Question: When you lower the threaded shock body, does it expose more of the shaft, or does it stay the same?

 

You are correct that the spring will always be compressed by the same amount (total) regardless of preload, but this may be more clear.

Assume that a particular spring will be compressed 2" by the weight of the car, and the damper only has 2" of total stroke. If you don't preload the spring, then the spring AND the damper will be compressed by 2". The damper will already be at the bump stop. However, if you preload the spring by 1", then the weight of the car will only compress the spring by 1" more (2" total), and the damper will only be compressed by the same 1". This will put the damper right in the middle of its stroke with the car resting on it.

 

Lowering the threaded shock body does nothing to the damper. You are really just raising and lowering the bracket that supports the coilover from the lower control arm.

EDIT: It does nothing to the spring either.

 

OK... I think I'm almost there with you. One more question: When you preload the spring, does the shaft extend beyond its 'at rest' position? Thereby giving you more downward movement before the damper bottoms?

EDIT: Or is the "at rest" position of a damper 'fully extended'?

 

I'm not sure I understand the questions. If you preload the spring too much (more than the amount that the car's weight will compress it), then the damper will be fully extended even with the car's weight resting on it. You would have plenty of compression travel, but no rebound travel. The car would ride and handle like complete crap.

I really don't know what the optimum amount of preload will be, since I don't know where damper's "at rest" position should be. That is why I chose the middle of the stroke as my target.

 

So, basically, I guess my question is:
1) What is the total amount of travel for the JIC dampers?
and
2) Is the shaft fully extended with no spring pre-load (when the car is up in the air). Or does the shaft extend further when you add pre-load (i.e., force the shaft to extend)?

Optimally, you would think JIC would design the dampers such that the compression caused by the vehicle's weight would put the damper in the middle of its stroke (based on the spring rates provided with the kit, and the estimated known weight of the vehicle). Or, if they couldn't design it with that much travel, it should be relatively easy to determine exactly how much pre-load is required, using the same information, to put the dampers at their optimal "at-rest" position.

 

The total amount of damper travel is about 2", and the shaft is fully extended when there is no weight on it, so increasing preload does not extend the damper any further.

I doubt the JIC damper is specific to any particular car or spring. The only S2000 specific components to the kit are probably the top and bottom mounts. There is also a wide assortment of springs available with higher and lower rates, and different lengths. Every application would have different preload requirements.

 

Ahh.. I see.
So, given an approximate corner weight of 700 lbs per corner, and spring rates of 7kg/mm front & 8kg/mm rear, and the stroke of 2", you would need 0.7857" of pre-load front, and 0.5625" of pre-load rear, to put the damper at the middle of the stroke.

Approximately.

EDIT: If we get more precise measurements for the stroke, we could re-calculate this to give a more accurate measurement, since this appears to be about 1/4" off of what you said that you used.
Maybe I should increase the corner weights, too... to take into account driver & fuel weight.

 

Right On, Bro! When I said 1", I was just rounding up from .785723".

Actually, I was not so scientific about it, and didn't calculate anything. I guess I have a bit more compression travel than I have rebound travel.

 

Maybe we should pass this info on to the distributor, for the benefit of future buyers. Does anyone have contact info for the distributor?