Just twenty years ago Paul Turner and RockShox introduced the modem mountain bike suspension fork. Known as the RS-1, this fork would prove its worth on the bicycle of 1990 World Champion Ned Overend. Now, as then, mountain bike suspension is still a little misunderstood, under maintained and under utilized. In order to get best performance from your mountain bike, it is important to take the time to learn the different adjustments of the suspension and tune it properly.
Bicycle suspension forks and rear shocks consist of two basic parts. The first part is a spring of consisting of air, a metal coil or an elastomer. The second part is a system of oil or air to aid the compression and control the rebound of the spring.
When adjusting the suspension, the place to start is with the spring. Metal springs have a weight rating in pounds. Air shocks are almost infinitely adjustable. The suspension operates best when it is under a certain amount of preload, or pre-compression. This preload is indicated by the amount of “sag”, which is measured as a percentage of suspension or shock travel. Recommended sag varies by manufacturer, but an amount approximately 20-25% of total suspension or shock travel is a good starting point.
Measuring the sag requires a couple plastic wire ties, tape measure, a shock pump (for air forks and shocks) and maybe a helper. To adjust the suspension, first measure the length of upper fork tube or shock shaft to determine the amount of shock stroke. This information may also be in the owner’s manual. Next, for air shocks, place a wire tie on the upper fork leg or shock shaft for the rear. Then, ride the bike around the street or yard, making sure the suspension is moving while riding. Stop, staying on the bike balancing yourself upon a stationary object. While sitting still, carefully slide the plastic tie down against the rubber seal on the shock. Now gently dismount the bike without disturbing the ties. Measure the distance between the seal and the tie. This is the “sag” Adjust the air pressure and ride again until this distance is the desired amount.
To adjust a rear shock with a metal spring, measure the shock eye-eye distance in an un-weighted position. The shock stroke will often be printed on the spring. Activate the suspension. Next, have your helper take the measurement again in the weighted position. The difference between the measurements is the sag. Adjust the sag by tightening or loosening the spring collar. If the desired sag cannot be reached, the spring may need to be replaced with one of a different weight rating.
In most shocks oil is used to control the action of the spring. When the fork or shock is compressed, this oil is forced through a valve. Depending on the valve size and compression speed the oil may or may not have any effect on the compression. For example, when riding slowly up a driveway curb the compression speed is very slow and the oil provides little resistance. If you jump off the house, however, the compression speed is very fast. Therefore, the oil is not able to move through the compression valve quickly and helps to prevent the shock from bottoming out. The opposite occurs in the rebound circuit when the spring extends the shock back to its neutral or unweighted position.
When adjustable, the rebound speed is the easier to adjust. When the rebound is too fast the shock feels too bouncy. There may be a firm clunk called “top-out” when the shock is unweighted. An easy way to adjust the rebound is to start with the valve all the way open. This provides the fastest, bounciest rebound. Ride around and feel how much the shock moves. Gradually slow the rebound damping to an acceptable level. Test it by riding off a curb slowly. Aim for a one bounce curb drop.
Conversely, the compression is easier to adjust from maximum. In this position, riding over obstacles will be a relatively harsh ride. A good place to test the compression is over some fast washboard or braking bumps. Reduce the compression damping until the harshness over the bumps is minimized. Also test it on some small jumps to make sure it does not bottom out too hard.
The tricky thing about adjusting suspension is getting everything to work in harmony. For example, when one spring is softened, the other is a little stiffer and vice-versa. If the rebound damping is too slow, the suspension will not have time to fully extend before the next impact. The shock just keeps compressing slowly giving a harsh ride or bottoming out. This is called “packing up”. If the compression damping is too little the shock may bottom out too often.
The properly tuned suspension will be smooth in most conditions and bottom out once in a while. Reaching this goal can be a little time consuming. But it is however, very rewarding. That’s because tuning suspension requires one of our favorites things, riding. And once it’s done, riding is more fun because the bike performs smoothly and predictably. In fact, maybe it will have you riding like a champion.
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