Using Chevette Springs to Re-gain Original Ride Height

[Hardway]

You can't just cut one corner and expect a major change on an end of a car. What's happening is the right rear is helping to hold the right front up (sounds crazy but its true - diagonal corner weights and all that).

You were spot on John. I got the passenger side spring cut down and everything reinstalled and the car did level out. Now I have a problem with too much positive camber. I posted pics of it all on my resto thread -> http://www.classiczcars.com/forums/open-zcar-discussion/47863-lime-green-machine-restoration-thread-1972-240z-5.html#post432836 I am not sure what I need to do to go about correcting it.

[Hardway]

As I cruise through the threads I noticed Grannyknot's thread regarding his suspension and the numbers he posted. He is running a set of illumina's along with new Eibach springs. He posted the following numbers.

driver front - 26.125"

driver rear - 26"

passenger front - 26.250"

passenger rear - 25.5"

On the Eibach's I know a lot of guys cut one of the front coils to help level their car out so Grannyknot may need to do the same. However, below are my numbers taken from the same point, in the middle from the ground to the edge of wheel opening. Now I am really lost.

Driver front - 25 3/4"

Driver rear - 25 7/8"

Passenger front - 25 2/8"

Passenger rear - 25 1/4"

As of right now I am sitting lower than what Eibach's would give me but in my opinion the car is still sitting high. I have considered taking the front springs out and cutting another coil out but this would result in the spring being very loose in the upper spring pocket when not under load and possibly coming out completely when the car is jacked up. I am beginning to think the coils are just too thick and spaced too far apart to get the end result I want. Right now on the rears I have cut down 1.5 coils and on the fronts I have cut down 2.5 coils. Any ideas besides ditching these springs and buying a set of Eibach's and cutton one coil off the fronts?

[jer305]

If you take the car out for a quick drive the suspension will settle down a little compared to how it sits after jacking the car up in the air.

I'm not sure how big of a difference it will make but it's worth a try to see if that front will come down.

[John Coffey]

Although few will care... never use the wheel well arches to measure ride height. Use the bottom horizontal surface of the rocker panels.

[240260280z]

Jeff,

I just made an xls spreadsheet for estimating your front spring compression. Here are the numbers and spring constant changes:

Stock Chevette Spring Front

Compressed Spring Height 11.27"

Spring Constant 197.00 lbs/inch

1.5 Coils Cut from Stock

Compressed Spring Height 9.60"

Spring Constant 237.48 lbs/inch

2.5 Coils Cut from Stock

Compressed Spring Height 8.29"

Spring Constant 275.17 lbs/inch

3.0 Coils Cut from Stock

Compressed Spring Height 7.63"

Spring Constant 298.90 lbs/inch

3.5 Coils Cut from Stock

Compressed Spring Height 6.97"

Spring Constant 327.09 lbs/inch

4.0 Coils Cut from Stock

Compressed Spring Height 6.31"

Spring Constant 361.17 lbs/inch

I think you are at 2.5 coils cut from stock so the compressed height should be close to stock of ~8". I would only cut 0.5 coils at a time on the front until you get it where you want. The spring constants climb significantly.

FYI: I used a car weight of 2,244lbs (estimated as car and one 150lb driver) and a 47/53 front/rear weight distribution. If anyone can give better weight and distribution I'll plug it in.

[240260280z]

1 month old 72 for reference

here is a 71

Edited July 4, 2013 by Blue

[zKars]

Thanks Blue for reminding us that shortening a spring has the (perhaps) undesirable effect of making it a stiffer spring as well.

Since I've always had a hard time conceptually understanding why this is, so imagine others do as well, try thinking of it this way.

Instead of a coil spring, think just of a long retanglar straight bar of steel. Perhaps you have a springy 36 inch ruler in your shop to try this with.

Clamp one end to your bench with a c-clamp and let the other end extend off of the bench toward you. Press down on the end of the ruler. It takes a certain amount of force to push it an inch downward.

Now press on the bar at a point half way along it's length instead of the very end. It is much harder to push down. You have in effect a shorter lever arm so you need more force to get the same deflection

(at that position on the ruler, not at the end. Feel free to cut your ruler in half to confirm...).

So we see that compressing a spring is just deforming the steel with a given force, and if you have more length of steel to push on, you have more leverage to do the deforming. Cut coils off and its harder to deform.

To further cement the understanding that is not just a function of the material that the spring is made of that determines the lb/in rating, but the leverage you are excerting to deform it, imagine putting your ruler on edge in the above experiment and pushing down on that.... Its a combination of the material stiffness and its manufactured length that gives a particular lb/in rating.

