Trials Bikes For Dummies

By Ron Milam

Last month we started a discussion on suspension systems in which you were either dazzled by my amazing recollection of ancient history or bored silly. This month, we will get down to the nitty gritty. Some of you will think this part is boring too. You would rather me just tell you which knob to turn to make hopping easier. Others will understand that if you understand the principles and hardware, your eventual adjustments will be much better. If you are in the first category skip to the last sentence. For the rest of you, this is my opinion of suspension principles.

So how does it work? Well Grasshopper, an old and well known system is utilized to turn the energy from hitting bumps and such into heat. You may not have thought about it in that way, but you cannot create or destroy matter and energy. You can change the form of either, but you cannot destroy it. If you were to hit a big bump on a bike with no suspension and solid tires and a rigid frame ( as in not springy ) and you were sitting down so you could not absorb the impact with your legs, you and the bike would be tossed into the air. How high would depend on how fast you were going, how much you and the bike weigh ( thus determining the amount of kinetic energy available ), and the size and shape of the bump. The point is that the energy you have riding down the trail is redirected by the bump. Some energy is used to lift you into the air. Since you do not really want to be launched from every bump you hit, you have elected to do your riding on a modern trials motorcycle, complete with pneumatic tires and an oil damped suspension system.

If you were to ride your trials bike over the same trail and hit that same bump ( hey, you were dumb enough to ride that rigid framed solid tired thing over it sitting down so you might be dumb enough to hit it again! ), some of the energy that you and the bike have available is absorbed by temporarily squashing the tires flat. Some of that energy is converted to heat in the sidewalls, but it doesn’t make a lot of heat so you never notice it. Some more energy is used to compress your fork and shock springs. But remember what we said in paragraph 2. You can’t destroy the energy and you are not doing anything that could convert it to matter, so what happened to it? You are just storing it for a short time. After you get past that nasty bump thing you hit, you can let go of that energy slowly and it will not have much effect on your progress down that hypothetical trail. The tires are going to rebound and there is nothing you can do to control that. Just like those big hoppity balls kids used to sit on and hop, your tires are going to rebound and push you upward. How much and how hard depends on many factors such as tire pressure and how much of the energy gets absorbed by the forks, shocks, and legs. There is a complex interaction between each component that pretty much adds up to an infinite number of possibilities. That is why suspension development continues almost 30 years after the “suspension revolution” and is also why it will never end. Now that I have made the point that tires and legs are part of the system, let’s forget about them for now and concentrate on the forks and shocks.

As we hit the aforementioned bump, the forks were compressed. The energy that was expended to compress the spring is stored in the spring. But not all of the energy that the bike/rider unit gave up was used to compress the spring. Some was used to overcome mechanical friction in the fork. This was converted to heat. If the bump was sharp, the compression damping of the fork also came into play. This means that the compression speed of the fork was slowed by pumping the oil in the fork through restrictions of some sort. Most folks don’t think of it in this way, but forks and shocks are simply hydraulic pumps that only pump their oil internally. Everybody knows you can’t pump something without using energy of some type. When the fork compressed and pumped oil through the restrictions, energy was used. Since it did this pumping fairly quickly, there was a lot of fluid friction involved as the oil was crammed through the holes much faster than it wanted to go there. In overcoming this friction, some of the energy was converted to heat.

So now we have rolled passed the bump and the forks are compressed. Some of the energy has been given up as heat, but most is still stored in the spring just waiting to pogo you into the air. But now the rebound damping comes into play, and the extension of the fork is slowed by pumping oil through some more restrictions. Energy is again given up as heat, and still another portion of it is used to return the bike to the attitude that it was in before all of this unpleasantness started.

The point of all of this is that the characteristics of your suspension are simply the way that it stores and releases energy, and how much of it is given up as heat, and when. Next month we will get into the hardware and adjustments. For those bored folks who skipped to the end, turn the blue knob to the left.

Ron