Professor Morris in the laboratory. Photo: Joe Bonnello
This Month’s Lesson: Position-Sensitive Internal Bypass Shocks
Welcome back to the 17th class in Dirt Sports University’s Shock and Suspension Series. As we wind down the last few classes revolving around off-road shock absorbers, I felt it only right to take a class to show you the inside world of a version of shock that most of you will not have the opportunity to learn about or see in person. In this month’s class we will look into the relatively unknown and mystical position-sensitive internal bypass shocks. While just about everyone is used to seeing bypass shocks on off-road race vehicles with adjustment tubes all over the outside of the bypass shock and magazine advertisements professing why you need to use their shock to race on, there is more to it than just that. Little is known about the numerous races won by some of the top off-road drivers with the hidden tuning secrets only available inside the shocks where you cannot see them. While not very many teams or drivers have taken the time to set up their vehicles with internal bypass shocks, you would more than likely be surprised to know that many off-road races have been won in Class 8, Class 1 and Trophy-Trucks, as well as overalls, with them. Without sounding like we are trying to leave out several different race organizations by not calling their classes by the organization’s proper names, we are using a basic class distinction generically here for all.
A FEW COMMENTS BEFORE WE START: You should always look first at using an externally adjustable position sensitive bypass shock over the internal bypass version. This is said with a few exceptions, such as having extreme space limitation issues, unlimited budget along with limited common sense, more free time than you know what to do with, or I just want to be like that guy, or where once the vehicle is tuned you do not want anyone messing with it. I would not recommend using the internal bypass shock design unless you really needed to. While there are several advantages to them, which we will discuss here later, the downside of the time needed to tune them properly is outside of the time constraints and patience level of most people. Most shocks are called out on size by the diameter of the outside of the shock canister tube, regardless if it is a coilover or bypass shock. So if you are measuring your shock at three inches in outside diameter, the shock piston is going to be about 2.8 inches in diameter depending on the tubing thickness that the brand of shock is using. If this were an internal bypass shock there would be an internal cylinder of 2.5 inches in diameter. The inner cylinder with its compression side shim stacks will just fit inside the outer shock cylinder and will use a shock piston of 2.3 inches in diameter. This type of inner cylinder bypass design will require a smaller shock piston no matter what the main outer shock can is, with similar proportions in diameter sizes. All is not lost with a smaller shock piston, as you can still generate enough force to destroy any shock mount if you wanted to, just like an externally adjustable shock style. You carry just about the same amount of shock oil with both designs, so cooling will remain about the same between internal and external adjustable shocks.
A basic exploded view of an internal bypass shock.
INTERNAL BYPASS SHOCKS’ WINNING HISTORY: A lot is unknown about all of the drivers and teams that raced with position-sensitive internal bypass shocks due to the secrecy of the development and the thought process being used to tune them. The internal bypass shock actually started being used just about the same time as the first external bypass shocks were popping up several decades ago. Ivan Stewart, Larry Roeseler, Robby Gordon, Bob Gordon, Scott Taylor, Rob MacCachren, Roger Norman, Jerry Whelchel, Mark Post and Nick Baldwin, as well as possibly a few others have all won off-road races in the top classes, as well as Trophy-Truck, with them. Many of these vehicles were outfitted with just a single internal bypass shock with a coil spring over it at each corner. You most likely never knew it happened because that is how competitive and secret some things were then, and are now.
An example of a 3-inch internal bypass shock complete with coil spring.
EXAMPLES TO SHOW YOU: What we have here to show you for reference is from some shorter wheel-travel vehicles. The concept and design is the same, just a little shorter in travel and smaller in diameter than a current Trophy-Truck would use. As mentioned earlier here, no one wants to share what they have been competitive and winning races with. The internal bypass design takes quite a while to tune, and different ideas have been implemented to get each vehicle to work in the particular terrain for which the driver wants to tune the vehicle. Most of the teams made their own shocks as the technology was not commercially available early on. Hence, these shocks are not just lying around to take a close look at. If the opportunity arises when the shocks are apart, the door is closed and you are usually told to stay out of there. Moving forward in time, Fox Shox was big into the mountain bike market while at the same time making large strides in the off-road vehicle market. At Fox Off Road, John Marking was running the show and realized that there was a need for an internal bypass shock for certain applications on off-road race vehicles. After thinking of a few designs, he went ahead and patented the internal bypass shock design that we will be reviewing here. There are constant upgrades being made to the original concept on a regular basis. One of the most noted examples of recent production vehicles using the internal bypass Fox shock is the Ford Raptor. This patented Fox technology has also found its way onto many military vehicles, both large and small. The durability requirements for these sorts of vehicles have led to improvements in designs and materials in many areas of the shock.
WHERE YOU NEED IT: Depending on the vehicle you are working with there might be space or fitment limitations. If a shock absorber needs to be used with a coil spring fitted around the outside of it, there is no place to have external bypass tubes unless you are able to fit a very long coilover shock that has an external bypass area above the coil spring. This is sometimes referred to as a coil-pass shock and works fine for some buggies that have the room to fit the very long length of them, but this style is not very common. In most cases it is best to use a coilover shock along with a separate externally adjustable bypass shock. The internal bypass design really works well for applications where you are already going to use a coil spring around a shock and there is no room to fit a second shock, such as an external bypass shock. Your design can also be one of simplicity, where using one large shock at each corner of the suspension leaves more room for other things in the design.
An example of the different styles of inner cylinder bypass holes.
