Steering Geometry

Quoted from the Ontario Federation of Trail Riders 2002 Tech Session - Why concern ourselves with suspension tinkering and alterations? Strong running engines are wasted if they cannot transfer power efficiently to the ground. Rider talent and training will be minimized by a machine that wallows or rides too harsh. Modern motorcycles have a vast assortment of adjustments but a rider must understand the basics before experimenting.

SUSPENSION TERMINOLOGY and POINTS OF INTEREST

RAKE: Draw a line down the forks to the axle center then draw a line straight up from the axle center. The angle created by these two lines is your bikes rake measured in degrees.

• Less rake = quicker steering and less stability at speed
  
• More rake = reluctance to turn and slow steering at lower speeds

TRAIL: Draw a line straight down the steering head centerline and then draw a line straight down from the axle center. The measurement of those two points is your bikes trail. Trail influences the self steering ability of motorcycles.

• Less trail = quick steering
  
• More trail = slower steering
  

OFFSET: Look at your top triple clamp. The distance from the center of the steering head and a line drawn through the center of each fork tube (measure straight out from the steering head) is the fork offset.

• More offset decreases trail
• Less offset increases trail

SPRINGS:

• Functions: To suspend the weight of the combined bike and rider.
  
• Determining your needs: Simply contact a suspension shop. Give them the year, model and your weight when fully dressed for riding (including fanny bag, helmet, boots and drink system).

SPRING WEIGHTS: e.g. 4.5kg spring will depress 1mm when 4.5kg is placed on it. An additional 4.5kg will depress it a further 1mm.

STATIC SAG: "Free Sag" is the amount the unloaded bike settles on it's suspension when lowered from it's stand. This is altered by adjusting the large preload ring on the threaded section of the shock. (Should be approx. 10-25mm)

RACE SAG: "Rider Sag" is the amount the bike settles when lowered to the floor and a fully equipped rider mounts it.

• have a helper steady the bike and sit in a normal riding position.
  
• measure from the axle center to a point on the body with the wheels off the floor.
  
• lower the machine, mount it and have an assistant measure from the same two points.
  
• Measurement should be 1/3 of total suspension travel (usually 95 - 105 mm).
  
• if static sag is correct but race sag is LESS than 1/3rd, you need softer springs.
  
• if static sag is correct but race sag is MORE than 1/3rd, you need stiffer springs.

PRELOAD: The loading of the spring from its static length. Preload adjustment is used to obtain correct sag readings.

• incorrect preloads will affect steering qualities and ride.

DAMPENING: The term used to describe the controlling of the shock or fork movement during both compression and rebound.

LOW SPEED COMPRESSION: Refers to the dampening of the shock or fork during SLOW SPEED compression such as when encountering gentle whoops at low speeds. (Not the machine movement but the suspension component speed)

HIGH SPEED COMPRESSION: Refers to the dampening of the shock or fork during fast movements, such as a hit from a square edged hole or log.

LOW SPEED REBOUND: Refers to the dampening of the shock or forks during a slow return stroke.

HIGH SPEED REBOUND: Refers to the dampening of the suspension during quick return strokes.

NOTE: High speed compression and rebound requires the channeling of large amounts of oil in a short time period. Therefore, the component (shock or fork) opens larger passages than used for low speed actions.

STICTION: Refers to the resistance to suspension movement or "sliding friction". Can be attributed to fork seals, bushings, linkages, misalignment, wear and damage.

PACKING: Refers to the situation were the suspension component fails to rebound to a useful length before being compressed again. It results in less and less travel with each compression of the fork or shock. Too much high speed rebound dampening is the usual cause.

G-OUTS: refers to the extreme compression of the suspension components in a low speed manner. The bike will be moving quickly but the suspension compresses slowly as in a fast bermed corner. This is an extremely difficult action to control because the suspension must move a large quantity of oil through the low speed compression valving, which usually handles only small quantity of oil. Due to the low speed action, the high speed valving does not come into operation.

SIDE EFFECTS OF IMPROPER SETUP:

STINKBUG?: Refers to a "tail low" chassis set up. Is the result of insufficient preload or fork springs too heavy. The bike may "swap" and the cornering will be slow to react. The front wheel will tend to climb over bermed corners. Short riders often create this chassis as they attempt to lower the bike for a more secure, "foot -down" seating position.

FAILS TO TURN: See above. Sometimes, raising the fork tubes slightly in the triple clamps will cure this ill. Also, too much compression dampening will prevent the forks from settling during breaking. That resultant increase in rake allows quicker cornering and weight transfer to the front wheel for traction.

REAR WHEEL KICKS ON BRAKING: Excessive compression and insufficient rebound dampening on either the front or rear will fail to control the rear wheels action as the wheel brakes and reacts to trail bumps. Should the front wheel dive excessively, or too quickly, it will upset the chassis loading and cause a loss of rear wheel/trail contact.

HANDLEBARS SHAKE AT SPEED: Caused by too steep a steering angle (rake) or loose front components. The rear preload may be excessive and preventing the rear from settling slightly during acceleration (there is that built-in increase in rake, again).

WALLOWS IN WHOOPS: Too soft a ride will create an increasingly imprecise ride through deep whoops. Try more compression and rebound.

HARSH INITIAL TRAVEL: Too much tire pressure, too high a spring rate, stiction, preload or low speed compression dampening will prevent suspension reaction.

HARSH TRAVEL FURTHER INTO STROKE: Too high an oil level in the forks or excessive air trapped in forks will "lock" the front end. Mid and High speed compression dampening may be too high.

SWAPPING: The rear wheel is not reacting to the trail surface quickly enough and this sends the rear from side to side in an increasing angle to the front wheel. Not a good thing and "packing" of the shock is the usual cause. Swapping in sand can be the result of too little low speed compression.

KICKING: Should the front tire pressure, spring rate, preload or compression dampening be excessive, the front wheel will not react to trail obstacles quick enough and the wheel will bounce or "kick" back and loose contact with the trail.

WHAT IS THE IDEAL SITUATION FOR TESTING SUSPENSION CHANGES?
Set out a short trail loop containing all the usual obstacles and trail conditions you encounter during a normal ride. Turn each clicker to it's end stop and back to the original setting to determine your initial setting. Set the Rider sag and Static sag, as best you can. Ride the circuit and mentally evaluate suspension deficiencies. Return to your start point and make setting changes (Turn clickers 3 - 4 stops, too ensure a noticeable change, rather than single stops that may not be distinguishable). Ride the circuit again and reevaluate the ride. Record your changes in a note book and don't be afraid to experiment with the extremes.
REMEMBER: The machine SHOULD BOTTOM SOFTLY, ON THE MOST EXTREME OBSTACLES. Why have suspension travel you do not use? Double check your results after a riding break and be critical. You may end up back at the original settings but that is more desirable than not utilizing your stock suspension to it's fullest.

I'M TOO SHORT FOR MY BIKE! WHAT CAN I DO?
Just because you are short in the inseam does not condemn you to a life without trail bikes or boots with "lifts". The suspension units can be correctly lowered to suit your height and confidence level. It is better to surrender 2 - 4 inches of suspension travel and ride, than not ride or dread the though of stopping.

While it is a task best left to the trained, suspension expert, it is worth the cost. Don't grab the bread knife and cut foam out of the seat or slack off the preload and slide the forks up the clamps excessively, disturbing the bikes balance.
Talk to an expert and let them evaluate the dilemma. A properly lowered machine will have a good suspension travel and action. You do not have to ride under-suspended, underpowered beginners bikes, or ride machines too tall for comfort.