"Today's sportbikes offer ... suspension componentry that's better than what frontline Superbikes had less than a decade ago. The downside is that the myriad spring and damping adjustments can be confusing, and trying to figure out which adjuster affects what can make you feel like you need a Ph.D from Caltech. The factory manuals only go so far; recommended settings don't give you an idea of what to look for in order to decide if the setting you've chosen is the right one."
We wrote those words 10 years ago, but they remain true today. If anything, getting a grip on correct suspension settings is harder due to the significant jump in suspension technology over the last decade.
We showed you how to set correct spring sag last month. A correct sag setting will help keep your bike balanced front-to-rear from a spring/ride-height perspective, which will help it steer more neutrally and banish any chassis imbalance.
This time around we'll try to explain in relatively simple terms what rebound and compression damping are all about; how each system functions and how each affects your bike's handling. We'll also include troubleshooting strategies to help you diagnose problems in your bike's damping setup.
First, be sure to set spring sag before messing with damping adjusters. Spring preload affects damping settings, and incorrect sag settings can lead you in the wrong direction. Also remember that your bike's suspension needs to be in good working order. If your bike's fork and shock haven't been touched in more than 15,000 miles, they probably need a rebuild. Chasing the proper damping setup with a worn-out suspension is a losing proposition. Make sure your bike's suspension isn't binding anywhere, and that your frame is straight (if it's been crashed). Also be sure your tires are relatively fresh; worn and/or mismatched tires can cause handling gremlins that will likely confound the process.
Lastly, remember that every rider is different; damping and spring rates that work great for one rider may not work as well for another. It all depends on how it feels to you. After all, you're the one riding the bike.
Rebound Damping
The main function of rebound damping is to control the extension of the fork or shock after it compresses over a bump--hence the term "rebound." (This is also called tension, which is why you often see "tens" or "ten" on a fork rebound clicker.) Rebound damping works by dissipating the fork or shock spring's energy; once a spring is compressed, there is force--or energy--released when it springs back.
If your bike's suspension was composed of only springs (no rebound damping), the wheels would continue to bounce until the energy was dissipated through friction. An ideal example of this is a car with worn-out shocks that hits a bump in the road and continues to wallow up and down.
Too little rebound lets the suspension spring back too quickly, sacrificing wheel control and upsetting the chassis. Too much rebound causes the suspension to "pack down," which means it doesn't have the chance to return to its starting point before another bump is encountered.
Compression Damping
Compression damping controls the initial "bump stroke" of the suspension. As the wheel is forced upward by a bump, the compression circuit controls the speed at which the suspension compresses; it helps the spring keep the suspension from using an excessive amount of travel and/or bottoming out.
The spring cannot control the bump stroke by itself; without compression damping, the wheel's considerable inertia after hitting a large bump would carry it through a large portion of the suspension travel, with the spring only building up enough resistive force toward the end of its travel to stop it.
Conversely, if the spring was stiff enough to deal with the big hits, the overall ride on smaller bumps would be very harsh. This is why a careful balance must exist between compression damping and spring rate to provide good control and resist bottoming while still allowing enough compliance for smaller bumps.
Suspension Troubleshooting Symptoms
Here are some basic symptoms of damping problems that might be affecting your bike. Remember, these are extreme examples; yours may be more subtle. You may have to find an acceptable compromise on either end of the adjustment spectrum. It all depends on how the bike's handling feels to you.
Lack Of Rebound, Fork
* The fork offers a supremely plush ride, especially when riding straight up. With higher speeds, however, the feeling of control is lost. The fork feels mushy, and traction feel is poor.
* After hitting bumps at speed, the front tire tends to chatter or bounce, and the fork has a wallowy, loose feel.
* When flicking the bike into a corner at speed, the front tire begins to chatter and lose traction. This translates into an unstable feel at the handlebar.
* As speed increases and steering inputs become more aggressive, a lack of control begins to appear. Chassis attitude (sudden changes in pitch) becomes a problem (front-end wallowing), with the front end refusing to stabilize after the bike is steered hard into a turn.
Too Much Rebound, Fork
* The ride is harsh. Rough pavement makes the fork feel as if it's locking up with stiction and harshness.
* Under hard acceleration exiting bumpy corners, the front end feels like it wants to "wiggle" or "tankslap." The tire feels as if it isn't staying in contact with the pavement when on the gas.
* The harsh, unforgiving ride makes the bike hard to control when riding through dips and rolling bumps at speed. The suspension's reluctance to maintain tire traction through these sections erodes rider confidence.
Lack Of Compression, Fork
* Front-end dive while on the brakes is excessive.
* Rear end of motorcycle wants to "come around" when using front brakes aggressively.
* Front suspension bottoms, with a solid hit under heavy braking and after hitting bumps.* Front end has a mushy and vague feeling, similar to lack of rebound damping.
Too Much Compression, Fork
* Harsh ride, especially when bumps and ripples are first contacted by the front wheel.
* Bumps and ripples are felt directly; the initial hit is routed through the chassis instantly, with big hits bouncing the tire off the pavement.
* The bike's ride height is affected negatively; the front end rides too high in the corners; bike may want to drift wide in corners.
* Brake dive is reduced, though the chassis is upset significantly by bumps encountered during braking.
Lack Of Rebound, Shock
* The ride is plush at cruising speeds, but with increased speeds the chassis begins to wallow and weave through bumpy corners.
* Poor traction over bumps under hard acceleration; rear tire starts to chatter due to reduced wheel control.
* Excessive chassis pitch through large bumps and dips at speed; rear end rebounds too fast, upsetting chassis with pogo-stick action.
Too Much Rebound, Shock
* Harsh ride; rear suspension compliance is poor and "feel" is vague.
* Poor traction over bumps during hard acceleration due to lack of suspension compliance.
* Bike wants to run wide in corners since the rear end is packing down; this forces a nose-high chassis attitude, which slows steering.
* Rear end wants to hop and skip when the throttle is chopped during aggressive corner entries.
Lack Of Compression, Shock
* Too much rear end squat under acceleration; bike wants to steer wide exiting corners (since chassis is riding rear low/nose high).
* Hitting bumps at speed causes the rear to bottom, which upsets the chassis.
* Chassis attitude affected by large dips and G-outs; steering and control become difficult due to excessive suspension movement.
Too Much Compression, Shock
* Ride is harsh, though not quite as bad as the too-much-rebound situation; but the faster you go, the worse it gets.
* Harshness hurts rear tire traction over bumps, especially during deceleration. There's very little rear-end squat under acceleration.
* Medium to large bumps are felt directly through the chassis; when these are hit at speed, the rear end kicks up.