock mount away from the knuckle (so it could articulate), making the "lever" even longer...

The post I made got kind of long so I decided not to cover that. My gut feeling is there might be a problem with loading on those parts but I really had not information to support it. In defense of C4 coil-over conversions, I'm not aware of anyr reports of fatigue failures of C4 shock mounts on cars converted to coils.

Agreed. I too have a skew side-to-side in the rear, maybe as much as 3/8 inch. I don't think the car is bent, just the way it is and by looking at more than a few C4s it seems not unusual.

I've seen C4s like that, too.

Here's where I would love to find adjustable end links. I don't have a perfectly level garage (but I don't think it's really far out), but I do get a bit of preload on installing sway bars. Since Mr Guldstrand has left us, his fine (although spendy) adjustable links dried up just as I was ready to bite the bullet and order them.

Sigh,
- Jeff

Russ addresses that above and I bet there are others making heim-jointed stabilizer bar links. You can even do it the po-white-trash way and just use washers between the stock end link spacers and the seats the link cushions sit on, but, yeah...heims look much cooler and sharpen bar response.