Shocks are an interesting quandary in the world of motorcycle design, construction and performance. We all want to look cool, have the correct travel and a ride that won't break our backs. The science of shock performance is not simplistic. You can't just grab any shock the length you want, and receive the other critical elements. We were in the process of building a pro-street FXR, cleaning up a 1961 Panhead and dealing with a stack of Dyna Shocks. Then Bad Brad rolled it with his classic and started complaining. “I found some 12-inch shocks. She sure looks cool, but won't corner.”
Bandit reached in his shock drawer and started pulling shocks. We found shocks the perfect length but with different spring rates. We found shocks with different mounting shaft diameters, different mounting stud positions and way too long. Plus the issue of Brad's exhaust pipe position needed to be dealt with if he wanted to run anything lower than stock. Nothing at the Bikernet Interplanetary headquarters worked except the women, so we reached out to progressive suspension built right into the Performance Machine building in La Palma, California. We figured they had to know. So here's the shock scoop PS:
Shock Tech 101
For the most part, the inner workings of motorcycle suspension seem to be shrouded in a great deal of mystery. Although the spring element of the up and down motion is readily obvious, the magic that happens within and which truly defines the difference between good and not-so-good suspension tends to elude most riders. Having a basic understanding of these critical components is not only important, but is ultimately the path to your becoming a better and certainly more comfortable rider.
The Basics of Motorcycle Suspension
Motorcycle shocks (as well as the suspension element of forks) can be divided into two basic components. The spring whose simplified function is to carry the load as well as absorb the impact of bumps and other road imperfections and a damper that dissipates the energy created by the spring’s compression and resulting extension (usually referred to as rebound).
Springs
The spring is (for the most part) wound steel and is defined by its rate. Rate is a measurement of the force required to compress the spring and is expressed in lbs per inch. For instance a spring with a 100 lbs/inch rate will require 100 lbs to compress it one inch.
Each subsequent inch of compression would require an additional 100lbs of force. This is referred to as a straight or linear rate spring. The alternative, is a progressive rate spring which allows a single spring to essentially exhibit multiple rates. By utilizing varied spacing spring coils, the initial rate may be 100lbs/inch, but each subsequent inch of movement would require more than an additional 100 pounds, and the third inch would require still more. Progressive-rate springs become stiffer as they are compressed. The advantage is a spring that is supple enough to soak up small bumps, yet firm enough to handle a big hit.
Dampers
Now that your motorcycle has “absorbed” a bump, the compressed spring has stored the energy of the hit and without a damper it would be released in a fury of rebound might, extending the spring beyond its original static length and repeating the cycle until the energy had dissipated. The resultant store and release sequence would cause your bike to pogo down the road. Imagine a basketball bounced hard onto the asphalt, not only does it bounce back, but it would bounce higher than your hands where it had started.
Now, imagine if you bounced it under water. This is the effect that a damper has on the stored energy of a spring. In fact, a damper, like the water analogy, uses a liquid (in this case oil) and forces it through a series of small holes. The ensuing resistance controls the return of the spring energy. The kinetic energy (motion) of the spring is transferred to the oil and dissipated as heat. Oil based damping is a clever solution, yet comes with its own set of challenges. For instance, rapid damper movement reduces the space within the shock for the fluid, thus a compressible medium must come into play or the shock would likely stop moving. The obvious solution would be to leave a bit of air space for the oil to move into, but air rapidly expands when heated (remember the shock has transferred the energy of motion to heat) and the damping characteristics of the shock would be ever changing. The solution is a separate chamber of a heat tolerant gas (in this case nitrogen) that allows the oil to expand without impacting the damping.
Shock Preload Adjusters
All of Progressive Suspension’s shocks are Preload Adjustable. This allows the user to easily tune the shocks for their specific combination of bike and rider weight. Although we design a shock to fit and perform on a specific motorcycle, we have no way of knowing if the rider will be 140lb or 240lb and the addition of cargo and passenger further complicates the issue. An obvious solution is an adjustment on the shock that allows the rider to quickly set up his shocks to match his bike/rider weight combination. This is where Preload Adjustment comes into play.
