USAsOnlyWay

Ok, looking for input from people FAR smarter than I... Thank you in advance.

Motion Ratio
MR= d1/d2
Where:
d1 is the distance to the spring mounting point (in this case coil overs being considered, so distance to the shock mounting point).
d2 is the distance to the ball joint/knuckle attachment point.

Apprx measurements:
Front:
d1= 11"
d2= 14.5"
MR= 0.759

Rear: (case 1)
d1=16"
d2=18.25"
MR= 0.877

Question 1: The rear shock is mounted physically on the knuckle, doesn't this mean that since the spring force is on the same piece of metal as the lower control arm attaches too, they have the same d1,d2... and thus a MR of 1? I don't think there is a possible difference in leverage despite length since they both act directly on the knuckle, right?

Rear: (case 2, which I think is correct)
d1=18.25"
d2=18.25"
MR= 1.00

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Spring Rates and Angles

I used a phone app to measure shock angle. Handy and hopefully for this exercise accurate, I came up with at full suspension drop (and yes these might be very slight underestimates but I think it should be ok as I don't think the shock angle changes that much...could be wrong here...)

Front: ~20-22*
Rear: ~10-12*

I'll just use 20 front and 10 front and a simple table for calculate spring rates gives:
20*= 0.88
10*= 0.96

So all spring rates will be adjusted accordingly:
Using very popular c4 rates:
Front:
400 = 352 lbs/in
Rear:
275 = 264 lbs/in

-----------------------------------

Wheel Rate (More appropriately: Ball Joint Rate)

WR = MR^2 * Spring (adjusted)

So first lets reference Hib Halverson's C4 suspension chart:
http://www.corvetteactioncenter.com/...usp_chart.html

For the "Ferrari (and lower back) beating" 84 Z51 Suspension it lists:
Spring Rate: (lbs/in converted by 5.710148 lbs/in per N/mm)
Front: 102 N/mm (582 lbs/in)
Rear: 87.5 N/mm (500 lbs/in)

Wheel Rate:
Front: 28.6 N/mm (163 lbs/in)
Rear: 37.5 N/mm (214 lbs/in)

Question 2: Now I am making the assumption here that this is the same wheel rate I am trying to determine and therefore comparable. If it isn't, this all goes out the window, but if it is, well this is a good baseline to build from. Sound correct? Or is it more likely these are wheel rates calculated out at the actual mid-point of tread of the tire, which usually are lesser than those at the ball joint. Thoughts?

Now using the common spring rates above and also the MR calculated, I get early c4 coilover wheel rates of:

Front: WR=MR^2 * (k-adj)= (.88^2)*(352)= 203
Rear: WR=MR^2 * (k-adj)= (1^2)*(264)= 264

Which is first of all, a lot higher than the 84 z51 wheel rates. This is surprising to me considering that I don't hear many complaints of ride quality on c4s with CO's, however I do admit I don't know about the specific shock valving and perhaps that is the reason why.

Secondly, ZR1 MK's values of 500/300 for racing look quite good to me here with WR's of of 253/288.

Thirdly, even using more conservative 25* front shocks and 15* rear, the WR with 400/275s are 188/256. Closer to front C4 84 z51 rates but still exceeding the rear by a good deal. If I am wrong about the mounting point, and the MR is no 1 for the rears, this becomes 188/203 and really quite close to 84 z51 springs.

So I guess the big thing first off, is this even correct at all?

If so I can use it and corner weights to get a better idea of what spring rates to go with... and also, I can start figuring out how in the heck I am going to get these to work on an early c4... I have some ideas but we'll see...

This is all working on the neglected red headed stepchild of the Corvettes... 84-87 C4s. Specifically an 86.

Thanks again for any confirmation, correction or help of any kind.

63Corvette

I have absolutely no idea.......BUT, for anything C4, I would call Kim Baker at Bakerracing. Kim won all of the Escort showroom stock races back in the 80s, and he is the absolute guru for C4 suspension setup.

Kubs

I have not messed with coilovers, but to answer question 1, yes the value 1 can be used for the motion ratio. We analyzed my C4 in my chassis class at school and while the exact MR is not 1, it is close enough and can be assumed to be 1.

Canam

keep this on going. I'm listening (reading) intently. I have an 85Z51.
The shocks are bilstein race ,the springs are composite and the only thing I know is that the rear has one from an automatic that is apparently a bit more compliant. Like to learn all I can about the suspension. Thanks.

USAsOnlyWay

Sweet. Sounds like a fun class.

Did you end up finding those leaf springs? (Part of my interest in coilovers since I'm still on NYRs...)

I'm guessing my angles are slightly off too as I was really doing this for the exercise and they are probably more like the latter example. I'll need to get some updated measurements and angles.

For now, I really just wanted to complete the exercise and make sure I'm even in the ballpark for doing this correctly.

I also need to get corner weights and start calculating spring rates based on that as well.

@63Corvette I'll look em up. Thanks.

@Canam I have custom valved bilsteins too. Like the valving for the most part but the springs are a complete mismatch.

