The original subtitle to this article was Learn how your Santa Cruz Suspension Works, but we thought owners of other bike brands might like some schooling on bounce as well. This series of articles was started by Joe Graney when he was the Director of Quality and Engineering at Santa Cruz Bikes, but now that he’s the COO, he asked Nick Anderson to update this article for us (complete with new diagrams!) which was originally posted in 2012.

This piece, dealing with shock rates, is not gonna be a walk in the park but hopefully it’ll give some of you a little more insight into the how and why of suspension workings, and how Santa Cruz designs in particular behave.

Levers

We have to start with this. Levers are one of the simple machines, of which there are five (or six, depending on how nerdy you are). Levers hold company with the inclined plane, wheel and axle, wedge, pulley and screw. In a nutshell, levers operate on the principal of the Conservation of Work. Work is the amount of force exerted times the distance over which the force is exerted. If you push on one end of a long lever arm you can exert a high force on the smaller end but the small end moves over a shorter distance (work is conserved).

With suspension bikes, Wheel Travel divided by Shock Stroke = Average Leverage Rate (eg, a 5 inch travel bike with a 2.5″ travel shock has an average leverage rate of 2.0).

Shock Rate is the inverse (opposite relationship) of Leverage Rate. Shock Stroke divided by Wheel Travel = Average Shock Rate. The aforementioned bike would have an average shock rate of 0.5. Why use shock rate instead of leverage rate? The hell if we know, but it’s common for bike people to use the terms falling rate and rising rate, both of which refer to shock rate. So at some point in the past we decided to use shock rate so it fits that nomenclature. It’s easy to convert – just remember that a high leverage is a low rate, and that rising rate means falling leverage. Ready?

Bike Rate

A rising rate on a bike means that at the beginning of the travel, the rear axle has a higher mechanical advantage or leverage than at the end of the travel. This typically implies that the bike won’t “bottom out” easily. Falling rate means the opposite; the bike may bottom out easily, as the mechanical advantage increases through the travel – it ‘uses’ the travel easily. A bike with a constant leverage, or shock rate, has the same leverage throughout the entire travel. A rate of +/- 3% can be considered constant.

Rate

Spring rates are a measure of how much the spring pushes back at you as you push into it. A constant rate spring is assigned a value, typically referred to as k. The k value of a spring is in units of force per unit length, or lbs per inch. Practically speaking, if you have a 500 lb. spring, 500 is the k value, meaning 500 lbs per inch compressed. At zero inches compressed, it pushes back zero pounds, which is handy because then it just sits there on your desk holding your pencils. At one inch compressed, it pushes back 500 pounds. At two inches compressed, it pushes back 1000 lbs, and so on. Coil springs on bike shocks aren’t perfectly linear, but they are damn close. Air shocks are different, though. Air shocks work by compressing air, which doesn’t push back the same way as a metal spring does. The air’s force pushing back has to do with volume, which is tough to make a linear. So you set the air pressure (the volume is typically constant, and set by the shock manufacturer), and when the volume is cut in half, the pressure is doubled (remember PV=nRT from chemistry?). So half way through the shock stroke, the force pushing back is doubled. Then, 3/4 of the way thru the stroke, the pressure doubles again. And so on. Hence, that ‘rampy’ feeling, since the spring rate skyrockets toward the end of the stroke as the volume gets cut in half over and over and the force goes way up.

If you increase the volume a lot, the spring rate doesn’t do a linear progression like a coil, and has what is sometimes called a cavitation, or a place where the spring rate drops below a constant k value.

The beginning of the stroke on an air shock also doesn’t start at zero force, due to negative springs and all sorts of trickiness that air shock designers could write a book on. Preload doesn’t change your k value, it just makes the start point go higher since you start compressing the spring, but you haven’t compressed your bike yet.

Wheel Rate

Wheel rate could be defined as the combination of Bike Rate and Spring Rate. In practice, it’s really how your bike is going to behave since it defines the amount of force that the shock exerts on the rear wheel at any given point in travel. Damping is of course also a big factor but this will be covered later on.

VPP Rates

The VPP platform is very flexible and can allow for nearly any shock rate to be designed into the frame. A typical linkage moves in a linear fashion, and the shock can be positioned to have a falling or rising rate, but not both. The VPP system has two links that rotate in opposite directions. The top link moves counter clockwise, the bottom link clockwise (from the drive side viewpoint). This is unique to VPP bikes, and is specifically protected by US Patent 6488301.

As the suspension compresses, the two links rotate, but not at constant rates. The upper link starts rotating quickly, then slows down mid-stroke, and then speeds up again. The lower link does the opposite, starting slowly, then faster, and then slows down later in the travel. Exactly how much they speed up and slow down can be manipulated by changing pivot points, link angles and lengths. Attaching a shock to one of the links makes it compress at different rates through the travel. On a VPP bike with the shock attached to the upper link, the shock starts with a high rate, decreases through the middle of the travel, and then increases again. This gives the rider a feeling of great bump absorption on small and medium size bumps, but then ramps up so the suspension doesn’t bottom out on larger impacts.

The V10 shock rate starts very low (it means a very high leverage at the beginning). This allows the rear wheel to move very easily when it drops away into a hole, and then gets hit hard and fast by an obstacle. Instead of ‘kicking’ the bike, the wheel easily moves back to sag which is where it should be when a rider is aboard.

As always, there is a balance here. Too much ‘falling’ in the middle in the shock rate can yield a bike that feels like it wallows in the mid-stroke, or mushes down in corners, without a nice snappy feeling on the rebound stroke. Too much ‘rising’ from mid-stroke to bottom out can give a bike that doesn’t use full travel under normal circumstances. Complicating the matter is that different shocks have different spring rates and damping characteristics and can change the way a bike feels.

Damping

So far we have only talked about spring rate and shock rate. Both of these properties are position sensitive. What this means is that if you push the bike into the travel and hold the rear wheel there it will exert a force. If you continue to hold the wheel there it will continue to exert the same force. Damping is speed sensitive. The faster you move the rear wheel the more damping force the shock exerts on the wheel. So if you move the wheel quickly into the travel the shock will exert more force on the wheel than if you move it slowly into the travel. When the wheel is not moving the damping forces are zero.

A damper is a complex set of valves that allows oil to flow from one part of a shock to another.

The faster you try and force oil through these valves the harder it is to compress the shock and therefore the rear wheel.

Developing good bike suspension requires an understanding of the shock rate of the bike, the spring curve of the shock and the damping characteristics of the shock.

Check out the dozen articles on the Santa Cruz web site in Joe’s Corner

This is not sponsored content; the presence of this article isn’t the result of cash flowing in either direction between NSMB.com and Santa Cruz. So thanks to Joe and Nick for their work on this. We often brag that we have the smartest audience in mountain biking (a self-serving opinion bolstered by anecdotal evidence), and we knew a portion of you would appreciate this nerdery. So congratulations; if you made it this far you are a genuine bike geek. CM

Any questions about – or do you have this all figured out?