How to evaluate shocks

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Dec 7, 2002
I am in the process of putting on new OEM shocks on the 1987 FJ60. Because it has about 130 K and rides sort of bumpy, I assumed that the shocks were due a change out. However, when I took off the fronts, it appeared that they had been replaced once. The rubbers bushings appeared fresh and there was anti-seize on the bolts. I went ahead and put the new ones on but decided to play with the old ones. What I found was that there was heavy resistance when I elongated the shocks but, in contrast, they collapsed very easily. Does this indicate that the shocks were worn out?
They are stock Toyota's for the FJ60.
should be close to the same resistance in and out

I agree. In fact, as I understand shock function, the resistances should be identical. At it's core, a shock is basically a piston with holes in it, right? Fluid moves thru the holes as the piston slides back and forth.
should be close to the same resistance in and out

I heartily disagree. I rebuild shocks all the time for motocross bikes and I have built some custom valved Bilsteins for my Cruiser. The difference in resistance is becuase it was built that way intentionaly. It has more rebound dampening than compression dampening, that's how it should be to counter-act the springs. And the compression dampening that you feel is only what is reffered to as "slow speed" compression dampening. High speed compression dampening comes into play when you hit the edge of a pothole or a rock, causing the suspension to compress rapidly. You wil never be able to feel that with your hands, only with the shock on the vehicle. Most over-the-counter shocks use just one set of valving shims for all the different shocks they sell regardless of the application. So you could end up with exactly the same valving on your 7000 pound Cummins powered Dodge as you do on a two-wheel drive mini truck. What works on one will suck on the other. Only Bilstein and Fox make shocks that can be re-valved. Bilstein will allow you to buy the parts and do it youself, Fox will not sell you parts. The best advice I could give you, unless you want to take a crash course on shock valving priciples and techniques, is to drive something and see if you like the way it feels. Of the pre-valved, non-rebuilable, over the counter shocks I tried on my 60, my favorites by far where the Skyjacker Softride Nitros. The Skyjacker Hydros were harsh although most sales people will claim that it's the other way around. Don't let anybody tell you that a hydraulic shock is softer than a gas charged shock, it all depends on the valving inside, not whether or not it is gas charged.
Interesting. I'm learning something here. Is the damping difference in each direction brought about by some sort of directional valving? Does gas charging play a role in it, too?
The way I always tested a shock for being bad is bounce the vehicle, then stop. If it keeps moving, after you stop the bouncing, then the shocks are bad or worn. If it stops bouncing/swaying when you stop the bouncing, (ie, standing on the tailgate and bouncing) then the shocks are in good working order. Ride harshness and handing characteristics are determined by the valving specific to make and model of shock as the above poster stated. I myself prefer a firm ride, and sacrafice a bit of luxury for stiff and predictable cornering/handling over cushy comfort.

Interesting. I'm learning something here. Is the damping difference in each direction brought about by some sort of directional valving? Does gas charging play a role in it, too?

There is a piston on the end of the rod. On either end of that piston there are shims arranged in sort of a pyramid. The mumber of shims, their diameter and thikness determine how much force is required to push them open. Since the fluid flows through different ports on compression then it doen on rebound, they are independantly tunable. Gas charging is really superior. As the piston moves into the shock body it displaces an equal amount of fluid. This fluid has to go somewhere. The gas chamber is separated from the fluid by a piston or in the case of a shock with a resevoir, there is a bladder that can collapse to compensate and still keep the oil and air (nitrogen usually) separate. Either of these arrangements ensures that the oil in the shock does not get emulsified with air bubbles that would affect the dampening in a negative way. Most cheap shocks are emulsion shocks, meaning they are valved assuming that the oil and air in the shock will foam together in an emulsion thus allowing compression of the fluid to compensate for the fluid displaced by the shaft. Gas charging (in a decent quality shock) will provide much more consistent dampening since the oil doesn't get foamy or emulsified. There is always pressure to extend, so some people have assumed that they are stiffer, but it all depends on the valving inside. I'm a huge fan of Bisteins because they are rebuildable (the 5100 series) and I can change the valving to suite me which I have done several times untill I got what I wnated, and I can replace a bad seal or dinged shaft instead of buying a whole new shock. Now rocks and tree roots are swallowed with impunity as I float along!

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