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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.
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