Shock Swap Myth
While I understand the reasons why people swap shocks from one bike to another, often the basic underlying engineering principals behind what makes a shock work correctly are often over looked or ignored. The following article is written in layman’s terms to try and give those that are swapping shocks around some basic understanding of the theory behind what they are doing and why it can be potentially dangerous. This is very difficult to do with out dynamic illustrations. It is not meant to be as a complete course in motorcycle chassis dynamics. It’s also not meant as a scare tactic, just a very basic description of a part of the motorcycle suspension system that most people, suspension engineers included, do not fully understand. The rates and changing ratios of movement are complex and very difficult to describe. Because they are mentioned several times, the term Leverage Ratio Curve is abbreviated LRC and Axle to Damper Speed Ratio is abbreviated ADSR.
The rear suspension system of a motorcycle is a very complex combination of pivot points, linkages, arc motions, linear motions, springs and dampers. The sum total of which create a rear wheel force curve. The rear wheel force curve is a measurement of force taken at the contact patch of the rear tire. It includes the spring force (multiplied through the LRC), the shock damping force (again multiplied through the LRC), spring coefficient of the tire, and any unsprung mass which consists of a portion of the mass of the swing arm and shock and spring, all of the mass of the rear wheel assembly and the rear brake caliper, and some of the mass of the chain. Since most motorcycles of the same genre have similar mass and construction, the rear wheel force curves are also similar. A rear wheel force curve that works well for one 600cc sport bike works well for all 600cc sport bikes if they share the same rider mass and application (such as track day riding or racing). How the engineers achieve that force curve can vary greatly how ever. This is why different bikes of similar mass and construction can require dramatically different spring and damping forces. A good example is the Honda Hawk and the Suzuki SV 650. Both bikes are 650 cc twin cylinder motorcycles of similar mass. Why then does a 180 pound rider need a 1200 lb/in spring on the Hawk and a 650 lb/in on the SV? The answer is in the Leverage Ratio Curve (LRC) and the Axle to Damper Speed Ratio Change (ADSR).
When motorcycles first developed suspension, the rear shocks were mounted at or very near the back axle. It was easy to manufacture and had few moving parts, but they had very limited wheel travel. As the need for greater travel became more important, racers began to “Lay Down” the shocks. The top shock mounts were moved forward towards the front of the seat and tank and the lower mounts were moved towards the swing arm pivot. By “laying down" the shocks, the engineers created a progressive leverage ratio between the shock and the rear axle. This allowed for longer wheel travel without the need for excessively long shocks. How ever the shock and spring now needed to be made stiffer. This was a result of the loss of mechanical advantage of the shock and spring over the rear wheel. When the shock was mounted straight up over the rear wheel, for each millimeter of wheel travel, the shock also traveled 1 mm. When the shock was laid forward the shock moved less than 1mm for each millimeter of wheel travel. This is the leverage ratio of axle to damper movement. What engineers discovered was that laying down the shock creates a fairly Linear Leverage Ratio Curve. In other words if the shock moves .5mm for 1mm of wheel travel at the top of the stroke, it also moved close .5mm at the very end of the stroke. This worked well for a while. As motocross bikes got faster and jumps got bigger, more wheel travel was needed to “catch” the bike and rider and keep it from bottoming out on hard landings. To achieve this, engineers created the linkage system. This allows the engineer to not only increase wheel travel, but also control the progression of the LRC by altering the geometry of the rocker and pivot points. This is used to add spring and damping force as the wheel approaches maximum stroke. This system was soon employed on road bikes as well.
You remember the old adage; there is more than one way to skin a cat? Well when it comes to LRC engineering, there is myriad of systems, rockers, linkages, leverage ratios, speed ratios, mounting positions, and damper articulation used. Some systems work better than others for a given application, some not so good in any application. So why then if two bikes have similar mass and construction, and have a similar spring rates could the shock not work properly? The answer lies in the Axle to Damper Speed Ratio. The ADSR is the difference is speed between the axle and the damper. Realize that we comparing the arcing motion of the back axle rotating around the swing arm pivot to the almost linear motion of the shock shaft entering the shock body. Since we are converting an arc motion to a linear motion, the LRC can not be exactly linear. It must progressive, digressive or both. (For you anal retentive engineer types, I realize that it can be, but the linkage system would be rather cumbersome in a motorcycle application). It has been found that a progressive LRC is desirable for most applications. How much progression depends on application (MX, road, road racing etc.) road or track conditions, chain pull moments, rider mass, horse power and torque out put and rider preferences. Because the LRC is progressive, so is the ADSR. (The exception to this is found on Suzuki’s TL and TLR models, where the spring and damper are separate. On the TL’s there is a LRC between the spring and the axle, a LRC between the damper and the axle, and a LRC between the spring and the damper.) So not only does the shock move farther per unit of wheel movement as the axle travels through it’s arc, the shock also moves faster per unit of wheel travel as the axle travels through it’s arc. This is important because some motorcycles get the progressive rear wheel force curve through displacement and other through velocity.