This guide has been a learning experience for me, because prior to writing it I knew little about motorcycle suspension. It is intended for beginners, and as such it describes some things that may be obvious to everyone else. In any case, I hope you find it useful, and please don't hesitate to contact me on ApriliaForum in case of errors or additions or if you just want to say hi. Happy trails! —altaic
- Springs are mechanical devices usually in the form of a coil of metal. They suspend the bike above the wheels, which is very effective at temporarily absorbing kinetic energy (bumps). The mass attached to one side of a spring is effectively isolated from the mass of the other side. The problem with springs is that bumps (and other anomalies) aren't tuned to the spring rate, so it will inevitably oscillate (bounce up and down). To mitigate that problem, suspension systems employ a damper.
- This is usually a tube (housing) which contains oil, with another tube (piston) which moves inside it. The piston has what's called a wiper on the end, with holes in it, so oil can pass from the housing into the piston when the piston moves up and down. But the holes are small enough that forcing the oil through them creates resistance. This resistance is called damping. It is primarily used to prevent the springs which suspend the bike from oscillating. There are extensions to this basic design which I'll go into later.
- This is the near-vertical suspension member which is attached to the front wheel. It almost always consists of a damper to either side of the wheel with an internal spring in each damper.
- This is the small cylindrical suspension member which isolates the bike from the rear wheel. It almost always consists of a damper with a spring coiled around the outside of it, and attaches in some manner (a linkage) to a pivoting arm (swing arm) which is attached to the rear wheel.
- Triple Clamps
- Also called triple trees, these mount both of the legs of the fork to the head tube on the chassis. The handlebars then are attached to the top of the triple clamps via a handlebar clamp.
- High-Speed and Low-Speed Damping
- When the suspension is subject to force, damping creates a resistive force. If the force is small, for instance from taking a turn, the damper uses a set of holes which controls the low-speed damping. But if the force is powerful, from a large bump or something, another passage opens up allowing the suspension to move somewhat faster, which would be high-speed damping. The amount of force needed to open the high speed damping passage and the size of the high and low speed holes can be adjusted, but may require a damper rebuild depending on the capabilities of the damper.
- Compression Damping
- As the suspension is compressed by a bump (or something else), this causes resistance. It's sometimes called bump damping.
- Rebound Damping
- After the suspension has been compressed, the spring rebounds, forcing the suspension to uncompress. Rebound damping resists this.
- Spring Rate
- The spring rate determines how stiff the spring is. The idea is to have the spring stiff enough not to bottom the suspension out, and not so stiff that the suspension tops out. The spring rate is crucial for balancing comfort and performance, as well as properly setting sag.
- Spring Preload
- The springs can be preloaded, which increases the force (proportional to the spring rate) the spring exerts on the bike and rider. This usually is adjusted via locking collars where the spring perches on your shock and nuts in/on your fork (see your bike's manual for details).
- Static Sag
- Also called free sag, this is simply the distance between full extension of the suspension and the suspension at rest without a rider. In other words, it's the amount the bike sags under its own weight.
- Race Sag
- Similar to static sag, this is the distance between full extension of the suspension and the suspension at rest with a rider and full gear.
- Fork Offset
- Sometimes called steering offset, this is the distance from the head tube axis to the center of the front wheel. Note that it is NOT the distance from the center of the fork legs to the center of the wheel. The term comes from bicycle lingo, where forks are usually not straight like on motorcycles.
- Handlebar Offset
- This is the distance from the axis drawn through the head tube to your handle bars, like stem length on a bicycle.
Measuring Sag, Setting Preload, and Choosing the Correct Springs
It 's good to have a helper or two to measure and hold the bike upright for you. Here's the general procedure for measuring sag:
- Set the motorcycle on a stand, ensuring both wheels are off the ground and the suspension is fully extended. Measure F1 and R1 as shown and note them down.
- Set the motorcycle on the ground and hold upright. Record F2 and R2.
- Mount the motorcycle while wearing full riding gear and sit in normal riding position (feet off the ground!). Record F3 and R3.
The static sag is F1 - F2 and R1 - R2, and the race sag is F1 - F3 and R1 - R3.
Note: For extra precise measurements, compensating for the static friction in the dampers, many people do the following for both ground measurements:
- Load the suspension by having someone press down on the bike, and then have him very slowly release pressure. When the suspension stops moving, measure and record it.
- Have someone lift the bike, slightly extending the suspension, and then very slowly lower it. When the suspension settles, record the measurement again.
- Average the two measurements.
Now adjust the preload (and remeasure the sag) on the front and rear such that the static sag for the front and rear is roughly 30mm for supermoto, or 35mm for enduro. The race sag should be roughly 65mm supermoto and 105mm for enduro. If the race sag is way off, you'll need to change your springs to something more appropriate for your weight.
