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 the 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.

Engineers design LRCs and ADSR to fit a particular chassis and use. Since no two engineers can agree on a single solution to a given problem, two makes of 600cc sport bikes can have very different LRC and ADSR. This is the key reason why swapping shocks from one model to another does not often work out. The only way to analyze the LRC and ADSR, is to measure the critical points of the rear suspension with a three dimensional coordinate measuring machine (3D CMM), and import the X, Y and Z coordinates in to a LRC analysis program. Then the LRC and its progression can be analyzed. Next the ADSR can be analyzed and damping curves configured. How the LRC and ADSR relate to back axle motion will determine spring rates and damping curves. If the LRC requires a stiff spring, higher shaft velocities might be used to allow lighter valving. Conversely if the LRC requires a soft spring rate, lower shaft speeds and stiffer valving might be used. But in many cases a stiff spring indicates stiff valving and low shaft speeds and a soft spring indicates lighter valving and higher shaft speeds. Which system is used can be inferred by looking at swing arm length and shock stroke. If we assume the same LRC, a longer swing arm will need a stiffer spring. A good example is the swing arm extensions used for drag racing. By extending the length of the swing arm, the mass of the bike and rider has more mechanical advantage over the rear suspension making the rear feel soft. So why then does replacing the spring with a stiffer one not yield good results? The reason is the ADSR. With a longer swing arm, the travel of the rear wheel through the same 120mm of arc results in a smaller angular movement of the swing arm. (Think of a long pendulum versus a short pendulum, both going through 45 degrees of rotation. The end of the long pendulum will travel farther than the short pendulum.) By rotating through a smaller arc in the same amount of time, the shock stroke, and the shaft speed of the shock, is reduced and therefore the damping force is reduced. Too little compression damping, and too little rebound damping is the result. Just changing the spring does not work.

So this brings up the case for revalving a shock from some other model and changing spring rates. While this is possible and will work, the economics don’t make sense. First, the donor shock needs to have two similarities to the stock unit, length and Stroke. Both must be the same, or if geometry changes are desired, at least the stoke needs to be the same. In some cases the donor shock can be “drooped” or shortened to bring it in to spec. No stock shock that I know of can be easily lengthened without replacing the shaft and then you need to be sure the piston will not hit the top of the shock body on the inside at full compression. So let’s say you have found a shock of appropriate length and stroke, next you will need to revalve it. Given the restrictive nature of stock pistons, a race Tech Gold Valve is recommended. Now you will need some one with the tools and experience to specify a valving configuration to suit the application. Race Tech does not have instructions to valve a ZX6R shock for an SV 650. The shop will need a Shock Dynamometer to test the damping curve of the revalved damper as well. Last you will need a spring of appropriate rate for the bike and rider. While many websites have spring generators, there are two things to consider here. First is I have found most web based spring generators to recommend spring rates that 50 to 100 lbs/in too soft. We can debate this all day, and I’ll respect other’s opinions, however almost two decades of real world experience have shown that a slightly firmer rate with less preload gives the best ride. A “stiff” ride is the result of the valving being too stiff, not the spring. Second is, can you get a spring that will fit the shock, in the rate required. In most cases that answer is yes. So let’s add up the prices:

Donor Shock $35

Gold Valve $190

Spring $130

Labor $150

Shipping $50

$555

While this is less than the $599 retail for an aftermarket sport shock like the JRi 35PRO or the Penske Racing Shocks 8975, consider this. First most retailers sell a sprort type shock for under $600. Second with an aftermarket shock, you get a much better quality unit. The fit and finish of all parts is critical to the performance of the damper. Showa, KYB and other OEM manufacturers can not produce the same quality for a price the motorcycle manufacturers are able to pay and stay with in the MSRP for the bike. In most cases, you also get a ride height adjuster, critical for dialing in chassis geometry. So for less than a $50 savings, you give up quality, adjustability, engineering, and proper testing. To me that’s a false economy. My advice is to save a little more and get the right shock, properly engineered, from a reputable aftermarket manufacturer that has reputation in the business for quality products and outstanding customer service.