David or Anyone on Spring rate - Team Camaro Tech
Brakes, Suspension & Steering Conversion questions, Steering & Handling

 
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post #1 of 7 (permalink) Old Nov 25th, 01, 11:36 AM Thread Starter
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Ok so I sat on my car (I weigh about 170 lbs) and took a measure before and after and it went down 1 inch. Does this mean they are rated at 175? If so they are pretty weak.

If I cut them to lower them what will the rate be?

Tony

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obnoxious 67 camaro under construction. TH350/383 stroker w/ aluminum Edlebroc RPM heads,cam, and carb.This is a "Daily Driver" that need A LOT of work. But has new quarters and new floor pans.
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post #2 of 7 (permalink) Old Nov 25th, 01, 12:54 PM
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There's no accurate way to measure spring rate while the spring is installed in the car.

Friction from bushings, tie rods, ball joints, sway bars, tire scrub, etc. all add up.

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post #3 of 7 (permalink) Old Nov 25th, 01, 05:39 PM
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David Pozzi
 
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If you weigh 170 and sit on the front of the car, your weight is split between each side.
That's 85 lbs per side.
there is also leverage of the A frame and wheel to consider.
A stock spring rate should be around 300 lbs per inch for a first gen camaro.

If you cut10% of the free height off the spring it will stiffen by 10%, a 10% increase in rate won't make much improvement in handling performance. You need nearly a50% increase in stiffness.
To make a significant change in rate, you need a new spring.
David

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post #4 of 7 (permalink) Old Nov 25th, 01, 05:54 PM Thread Starter
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Thanks David,

So the only thing cutting the coils will do is lower the car so I can align it properly so when I do get the springs I want it will be aligned at the height I want.

Tony

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obnoxious 67 camaro under construction. TH350/383 stroker w/ aluminum Edlebroc RPM heads,cam, and carb.This is a "Daily Driver" that need A LOT of work. But has new quarters and new floor pans.
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post #5 of 7 (permalink) Old Nov 27th, 01, 12:27 PM
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Scott
 
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David,
I don't understand the dynamics of increasing your spring rate by cutting one coil off the spring. My logic tells me if a front heavy 1st Gen. car weighs 1800lbs or 900lbs per front quarter, then the front springs with 9 coils each are supporting the front half, and each coil has a spread load of 100lbs per coil. Cut one coil off, then the same weight must be borne by the remaining 8 coils per side, yielding 112.5lbs load per remaining coil. Not only will the front height of the car be shorter due to cutting one coil, but it will also be additionally shorter due to the added load per coil. How does cutting one coil then increase the stiffness? If anything, the front should be softer. Maybe the terminology is confusing me?

[This message has been edited by bonecrusher67conv (edited 11-27-2001).]
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post #6 of 7 (permalink) Old Nov 28th, 01, 08:52 AM
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David Pozzi
 
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Well, you are thinking of the loads in a little different way.
A coil spring is nothing more than a coiled up torsion bar. The sway bar arms are equivalent to the ends of the springs dia which cause the spring metal to twist like a sway bar when compressed.

You are not allowing for the added travel shared by the remaining coils, they will not only be loaded more per coil but will be compressed more for each coil than before.

If you take a 300 lb spring and cut it in half, you will have two springs with 600 lbs rate PER INCH OF COMPRESSION each.
That is due to the remaining coils being compressed twice as much as before, half the number of coils are recieving the same load and travel as the whole spring did.
In other words, the inch of compression you would use to "rate" a spring is shared by half as many coils as before.

The end coils don't contribute as much to stiffness, so cutting off the first coil doesn't make as large a change to stiffness as the second coil cut does on a Camaro spring.
Hope this helps.
David

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Check my web page for First Gen Camaro suspension info:
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67 RS 327
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post #7 of 7 (permalink) Old Nov 28th, 01, 12:51 PM
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David My 69 rs z seems to sit too low. I have compared my car with other orignal survivor z28s.We have measured from the center cap to the fender and mine seems to be about 1" lower. My car weight is 3360. The orignal springs were # 3925814. The guy I bought them from says they are correct for Z28s.Mine is a RS Z X33 with some options that add weight. What do you think
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