Suspension Sag Calculator
For manual testing and adjustment of a motorcycle’s suspension sag, Racetech’s method is the most accurate. The geometry in their formula eliminates any marginal grey area by averaging results against one-another from both directions. With springs, what goes down wants to come up and vice versa, so we take note of the ceiling and floor before predictively calculating the horizon.
Physically, this process is easier with two or three people. Our instructions assume that you’ve got friends on hand to help out. With that in mind, pizza and soda are a wise investment.
You’ll need to measure and write down three figures per axle (front/back). We’ll call them F1, F2 and F3. For accuracy, you should only collect one axle’s figures at a time.
F1: Place your bike on a stand so that the wheel is off the ground and the target spring(s) are fully extended. For the rear, measure directly upward from the axle’s center to a fixed point under the subframe. Up front, measure inline with the fork tubes … axle center to dust seal for USD, dust seal to lower triple tree for standard. Be sure and remember the points you use to measure!
F2: Remove the bike from it’s stand and have a rider get on board, seated in riding position. They should hold still for a more consistent center of gravity. While one person keeps the bike vertical, another must push down on the target spring(s) by 25mm (about an inch). Slowly release pressure until the suspension stabilizes. Measure the distance between the same points used to obtain F1.
F3: Repeat the procedure used to acquire F2, this time lifting upward instead of pushing down. Enter your resulting figures into their designated form boxes below and hit “Calculate”.
Important: If you’re measuring in inches, convert to mm first using our calculator …
The formulas for calculating static sag and stiction are as follows:
Static Sag = F1 – ( ( F2 + F3 ) / 2 )
Stiction = F3 – F2
For street, sport-touring and standard bikes, the ideal sag is 1/4 to 1/3 of the suspension’s total travel. That’s between 30 and 35 millimeters for sport, sport-touring and standard bikes or 25-35 millimeters for road race bikes. The FREE sag, which applies only to the rear suspension and uses the same system of measurement, should be between 2 and 8 millimeters. The stiction should be between 2 and 5 millimeters.
How Do You Tune Your Motorcycle Suspension?
Other techniques are known to work. Which methods have you tried? What do you like about each and why? Your input is invited. Post an article!