Professor Morris in the laboratory. Photo: Joe Bonnello
This month's lesson: More Bump Stops and Updates
Welcome back to the 16th class in Dirt Sports University’s Shock and Suspension Series. In this month’s class we will continue on with some additional choices for stopping your suspension travel before components go metal to metal — more commonly referred to as “bottoming out.” Along with finishing up with bump stops, we will touch on some other items that will also help you with the choices on your off-road racer or project, so sit back in a quiet place and join us.
WHERE IS YOUR PROJECT TAKING YOU? While some of you are working on your newest creation to win an off-road race overall, the vast majority of you are working your way up through various classes that either fit the budget you have to work with or the time available to devote to it aside from your normal job. Others also understand that polishing your driving skills by driving a limited class vehicle will actually help as you seek to race in the upper classes such as Unlimited cars or the coveted SCORE Trophy-Trucks and BITD Trick Trucks.
Take Rob MacCachren for example. He recently won his 200th off-road race and came from the very humble beginnings of what was available for him to drive and use to hone his skills. He started racing as a very young teenager and now, almost three decades later, is still at the top of his game. Rob did not start off with a full-blown Trophy-Truck program, but in limited-class buggies with less horsepower than you can imagine. He was able to learn at an early age that it was not how fast you could go at any one time, but that the combination of maintaining his momentum and slowing down as little as possible through every corner and bump was more important. These skill sets have served Mr. MacCachren well in becoming a desert champion as well as short-course champion many times over. My point here is that not every vehicle you are going to be driving or racing will have a big V8 engine or big bypass shocks on it along with hydraulic bump stops. Many of the limited-class rules dictate what you are allowed to use. In the end, this helps keep costs down while at the same time making it more of a driver’s class rather than a checkbook war. Making the correct decision on what sort of off-road vehicle you are going to dedicate yourself to is going to have a big affect on how quickly you will put yourself in the poor house, or not. With this thought in mind, we sometimes need to take a step back to some of the simpler items that many are working with for the particular vehicle they have. Ultimately, the choices and the decisions of what you are going to move toward and focus on is yours and yours alone. Where do you want to end up, and how can you best plan now for getting there?
TIMES ARE CHANGING: Many of you may be reading Dirt Sports for the first time as it has just recently become available on newsstands. The partnering with Source Interlink and GrindMedia will bring a new depth of information to you. If all works out to plan, you will also be able to get electronic versions of past issues of Dirt Sports magazine along with the Dirt Sports University classes you have missed. There are also loads of Masterpiece in Metal vehicles that have been showcased in this fine magazine that you might have missed. Hopefully you will make the time to get your hands on these previous issues and enjoy all the hard work that has been done to provide you with the absolute best there is in the off-road universe here in Dirt Sports magazine.
SIMPLE BUMP STOPS: Not every vehicle has the need or availability of using hydraulic bump stops on either the front or rear. In our last class we showed several versions of the hydraulic bump stops that are available, but if they are not what you can use, there are some styles of simple bump stops that might do the job for you. Remember, the goal is to prevent metal-to-metal contact of the suspension system in a full compressed travel situation (full bump) as well as while fully extended (full droop). This class will focus on the versions of simple bump stops, while the use and fitment of limiting straps for droop travel will be discussed in a later class.
The simplest version of front suspension that comes to mind is the Volkswagen-based front beam that has been used from the beginning of off-road time in the 1960s and at the original NORRA races in Baja, Mexico. This simple VW beam front is still commonly used today on limited-class buggies as well as Baja Bug style vehicles. In most cases the little cone-shaped original rubber stop has either been eliminated completely or a redesigned to incorporate aftermarket polyurethane bump and droop stops.
Polyurethane bump stops are available in several smaller sizes.
POLYURETHANE BUMP STOPS: To simplify things, we will just call these “poly.” There are many sizes, styles, shapes and brands of bump stops made from polyurethane available. Many of the engineers have been very creative in what they think will work, so the variety is quite large in what is out there. For the majority of applications in off-road, smaller is actually going to be better. The energy contained in compressing any formed bump stop is quite large and you will be doing yourself a favor if you can control this last part of your wheel travel with your shocks. If you feel you need a poly bump stop, use it for the very last bit of travel to avoid any metal-to-metal contact. Using any more than an inch of a hard polyurethane bump stop will cause rebound issues from the energy stored during compression that will want to kick up the suspension when not properly dampened. The idea is to be able to use effectively all of the wheel travel you have available, and if you are trying to compress a poly bump stop a couple of inches, more than likely the results will not be appealing to you.
When looking at all the poly bump stop sizes available, keep in mind that there is a whole world of street-based or limited-use off-road vehicles that the poly bump stop manufacturers are catering to, and that some of the mentality is “bigger is better.” Pure race vehicles and prerunners will only be choosing from a couple of the smaller sizes, and only if your particular vehicle application really needs them.
