Off-road Essentials – Shock Types
Story and photos by Budd Stanley, additional photos by FOX Racing
Just what are remote reservoir, internal bypass and external bypass shocks?
We all know that the suspension is one of the most important parts of a 4WD’s setup, and we take great care and spend large sums of money to get the perfect lift, as well as the perfect amount of tire clearance and axle articulation. However, once the springs are set up to our liking, more often than not, the trend is to slap a set of what we like to call “White Can” shocks on, to fit the new suspension travel specs.
Check any forum and you will find an abundance of people asking, “what shocks should I put on my freshly lifted rig?” The usual answer that gets thrown back is, “what you want to do is drive one wheel into a ditch with no shocks mounted, measure the full suspension droop to the full compression, take that measurement down to Johnny’s Auto Parts and find a set of shocks that happens to have the same travel specifications.
We’re not the biggest fans of “That’ll do pig, that’ll do” type of thinking (from the movie Babe for those of you that don’t have kids). If you’re going to make the time and financial commitment to change the springs and geometry of your 4WD, you should take the same care in choosing suitable shocks. The results from simply buying shocks that have the correct travel specifications could completely destroy the performance gains you’ve achieved with a good lift, and even be down right dangerous. If you are throwing the shocks from a Dodge Ram HD onto a Toyota truck, or a Jeep JKU onto a Samurai, you will have massive valving differences. Sometimes you might get lucky and find something that matches decently to your setup and driving style, but chances are you’ll have completely inadequate tuning that will either produce a poor ride or horrible handling characteristics. Over valved shocks will put undue forces on mounts, as well as counteract the springs, while under valved shocks will not keep the springs in check, causing over exaggerated actions that will likely see your rig either in a ditch or upside-down.
Coming from the world of motorsports, rally racing in particular, I know that the value of tuning good quality shocks to a particular vehicle and style of driving. It is one of the most important investments one can make when setting up a vehicle to perform to the best of its abilities and unfortunately, shocks are one of the most overlooked pieces of equipment on a 4WD.
The choices for quality shocks are nearly as large as the choices for crappy shocks. More often than not, a good quality shock can be spotted easily, simply by the material, build quality and finish, and they look nothing like their White Can counterparts. They’re also likely to be sprouting all sorts of tubes, canisters and fluid lines. These are the highest of the high tech in shock development. All these growths coming off the main shock body are meant to move and redistribute shock fluid in order to give the optimum dampening performance. But what is really going on behind all that shiny aluminum and machined steel? To get the rundown on the different forms of shock technology, we went to the specialists and got together with John Brindell of Fox Racing Shox to explain.
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Off-road Essentials – Shock Types
These will keep your rig up, but they are most likely improperly valved for the weight and balance.
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Remote Reservoir
Remote reservoirs can be solid mounted to the shock body, or run remotely with a fluid line.
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Remote Reservoir
Remote Reservoir - Nitrogen charged reservoir ensure equal pressure feedback to the shock.
Remote Reservoir Shocks
Remote reservoir shocks are of a typical shock design that incorporates an additional canister. There are two types of remote reservoir shocks; fixed reservoir (aka piggyback) shocks have the additional canister mounted directly to the shock body, whereas a detached canister is connected to the shock body via a high-pressure fluid line. This allows the canister to be mounted remotely in an idea position out of the way of moving suspension parts and flying rocks.
Inside the remote reservoir, a second floating piston separates the shock fluid with compressed nitrogen, tuned to supply an equal amount of pressure on the shock oil so that the shock itself does not lose its original intended performance.
As the shock piston cycles within the shock body, it is creating heat. That heat makes the hydraulic oil expand, changing the characteristics of the shocks performance. By plumbing in an external reservoir, you accomplish two things. You provide the shock with extra oil, which in turn aids cooling. But more importantly, by providing an oil reservoir that is pressurized with nitrogen, you combat cavitation. Cavitation is the frothing of the shock oil inside the shock. Oil with bubbles can’t do its job, rendering your shocks dampening abilities ineffective. This equates to heavy drop and bottoming out of the shock, and will likely damage other suspension parts.
