The modern motorcycle uses suspension to accomplish several things, it provides a smooth comfortable ride absorbing bumps and imperfections in the road. It also allows the rider to fine tune the machine to give him/her better control over the machine when riding.

Motorcycles built in the 1950's or later, will most likely have some form of damped-fork suspension at the front and a swingarm and shock or shocks in the rear.

Springs

The main function of your suspension springs are to support the motorcycle above its wheels and to isolate the chassis and rider from the up and down motion of the wheels as they roll over bumps.

Not every motorcycle is the same and neither is every rider or the conditions he will subject his motorcycle to. Take two riders one weighing in at 150 lbs and another one at 250 lbs; putting either one on the same motorcycle, and they will come away with different opinions about how the motorcycle handles and feels. This is all based on the ability of the suspension to perform it's function.

Obviously the lighter rider may feel the suspension to be rather stiff and the heavier rider may complain that its too soft. Most motorcycles use springs in their suspension and of course they are set up for the average rider what ever that is according to the manufacturer.

A spring's rate is a measure of how much force is required to compress the spring a given distance. The higher the rate, the more force it takes to compress it a given distance, and the less it compresses under a given force.

Spring rate is normally expressed in pounds per inch (in the U.S.). a common spring rate is 100 pounds per inch; this means it takes 100 lbs to compress it one inch; 200 lbs to compress it two inches, and so on until its completely compressed.

Ideally the spring rate on your suspension is stiff enough to keep the bike from bottoming out (fully compressing the suspension) on all but the very worst bumps; yet soft enough to provide a comfortable ride. Our 150 lb. rider will not compress the springs as much as the 250 lb rider will even though the spring rate remains constant.

This is the reason why the ride will feel different to each one. Our lighter rider didn't compress the springs as much as the heavier one did; he rode the motorcycle at its upper limit, it may have been a harsh ride and the suspension could have topped out every bump. When the heavier rider rode the motorcycle he compressed the springs further than the lighter rider did, the ride might have been soft and sloppy feeling with the suspension bottoming on each bump.

Springs come in two basic types; straight-rate and progressive-rate. Straight-rate springs compress at one rate until coil bound. Progressive-rate springs are built to have a spring rate the increases as the spring compresses. (For example, from 100 lbs per inch at the beginning, to 300 lbs per inch when near coil bound.)

With progressive-rate springs the result is a spring (and suspension) that reacts well to light hits but becomes increasingly stiff when a big jolt occurs.

Your motorcycle may be equipped with either straight or progressive-rate springs. There are also lots of aftermarket springs available to customize your suspension to your requirements.

Preload

Preload is a load placed on a spring before its used. On your suspension you may have means to apply pressure to compress the spring with out effecting suspension movement. It can be an adjuster in a front fork, or a collar on rear shocks. This loads the spring to alter the point at which the spring begins to compress. If you preload a 100 lb per inch spring one inch, it will take 200 lbs of force to make the shock spring compress it's first inch from full extension. Preload does not change the springs rate.

Preload is useful; because the correct spring can be chosen with the right rate for a comfortable ride and then using preload the weight of the rider and the motorcycle doesn't compress the suspension (sag) more than is desirable.

This sag should be about one third of the suspensions total travel. Leaving equal amounts of spring travel up as well as down to prevent the suspension from bottoming or topping out. Using preload suspension can be set up for riders of different weights on the same motorcycle and setting ride height.

Tthe first step in setting your suspension is to set sag. On some motorcycles sag may not be set easily without dismantling the front forks to add shims or it may require replacement of the springs themselves.

If the sag of a motorcycle's suspension for a given rider cannot be properly set using pre-load adjustments, typically the spring must be replaced with one of a different rate. If the sag is too great, a higher rate spring must be used, and vice-versa. Even when the sag is set correctly sometimes the springs have to be replaced. This is dependent on the weight of the rider and motorcycle. If the rider is too light for the design of the springs the ride will be harsh, even when sag is correct. If the rider is too heavy the ride may be mushy, brake dive may be excessive, etc.

Damping

With springs alone the motorcycle would yo yo up and down depending on the conditions and would not settle down until the springs had lost all of their stored energy. Your motorcycle's suspension is designed to absorb or damps (not dampens; which means to make wet) much of the energy transferred to it from the wheel when it rolls over a bump.

