Dirt Bike Suspension -- Terminology
A good dirt bike can’t turn a bad rider into a motocross professional, but keeping your dirt bike at its best will keep you at your best. Your dirt bike’s suspension plays a big role in how the bike performs and responds.
Fortunately, today’s suspension allows for a lot of adjustment and tuning and, even better, you can learn to do it yourself. The most important thing is small steps – do everything just a little bit at a time.
Before you get started, it helps to be familiar with the terminology. Following are some common terms and quick explanations. If I’ve missed some important ones, post a comment and I’ll add it.
Compression dampening controls how the suspension – you guessed it -- compresses. An adjustable valve allows a specific amount of oil through the suspension and controls the speed at which it travels through its stroke.
High speed Compression Dampening
High speed compression dampening allows you to adjust the suspension rate for times when the suspension is moving at high speed. For example, hitting a large, flat, near vertical surface will cause the forks or rear shock to compress at a higher speed. It’s all about how fast the suspension is moving – not your dirt bike.
Low speed Compression Dampening
Low speed compression dampening adjusts the flow of oil through the base valve and comes into play when attacking obstacles that are more rounded – times when the suspension is moving though its travel at a slower speeds. Again, it’s about how fast the suspension is moving – not your dirt bike.
After your suspension is compressed, it has to return to its starting point. This is called rebound, and rebound dampening controls what that speed will be. If your rebound dampening is set too fast your dirt bike may kick up over braking bumps or when accelerating out of corners. If it's set too slow the suspension won’t return far enough to absorb the next bump – and a series of bumps can lead to the suspension bottoming out.
Spring rates are numbers used to indicate the stiffness of the fork springs or shock spring.
Wheelbase is the distance between the front wheel and back wheel – measured from where each wheel touches the ground. Bringing your rear wheel forward in the swingarm will decrease your bikes wheelbase and will make the bike quicker to turn in corners. Increasing your wheelbase makes the bike more stable at high speeds. Bear in mind, you’d have to be a fairly aggressive or experienced dirt bike rider or motocross racer to notice any real difference. (You WOULD notice the difference between a 50cc pit bike and 500cc race machine though)
Fork Leg Height
In simple terms, fork leg height changes how far your handlebars are from the ground. It’s adjusted by moving your fork legs either up or down in the triple clamp. The triple clamp is that part connecting your forks to the handlebars and the dirt bike frame. Adjusting the fork leg height also affects your wheelbase. If the fork legs are high in the triple clamps, the wheelbase is effectively shortened with the bike turning in faster. Dropping the fork legs down makes your dirt bike more stable at higher speeds.
Fork Leg Spike
Fork leg spike is used to describe a rough, uneven feeling as the forks or rear shock travel. The cause is usually a mismatch in front and rear suspension tuning. As one end tries to move at a different rate than the other it feels like the dirt bike is fighting itself (and you).
Fork Leg Stiction
Other than just sounding neat, stiction is a word used to describe the friction of the suspension components moving against each other. The goal, through special coatings and seals, is to reduce stiction, allowing the components to move freely.
Bottoming is when the suspension reaches the end of its travel. The ideal situation occurs when you almost, or very lightly bottom out – that means you’re using all of the suspension's travel and reducing the impact as much as possible.
Preload refers to adjusting the length of the shock to match your body weight. The springs that you see on them control your preload.
Free Sag or Static Sag
Free sag or static sag is how much your dirt bike suspension compresses under its own weight.
Race Sag or Laden Sag or Rear Ride Height
Race sag, or laden sag, is how much your dirt bike sags while you’re on it – sometimes called rear ride height as well.