How Suspension Works
Requested by: Dave Hung, Kansas City, MO
What is Valving? Valving is the slowing down and speeding up of oil movement using valving shims, size of holes that bypass the valving piston, which sometimes have a metering rod to tune this hole, and adjustable tuning knobs with different size holes located on the shocks reservoir.
What is Compression? Compression is the upward movement of the shocks shaft(See Figure 1). It is controlled by Spring Rates, Oil Moving through an orifice, and oil moving through a valving shim stack. The compression valving is located on the underside of the valving piston.
What is Rebound? Rebound is the downward movement of the shocks shaft(See Figure 1). It is controlled by: Spring Rates, Oil Moving through an orifice, and oil moving through a valving shim stack. The rebound valving is located on the top side of the valving piston.
These pictures were used from the Custom Axis Shock manual
What is the difference between High and Low Speed Compression Valving? First of all you need to break a shock down into shaft speeds. Because this is what the High and Low speed means. Lets just say that the shaft moves at under 2 inches per second and this is your low speed side where as the High speed is at 10 inches per second and up. And you may be asking, what about in between 2 and 10? I call this the medium speed valving. What you are trying to achieve when setting up your suspension is a good transition from Low to Medium to High speed shaft travel yet have a good initial ride that rides nice and corners well yet is resistant to bottoming out harsh. Examples of Low Speed Valving are: going around corners and hitting the downside of a jump perfectly. High speed valving is mostly confused because it is thought that this is only used when hitting a large jump. This is where the breakdown between High and Low Speed comes into play. A shock doesn't know the difference of how large of a jump you are hitting, it only knows the speed of the shaft movement and controls this with the internal valving. So, with that said, High Speed Valving is used when: hitting deep holes, overjumping a downramp, landing flat landings, and slamming through a section of whoops.
What is actually happening when I turn a knob on my shock? What is actually happening is that you are closing or opening an orifice to let more or less suspension fluid through in a given amount of time. These are only used to fine tune your suspension! On the shocks with a knob on the reservoir, this is your High Speed Valving adjustment (See Figure 2). The knob will either have about 8 different size holes on the back side of it or you will be screwing a needle in or out causing more or less fluid to come in. This is only your high speed adjustment because when the shock shaft moves into the body of the shock, this takes up space inside of the body and something has to give. This makes the fluid move into the reservoir and go through the orifices in the compression knob. When adjusting the Rebound knob or screw, located on the bottom of the shock, you are adjusting the low speed rebound valving and some low speed compression valving. As you move the screw in or out, you are actually moving a damping rod up and down in the shocks shaft, which is hollow, to a jet in the end of the shocks shaft. This is a bypass around the valving stacks on the valving piston.
Reservoir Rebound Adjust Needle
What is a leverage ratio? A leverage ratio is the ratio of tire movement to shock shaft movement. If have a leverage ratio of 2:1 and your tire moves 10 inches then your shocks shaft will move 5 inches. This is a good reason why not to purchase a shock that was set up for another brand of A-Arm. Usually an A-Arm manufacturer will set the leverage ratio between 2.5:1 to 1.8:1. How do you figure your leverage ratio? Take the shocks off of your bike and measure the extended length of your shock from eye to eye where they mount to the frame. Insert in graph below. Now measure from the bottom of the seal head to the bottom of the bottom out bumper, this is metal to metal shaft length and insert into graph. Now set your quad on a stand and raise the tire until the eye to eye measurement where your mount your shocks is the same as your calculated compressed shock length. Then measure from the floor to the center of the wheel hub and put it into the graph below. Now lower your tire until the eye to eye length is the same as your extended shock length and measure from the floor to the center of the wheel hub again and insert into the graph below.
What exactly does a linkage do? A linkage changes the leverage ratio on a shock throughout the wheel travel. At the beginning of the wheel travel you may have a leverage ratio of 3:1 whereas at the end of the shocks travel it will be more like a 1:1. If you are running a No-Link rear end then you have very little change in the leverage ratio and it is much harder to set the shock up because all of the progression is in the springs and crossovers on the shock just like on the front shocks. Some linkages can hurt you more than help you like on the 400ex and TRX450R because it goes from a good ride to a stiff leverage ration way too fast causing the rear end to buck. There isn't a smooth progression through the wheel travel causing the harsh ride. A longer shaft length is very good because it will not wear your shocks oil out nearly as fast as well as not heat up during a moto causing shock fade.
Why does your Extended and Compressed lengths on your shocks matter so much? They are very critical because for instance if your compressed length of your shock is too short, then your frame will hit the ground before your shocks bottom out. Or if your extended lengths are too short, you won't ever be able to have the most travel possible out of your suspension.
What is the difference between Standard and Long Travel Shocks? The only difference between these two are the lengths of bodies and number of springs used. There is NO travel gained with a long travel shock over a standard travel. If you measure a standard travel shocks shaft length with the same leverage ratio A-Arm as a Long Travel shock I bet they are the same! However, there are some slight advantages to a Long Travel shock. The shock manufacturer's are able to add more spring rates to the shock since it is longer which helps to give more of a progressive spring rate curve and tuneability is a little easier.
Why is Nitrogen used in the reservoir rather than Compressed Air? The only reason that Nitrogen is used in the reservoir instead of compressed air is because as the temperature of the shock rises while in use the Nitrogen doesn't expand and create a higher pressure, which would cause the shocks valving to change. If compressed air where used then the reservoir pressure may go from 150 psi to 200 psi when heated up. Some of the pro's even run Nitrogen in there tires so that there pressure doesn't change throughout a race. Believe it or not, if you measure you tire pressure before and after a race I bet you will be surprised at the difference in pressure!
Why should I have my shocks serviced? You should service your shocks because the oil breaks down over time which will change your valving. The oil is worn out because it heats up and cools down and is also ran through small orifices and the vavling stack every time your tire hits a bump. If you've ever seen used motor oil, then you know about what used shock oil looks like, except used shock oil smells about like gear oil. If your shock oil is worn out then the oil will be able flow more oil through the same sized hole in the same amount of time. There is no set time to change your shocks oil, but I would recommend changing it at least 1 time per year. And if you are racing most of the time then I would recommend 2 times a season. Go here for shock service pricing
Why are stock shocks like poggo sticks? The reason for the poggo stick feeling on stock shocks is because the OEM manufacturers don't want your shock oil to wear out as fast, so they will put less valving restrictions interanlly so that the oil last longer. They will also use too heavy of a spring rate to compensate for the undervalved shock.
Here is a shock cutaway that I made so you can see what the internals of a shock look like
Click on pictures above to enlarge!
If you would like to see another question answered or for me to go deeper into any of these current questions, please feel free to e-mail me at email@example.com.