The force of the front wheel hitting a bump is transferred to the suspension. To damp this shock which stores energy in the spring, we use fluid (oil) as a shock absorber; by pumping oil through small orifices as the suspension goes through its motion. For the same sized orifice a thin oil gives less damping action, and a thick oil more. Similarly, for the same viscosity oil , a large orifice gives less damping and a small orifice more.

Today's dampers use valving instead of orifices, which allow them to be compliant over small bumps and yet provide enough damping for large ones.

Modern suspensions also damp in both directions when the wheel strikes an object and is driven upward (compression), and when the wheel is forced downward by the springs and gravity to its normal position (rebound).

Ideally you want rebound damping to be four times as powerful as powerful as compression damping. Overly stiff compression rates prevent the wheel from following the pavement, so it's better to let the spring rate control the majority of upward wheel movement, rather than oil flow.

Motorcycle suspension uses springs and damping together, in the form of forks up front and shock absorbers in the rear to soak up bumps and keep the wheels on the ground. Damping and spring rates must always be in balance.

If the motorcycle has too much damping, it will overwhelm the spring and the suspension will be unresponsive, forcing the wheels to hop and bounce over every little bump. If there is too little damping, the motorcycle is said to be oversprung, and it wallows and pitches through the turns and after the least little bump. In either case, both ride and handling suffer.

Front forks

Almost all motorcycles are fitted with some form of telescopic front forks. The most common of these consists of a pair of male tubes fixed to the triple clamp and a pair of female sliders affixed to the wheel by the axle.

The slider telescopes up and down over the tubes as the wheel moves over bumps. Springs, damper rods, and damping oil inside the tubes absorb the shock and damp the movement. Seals in the slider keep dirt out and the damping oil in. More modern forks include adjustable dampers, adjustments for spring preload and other refinements.

Two variations of the telescopic fork design are the male slider or inverted forks (upside down forks USD). Standard forks put the small diameter tubes in the triple clamp, right where the bending load is the greatest. This makes them prone to flex under strenuous riding conditions, such as heavy braking or rough terrain at high speed. USD or inverted forks place the larger diameter outer tube in the triple clamp; giving the front end more support to resist flexing.

Another variation is the cartridge forks, so called because they feature small self contained damper units called cartridge dampers in each fork leg. USD or inverted forks are sometime referred to as cartridge forks because they all use cartridge dampers. Many conventional telescopic forks also now feature cartridge style dampers.

Telescopic forks have proven the most viable in the long run; they aren't without their problems. Telescopic forks are prone to flex and lack sufficient rigidity. This was true until the advent of large diameter fork tubes and USD or inverted forks. Telescopic forks compress during braking and whenever a bump is encountered. This reduces rake and trail at a time when both would be useful to have. To address these issues some manufacturers built forks with anti dive valving and others used alternative fork designs.

Under heavy braking telescopic forks compress and shorten the forks overall length; changing rake (steering angle) and trail quickening the steering of the motorcycle. Once the braking load is removed the forks extend to the design geometry.

Heavy braking causes weight to transfer to the front wheel that along with friction of the front wheel causes the forks to compress as the forks become shorter this lowers the front of the chassis changing the steering head angle. This changes the front end geometry suddenly. Steering becomes quick and unstable until the weight from braking is removed.

When Anti dive valves are used in the forks, they close under heavy breaking, and block the movement of the fork oil and attempt to keep the forks from shortening. If the road surface is rough the forks are not able to absorb bumps and can cause wheel chatter as the front wheel loses contact with the surface. Not the best of all worlds.

BMW Earles forks

Some fork designs limit front end dive under heavy braking, eliminate it, or even reverse it without affecting the front suspension adversely. The Earles fork is among the latter; when braking the front brake hard, the front end of the motorcycle actually rises. BMW's Telelever fork is designed nearly to eliminate dive. Leading link front forks, such as used on some Ural motorcycles, can also be designed either to reduce or eliminate dive.

Saxon-Motodd (Telelever) fork

The Saxon-Motodd (marketed as Telelever by BMW) has an additional swingarm that mounts to the frame and supports the spring. This causes the trail and castor angle (rake) to increase during braking instead of decreasing as with traditional telescopic forks.