BYPASS HOLE VARIATIONS: The picture with four inner cylinders (above) shows what compression bypass holes look like uncovered from the compression blow-off side shims as well as with the shims installed. You can use as many bleed holes as you want in whatever location that you think you need, as well as having them covered with the side blow-off shims that act like check valves for compression flow only. You can also use holes without the side shims, which will allow the hole to free-flow the shock oil in both compression and rebound directions. Take note that if you are going to use the side shims that the inner cylinder must have a machined flat area along with two 10-32 tapped holes in which to mount the side shims. Before the advancement of the threaded holes for removable side shims, teams would machine a flat area over the hole that they wanted to have flow one way and weld a single feeler gauge over the hole. Not very much adjustability was provided with this welded setup.
The compression blow-off side shims.
COMPRESSION BLOW-OFF SHIMS: These are what give the holes in the inner cylinder the ability to work like a one-way check valve. The shims can also be used as a tuning tool and are available in several thicknesses that will change the inner cylinder hydraulic pressure that opens the side shim. The shim thickness, along with using different hole sizes, gives a wide range of tuning options as well as placing these holes or side shims at whatever position in the inner cylinder that you want. By using different side shim thicknesses you can change the low-speed valving adjustment completely separate from the main shock piston valving stack.
INTERNAL, THE UPSIDE: The tuning possibilities of the internal bypass shock are endless. Once you have the shock fit on your vehicle, all that needs changing or modifying are the inner cylinders and shock piston valving. Add holes or blow off shims in the inner cylinder as you see fit to have an extremely progressive bypass dampening throughout the entire wheel travel. There is no rule on how many holes that can be placed in the inner cylinder or the distance between them. Many times these holes are one inch apart or less, which helps to provide a stepless feel to the shock stroke action. If you have an external bypass shock you are limited to where the bypass tubes have been welded to on the outside of the shock. Depending on how many tubes you have between compression and rebound, these are the zones of control that you are limited to in adjustment. However, the tubes that you do have are very quick and easy to adjust at a moment’s notice. As with a regular external bypass shocks, there is no cavitation in the compression stroke due to the inner cylinder being capped off from shock oil flow to reservoir area. Base valves can be fitted to this inner cylinder design so that if any cavitation is occurring during the rebound cycle it can be eliminated. This also allows for very low reservoir nitrogen gas pressures to be used that will reduce seal wear as well as let the shock shaft move very easily at low shaft speeds, giving a little more comfortable ride over the small rocks. Now for the best news of all, there is no check valve noise. All that clatter is gone. If you ever wanted to build the ultimate quiet prerunner, the internal bypass position-sensitive shock is what you want. That’s all there is to it. If you have the budget to build a first-class vehicle, then look into what Fox Shox has spent time, resources and patents on.
Welding up and changing holes is a constant with internal bypass shocks.
INTERNAL, THE DOWN SIDE: As mentioned earlier, with all of this tuning ability inside the shock absorber comes a lot more time and work needed to tune the vehicle properly. Having lots of extra inner cylinders to work with in trying different hole combinations, positions and hole diameters is mandatory unless you want to drive yourself totally crazy. Even then you will find yourself welding up and changing hole sizes or moving their location. Tuning time is measured in weeks for internal bypass shocks if you are very good and lucky at the same time. Sometimes it can take months, with many different inner cylinder combinations being tried. With external bypass adjustment shocks mounted on your vehicle, you can usually get very close to where you need to be in a few days of testing. Take a look at the picture (above) of the welded cylinder with the green and yellow pencils pointing at holes. The green pencil points to where the free bleed holes were welded up and moved to drill a new hole that needed to be smaller. The yellow pencils points to where the compression check valve hole was too large and welded up, machined flat and a smaller hole drilled in its place. The welding is done with silicon bronze rod, also known as Evedure or OX 26. This brazing process is used in order to keep the welding heat low to avoid distorting the inner cylinder. Imagine having a 20-inch Trophy-Truck shock inner cylinder that needs to have several holes modified. Most of you will have a common fractional drill set. On top of this you will need a number drill set to go along with it. Next up are the special de-burring tools that need to be used to de-burr the inside of the inner cylinder wall properly after you drill holes in it. Without the holes inside the inner cylinder bore being properly chamfered and polished, you risk damaging the piston band and piston. These handy little de-burring tools cost about $50 each, and each size only fits a couple of hole sizes. The de-burring tool will chamfer the outside and inside of the hole in one operation. The cutting blade is spring-loaded and will pass through the hole with its flat tip without enlarging the drilled hole size.
A drill set and special de-burring tools for piston work.
A special de-burring tool in use.
IN CLOSING: While the internal bypass shock might not be what you desire for your particular vehicle application, you should have some basic knowledge on what they are all about. Some of the best drivers in the business have raced and won with them, as well as taken advantage of the extreme tuning available inside them. In the end, most teams have gone to the external bypass shocks on their new race vehicles to greatly reduce the testing time needed to develop the vehicle’s shock tuning.
HOMEWORK For those of you who have followed us from the start of this journey, we hope that you have been able to put the knowledge that you have gained from this series to good work. Many of you are new to this magazine now that it is on the newsstands, and have missed all the good information, so get your hands on the past issues you are missing. The University Series on shocks and suspension along with the entire Masterpiece in Metal collection of current and past vehicles has a lot to offer. Remember that knowledge transforms itself into speed that you need.
Class Dismissed. Professor Tom Morris Dirt Sports University and School of Hard Knocks