coil over spring rate ?

[SDMAN]

Have a question on coil over shocks. Specifically, a spring rate question. My rear coil overs are mounted in a vertical orientation (0 degrees). No angle on them at all. They have 200# springs on them, and since they are mounted vertically, the spring rate of the spring is 200#.

My front coil overs are mounted at an angle. For this question, lets assume that they are 30 degrees from vertical (0). The springs on these coil overs are 500#. What I have been told is, because these coil overs are mounted at an angle, the actual spring rate will be something less than 500#, because they are not mounted straight up and down.

I am also told that there is a formula I can use, that takes the angle of the coil over into consideration when calculating desired spring rates for coil overs mounted at an angle other than 0.

Anyone know if this is true, and if so, where can I find this math formula for calculating the correct rate spring?

[MrQuick]

You need the Cosine chart....
angle cosine
5* .996
6 .995
7 .993
8 .990
9 .988
10 .985
11 .982
12 .978
13 .974
14 .970
15 .966
16 .961
17 .956
18 .951
19 .946
20 .940


I have the calc here but its hard to type..... try here. my eyes are tired to move it over. http://www.pirate4x4.com/tech/billavista/coilovers/Part_1/

[JRouche]

You are on the right track. The canted springs are going to be less than a full rate. But dont forget wheel rate as opposed to spring rate. The placement of the spring mounts (talking leverage action here) will have just as much to do for the actual wheel spring rate (and thats what matters, what the wheel sees) as the adjusted spring rate when its canted. You cant apply the simple formula for spring rate when the springs are canted over cause the location of the ends of the spring (in relation to the control arm pivot points) can throw it all out the door. JR

[MrQuick]

whoa....this was from back in the day....might help. Thought i'd post it anyways. https://www.pro-touring.com/forum/showthread.php?t=744

[Norm Peterson]

I am also told that there is a formula I can use, that takes the angle of the coil over into consideration when calculating desired spring rates for coil overs mounted at an angle other than 0.

Anyone know if this is true, and if so, where can I find this math formula for calculating the correct rate spring?
Ultimately, you're looking for the 'wheel rate', which for a "coil spring on control arm" independent suspension includes three separate terms. One is the ratio relating the spring attachment position on the control arm to that arm's length between pivots. The second is the cosine of angle that the spring makes with a line drawn perpendicular to the control arm at the spring point. And the third deals with where the wheel and the ball joint on that control arm are, relative to the front view instant center (usually this is a minor effect, and many references don't even mention it). All three of these ratios, including the cosine one, are squared when you convert a spring rate to a wheel rate.

Example (no particular car here, just the first numbers that popped into my head):
600 lb/in spring at midpoint of control arm, mounted at 10° away from perpendicular (cos = 0.9848). Ball joint 100" from front view instant center, middle of wheel 103" (camber assumed zero).
Wheel rate = 600 x 0.5^2 x 0.9848^2 x (100/103)^2 = 600 x 0.2500 x 0.9698 x 0.9426 = 137 lb/in


On edit, you can then use that 137 lb/in as the front suspension stiffness part of the input for checking pitch (flat ride) behavior.


Norm

[79T/Aman]

here is a calculator that give you the rate you need based on angle, it also has a chart that gives you the percantage multiplier to calculate rate lose according to angle http://www.proshocks.com/calcs/anglefirst.htm