Getting damping right can take some experimentation since road conditions vary greatly and many riders prefer stiffer or softer settings. Rebound has a more noticeable effect than compression, so you should fiddle with rebound first. The following settings are given by setting the suspension to fully closed (stiff), and then slackening by some number of clicks. Here are the standard ones as suggested by Aprilia:
|Fork Rebound||10 clicks||12 clicks|
|Fork Compression||10 clicks||12 clicks|
|Shock Rebound||13 clicks||23 clicks|
|Shock Compression||16 clicks||fully open|
|Shock Bypass Adjuster||fully open (-)||fully open (-)|
I will post some popular settings as I learn them from others.
Changing Fork Oil Level and Using Subtanks
Modifying the fork oil level changes the amount of gas in the fork, which acts as an air-spring. A subtank is basically an external cylinder which adds gas volume in certain conditions, changing the air-spring characteristics. More info is forthcoming.
Tuning and Troubleshooting
This is taken directly from Sport Rider Magazine, hopefully helping you embark on your suspension diagnostic journey. A lot of these symptoms are more subtle than they sound, and may require close attention and diligent experimentation to get your bike in tip-top shape.
LACK OF REBOUND DAMPING (FORK)
- The fork offers a supremely plush ride, especially when riding straight up. When the pace picks up, 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.
- 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 bars.
- As speed increases and steering inputs become more aggressive, a lack of control begins to appear. Chassis attitude and pitch become a real problem, with the front end refusing to stabilize after the bike is countersteered hard into a turn.
TOO MUCH REBOUND DAMPING (FORK)
- The ride is quite harsh—just the opposite of the plush feel of too little rebound. 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 DAMPING (FORK)
- Front end dive while on the brakes becomes excessive.
- The rear end of the motorcycle wants to "come around" when using the front brakes aggressively.
- The front suspension "bottoms out" with a solid hit under heavy braking and after hitting bumps.
- The front end has a mushy and semi-vague feeling—similar to lack of rebound damping.
TOO MUCH COMPRESSION DAMPING (FORK)
- The ride is overly harsh, especially at the point when bumps and ripples are contacted by the front wheel.
- Bumps and ripples are felt directly; the initial "hit" is routed through the chassis instantly, with big bumps bouncing the tire off the pavement.
- The bike's ride height is effected negatively—the front end winds up riding too high in the corners.
- Brake dive is reduced drastically, though the chassis is upset significantly by bumps encountered during braking.
LACK OF REBOUND DAMPING (REAR SHOCK)
- The ride is plush at cruising speeds, but as the pace increases, the chassis begins to wallow and weave through bumpy corners.
- This causes poor traction over bumps under hard acceleration; the rear tire starts to chatter due to a lack of wheel control.
- There is excessive chassis pitch through large bumps and dips at speed and the rear end rebounds too quickly, upsetting the chassis with a pogo-stick action.
TOO MUCH REBOUND DAMPING (REAR SHOCK)
- This creates an uneven ride. The rear suspension compliance is poor and the "feel" is vague.
- Traction is poor over bumps during hard acceleration (due to lack of suspension compliance).
- The bike wants to run wide in corners since the rear end is "packing down"; this forces a nose-high chassis attitude, which slows down steering.
- The rear end wants to hop and skip when the throttle is chopped during aggressive corner entries.
LACK OF COMPRESSION DAMPING (REAR SHOCK)
- There is too much rear end "squat" under acceleration; the bike wants to steer wide exiting corners (since the chassis is riding rear low/nose high).
- Hitting bumps at speed causes the rear to bottom out, which upsets the chassis.
- The chassis attitude is affected too much by large dips and G-outs.
- Steering and control become difficult due to excessive suspension movement.
TOO MUCH COMPRESSION DAMPING (REAR SHOCK)
- The ride is harsh, though not quite as bad as too much rebound; the faster you go, the worse it gets, however.
- Harshness hurts rear tire traction over bumps, especially during deceleration. There's little rear end "squat" under acceleration.
- Medium to large bumps are felt directly through the chassis; when hit at speed, the rear end kicks up.
Modifying Suspension Geometry
Fork offset changes the trail dimension, which greatly affects straight line stability and turning. Too little trail will make steering less affected by lean angle, which translates to less stability at high speeds. Too much trail will cause the steering to be more affected by lean angle (fork flop) and bumps, making low speed turning take more effort. In other words, change fork offset in small increments and pay attention to low speed steering effort and high speed stability. There are a couple options to modify offset: different forks, different triple clamps (note that some of these offer adjustable handlebar offset, which is different), or the Rekluse e-Axle. Check the brakes, steering, and suspension mods section for info on the e-Axle.
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