Dirt Bike Suspension
No matter how great a dirt bike and its suspension is, it won’t give you great performance if don’t know how it works, understand how to adjust it, or keep it well maintained. Getting your suspension right can be slightly tricky, even if you’ve been riding for a while. Problems are easy to misidentify, and sometimes a change that works well for one problem makes another problem worse. So, check out these basics before your next ride to improve your suspension and performance.
Sag and spring rates
Your springs are there to hold both you and your bike up, so make sure you have the right spring rate. This will depend in part on your size and in part on your riding style and preferences. Right out of the box your spring rates are usually set for someone of average size — this typically means 75 to 80 kilos or 165 to 176 pounds. If you are lighter or heavier, that spring rate won’t perform as well for you.
Next, determine your static sag and laden sag. Assuming your bike is 125cc or more, the static sag should be around 25 to 40 mm. Smaller bikes can handle a static sag between 8 and 10 mm. If your static sag is too low you need a stiffer spring; if it’s too high you need a softer spring. Check the static sag like this:
Put your bike on a stand, wheels off the ground. Measure vertically from the rear axle bolt to any point on your bike for references, like the rear mudguard or the side plate. Now take the same measurement with the bike off the stand, upright, with the clickers set to soft. Subtract that second measurement from the first, and you have the static sag.
Check the laden sag by having someone take that same measurement while you sit on the bike. Subtract that new measurement from the on-stand measurement and you have the laden sag; for bikes 125cc and larger this should be about 95 to 105 mm, and for smaller bikes look for 65 to 85 mm. Check your owner’s manual, though, because every bike is unique. Just like the static sag, a low laden lag means you need a stiffer spring; a high laden sag means you need a softer spring.
If the laden sag is wrong, adjust the spring preload. Use a hammer and punch to release the shock spring’s top collar. Then tap the collar to compress or release it depending on whether you need the spring to be stiffer or softer. After you’re done, remember to tighten the top collar so it’s locked into position.
Compression and rebound
If your bike kicks a lot that may be because it has too much compression, too much rebound, or even not enough rebound. Try more compression first; if that doesn’t work, move on to checking rebound.
Top of the front forks compression
Start out by setting your rebound and compression adjusters to standard clicks based on the standards for your bike. In fact, it’s a good idea to mark the standard setting on your bike so you can default back to it quickly as needed. If you can’t figure out what the standard suspension settings for your dirt bike are, count how many total clicks it has and then set them at 2/3 out or open.
Bottom of the fork (rebound)
Now that you’re set at standard, ride to get a sense of what needs adjustment. Remember, only adjust one thing at a time, and never more than 3 clicks in one direction at once. The screw at the top of the fork adjusts compression; the one at the bottom adjusts rebound. Both have “S” and “H” labels which mean soft and hard. Winding the compression screw clockwise toward H makes the fork harder, and vice versa. Winding the rebound screw towards H slows down your rebound speed for rolling terrain with larger bumps; winding it towards the S increases the rebound speed for rougher, smaller bumps.
Oil and suspension
Remember to change your shock and fork oil every 20 hours. (You should replace parts that wear down such as seals and bushings every 40 hours.) Oil gets contaminated and breaks down over time, and this causes fading in your suspension. Excess play can also cause worn bushings which in turn lead to friction and leaky seals.
Most dirt bikes today call for 5wt fork oil because they need to cope with higher temperatures and mount more resistance to breaking down and fading, shocks typically use lighter 2.5wt oil. Changing the height of your fork oil alters damping and changes the amount of air space in the fork. Adding more oil stiffens the fork while taking oil away softens it; add or remove oil in very small amounts — no more than 10ccs at a time — until you have the effect you’re after.
Getting your suspension exactly where you want it to be is a trial and error process. It’s labor- and time-intensive, but if you don’t like how it’s going at first, just keep on trying because it’s the one thing you can do to get your bike truly dialed for your height, weight, and riding style. You can always restore your stock settings and give it another go. Your effort and attention will be rewarded in the long run with better suspension settings, more effective mastery of obstacles, and improved riding techniques.