BMW Duolever forks

Hossack/Fior (Duolever) fork

The Hossack/Fior (marketed as Duolever by BMW) separates completely the suspension from steering forces. It was developed by Norman Hossack though used by Claude Fior and John Britten on racebikes. Hossack himself described the system as a 'steered upright'. In 2003 BMW announced the K1200S with a new front suspension that is based upon this design.

Single-sided

Yamaha GTS RADD front end

The only production motorcycle to use a single-sided front swingarm suspension was Yamaha's GTS1000, introduced in 1993. The GTS used the RADD front suspension designed by James Parker. However, a single sided girder fork was use by the German firm Imme between 1949 and 1951, and the Vespa scooter has a single-sided trailing-link fork.

Coaxial steering front suspension

Developed by MotoCzysz for their C1 and awarded United States Patent 7111700 on September 26, 2006. It is a fork with "coaxial steering and suspension components, and having telescopic forks. Swing weight of the forks is dramatically reduced by removing their suspension components to the central location, coaxially within the steering tube. Ride height can be adjusted without loosening the forks in the triple clamps. A shock tube disposed substantially coaxially within the steering tube wherein the shock tube includes a passage therethrough substantially coaxial with the steering axis; an upper triple clamp and a lower triple clamp coupled to the shock tube; a pair of sliding-tube forks each having an upper fork tube coupled to the upper triple clamp and to the lower triple clamp, and a lower fork tube; a coil-over shock disposed within the shock tube; a front wheel rotatably coupled to the lower fork tubes; a pair of bearings rotatably coupling the shock tube to the steering tube; and a top bolt coupling the shock tube to the upper triple clamp and having a passage therethrough substantially coaxial with the steering axis; wherein the coil-over shock includes a setting adjustment mechanism which is accessible via the passages through the top bolt and the shock tube."

Rear Suspension

During the late 1970s and 1980s, motorcycle rear suspension design and performance underwent tremendous advances. The primary goal and result of these advances were increased rear wheel travel, as measured in how far the rear wheel could move up and down. Before this period of intense focus on rear suspension performance, most off-road motorcycles had rear wheel travel of about 3.5 - 4 inch (9 -10 cm). At the end of this period, most of these motorcycles had rear wheel travel of approximately 12 inch (30 cm). At the beginning of this period, various rear suspension designs were used to reach this degree of performance. However, by the end of this period, a design consisting of using only one shock absorber (instead of two) was universally accepted and used.

Motorcycles with only one shock absorber are called monoshock motorcycles. The performance of monoshock motorcycles is vastly superior to twin shock motorcycles. It is important on twin-shock motorcycles that both shocks be the same, and that if they are adjustable, that the adjustments on both sides be the same. Otherwise, there can be a torque to the swingarm which may cause dangerous handling and braking characteristics.

Many motorcycles use twin shocks, one on each side of the rear wheel. With one end of the shock absorber attached to the frame and the other end to the swingarm. Some motorcycles use only one shock mounted behind the engine to the frame to the swingarm; or indirectly through linkage that provides the effect of a progressive rate for both the spring and damping. Others use one or more shock absorbers, mounted underneath the engine or transmission.

The shock absorbers consist of a sealed damper unit with a coil spring over the damper body. Oil and damper units inside the shock provide the damping. Some are adjustable for damping and spring preload.

Most all shocks are adjustable for spring preload by use of a rotating collar with ramps of different heights around the collar. Some shocks use pressureized gas (or even air) to augment the spring. The pressureized gas is usually nitrogen and pressures are in the 300 psi range. Some even have a piston in the shock or in an external reservoir to keep the gas and oil seperated. The latter are know as DeCarbon type shocks.

Ride height

Consider your bike as just a frame and subframe hanging in space. Ride height is a measure of how high the steering head (front) and subframe (rear) are above the ground, and changing front and rear ride heights will change your bike's geometry.

Tipping the bike forward with less front ride height will reduce rake and, more importantly, trail. This will quicken your bike's steering, but reduce stability. Raising the front of the bike or lowering the rear will lengthen trail, slowing steering but benefiting stability. You can change front ride height by moving the forks in the triple clamps, and rear ride height can be altered by lengthening or shortening the shock.

