the primary goal of motorcycle suspension is to keep vehicle tires in contact with the ground. without proper suspension, tires would lose traction when encountering bumps, dips, or other ground imperfections. we can’t forget about braking, acceleration or cornering forces either.
motorcycle suspensions use a spring and damper combination to isolate the chassis and rider from road imperfections. on-road motorcycle suspension systems work to minimize the effect of potholes, bumps, cornering, and acceleration/deceleration forces. off-road motorcycle suspension systems handle roots, rocks, jumps, ledges, and more.
without suspension, any impact between a vehicle tire and a ground imperfection is at best uncomfortable, and at worst, the cause of a dangerous crash.
basic motorcycle suspension lacks adjustability. ıt works fairly well in a wide variety of circumstances, whereas more premium suspension is tunable to rider weight and intended riding type. cruisers or dual-sport motorcycles have vastly different needs than a dedicated sportbike.
adjustability can include ride height (under load), fine-tuning how quickly springs compress or rebound as well as preloading spring tension to accommodate differing weight for different riding styles, such as riding with a passenger and/or luggage. the most common suspension systems found on motorcycles use a coil spring and hydraulic damper setup. air springs and other types of suspension exist, which will be covered more in-depth in another article.
springs allow a motorcycle wheel to move independently from the chassis, and dampers control and manage the movement of the spring. a motorcycle riding only on springs would bounce continuously and dangerously after every road impact. springs are coiled steel wire that compresses or stretch when acted upon by an external force. spring rate is the measurement of the force required to compress it a certain distance, which is typically measured in pounds per inch. spring rate varies with material thickness and the number of coils. heavier duty springs will have relatively thicker coils spaced further apart from one another.
- linear rate springs offer consistent resistance throughout the spring travel. if 10 pounds will compress the spring one inch, 20 pounds would compress it two inches, and so on.
- progressive springs require more and more force to achieve the same travel. progressive springs are essentially two (or more) springs in one, with both widely and narrowly spaced coils. initially, a lighter force will compress the first coils, and then a greater force compresses the remaining coils.
damper (shock absorber)
in its most basic sense, a damper slows and controls spring action. dampers control spring action using hydraulic fluid, which travels through a series of passages and restrictions.
a piston with a precisely measured passage (orifice) travels within the shock body in a bath of hydraulic fluid. the weight of the fluid and the size of the passage determines the piston’s travel speed. when a motorcycle encounters a bump, dampers slow spring compression and rebounds as the fluid slowly travels through the passages within the shock body.
kinetic energy from spring movement turns into heat energy within the damper, and the hydraulic fluid dissipates the heat. rear motorcycle shocks generate much more heat than front forks, due to the additional loads they support.
compression and rebound damping
compression damping is the intentional slowing of spring compression (hitting a bump) travel. rebound damping is the intentional slowing of the spring expansion as it resumes to its natural state.
some motorcycles will have both high and low-speed adjustments to compression and rebound damping. sport bikes and off-road motorcycles typically offer greater adjustability than entry-level or cruiser-style motorcycles.
high and low-speed damping refers to the speed of the suspension travel, rather than the speed of the motorcycle. high-speed damping affects suspension behavior when hitting a sudden pothole on the street, or an individual rock on a trail. low-speed damping affects behavior such as braking-related dive or cornering changes.
- damper rod: fluid travels through a fixed orifice to control the compression and rebound of the suspension. damper rods are inexpensive to produce but are limited in their effectiveness. low-speed damping is too soft and high-speed damping is too harsh.
- cartridge forks: fluid travels through a set of stacked shims to control suspension movement. shims bend and flex as the fluid travels past them. cartridge forks are more tunable and offer better damping from low to high speed. they are more complicated and more expensive to produce.
in the image below, you can see how the shims bend and flex as oil travels between the upper chamber (a) and the lower chamber (b).
motorcycle springs are always under tension, even when stationary. vehicle weight causes compression at all times. add a rider or two and luggage, and the suspension compresses even further.
sag is the percentage of suspension travel utilized while stationary. if the suspension sags too much when at rest, the bike may bottom out when encountering bumps once underway. too little sag can cause a stiff, harsh ride.
some motorcycles offer suspension preload adjustability. preload is the amount of tension on the springs when the bike is at rest. increasing preload will decrease sag, and vice versa. since a single motorcycle is often used for solo riding, riding with a passenger or riding with luggage, preload adjustment allows a degree of adaptability for multiple use cases.
although not recommended, adjusting preload can increase ground clearance for off-road travel or decrease seat height for shorter riders. some novice riders use preload as a ‘band-aid’ for overcoming incorrect spring stiffness relative to their height and weight. while not ideal, this is a common practice, as changing springs is expensive and labor-intensive.
the image below shows a typical rear suspension preload adjuster. by turning the bottom adjuster collar, more or less preload force is applied to the spring, while not changing the overall length of the spring. increasing preload will result in less suspension sag once under rider load.
(ic: progressive suspension)
motorcycle front suspension
motorcycles use suspension forks. in some systems, both springs and dampers work together within each fork leg. in others, one leg contains the spring and the other contains the damper (separate function forks).
the front wheel axle mounts to the lower end of the suspension fork, and the triple tree secures the top end.
motorcycle rear suspension
outback, it is common to see a heavy-duty coil wound around a damper, with external adjustments for preload and damping. motorcycle rear suspension carries more weight than the front, so heavier duty springs are common.
on some motorcycles, dual rear shocks mount between the frame and the swingarm directly. on others, a linkage and single-shock system handle suspension needs.