[240260280z]

jim we have the same geek gene Love your lesson... spot on.

btw Pics of that series one please Hopefully I'll have the metal roof on the shed this weekend and pics to counter

[240260280z]

Jeff,

Revised with the following:

Vehicle Weight: 2332 lbs (72z and two 150lb drivers per 72 FSM)

Weight Ratio (F:R) 48.2:51.8 per 72 FSM

This is for the front:

[TABLE=class: grid, width: 500]

<tbody>[TR]

[TD=align: center][/TD]

[TD=align: center]Uncompressed

Height (Inches)[/TD]

[TD=align: center]Compressed

Height (inches)[/TD]

[TD=align: center]Spring Constant

(ft-lbs)[/TD]

[/TR]

[TR]

[TD=align: center]Stock[/TD]

[TD=align: center]14.25[/TD]

[TD=align: center]11.40[/TD]

[TD=align: center]197.00[/TD]

[/TR]

[TR]

[TD=align: center]1.0 cut[/TD]

[TD=align: center]12.63[/TD]

[TD=align: center]10.10[/TD]

[TD=align: center]222.26[/TD]

[/TR]

[TR]

[TD=align: center]1.5 cut[/TD]

[TD=align: center]11.82[/TD]

[TD=align: center]9.45[/TD]

[TD=align: center]237.48[/TD]

[/TR]

[TR]

[TD=align: center]2.0 cut[/TD]

[TD=align: center]11.01[/TD]

[TD=align: center]8.81[/TD]

[TD=align: center]254.94[/TD]

[/TR]

[TR]

[TD=align: center]2.5 cut[/TD]

[TD=align: center]10.20[/TD]

[TD=align: center]8.16[/TD]

[TD=align: center]275.17[/TD]

[/TR]

[TR]

[TD=align: center]3.0 cut[/TD]

[TD=align: center]9.39[/TD]

[TD=align: center]7.51[/TD]

[TD=align: center]298.90[/TD]

[/TR]

[TR]

[TD=align: center]3.5 cut[/TD]

[TD=align: center]8.58[/TD]

[TD=align: center]6.86[/TD]

[TD=align: center]327.09[/TD]

[/TR]

[TR]

[TD=align: center]4.0 cut[/TD]

[TD=align: center]7.77[/TD]

[TD=align: center]6.22[/TD]

[TD=align: center]361.17[/TD]

[/TR]

</tbody>[/TABLE]

This is for the rear:

[TABLE=class: grid, width: 500]

<tbody>[TR]

[TD=align: center][/TD]

[TD=align: center]Uncompressed

Height (Inches)[/TD]

[TD=align: center]Compressed

Height (inches)[/TD]

[TD=align: center]Spring Constant

(ft-lbs)[/TD]

[/TR]

[TR]

[TD=align: center]Stock[/TD]

[TD=align: center]14.25[/TD]

[TD=align: center]11.18[/TD]

[TD=align: center]197.00[/TD]

[/TR]

[TR]

[TD=align: center]1.0 cut[/TD]

[TD=align: center]12.63[/TD]

[TD=align: center]9.91[/TD]

[TD=align: center]222.26[/TD]

[/TR]

[TR]

[TD=align: center]1.5 cut[/TD]

[TD=align: center]11.82[/TD]

[TD=align: center]9.28[/TD]

[TD=align: center]237.48[/TD]

[/TR]

[TR]

[TD=align: center]2.0 cut[/TD]

[TD=align: center]11.01[/TD]

[TD=align: center]8.64[/TD]

[TD=align: center]254.94[/TD]

[/TR]

[TR]

[TD=align: center]2.5 cut[/TD]

[TD=align: center]10.20[/TD]

[TD=align: center]8.01[/TD]

[TD=align: center]275.17[/TD]

[/TR]

[TR]

[TD=align: center]3.0 cut[/TD]

[TD=align: center]9.39[/TD]

[TD=align: center]7.37[/TD]

[TD=align: center]298.90[/TD]

[/TR]

[TR]

[TD=align: center]3.5 cut[/TD]

[TD=align: center]8.58[/TD]

[TD=align: center]6.74[/TD]

[TD=align: center]327.09[/TD]

[/TR]

[TR]

[TD=align: center]4.0 cut[/TD]

[TD=align: center]7.77[/TD]

[TD=align: center]6.10[/TD]

[TD=align: center]361.17[/TD]

[/TR]

</tbody>[/TABLE]

You may wish to cut 1/4 less coil off the rear to keep it higher than the front.

3 coils cut from the front and 2.75 cut from the rear would be ~ .6" below stock.

Edited July 5, 2013 by Blue

[Hardway]

Thank you Blue! This definitely explains why I can barely push down on the front end as it is. If I cut another half coil off I will be looking at almost 300lbs of spring pressure. I would imagine the car would corner flat but the overall ride experience would be something reminiscent of an old heavy duty farm truck. I am thinking the Eibach's are looking better and better at this point.

Edited July 5, 2013 by Hardway

[240260280z]

It would be interesting to see how it works out if you complete the experiment. I am curious to see the ride height (compressed height of spring) vs number of cut coils and how it relates to the xls tool I made.

I did some spring research yesterday and John Coffey mentioned in a post that shocks can have more of an effect on the perceived roughness than springs when driving. He knows suspension so it could be fun to see what happens.

Edited July 5, 2013 by Blue

[240260280z]

hmmm looking at the FSM again and it is more precise than at first glace....it gives different corner weights for the front so a bit more care is needed. Nissan seems to address this by having a 1/2" longer spring on the passenger side 15.2" vs 14.7"