Adjusting the Suspension

When making suspension changes, it's vital to work in a methodical manner. You need to work from known base setting and work in increments making one change at a time.

To begin you will need the bikes basic took kit and owner's manual. If a special spanner or wrench is needed to adjust the rear shocks preload have it in hand. You will also need a tape measure preferably a metric tape measure, possibly a calculator, your owners manual, and lots of patience. Having a couple of friends handy won't hurt either, you'll be getting on and off the bike several times in full riding gear; extra hands to steady the bike and take measurements can make it a lot easier.

It's also necessary to take notes on the changes you make. Mental notes won't be much use to you later, so write your changes down; you can always go back to the starting settings if you go too far.

The first item to address for any good suspension setup is setting preload for static sag. Static sag is the amount your suspension compresses from full extension when you sit on the bike.

Setting your sag

Rear end

Extend the suspension completely by getting the wheel off the ground. It helps to have a few friends around. On bikes with side stands the bike can usually be carefully rocked up on the stand to unload the suspension. Most race stands will not work because the suspension will still be loaded by resting on the swingarm rather than the wheel.

Measure the distance from the axle vertically to some point on the chassis Mark this reference point because you'll need to refer to it again. This measurement is L1. If the measurement is not exactly vertical the sag numbers will be inaccurate (too low).

Take the bike off the stand and put the rider on board in riding position. Have a third person balance the bike from the front.

If accuracy is important to you, you must take friction of the linkage into account. This is where our procedure is different: We take two additional measurements. First, push down on the rear end about 25mm (1") and let it extend very slowly. Where it stops, measure the distance between the axle and the mark on the chassis again. If there were no drag in the linkage the bike would come up a little further. It's important that you do not bounce! This measurement is L2.

Have your assistant lift up on the rear of the bike about 25mm and let it down very slowly. Where it stops, measure it. If there were no drag it would drop a little further. Remember, don't bounce! This measurement is L3.

Step 4: The spring sag is in the middle of these two measurements. In fact, if there were no drag in the linkage, L2 and L3 would be the same. To get the actual sag figure you find the midpoint by averaging the two numbers and subtracting them from the fully extended measurement L1: static spring sag = L1 - [(L2 + L3) / 2].

Adjust the preload with whatever method applies to your bike. Spring collars are common, and some benefit from the use of special tools.

If you have too much sag you need more preload; if you have too little sag you need less preload. For road race bikes, rear sag is typically 25 to 30mm. Street riders usually use 30 to 35mm. Bikes set up for the track are a compromise when ridden on the street. The firmer settings commonly used on the track are generally not recommended (or desirable) for road work.

Front end

Extend the fork completely and measure from the wiper (the dust seal atop the slider) to the bottom of the triple clamp (or lower fork casting on inverted forks). This measurement is L1.

Take the bike off the sidestand, and put the rider on board in riding position. Get an assistant to balance the bike from the rear, then push down on the front end and let it extend very slowly. Where it stops, measure the distance between the wiper and the bottom of the triple clamp again. Do not bounce. This measurement is L2.

Lift up on the front end and let it drop very slowly; where it stops, measure again. Don't bounce. This measurement is L3. Once again, L2 and L3 are different due to stiction or drag in the seals and bushings, which is particularly high for telescopic front ends.

Halfway between L2 and L3 is where the sag would be with no drag or stiction. Therefore L2 and L3 must be averaged and subtracted from L1 to calculate true spring sag: static spring sag = L1 - [(L2 + L3) / 2].

To adjust sag use the preload adjusters, if available, or vary the length of the preload spacers inside the fork.

Street bikes run between 25 and 33 percent of their total travel, which equates to 30 to 35mm. Road race bikes usually run between 25 and 30mm.

This method of checking sag and taking stiction into account also allows you to check the drag of the linkage and seals. It follows that the greater the difference between the measurements (pushing down and pulling up), the worse the stiction.

A good linkage (rear sag) has less than 3mm (0.12") difference, and a bad one has more than 10mm (0.39"). Good forks have less than 15mm difference, and we've seen forks with more than 50mm. (Gee, I wonder why they're harsh?) It's important to stress that there is no magic number. If you like the feel of the bike with less or more sag than these guidelines, great; your personal sag and front-to-rear sag bias will depend on chassis geometry, track or road conditions, tire selection and rider weight and riding preference.

