When the time comes and you have realized that your vehicle has exhausted the capabilities of a standard smooth body shock, where do you turn? Usually when that time comes you are in need of something that will not only soak up more abuse but give you infinite adjustability controlling the way your suspension moves through even the toughest terrain. What you need is a bypass shock. They come in various lengths and diameters to suit your needs from mild to extreme. You can even custom order how many bypass tubes come with your shocks allowing you to valve the control of your shock externally. The only real problem with bypass shocks is most of us look at them as a complex equation of mechanical goodness that we couldn’t possibly try and take apart ourselves. One of the number one benefits to a shock of this caliber is that it shy of totally destroying it you can rebuild it and replace every part making it last a lifetime.
Fox Racing Shox is one of the leaders among the high performance shock manufacturers in today’s automotive and racing scenes. You will find their shocks in OEM replacement kits, on mountain bikes, aftermarket long travel suspension and running on some racing winning cars and trucks in Baja. They opened their doors and their tool chest for us to come in and pick their brain and learn just what goes into building and rebuilding their high performance bypass shocks. Hopefully, this article will help you realize as we did that although it seems complex, once you start turning wrenches it’s actually a fairly easy process.
So what is the basic operation of a bypass shock?
The external bypass shock functions by allowing a metered amount of oil to flow around the valving piston by way of the tube(s). By allowing some oil to flow around the piston, the damping of the shock will only be a fraction of the total amount of valving on the piston. By controlling the bypass amount with the adjuster, the shock can be broken into small zones, each zone with a different amount of control than the previous. This is only true while the piston is passing the thru the bypass circuit or tube. Once the piston has reached the end of the circuit or tube, the effects of that adjuster are no longer effective. If the piston has entered another bypass circuit, then that adjuster is now controlling the bypass. If the piston is passing thru any bypass tube, then there is no bypass, and therefore 100% of the control is coming from the valving on the piston. Typically this area of zero bypass is common in the last 25-30% of travel. As the shock is compressing, bypass only occurs from the blue compression tubes and as the shock in extending, bypass only occurs from the red rebound tubes.
What do those bypass adjusters do and how do they work?
The bypass unit adjuster consists of a lock nut, adjusting screw, plunger, and check valve. When oil is passed thru the bypass tube, the check valve is pushed back against the plunger, whose depth is determined by the adjusting screw. With the valve in the open position, oil can bypass the piston, the greater the distance between the plunger tip and the valve, the greater the amount of bypass. The adjuster itself can be adjusted by using a 9/16-inch wrench to loosen the jam nut, and a 3/16-inch Allen wrench to turn the adjusting screw in or out. Rotating the screw clockwise will decrease the amount of bypass, by decreasing the travel of the check-valve. Likewise, rotating the screw counter clockwise will increase the amount of bypass. To determine how far open the adjuster is, loosen the jam nut and count the number of revolutions until the screw is in the full closed position. The adjusting screw may be set in any position to function; however it is easier to keep track of the adjustment if the screw is adjusted in full or half turns. When opening the adjusting screw, the screw will stop when the adjuster reaches the full open position. The bypass valves are designed to allow flow in only one direction, and therefore can only control flow in one direction. For this reason there are two types of tubes on a bypass shock, a compression tube with a blue cap, and a rebound tube with a red cap. Any adjustments to the compression tubes will not affect the rebound control, and vice-versa.
It’s time to service my shock, how do I do that?
The following is a basic guide on how the Fox Racing service department walked us through rebuilding a 3.0 diameter bypass race shock. Please consult your included instructions if you plan on doing this yourself. If you feel at any time you are not confidant tearing apart the shocks yourself Fox is just a phone call away.