gkmccready

I've been staring at this for a while now. Here're my assumptions:

Leafs

• C6 Front spring motion ratio: 0.55
• C6 Rear spring motion ratio: 0.4
• Effective spring angle: 90deg

Coils

• Front motion ratio: 0.69
• Front spring angle: 61deg
• Rear motion ratio at droop: 0.70
• Rear spring angle at droop: 53deg
• Rear motion ratio at bump: 0.53
• Rear spring angle at bump: 40deg (?)

Stock C6 Z51 springs

• Leaf rate: 526# front, 645# rear
• Leaf wheel rate at droop: 159#, 103#

It's my belief that as the leaf springs experience more load their rate effectively goes up. The springs are a fixed length and as the lower control arm moves upward the spring pad will slide further out the LCA which has a little ramp along with the effective lever ratio increasing. (True? False?)

Equivalent coil springs

• Spring rate: 383#, 265#
• Wheel rate at droop: 159#, 104#
• Wheel rate at bump: 159#, 48#

As you can see, while the wheel rates were equivalent at droop/static ride height as the rear coilovers move through the bump travel they lose a tonne of wheel rate. And the front rate remains approximately the same through travel, while I contend that the leafs have a rising rate.

There's lots of successful folks with coilovers, and with leafs, so they both obviously work, but it's interesting to think about the various ways you can get there. Does everybody just aim for the "sweet spot" of the travel to get the rates they want? Meaning coils run a higher rate at static ride height and leafs a lower so they meet in the sweet spot?

Something to think about, anyway. Curious if anybody else has thought this through any further or can put some real numbers to the leafs?

WBHighwind

One post shows me that I know nothing about suspension setups. I guess its finally time to start studying up on this stuff.

Its good to look at things in a different way.

trackboss

That's great info. Thanks for posting. Now, if the numbers are correct, I would say many are running way too soft a front spring on their coilovers. Especially if the cars are street cars carrying well over 3000lbs.
I once did a complete setup on an AIX mustang that was very well prepared and suggested front springs about 300lbs firmer than what were on there. The driver ended up taking something like 6 seconds off his previous lap time. It wasn't all in the spring as I did some major changes to the setup, but the car just flat out cornered better. The driver reported to me how easy it was to drive fast.
I personally use leafs on my C5 and the front is well over twice as stiff as the original z51 spring. Ride is not any harsher and the car completely transformed for the better. I think many are afraid that running a stiff spring will make the ride horrible. In my opinion that is not the case. While a firmer spring will slightly change the ride I personally believe that ride quality is more associated with dampening. If I did it again I'd probably use an even firmer front spring although I am not sure there is anything available above what I use. That is one big advantage of coilovers. Springs are very readily available in a large variety of sizes and rate, and they are relatively cheap. In addition they are easy to swap out and with the exception of possible slight ride height changes they keep the corner weights close to where they were.

igo380

Maybe Aaron Pfadt can chime in here? It would be good to hear the Pfadt insights.

mousecatcher

"well known".

curious, how did you measure or where did you get the MR? i ask because you stated them as "assumptions".

great post with lots to think about.

gkmccready

I measured from the imaginary line between the LCA inboard mounts to (1) the LBJ, (2) the shock mount, (3) the spot where the spring pad hits. I did this at static ride height.

(D2/D1)^2 = Coilover MR
(D3/D1)^2 = Leaf MR

I measured the spring angles at the same time. Those are the droop numbers.

I based the rear "bump" MR on a previous post on the forums, and the 40deg based on the 13deg number ghoffman keeps saying about the shock angle change since I measured 53deg myself. I was planning to pull my coilovers, put on a stock shock, jack up the corner and measure every inch of wheel travel some time soon. So that's why I said "assumptions."

I verified the "droop" MR numbers with math as I set the ride height on my car...

GrantB

On a car without significant aerodynamic downforce, wouldn't a rising rate in droop and a falling rate in bump be desirable? Corner exit is usually the most important part of a track to be fast on, where Corvettes are typically limited by the grip of their rear tires. If squat caused the rear springs to soften up and the fronts to tighten, that should lessen the weight transfer at the rear of the car, aiding in track-out traction.

gkmccready

"Maybe?" Remember a "rising rate in droop" doesn't imply rebound damping. Also, wouldn't you prefer a lower rate for initial chassis movement, not to mention the nice ride, and then a higher rate as you lean on it?

Wayne O

I don't know if this will add much to the technical aspect of your post but the following web page delves into the Corvette transverse leaf spring/coil-over subject. I found the following page (and associated links) of interest.

http://www.ultimatecarpage.com/forum...f-springs.html

Thanks for sharing your thoughts and observations.

kmagvette

I have to agree with the statement about most people running too soft a spring up front. The 02 Z below has stock springs. The T1 front spring is only about 60 lbs heavier while the VB&P front springs (extreme touring kit) is just about 1000 lb. I do not believe that a 60 lb increase to the T1 spring would materially reduce body roll. The extra 450 lb that the VB&P springs or equivalent spring, coil-over, etc, would likely be a more noticeable improvement. The stock rear seems to be doing a decent job.

