Professor Morris in the laboratory. Photo: Joe Bonnello
This Month's Lesson: Bump Shocks, Stops and Cushion
Welcome back students to your 15th class in Dirt Sports University’s Shock and Suspension Series. In this month’s class we will be reviewing options for stopping your suspension travel before components get to a metal-to-metal state, or more commonly referred to as “bottoming out.” Several factors of how you have designed and built your vehicle will come into play here, as well as the components that you chose to do the correct job of minimizing this situation. It is not as simple as just stopping the suspension travel in the last couple of inches before maximum compression has been reached.
UP UNTIL NOW: From our past classes you have learned that your springs hold up the vehicles’ basic weight as well as controlling ride height. The spring rate and rise of that rate through the suspension travel also has a large effect on front to rear vehicle pitch as well as body roll side to side. Shock dampening should take care of just about all of the energy and forces of the vehicle’s weight compressing the suspension through hydraulic dampening. This dampened energy is then turned into heat in the shock oil. As air moves over the shocks it carries the heat away from the shock parts and life is happy ever after, or so you hope. Your choices along the path of building and tuning your vehicle will dictate the final outcome of how your time and money was spent. All the good along with bad will show itself in the end.
A vehicle’s suspension system is the sum of all the various suspension components working in harmony together. No one single piece of the puzzle can completely make up for poor choices in the other components. Over the years there have been many options of different products to consider when dealing with this unpleasant and often parts breaking, banging and clanking situation of “bottoming out.” This happens as your suspension, compressing upward, collides with the frame or other immovable object as your vehicle comes back down to earth and completely bottoms out all of the available compression suspension travel from your sky shot attempt or similar feat. You drive hard, race hard and ultimately want to be the fastest person out there and take that coveted first place finish at the checkered flag. Then again, some of you are not thinking this all the way through and are trying to make that big first impression for your new date or your friends and observers taking pictures and videos. It does not always end up looking pretty, as your poor choices can easily end up on YouTube for all to enjoy. We will do our best to provide you the information that you will need to make the correct decisions for your application.
Two different nitrogen bottle charging setups.
PROPER TOOLS AND EQUIPTMENT: It is not all about the cool suspension parts and pieces that you want to buy to put on your off-road vehicle. Do not expect just to hop in it and race to the checkered flag. As you have hopefully learned from our classes here in the Dirt Sports University Series, there is a lot of tuning of your suspension to be done to match the vehicle and the suspension components to the terrain that you will be in. There is always the aspect of your driving style or lack thereof. Just as a note: If you are new to this, you should consider setting your suspension up a little on the firm side to help stabilize the vehicle while you get accustomed to its balance.
Example of a simple pressure gauge.
Aside from common hand tools, there are special tool items that you will need to set up and tune your vehicle with. Every nitrogen-charged shock absorber, bump shock or what have you, that has the ability to adjust gas pressures must have the nitrogen gas pressure checked on a regular basis to be sure that an O-ring or Schrader valve has not leaked. There are also tuning advantages to using different gas pressures, especially if you are using bump shocks. Therefore it is mandatory that if you plan on having any sort of success in driving, racing and tuning your vehicle, you must have a nitrogen bottle and pressure regulator setup as well as a small pressure checking/setting gauge.
Remote bypass shock cooler system.
While we are talking about tools, in the last class we discussed keeping tabs on your shock temperatures while testing as well as racing. The need to keep your shocks at reasonable temperatures is dictated by the quality of shock oil you are using, along with the temperature limits of the seals and O-rings. Depending on the combination of shock oil and/or the seal parts, the temperature limit could be as low as 300 degrees or as high as 450 degrees. Check with your shock brand manufacturer for their recommendation. Based on the temperatures that you are seeing, you will need to get better air flow directly to your shocks to help cool them or look at the addition of a remote shock cooler system for your bypass shocks.
A simple and accurate pyrometer.
While there are real nice hand held laser temperature guns available that do a good job of a simple point and shoot temperature read ing, be careful as I have seen large temperature variations depending on the surface finish and material that you are pointing at with the red laser dot on the shock end and body. Call me old school if you like, but I sometimes like to backup the laser temperature reading with a thermocouple-style pyrometer when questions arise.
A normal shock shaft with bump rubbers.
