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Old May 10th, 2004, 22:27
Ed A. Stevens's Avatar
Ed A. Stevens Ed A. Stevens is offline
NAXJA Member
Join Date: Jun 2002
Location: Desert Beach So Cal
Posts: 1,105
Re: Pre-runner type suspension stuff

Originally Posted by RobertF
This is in reference to quality, rebiuldable, revalvable, mono-tube, reservoir shocks.
What makes a gas charged shock extend after compressing it, is the gas pressure working on the cross section of the shock shaft.
This may help?

Blow up a balloon about half full, and poke your finger into it. The pressure will lightly push out your finger with a resistance force related to the cross section area of the finger. Fully inflate the balloon, and it will push back with more force (as hard as you are willing to press before popping the balloon).

The resistance force you feel expelling your finger is isolated on the cross section area of the penetration (same as the cross section of the piston shaft), it does not force the finger sideways, even as you feel the pressure on the sides of your finger because the force is equalized on all sides of the finger (same as the shock shaft). The side force is equalized, but the extra pressure still restricts the blood flow to your fingertip, the same way the extra pressure adds friction at the shock seals (stiction).

Finding the pressure to achieve a balance between stiction (maximum pressure without excessive seal friction) and cavitation (minimum pressure needed to prevent fade) is trial and error unless you have considerable experience tuning shocks (IMO).

Originally Posted by RobertF
The valve stack can be designed to give different low and high speed damping. This is not the speed the vehicle is traveling, but the speed the shock is moving.
Valving can be progressive, digressive, single stage, dual stage (low speed, high speed), and triple stage (low, mid, and high speed). You do not have to limit yourself to the five or so generic stacks Bilstein sells once you understand how to configure them.

I have not rebuild a shock in many years, and never to the detail you describe, but what you describe is much of what we discussed when we were getting our 1600 car race shocks supplied and tuned by Curnutt. The review discussion after each race was focused on how the chassis performed and when it failed to perform, the shock speed when they did not work to an advantage.

The conditions were a challenge to describe and relate to shock speed: rear pogo kicking only in medium speed whoops or front lockup on high speed corregation ripples when easy on the brakes entering a whooped turn, as much as when the problems occured (early after a stop, or long into the lap). The valving changes (and a few spring rate and shock mount changes after monitoring temperatures) after each race debrief provided progressive improvements that resulted in a significant improvement in the ride and the sustained speed we could race without pain and breakage. Curnutt built bypass ports into the shock bodies to progressively alter the position sensitive dampening (long before Lightning Rod external bypass shocks) but the improvement isolated to changing the valve stack, by someone as experienced as Curnutt, was enlightening (to the point that I ignored learning how to improve on what he gave to us before each race).

Have you found a good reference guide on how to tune the valving?

My XJ modifications have been fairly static in the past few years, and a multipurpose crawler that runs well at speed, with more sophisticated shocks, may motivate me into playing with the XJ more (at least until I start building motorized toys for the kids). The direction you guys are taking (to higher speed performance as well as crawling) is where my interests have been for a while (my interests have been isolated with minimal feedback other than reviewing Jeepspeed forums).
is more than quality web forums and organized events, support the collective XJ/MJ voice and join the club.

Happy Trails!
Ed A. Stevens
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