September 24, 2006

The Bounce



On Snowmobile Suspension

(ED—Suspension on today’s snowmobiles have been sophisticated units over the past half-dozen or so years. There’s no doubt that suspensions today offer a lot better ride than those even eight, 10 or more years ago. When we hear talk about suspensions, jargon like shock damping, compression, rebound, spring rate and a whole list of terms surface. Yamaha Motor Corp. has put together a list of suspension terms and components and with the company’s permission, we’re sharing those with our readers. While some of the information may be Yamaha-specific, you will be able to get the general idea of the intended explanation and what’s happening with your suspension during different riding conditions.)


Shock Damping
A shock functions by controlling the movement of the suspension as it compresses and extends. When the shock is moved, oil inside is forced through calibrated openings. This is called damping and it works to slow the movement of the shock and help control the energy force that is being applied to the suspension. Compression damping occurs when the shock is being compressed. Rebound damping occurs when the shock is extending. In general terms, compression damping works with the spring to control the force of the bumps and other forces that compress the suspension. Rebound damping works to control the force of the spring as it extends the suspension after it’s been compressed.
Damping is calibrated to match the sled’s weight, the suspension’s spring rates and the intended use of the machine. Some shocks offer adjustable damping. The rider can turn a screw or dial or use an electronic switch to adjust compression or rebound damping.

Compression Damping
Compression damping works with the spring to control the force that compresses the suspension. When riding over bumps, landing off jumps, etc., energy force compresses the suspension spring and the shock’s compression damping helps absorb and dissipate this energy force. Proper compression damping should allow the suspension to move easily over smaller bumps to provide good ride comfort, but also control the severe energy forces of bigger bumps to prevent excessive suspension bottoming.

Compression Damping Adjustments
Compression damping adjustments will affect the ride comfort of the suspension along with anti-bottoming performance at higher speeds or in rough trail conditions. Correct compression damping (along with the spring settings) should allow the suspension to bottom occasionally in order to use its full travel, but should not be harsh or jarring to the rider.
Too little compression damping and the shock will not be able to control the energy of severe impacts caused by bigger or sharper bumps, jumps, etc. The ride will be smooth over smaller bumps or at slower speeds, but will bottom frequently and harshly with more severe use. If maximum compression damping does not control bottoming, it is likely that a heavier spring is needed.
Too much compression damping and the suspension will not be able to compress and move easily when going over smaller bumps. The ride will feel stiff at slower speeds and over smaller bumps, but will usually perform better at higher speeds and in bigger bumps.

Rebound Damping
Shock rebound damping works by controlling the force of the spring as it extends after being compressed by a bump or other force. When the snowmobile travels over a bump, energy force compresses the spring. After the bump, the energy releases by extending the spring. Rebound damping in the shock controls this energy release. As the spring extends, the damping in the shock keeps the spring from extending too quickly and bouncing the rear of the sled upwards.

Rebound Damping Adjustments
Rebound damping adjustments will have a noticeable effect on the sled’s ride comfort over multiple bumps by allowing the suspension to return to its extended position properly after each bump. Rebound damping will also affect pitching or the tendency to kick up the rear of the sled when going over bumps.
Too little rebound damping and the shock can’t control the spring force as it extends after the bump. The spring will rapidly extend the suspension causing rear kick-up. This causes pitching of the sled, or excessive up and down movement from front to rear, and will make the ride feel bouncy or springy.
Too much rebound damping and the spring will not be able to fully extend after hitting the bump. When hitting several bumps in a short time, the suspension will “pack down” and remain partially compressed. This results in a stiff, choppy ride and transmits the bump force directly through the suspension to the rider.

Springs
The springs of the suspension work to hold the weight of the sled and rider up, but must also compress and extend to allow the suspension to move and absorb impact forces so they are not transmitted to the chassis and rider. When a spring compresses, it absorbs the energy of the impact and stores it momentarily. After the bump, the energy releases by extending the spring. Suspension springs are calibrated to hold up the weight of the sled with the rider at the correct ride height and at the same time allow controlled movement over different types of terrain. The springs and their settings will affect the weight balance of the sled or distribution of the weight of the sled between the track and skis. It also affects the weight transfer of the sled, which is the action of the weight of the sled shifting from skis to track on acceleration and from track to skis on deceleration. Ride height, weight balance and the amount of weight transfer are important parts of snowmobile suspension tuning.

Spring Rate
Spring rate indicates the amount of force required to compress the spring. A higher spring rate feels stiffer because it requires more weight or other force to compress it. A lower rate spring requires less force and is easier to compress so it will feel softer and more compliant. The spring rate specification tells you how much force it will take to compress the spring a certain length. For example, a 4.5 kgf-mm spring rate means that the spring requires 4.5 kilograms of force to compress it one millimeter. It will require an additional 4.5 kgs for every additional millimeter compressed. Keep in mind that this is the amount of force required to compress the spring and is not referring to the actual rider weight.

Spring Length
For Mono Shock Suspension Tuning, the spring is measured in free length, installed length and “1g” length. These measurements will allow you to set the correct spring preload and spring sag, which in turn will set the correct suspension ride height. Proper ride height is necessary for maximum suspension performance.
Free length is the length of the spring with no load. Free length is referred to in the specifications of the spring.
Installed length (see figure 1) is the length of the spring compressed to its installed position on the shock, with no load on the suspension. Compressing the spring to its installed length is referred to as spring preload and results in spring preload force.
“1g” length is the length of the spring with the weight of the sled and rider on it. The difference between the installed length of the spring and its 1g length is referred to as spring “sag.” The amount that the spring sags with the sled and rider on it will determine the ride height of the rear suspension.

Preload Force and Ride Height
Compressing a spring to its installed length results in preload force. The further the spring is compressed the more preload force will determine the amount of spring sag and resulting ride height. Factory preload force is set for a rider weight of about 200 lbs. Different weight riders will need different preload force to maintain the correct sag and ride height. The preload adjusters on the shock allow you to adjust the spring installed length and set the correct amount of preload. Heavier riders may require a stiffer rate spring to achieve the correct ride height.

Suspension Ride Height
Correct ride height allows the suspension to perform as it was designed and tested. It puts the suspension and its components at the proper position so it can compress and extend properly to absorb bumps and impacts. At the same time, it keeps the chassis at the proper angle and the sled’s weight distributed between the track and skis for maximum stability and handling performance. For example, if there is too much rear suspension sag and the ride height is too low, you lose valuable suspension travel, the suspension may feel stiffer and you will reduce ski response. If there is not enough sag, the ride height will be too high, the sled will feel tall and it may feel stiffer in smaller bumps. Too much ride height may also cause heavy steering and steering feedback.
Confirming correct ride height (see figure 2) is as simple as measuring spring sag and adjusting the spring preload until the correct height is achieved. A heavier rider will require more preload force to hold the sled at the correct ride height; a lighter rider will require less preload.
The suspension springs also affect the weight balance of the sled. To simplify weight balance, think of the weight of the sled being divided between three points: the skis, the front arm of the rear suspension and the rear arm of the rear suspension. Adjustments to the preload force of the spring at one point will change the amount of weight balance, which in turn affects the handling characteristics of the sled. 







Dyno Port Racing
Best Western Sicamous Inn


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