Words by: Joe Graney, SCB Engineering
With each of these columns, the areas we are talking about have become more complex. This one, dealing with shock rates, is not gonna be a walk in the park. But hopefully it'll give some of you a little bit more insight into the how and why of suspension workings, and how our designs in particular behave.
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). They hold company with the inclined plane, wheel and axle, wedge, pulley and screw. In a nutshell, this is what leverage is: you get a longer lever arm, and through the principal of torque (force x lever arm) you can exert high forces via a small lever arm by exerting less force on a longer lever arm.
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 ratio of 2.0).
Shock rate is the inverse (opposite relationship) of leverage. 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 I know, but it's common for bike people to use the term "falling rate" and "rising rate", both of which refer to shock rate. So at some point in the past, I 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? I didn't think so.
Rising rate on a bike means that at the beginning of the travel, the rear axle has a higher mechanical advantage 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. I usually figure that +/- 3% can be considered constant.
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 air the 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 could be defined as the sum of Bike Rate and Spring Rate. In practice, it's really how your bike is going to behave. I don't typically use wheel rate, since it means picking only one shock, and I like having different options for shocks. Typically we're going for a bike shock rate that behaves well with various spring rates that the bike will be used with. So let's talk about bike rates.
I don't know of another current design that has a mechanism that allows for a change in shock rate that can be manipulated like VPP can. Typically a 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 w/ 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 mid stroke 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 can change the way a bike feels. A Fox DHX Air shock, for instance, has a different spring rate than a Fox Float. In fact, different Floats have different air volume canisters to manipulate this. Santa Cruz worked with Fox during the development of the first Blur to offer adjustable air volume (AVA was the Fox acronym) and later Fox fixed the larger air volume to save cost and weight calling it XV (extra volume).
Finally, A Note On Super Low Leverage
Low leverage rates also get big play these days, and we've tried to figure out why. Our latest V10 uses 2.75" of stroke to get 10 inches of vertical wheel travel. Blind back to back testing with longer stroke shocks on prototypes wasn't enough to convince anyone it was worth the weight penalty. Shock durability hasn't been a problem for our bikes, even for Nathan Rennie (although, one potential problem with high leverage ratios is for riders that are up in the well over 200 lb. range, and can't get springs that are large enough weight to fit in there). Our latest Heckler redesign changed from a 200 i2i, 57mm stroke shock to a 216 i2i, 63.5mm stroke shock because it really made a substantial difference in bump feel. The additional half inch of shock length apparently lets the shock designers get stroke over two inches that feels better for a 145mm travel package. It can make tuning of damping characteristics more difficult for lightweight riders (under 140 lbs) however, as the spring rates drop very low, and the damping adjustment range available with the shock may not be sufficient.
We continue to try different shock lengths on different bikes, but won't use something just because it's the cool thing to do.
No matter what Roskopp says.