Trials Bikes For Dummies

By Ron Milam

 

    This is Part 5 of suspension basics. Last time we discussed how spring settings  affect your steering geometry. This time we will talk a little about  making adjustments. Before we get into that though, let’s talk about springs. Let’s take two hypothetical trials bikes

( HTB ) and a hypothetical rider named Vern. These two bikes have identical damping characteristics in the forks ( The nice thing about hypothetical bikes is that you can make them work any way you want ). They are the same brand so the geometry is identical. Vern is offered a test ride on  HTB-1 and notices that when he is standing in a balanced position, the forks are compressed 1.5 inches. He rides off of a drop off approximately 6 Crawford units high ( about 3 feet for those of you who have not heard Scott describe the height of obstacles ). He notices that the forks just barely bottom out. Next Vern takes a ride on HTB-2. As he is standing balanced, he notices that the forks are compressed 2.25 inches. He thinks to himself that those forks feel cushier, but will probably bottom out too easily. When he rides over that 6 C.U. high drop off; again, the forks just barely bottom. He also noticed that on the small bumps after the drop off, the forks seemed to absorb the bumps better and kept the front wheel on the ground better. Wow, he thought. Those softer springs really work better. He goes back to HTB-1 and reduces the spring preload until the forks also compress 2.25 inches while he is standing balanced. He takes it out for a test and rides off that same drop off. The forks bottom harshly and he nearly eats dirt for lunch. What could cause this?

    The answer is that HTB-2 has stiffer ( higher spring rate ) springs than HTB-1. Wait a minute! Wasn’t HTB-1 the one that compressed less when he stood on it. Yep. And that is the whole reason for this quaint tale about Vern and his hypothetical bikes. In order for both to just barely bottom on the same drop off, both sets of forks had to develop the same spring force as the forks reached the end of their travel. But for the two sets of forks to compress different  amounts while at rest, the spring force must be different. Hum. Here’s the explanation. HBT-1 with the softer ( lower spring rate ) spring has more preload than HTB-2 with its stiffer spring. Spring rate is a measure of the force required to compress the spring a certain amount. I haven’t measured any springs lately, but I seem to recall from the good’ol days, numbers around 40 pounds per inch. This means a 40 pound force will compress it 1 inch. 80 pounds would compress it 2 inches and so forth. If the spring rate is constant ( all coils are the same distance apart ) and not progressive ( the coil spacing varies along the length ), then this linear relationship holds true throughout the travel. Most trials bikes have constant rate fork springs. I am going to do a little engineer stuff here that any kindergarten engineer could do and show a little chart that demonstrates the effect of preload versus higher spring rate as a way to adjust forks and shocks. It will compare a 40 lb/in spring with 2 inches of preload to a 45 lb/in spring with 1.1 inches of preload at various compression amounts. Total travel is 6 inches.

 

Compression    40 lb/in       45 lb/in

                0              80                    50

                1              120                   95

                2              160                 140

                3              200                 185

                4              240                 230

                5              280                 275

                6              320                 320

 

As you can see, the heavier spring with less preload is 1/3 softer at rest, but the same at the end of the travel.

      If this is too technical and boring for you, too bad. Trials is a technical sport that is won or lost with tiny details. I for one think all this spring stuff is really exciting! ( Not ).

Just file this away for later and for now just consider that if you are running high preloads to compensate for weak springs, you are hurting yourself. By the way, long preload spacers do not necessarily mean you have a high preload. Some bikes have short springs and a lightweight spacer to save weight. If  you need heavier springs and your dealer doesn’t have any, you can talk to a performance suspension service and they can supply whatever you need. Scott – if you are reading this you know we love you and I was just kidding about the C.U. Until next month, keep your feet up. And your weight centered. And your outside elbow up. And your shoulders….. Bye!

 

Ron