By compressing or uncompressing the spring a small amount, a shock can be perfectly tuned to suit the conditions for which it will be used. In addition to this level of tuning, most PSI shocks are also offered in Heavy Duty applications, recommended for bikes that are operated at or near the manufacturer’s maximum load rating over 50% of the time.
Shock Sag Adjustment
OK, so now you know your shocks can be adjusted to fit your bike and weight, but where do you start? Naturally from the beginning and with an understanding that shocks work in both the up and down movements of the bike. If all pavement imperfections where bumps, shocks for the most part would focus on compression. Unfortunately, there are potholes and other sorts of gaps that require a shock to function in the other direction, as well. To achieve this, a shock compresses a small amount under the weight of the bike and the rider, before it has even been asked to do any work. This is called Sag, and is adjusted via the handy Preload Adjusters we include on each PSI shock.
There are two key Sag measurements, the first is Free Sag, ideally measured from the center of the rear axle to an arbitrary point directly above the axle (a fender bracket, for instance). This is done off the stand and with a helper holding the bike straight up. A solid lift of the rear end (simply pick up the bike by the fender or rack as far as you can) will unladen the shocks and a Free Sag measurement can be taken.
The next critical measurement is Rider Sag and this is done with the rider on the bike (hands on the bars and feet on the pegs) and a helper or two supporting it. A measurement is again taken from the center of the rear axle to the same point as before. The difference between the two measurements is your final Sag and for cruiser and touring bikes is generally expected to be .75” to 1.25” (depending on application). If your number is less, then the bike will require less preload, and if it is higher, more preload is needed. Twin shock bikes should always be adjusted with the equal preload on both sides.
Suspension as a Rider Fitment Tool
Why would anyone change the suspension of a bike before it even leaves the showroom? Perhaps, a better question is why not, as proper bike setup is paramount to handling, comfort and safety. Riders above and below the baseline weight of OEM manufacturers are prime candidates for a suspension upgrade.
Assuming you wouldn’t ride with a low tire, why would ride at the bottom end of your rear suspension travel or unable to comfortably reach the ground? The custom market is notorious for swapping out seats for thinner units to acquiesce smaller inseam riders, often resulting in more of a reduction in comfort than height. A more effective solution is to utilize one of the many high performance suspension lowering kits available.
A good example is the product line from Progressive Suspension. They currently produce lowered shocks for most H-D models in half-inch increments, as well as fork lowering kits to match.
How can a lowered suspension feel better than a stock setup? Oddly enough, when it comes to front suspension, a set of lowered aftermarket springs are often longer than their stock counterpart. In an effort to keep cost down, the OEM’s will utilize a spacer on top of a relatively short spring. The aftermarket folks realizing they could make better use of the real estate run less spacer, more spring and still manage to reduce height and better the overall ride quality. A better fitting bike and a more comfortable rider is not a bad way to head down the road.
Okay, so we dicked with Brads bike which uses 13.5-inch stock shocks. He has a set of 12-inch shocks that were too low, but PSI came up with a set of 12.5-inch shocks, but with only ½-inch mounting shaft holes. Evidently there are ½-inch kits available or a lathe to turn down the existing 5/8-inch shafts. Here's what Brad did,
“We picked up the new shock posts and grommets. After much head scratching and measuring decided that I might not have to turn down the posts. Instead went to your press with Frank and pressed out the steel inserts in the progressive shock and pressed in the original stock posts. It was a very tight fit and was fairly difficult and took many different sockets and washers, but it worked. So now I have all of the meat on my stud and I used spare concave washers for the space difference instead of the spacers provided. Looks much better.
“It lowered the bike just right. Now have to see how it rides. We now have 4 inches of ground clearance at frame, instead of 3.5 inches. Where are my new Paughco pipes?”
There you have it. The Bikernet shock investigation. Spring rates are set up at the Progressive Suspension factory based on motorcycle weight, swingarm length, angle and travel. That's why the various shocks can look alike by won't work from a Sportster to a Dyna or touring bike. Watch out.
–Wrench