Rolling (over) the toe at WGI.

fatbillybob

All I can say is stiffer is not better. It is all part of an "engineering" package. I can understand spring, shock, sways as units but I don't understand the dynamics of the package. GM was amazing with the T1 suspension. I can't believe how fast the SCCA T1 racers are. Often they are significantly faster than the guys with hopped up coilover cars and more horsepower. The key IMO is can any of us engineer a coilover package? As another datapoint I'm pretty sure the T1 guys are well matched to the NASA ST2 guys with unlimited suspension goodies. Someone more into NASA will have to chime in on that.

gkmccready

Okay, spent some time with the car on jack stands, a measuring tape, and stock shocks with no springs on the car.

Front

• Shock travel: 102mm
• Wheel travel: 146mm
• Motion Ratio: 0.69
• Shock angle change: 68.7deg@droop, 60.5deg@bump, 8.2deg

Rear

• Shock travel: 94mm
• Wheel travel: 144mm
• Motion Ratio: 0.65
• Shock angle change: 62.4deg@droop, 48.9@bump, 13.5deg

I took measurements about every 25mm and calculated the MR, then calculated the average of all those measurements, and they matched the measured shock travel divided by the wheel travel.

Remember, I'm just some shmoe in his backyard with a measuring tape on his driveway. The car isn't completely level, and I'm sure my accuracy was easily +- 1mm, and 1mm makes a fair difference.

One thing I did -not- see was the falling motion ratio in the rear. My rear measurements netted me 10/17=.59, 14/23=.61, 17/23=.74, 14/25=.56, 16/22=.73, 12/23=.52, 11/14=.79 which averaged out to .65 which is the same as 94/144=.65 ...

And for the record, I did two passes on the front, 17/26=.65, 18/25=.72, 20/24=.83, 16/25=.64, 16/25=.64, 18/26=.69, average=.69 ... and the second, 18/28=.64, 20/28=.71, 18/24=.75, 13/23=.57, 17/23=.74, 17/21=.81, average=.69

So, going back to the original post:

Stock C6 Z51 springs

• Leaf rate: 526# front, 645# rear
• Leaf wheel rate at droop: 159#, 103#

And using the new motion ratios and angles:

Equivalent coilovers

• Spring rate: 360# front, 275# rear
• Wheel rates at droop: 159#, 103#
• Wheel rates at bump: 150#, 88#

Keep in mind that the stock shocks provide a tonne and a half of droop travel so in reality at ride height you're a good way into the measurements. The shock angle also changes with the overall length of the shock, I think, because my coilovers are much shorter shocks than stock, and at ride height they put the rear at 53deg and the fronts at 61deg.

gkmccready

Just waking this up again. Curious if there are more comments. Also very interested if anybody is willing to share their spring rates, be it leaf or coilover...

0Randy@DRM

We use right around 450-500 up front and 575-650 in the rear.
You can post the math if you would like to.

Randy

0Randy@DRM

Thanks for saying exactly what I was thinking. Well besides the part about a faster coilover car. Those T1 guys are hard workers and it shows in lap times.

Randy

gkmccready

Thank you for sharing, Randy! That's more than I would have expected from a vendor, I was curious about individuals, but this is excellent! I will note that spring rates alone don't make a set up and each vendor matches their rates with shocks and other components and set up information...

450 front ~= 187-199 wheel rate
500 front ~= 208-221 wheel rate

575 rear ~= 175-205 wheel rate
650 rear ~= 202-247 wheel rate

The "sweet spots" of these ranges line up to match F:R wheel rates, effectively.

(Sorry for the crummy formatting, this is F/R spring rate, F/R wheel rate, and the %age the F > R wheel rate)

The stock wheel rates appear to be ~30% greater in the front which (as we've all experienced) is meant to dial in a bit of push:

C6 Base 420 620 127 99 28%
C6 Z51 526 645 159 103 54%
C6 Z06 531 782 160 125 28%
C6 T1 582 850 176 136 30%

VBP rates look to be ~160+% greater in front, and also MUCH stiffer than the typical set up in the front:

VBP 1000 700 302 112 169%
VBP2 1100 800 333 128 160%

fatbillybob

I got a question. Is the C5 and C6 geometry regarding motion ratio the same? How do you calculate the wheel rate (WR)addition of the sway bars? This can really be significant because not only are people running all kinds of leafs and coils but sway use is all over the map too. I also wonder if you have two equal cars with different sways and leafs but total WR (spring + sway) equal what do those car feel like?

I think in nascar they run giant 2+" sways which must have a huge WR. I thought you tried to tune with springs first the used sways to fine tune then shocks to microtune. What do you guys think about that?

kflee

LG coilovers are 750# front and 600# rear and were designed to be used with T1 bars.

Randy,

DRM coilovers (450/600) are designed to be used with stock sway bars, right???

Kirk

BrianCunningham

The stock spring swing on, well it's not even an arch. so that effect the spring rate.

The contact point also changes as it moves, well my rear doesn't, but the front does.

Also the leafs interact left to right, which coils do not.


Higher in the back, that's interesting.

0Randy@DRM

We use the T1 bars for track applications. They get a little loud on the street and many don't like the extra stiffness and sound that comes from it. So for street cars we use just our coilovers and Z06 bars.

Randy