Using different sag front and rear will have a huge effect on steering characteristics. More sag on the front or less sag on the rear will make the bike turn more quickly. Less sag on the front or more sag on the rear will make the bike turn more slowly.

Increasing sag will also decrease bottoming resistance, though spring rate has a bigger effect than sag. Racers often use less sag to keep the bike higher off the ground for more ground clearance, and since roadracers work with braking and steering forces greater than we see on the street, they require a stiffer setup.

Of course, setting spring sag is only the first step of dialing in your suspension.

Rebound damping

Rebound damping can be initially set as follows: With the sag properly set and the bike at rest and off its stand, firmly push on the triple clamp (don't hold on the brake or push the handlebar) or seat. When you let go, the suspension should rebound quickly to its original position--but not beyond.

If it takes more than approximately one second for the suspension to return to position, less rebound damping is needed. If the fork or shock over-extends past its free sag, and then compresses again, more rebound damping is required. Street riding entails many different pavement characteristics, and the road is generally bumpy compared to a racetrack, so it's better to err on the soft side if you are unsure. This will also give you the added benefit of a smooth ride for daily use; you can always dial in a tad more rebound when you get to your favorite road where the surface is more of a known quantity.

Compression damping

It is difficult to set compression damping without riding your motorcycle and feeling how its suspension works. What feels nice and plush at a standstill may turn out to be too harsh at speed, and compression damping is sometimes set by personal preference as opposed to a definite optimum. Start with the compression adjusters in the middle of their adjustment range, and take your bike for a spin.

Working with the front and rear individually, soften the damping adjuster, and try your bike again over the same road. Is your handling better? Worse? The same? Try again, this time with the damping stiffer than what you started with. Continue experimenting, making adjustments accordingly. As with rebound damping, it's always best to err on the light side with compression, and for the same reasons.

One final check

With your bike off its stands, place your hands near the rear of the tank, and push down. A well-balanced setup will have both ends of your bike compressing and returning at approximately the same rate with this push. If the front compresses or rebounds different than the rear, attempt to match them, keeping within the parameters established individually.

Definitions

• Spring rate: the distance a spring compresses under load. Usually measured in lb/in or kg/mm. Conversion: kg/mm X 56 = lb/in.
• Sag: the amount the suspension settles from weight when the bike is at rest:
• - static sag: settling from the weight of the bike alone;
• - laden sag: settling from weight of bike and rider (this is the important measurement);
• - % sag: ratio of sag to maximum travel.
• Preload: the amount the spring is compressed when it's not supporting any weight. Some preload is necessary just to keep the spring in place when the suspension is fully extended. More preload is used to increase ride height and increase the apparent spring rate.
• Stiction: the tendency of the fork to "stick" when compressed or released because of friction.
• Ride height: arbitrary measurement of height above ground or some suspension component at front and rear of the bike. At the front, ride height is usually measured by a plastic zip tie around the fork tube. At the rear it can be the distance between the rear axle and a point you've marked on the fender or bodywork with a felt marker.
• Rake: the angle the fork makes with a vertical line through the axle.
• Trail: the distance between the extended steering head center line and the contact point of the tire. Trail is determined by steering head rake and fork offset. (Usually the steering head rake and the fork rake are the same.)
• Fork offset: the distance between a line across the centers of the fork tubes and the center of the steering head.
• Steering head: at the front most part of the frame, it's the tube that the fork's steering stem pivots on.
• Steering stem: the shaft that passes through the steering head, supported at top and bottom by bearings.
• Triple clamps: found at the top and bottom of the steering head. Each has three holes, one for the steering stem and two for the fork tubes. The fork tubes are set forward of the steering stem - see "fork offset" above.
• Coil binding: when a spring is compressed so that adjacent coils touch, it is coil bound. If all the springs are coil bound, it's no longer a spring but a solid steel cylinder. When this happens the ride suffers badly.
• Damping: resistance to movement of the spring. Usually damping is effected by forcing oil through small passages. This resistance tends to stop the spring from oscillating after it's been bumped. Insufficient damping can lead to pogoing.
• Pogo: to bounce (boing, boing, boing) like a pogo stick. Nothing to do with the famous possum of the same name