Calculating Unsprung Weight is needed for doing some of the math for suspension setup. So I will attempt to find out the actual weight on the C4.

At the same time, I wanted to measure the spring rate using the same process, here’s the methodology:




So the result was consistent 130lbs for the unsprung weight of the Right Rear. Details:
Apex S10 18x11 wheel
Nankang CRS 315-30-18 tire almost new
Wilwood brakes with no e brake and with aluminum hats (steel hats with e-brake may add 10+)
QA1 Mod Series Shocks mounted upside down,
titanium lugs, tubular lightweight hardware
probably should add 5lbs or so for the weight of the outer section of the spring.

For the wheel rate with Z51 rear leaf spring and averaging every half inch of suspension travel, through several cycles, the wheel rate for the NYU spring was about 155 lbs per inch.

Intercomp SW500 scale used. Tare weight for the bottle Jack was 36lbs. I will do the fronts sometime in the future.

 

Installed the VBP Extreme K=502 springs and re-did the exercise on wheel rates anyway. Same methodology as previous. Note that with the spring bolts captured (similar to the 1984 Z51 cars), the wheel sits fairly high instead of sagging down unsupported until it bottoms out on the shock.





From previous measurement, the original 1986 Z51 NYU spring (almost all later Z51's and Corvette Callenge springs were the same):
155 wheel rate

For the K=502 VBP or Van Steel rear spring:
223 wheel rate

 

I am sorry if you posted elsewhere but what is the weight of the Apex wheel by itself?

 

I guess you noticed I weigh everything :-) The APEX SM10 18x11 ET38 wheels were 24lbs and that was 4 years ago, just over advertised weight... I believe they are heavier now, and also they no longer knurl the seating edges on the flow formed wheels. I have those boxed up and spoken for, as I needed to change wheels for suspension modifications.

I bought a set of 10" wheels (18x10 SM10 ET30 front widebody C7 fitment) recently and they were advertised at 21.7 lbs and they were both well over 24lbs so 2-3 lbs more can be expected. I complained to Apex and they said if I'm concerned they would refund my money minus shipping and to buy the forged wheels... but the forged wasn't available in that size.

The Forged version SM10RS is 21lbs which is the same as the forged 11J for the Track Speck Forged front wheels in the later pictures (front is the same 11J the rear is 12.5J and weighs 22lbs).

 

That's why I am keeping my A-Molds for now but I am hoping to find some 18's and a more modern look eventually. My 17x11 Grand Sport rears weigh 22lbs and the front 17x9.5's weigh just under 20lbs. The knock off A-molds I had weighed 24lbs for 17x9.5!

 

Interesting thread! I would not worry much about the weight of the spring ends. Not only are those spring light overall, but they move very little: the working ends' mass is reduced by the motion ratio, and as you go inboard they of course move even less until you get to the frame clamp, at which point they don't move at all. To get a reasonable calculation for their contribution, you'd have to estimate the weight of just the parts between the clamp and ends, then basically halve that, and then also reduce it by the motion ratio.

 

Wouldnt the driveshaft and halfshats also be considered unsprung weight?

like the wheels, this type of weight would be the worst kind due to its inertiA

 

The title of the thread is Unsprung Weight but there are TWO separate things going on here. Also this is not the safest thing to do, be careful and have the transmission in first gear (or park for auto) with the e-brake set.

1. Unsprung weight -- as measured with the the spring and shock disconnected, and it's the actual weight of the rear wheel rig for the rear RH corner. The half shafts are connected and accounted for. That doesn't include the shock component and spring component, and the long bolt and hardware attaching the spring... or the ARB but I don't run a rear ARB anyway. I did it with the shock attached at first but that was a problem so I disconnected... this was measured in the first thread only, a couple of years ago. Figured 130lbs was a good number for suspension math (just make sure that the input is per corner not per axle which would be double). In theory, you can wight all of the components and then 1/2 of the ones attached to something else... and the main reason to know this weight is for calculating suspension frequencies (based on sprung weight only so unsprung weight are subtracted from actual corner weights) and also understanding control of the springs and suspension with shock valving.