Many times, companies will build adjustment controls into the reservoir, such as Fox’s DSC Adjuster system, which allows the adjustment of both high-speed and low-speed compression resistance. By either closing or opening valves in the reservoir, the ability to adjust the fluids flow through to the shock can firm or soften the shocks actions over select terrain.
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Internal Bypass
Internal Bypass shocks look much the same as any shock as the magic is all being done under the skin.
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Internal Bypass
Internal Bypass - The shimmed bypass valves allow fluid to flow into the secondary body until the piston pushes past, cutting the circuit.
Internal Bypass Shocks
There are many ways to achieve this type of actuation; several shock makers will build fancy valving into the shock piston itself to control the shocks behavior at different positions in its stroke. Fox utilize a second outer body that acts somewhat like a transfer tank.
When the shock is in an upward stroke, fluid is allowed to pass through shimmed bypass holes strategically placed within the upward stroke path. That fluid is pushed into the secondary cavity and allowed to bleed back into the main shock body below the piston via open refill holes. This allows the fluid to bypass the piston freely, offering softer dampening. This soft dampening allows the shock to soak up small and fast compressions with relative ease.
Once the piston has traveled beyond the bypass holes, fluid recirculation is blocked. The dampening is now restricted to the fluid trapped in the top of the shock passing through just the pistons compression valving, which offers much firming dampening. This firmer pressure begins to slow the piston as it reaches the top of the shock and prevents it from bottoming out.
The system works the same in reverse, or the rebound stroke. The blocked upper bypass holes momentarily hold the shock under compression to prevent recoil. Once the piston falls past the bypass holes, the fluid is allowed to flow around the piston, via the outer body, softening up the dampening force. Once the piston rebounds past the free flow holes at the bottom of the shock, fluid transfer is halted once again, restricting the valving to just the piston itself.
What you get is a shock that has variable valving rates throughout specific stages of compression and rebound actions. One can tune a shock to perform to their preferences by adding as many bypass holes as needed at select stroke compression lengths, either softening or firming up the shock stroke at specific points. The level of dampening can be tuned by the amount of shims added to each bypass hole. It is nearly as effective as the external bypass shocks, but comes in a tighter more compact package for easier applications on production 4WD’s.
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External Bypass
External Bypass - These flamboyant masterpieces work much like internal bypass shock, but fluid flows though tubes instead.
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External Bypass
External Bypass - These tubes each have an adjustable nozzle to fine-tune the rate of fluid bypassing the piston.
External Bypass Shocks
You see these funny looking monsters on all the top spec competition 4WD’s. They are the ultimate in variable dampening and adjustability. In essence, they do the same thing as the internal bypass shocks, but go about it in a different fashion.
Rather than using a secondary body overtop of the main body to transfer fluid, like the internal bypass shocks, external bypass shocks use tubes to control the flow of hydraulic fluid around the shock piston.
It works like this; when the piston begins its compression stroke, it will be as soft as the number of compression valved tubes connected to the body. Compression valve tubes will bypass fluid from above the piston to below, making for a softer dampening rate. As the piston pushes past each tube bypass valve, it then cancels out that tubes effect on the dampening, firming up to the effectiveness of tubes mounted still higher. Once the piston has pushed past the final tube, it is then under its own valving during the highest compression point.
However, once the rebound stroke commences, there are now tubes dedicated to the rebound stroke as well, providing the exact same service on the pistons downward stroke. Now, these shocks are more likely custom developed by the race teams that use them, so any number of bypassing tubes can be place at any length of piston stroke the user wishes, allowing for full custom mapping of the shocks travel through multiple lengths and dampening rates. If this wasn’t impressive enough, each and every bypass tube has its own adjustable valve. So, now each level of shock movement can be fine-tuned to get the absolute most out of the shocks dampening possibilities. Sounding complicated? The massive external bypass shocks sprouting 6, 7, 12 bypass tubes we’ll leave to the competition teams that have a shock technician on payroll, but Fox do make simplified versions for several production vehicles and custom projects.
And there you have it, a beginners guide to remote reservoir and bypass shocks. These are all the cream of the crop in shock technology and will allow the user to finally tune their dampening levels to perfectly match that of the spring and articulation characteristics of a lift kit. Now, one must know how to tune such shocks. Tune in (pun intended) to our next essentials feature when we dive into the basics of compression and rebound adjustments.