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Suspension Tech: How To Set Rider Sag | MC GARAGE

Measuring and adjusting rider sag are the first steps in setting up any bike’s suspension.

May 3, 2015

Measuring and adjusting rider sag are the first steps in setting up any bike's suspension. Bringing rider sag into the proper range (typically 30 to 40mm, front and rear) will place your bike's ride height, ground clearance, rake, trail, and other important factors into the right range and also help you to determine if the spring rates already on the bike are right for your weight.

We prefer the “Race Tech” method of checking sag because it takes into account stiction (static friction) within the suspension components. To check rider sag, you’ll need the rider (fully dressed in riding apparel) and a tape measure (preferably a metric one) as well as any tools needed to adjust your bike’s preload. Plus an assistant—checking sag properly is a two-person job.

To check free sag, repeat the following procedures with the bike unladen instead of with the rider aboard. Have a look at part one of our "Making Sense Of Suspension" story HERE to understand what free sag means and how to know if your bike's is in the proper range.

RELATED: Making Sense Of Suspension in the MC Garage

1. Start with the fork. If the bike has a centerstand, lift the bike onto the stand and then have the rider press down on the back of the bike to get the front wheel off the ground. Alternatively, you can lean the bike against your hip on the sidestand, as shown in the photo at the top of the page. Measure the length of exposed slider on the lower fork tube. For right-side-up forks, measure from the axle to the lower triple clamp. This measurement is called L1.

2. Put the front wheel back on the ground, and have the rider get on the bike and sit in the normal riding position. Use a wheel chock to support the bike in an upright position, or perform this procedure near a wall so that the rider can reach over and balance himself and the bike. Now compress the front suspension and then slowly allow it to rebound. Re-measure between the same two points you used before. This is L2.

3. Lift up on the front of the bike and then slowly allow it to settle. Re-measure between the same two points. This is L3. The difference between L2 and L3 is due to stiction in the fork. If there were no stiction, L2 and L3 would be equal.

4. Rider sag is calculated by averaging L2 and L3 and subtracting it from L1 using the following formula: sag = L1-((L2+L3)/2).

5. Rear sag is measured using the same procedure. Measure from the axle to a point vertically above the axle on the bike’s tail. It can be helpful to mark the upper measuring point with a piece of tape or a dry-erase marker.

6. If necessary, adjust spring preload to bring sag into the proper range. Adding preload reduces the amount of sag (both laden and unladen), while removing it increases sag. Handy tip: Marking the spring collar on the shock will help you track how many turns have been made.