2. Wheel Rate -- this is a function of the Spring Rate, and this was all that I measured yesterday, I didn't measure the unsprung weight this time as it hasn't changed enough and I didn't need a super accurate number (plus it's a lot more work with the more complex spring attachment shown in the picture) The methodology - Zero was 4 1/6" measured at the ram displacement where it could very easily be measured, and the first 1" measurement then was at 5 1/6" and the full weight scale measurement at 270lbs inclding the jack, that was the basis of subsequent two measurements an inch apart, as I didn't consider the first inch. Did this a couple of times.

The VB&P (then Van Steel) Spring Rate is measured by the manufacturer for the entire spring. Van Steel, per a phone conversation with me asking about the methodology for measurement: with the ends strapped down, and the center is jacked on a scale while measuring displacement, and then the springs are rated such as K=502 in this case. K is a unit of stiffness for the entire spring with the units being lbs per inch. That's the WHOLE spring with both sides working against the scale, but it's not a meaningful measurement until it's converted to Wheel Rate. If you call up Van Steele, they will have available roughly K=450 to K=650 range for the "Extreme' spring, as they are molded in multiple sets and many springs are then cut with a saw. The center ones tend to be stiffer by design so they can have multiple rates per batch. I also have a K=610 spring from them and expect that wheel rate to be around 280-290 range.

The motion ratio (of the spring not the shock) is pretty close to 1:1 as it's close to being aligned with the outboard halfshaft u-joint... so we expect half of entire spring to be the wheel rate on one side.

There's a measurement error when I do it against the tire, as the tire displacement and spring rate is measured along with the suspension displacement, and also the the spread spring pads meaning single spring measurement is stiffer displacing one side than it would be for both. If I measured the wheel rate with a bare wheel no tires, it would have been higher.

By the way, the motion ratio of the shock (for coilovers or force displacement calculations) is NOT 1:1, as there's an arm where it's attached at the bottom, and then there's an angle to it.

 

FWIW, my good friend who sold me the 96 I had (he went by MSR on this forum) measured motion ratios for that car when he owned it. Believe it or not, the rear spring contact point is not as close to 1:1 as one would think. I can't recall the number - I want to say it was around 0.8:1 - but I can hopefully dig it out at home this week.

UPDATE: MSR measured the rear spring motion ratio as 0.825:1. Even though it hangs from the rear knuckle, which is of course rigidly attached to the wheel and tire, I think the camber geometry ends up causing some difference in displacement between the wheel centerline and the spring contact point. There's a formula to account for this, and involves dividing the the upper lateral link length (the halfshaft) by the lower one and then something else which I have forgotten and seen very seldom. I don't know if the early cars like yours would be different, either.

 

Arey those vents off the shelf items or did you build them?

 

That's good info, I'll use that for now on. There's a long old thread here where the whole suspension kinematics class determined that the spring MR was 1.0 but I didn't trust their logic.

I could probably rig up a dial indicator to the hear spring bolt and a bare wheel and see what comes up... This is always a little bit of an awkward deal as motion ratio isn't technically measured to the wheel center, and wheel diameter center moves at a slight arc following the camber curve. Seems that it should be. In the front, it's to the LBJ which is well inboard to the wheel centerline.

For me, the wheel rate was the key information I needed... +/- 10 lbs rate was fine... and can't reliably go by how the spring was measured by VB&P... and not sure how to put a scale to the outer ujoint.

What's his calculated rear shock motion ratio (or coilover MR)? I did measure this but my process was just for a rough number.

 

Track Spec top (generic) and Race Vents side (C4 specific)

 

For some reason, he calculated the front shock mount MR*, but never the rear. Coilovers were never in the picture for him when he was actively developing the car (~2000-2006ish), so it probably wasn't a priority. The rear shock mount is very slightly inboard of where the spring bolt is hung from the knuckle, so I would guess the MR is slightly further away from 1:1, maybe 0.8:1 or thereabouts.

* 0.76:1 on the 96, which as you know is different for an early C4.