Determining Cross member height for coil overs

[Rhino]

Please bear with me as this is probably a simple question I'm over-analyzing.
I currently know the ride height I want my car, the length of the coil over, and the approximate mount point on my axle. While it should be simple to calculate cross member height, what I'm not certain of is how much I should expect the coil over to compress with full weight.

For arguments sake, let's say I'm using a 300 lb. spring with a 1:1 motion ratio mounted perpendicular to the ground. If I place 600 lbs on that corner, should I expect a 2" drop from a fully extended height?

Going a step further, lets say this coil over is mounted on a solid axle, perpendicular to the ground, 12" inward from that tire's center. I realize that as a shock moves inward, you begin to lose some control over that corners behavior. I would assume this distance from wheel center is something I need to account for in my spring selection as well. Would the wheel motion ratio be calculated using the opposite tire as the pivot? Should this difference be disregarded considering how small of an effect it would actually have?

[Bryce]

yes you would expect a 2" drop. The formula is F=kx.

F=force (weight of car)

K=spring constant (300lbs/in)

x=displacement

[Twentyover]

If the spring is preloaded at extended height, won't that change the force to move it 2 inches?

Say there's an inch of compresion in the spring with the coilover at full extention. You load @ 600 lb, the first 300 lob will just take up the preload, the next 300 lb will compress 1 ".

Or what am i missing?

[Eric Howell]

This is how I layout shock length at ride height for rear shocks.
Example: (VariShock part#11222-515) extended length = 16.1, compressed = 10.95, travel = 5.15.
60% of 5.15 is 3.09 so I set the shocks at 13.0 or 60% of total travel. You need more bump then droop.

[Bryce]

You are correct.

if you preload the spring that will be a subtraction from the load from the car.

example:
If you preload the C/O 300lbs and load the C/O 300lbs then you have a nuetral position on your coilover. it would not change.

[Norm Peterson]

This is how I layout shock length at ride height for rear shocks.
Example: (VariShock part#11222-515) extended length = 16.1, compressed = 10.95, travel = 5.15.
60% of 5.15 is 3.09 so I set the shocks at 13.0 or 60% of total travel. You need more bump then droop.
Don't you want to set it 60% up from compressed?


Norm

[Eric Howell]

Don't you want to set it 60% up from compressed?


Norm
Oh hell, I mixed the numbers up.
Should be 14.04". Sorry.

[Rhino]

That's the exact info I was looking for. Thank You.
Although I knew I was making it out to be more than it needed, I hadn't thought of the preload of the assembled unit.