Care needs to be taken with a poly bump stop, as any cuts in the smooth outer skin of the material will promote a split in the plastic and cause it to fail prematurely. The bonding of the poly to the metal base can also become marginal during hard use, and if abused excessively the polyurethane-based plastic will separate from the metal mounting plate. If you haven’t gotten the idea yet, these are fragile pieces and will need to be thought of as disposable. Monitor their condition frequently and replace as needed. Don’t get me wrong here, these poly bump stops can be used very effectively when needed. Just understand that they can be subject to very high loads if your shocks are not doing the job they are supposed to. If you are abusing the bump stops they will die off on you.
In the picture of the five red polyurethane bump stops with the very smallest version on the left, as you look to the right you can see that the height and size gets big in a hurry and will most likely not work well for your race or pre-run vehicle. The larger poly stops can be carefully cut down as needed on a band saw and tuned up on a belt sander to fit any odd fitments you may have. The bottom mounting stud on all five of these is 5/16-inch diameter. This will give you an idea of the relative sizing of them.
The most common poly bump stop used on a beam front end is the second from the left. This small rectangular stop should do a good job of preventing metal-to-metal contact. Many beam manufacturers will have their own mounts already welded in place that will work well with this stop, or will have a good suggestion of what pad seems to fit and work best on their product. In this case, look around to see who has a good reputation in providing a complete front beam assembly or the parts that you need.
Many of the limited-class buggies will also incorporate a bump stop on the rear suspension. While I prefer to use the shock absorber that travels the most on the rear suspension of a buggy with trailing arms as a stop by means of a small cushion on the shock shaft to help halt the suspension travel, many times this is not practical for a particular application. Do your best to use a motion ratio closer to 1-to-1 that has the shock eye mounted toward the end of the trailing arm to carry the load forces close to the stub axle. This will help to reduce the arm from wanting to twist in the middle as it could if you have something like a 2-to-1 motion ratio.
Two examples of larger sized poly bump stops.
LARGE SIZED POLY BUMP STOPS: When you have a truck, rock crawler or other larger-sized vehicle, there are bigger poly bump stops to work with. The picture above with two red poly bump stops represents two of the four sizes offered by one of the manufacturers. There are several brands available, so look into what will fit your application best. The poly bump stops shown here have a mounting base plate of 2x7 inches. The same applies with smaller or shorter being better so as not to store up too much energy. The openings in the poly material allow the material to fold around into itself so that you don’t get the feeling of hitting a big brick on contact.
Several versions of OEM rubber bump stops.
OEM RUBBER BUMP STOPS: Just about every automotive manufacturer uses rubber bump stops on the suspension. The rubber stops have a much better ability to bond to the metal mounting plates than polyurethane does. The challenge is that unless you are willing to sneak around parking lots and car dealerships sliding under vehicles to see what the bump stops look like, you will have a slim to zero chance of finding what you like. There is also the junkyard route to rubber bump stops, but you have no idea of what wear and tear they went through when grabbing used ones. You could get as much bump stop information as you can off of the junk vehicle and then go to a new car dealership to order new ones. Unless you have knowledge of a particular make and model of car or truck that has the exact rubber bump stop you are looking for, you will be out of luck standing at the parts counter trying to explain your needs to the smiling parts person.
THE BOTTOM LINE FOR BUMP STOPS: Do your best to get your shocks working correctly so that you can make use of the simple bump rubber that comes on the shock shaft, and hopefully you are able to use bypass shocks. If using bypass shocks is not possible, then the next step would be to outfit your vehicle with hydraulic bump stops (assuming this combination is an option). Next up would be polyurethane bump stops. Since the poly bump stops are readily available in a wide variety of sizes and shapes, this will be your last choice. As you probably figured out by now, getting your hands on a rubber bump stop that is mountable somewhere on your suspension system is not as much of an option. Remember that a bump stop does not travel very far, no matter how tall and soft it might seem. In the end, making use of a simple two-inch hydraulic bump stop on your suspension can work wonders. Pick a location that is as outboard, toward the wheel, as possible and make it happen. The sky hook structure needed to mount the bump shock on some vehicles can get complex, so be prepared. Hopefully in your case there is an easy solution.
IN CLOSING: You need some sort of method to slow down your suspension travel in the last few inches of compression travel before things get ugly and metal parts crash into each other. Evaluate your situation and decide how you can make the best use of the various products and components available to you. Bypass shocks properly set up and, if needed, complemented with hydraulic bump stops are the best to use whenever possible. It is not shameful to learn from what others who may be more successful than you have done.
HOMEWORK: I am going to repeat myself somewhat with the same homework from the last class as it is still quite fitting for your progress. 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. Another great resource is Dirt Sports magazine, with both the University Series on shocks and suspension, and 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 machine. Remember that knowledge transforms itself into speed that you need.
Class Dismissed. Professor Tom Morris Dirt Sports University and School of Hard Knocks