STANDARD BUMP RUBBERS: For most applications the standard short rubber bumper on your shock shaft that is around 5/8 to 3/4 of an inch tall does just what you need in preventing a metal-to-metal contact in your suspension. Some unique applications can benefit from an approximately three-inch long cream-colored foam rubber material that dissipates the full compressed loading over a little longer area than the short rubber bumper. The key point here is that you hopefully have already done a great job of setting up your shock dampening tuning and that there is a small bit of shock shaft travel left after a hard test run through big bumps. If this does not sound like you, then there are going to be issues. In a perfect world, all you should need is a adequately sized bypass shock on each corner, ones that you have meticulously tuned, valved and adjusted the bypass tubes so that even in the biggest bumps and jumps there is no feeling of the suspension bottoming out. All this and you do not even have bump shocks mounted on your vehicle. Class 1 cars and similar style limited class buggies that do not end up weighing too much can usually function well without the installation of separate bump shocks.
An exploded view of a bump stop.
BUMP SHOCK CHOICES: There are many brands, along with several styles, but they all have one thing in common and that is to help the last few inches of your suspension travel slow down very quickly in a very short distance. Hopefully this is done in a manner that provides a bit of comfort along with building the confidence that you are not going to get hurt or crash into the wild. Depending on the size and weight of your vehicle, the addition of a bump shock can provide you with another option with which to tune your vehicle. The last few inches of suspension travel can be very difficult to tune, while also maintaining a comfortable and compliant vehicle ride for the rest of the suspension travel. This is where a bump shock can help you, as it only has to do its work in the last few inches of the wheel travel. Depending on the bump shock stroke and position it is mounted on the vehicle, you could affect the proper operation if it is mounted in a goofy motion ratio to the wheel travel.
Various 2.0 bump shocks and mounts.
The most common bump shock to use is a 2.0-inch smooth body design that slips into a simple frame-mounted socket. One of the most important items to remember is that if using a pinch style clamp mount, don’t tighten the pinch bolt too much. If too much torque is applied, you will bind down the cylinder body enough to impair the proper movement of the shaft during its stroke. Also be sure that any welding has not burned through or distorted the mount so that the cylinder cannot be easily installed. Use a drum sander sparingly to clean up the inner bore of the weld on the mount to allow proper fitment and removal of the bump shock. Depending on the weight of your vehicle, you may need the larger 2.5-inch bump shock to work with. Step carefully here when going bigger as you would be amazed by what a seemingly small 2.0-inch bump shock can do. Fox, King, Bilstein, Sway-A-Way and other similarly designed hydraulic bump shocks are all able to be taken apart and valved very simply, as well as changing the gas pressure. Most should be close out of the box, but adjusting the nitrogen gas pressure can be a good tuning aid, depending on whether or not you feel a jolt when the bump shock makes contact. Most bump shocks come with 150 psi in them and can be lowered down to 60 psi to reduce the contact feel. There could be an issue of the bump shock recovery speed not coming out quick enough though at low pressures. This is where a GoPro can help see when the bump shock is in use. Depending on where you mounted the bypass shocks, you can usually make quick gas pressure adjustments when out testing. The simplicity in taking these apart, as well as the readily accessible Schrader valve, makes this design hard to beat.
A new bump shock has been advertised that uses some sort of foam rubber or similar material inside to dampen the bump shock. This is not the time to experiment with this. Stick to the proven hydraulic oil design that is used by just about every top Trophy-Truck and major class winner in off-road racing.
Light Racing makes an externally adjustable bump shock. The mounting system that needs to be used with these is not always user-friendly, as the threads and set screw can possibly rust if you are not careful. You must also be careful of rotation as the different body pieces are threaded together, as well as a reverse thread involved. The advantage of having the external adjustment is outweighed by the hassle of making a gas pressure change on these with the recessed valve that takes a special tool and tends to unscrew from the inner valve. I have successfully worked with these, and they do have certain advantages with their unique mounting design and use in particular vehicle applications. At the end of the day I prefer to use the more common smooth body design for most racing applications.
IN CLOSING I had hoped to get farther along in this class before we ran out of time. As you can see from the pictures here, and the details, there is a lot to go over. There is a discussion that needs to happen in the next class on how to balance the use of bump shocks with the bypass shocks as well as the rest of the suspension system.
HOMEWORK Get out to the races and see what the winners are doing for suspension. Look at vehicles similar to what you have or are building to get ideas. Take plenty of pictures and do not be afraid to ask questions. There is also a great resource with Dirt Sports Magazine in the entire Masterpiece in Metal collection of current and past vehicles. These are the best of the best and Dirt Sports has taken you deep inside of each vehicle to see the quality and workmanship it takes to build a first class vehicle. Also available from Dirt Sports University are the past classes that you might have missed, so get your hands on all that you can. Remember that knowledge transforms itself into speed that you need.
Class Dismissed. Professor Tom Morris Dirt Sports University and School of Hard Knocks