Kubs

No. I have a rear spring that I am going to "borrow" from my dad's '92 but Im still looking for an FHB front spring. Coilovers would be nice but my budget is extremely low at the moment.

Canam

I had a stock 92 LT1 and I put race spec bilsteins on it that I had gotten for free. The ride was absolutely punishing for the street and ok for track days. I ended up taking them off. I think the bilsteins had totally taken over the compliance of the springs.
I would surmise that my composites are probably mismatched also but I'll need to talk to the seller more and get more information.

Sgt.Gator

Check out this spreadsheet for doing suspension calculations, hope you find it useful. I didn't create it, I got this a few years ago in another forum:
https://skydrive.live.com/?cid=4f791...B484B3B%21678#
Click Download on the right once you are on Skydrive>

RacePro Engineering

Wow! Sounds pretty complicated.

Let's try this:

Motion Ratio - the amount the spring compresses (perch moves) when the axle moves up 1 inch.

I realize that probably seems too simple, so I'll write a little more, giving the IMPRESSION of complexity. If the spring compresses three-quarters of an inch, then your Motion Ratio on that corner is .75. If the spring compresses one and one-third inches, then your Motion Ratio is 1.33. Usually, we do a 2" raise of the axle to compensate for any initial slack that might be in the spring seat.

Spring angle does not need to be considered since one is measuring the actual compression of the spring along its centerline axis.

From this point, you can plug in various spring rates to determine what will yield the desired wheel rate.

Ed LoPresti

USAsOnlyWay

Sgt. Gator thanks for the ss, I'll check it out when I get home.

Ed, this is actually a perfect segue for what has been on my mind recently regarding this.

I will do your method measuring at the center of the tread. I think the wheel rate value I calculate will actually be less with this method than with the above as I believe my method is ball joint rate and yours is wheel rate... perhaps not, but if so it would make sense especially considering that people describe the 275 springs as fairly compliant but via my method it is near 84 z51 so if correct that would indicate the reported values are at the wheel rate and thus I should use that to compare to leaf rates.

I will update when I get the wheels back on after the arp studs are installed.

ZR1 MK

USA

The motion ratio is correct, but that’s for effective spring rate at the ball joint, as you indicated in you latest post. If you want the wheel rate, you need to do two more calculations.
I copied this from the web: The formula for determining wheel rate is outlined below, where "d1" is the distance from lower control arm's inner pivot point to the spring center, "d2" is the distance from the lower control arm's inner pivot to the ball joint, "c" is the distance from the lower ball joint to the front instant center, and "d" is the distance from the tire's contact patch to the front instant center.

Ill give you some info based on my car that has coils springs. If you take your measurements and plug them into this website Motion Ratio with a 300 lb ball joint rate, you get a 500 lb spring rate requirement. The 300 is the same as my rear that has a straight 1:1 ratio. This is a starting point with a car that has a 50/50 weight distribution ratio. Add in the coil angle and my cars 52/48 weight distribution and I need an approx 550 spring with my 300 rear. But, most of you with the little light weight engines are probably 1-2 % lighter in the front anyway.

Since I use the 32 mm solid anti sway up front that increases stiffness by a considerable margin, I use the 500 lb spring. I like this combo better as I get to change back to my 30 mm front bar for the street and its reduced my tire wear considerably. I eliminated the 2” outer edge wear band and the heavy angled wear pattern that most others have.

Since the two dimensions, shock angle and wheel offset determine wheel rate, you need to keep this in mind when comparing different year cars with different wheels. I use 12” and 13” wheels. My fronts have heavy negative offset and the rears have heavy positive offset, similar to a Trans Am car, and Im at 3400 lb. Some gutted C4 track cars are down to 3700 lb. The lower the weight the less spring you need.

Bottom line is, Chevy did a good with the 1990 Z51/1991 Z07 leaf springs. Any aftermarket spring thats stiffer is for extremely smooth tracks and you will have a bone crunching slot car. My coils are similar to the Z07. The coil spring specs for most race only C4s that Ive come across is 550 front and 325 rear. I had a friend with a ZR1. He had 600 front and 350 rear. We both agreed they were too stiff.

The early C4 had shorter A arms (I think), but the ratio is probably the same or so close its irrelevant when comparing.

And shocks. My shock settings from full soft to full firm make more of a diff in ride stiffness than my soft street springs compared to my track springs.

Hope this helps.

RacePro Engineering

Well, that might be part of the problem.

Wheel rate is not terribly difficult to calculate. There are 2 steps:

[1] Figure the motion ratio on a given corner. This is simply how much the spring compresses, in inches, when that axle moves up one inch. (Now in formula or sports racing cars, with pushrod suspensions, the bellcrank makes this calculation a little more complicated. That is not the case with coils or leafs.)

[2] Multiply the square of your motion ratio by the spring rate on that corner.

Done!

The next step here is to determine what rate spring will yield the Wheel Rate, and therefore the handling behavior one desires.

Now, if one wants to start calculating roll centers and roll axis ( a completely differrent discussion), then we can start using some of the measurements that Mike has posted above.

Ed