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.
“For manual testing and adjustment of a motorcycle's suspension sag, this method is the most accurate. Use our online calculator to generate your results.”
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 know your year/make/model's total suspension travel, then measure and write down three figures per axle (front/back). We'll call them TT, FE, SC and SE. For accuracy, you should only collect one axle's figures at a time.
TT: Total Travel - Consult your factory manual or search via the form below. It's typically around 100mm.
FE: Fully Extended - If you haven't already documented the max distance between your preferred measuring points, place your bike on whatever stand allows the target spring(s) to be fully extended, wheel off the ground, under zero load. 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!
SC: Sag Compressed - Remove the bike from its 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 FE.
SE: Sag Extended - Repeat the procedure used to acquire SC, this time lifting upward instead of pushing down. Enter your resulting figures into their designated form boxes below and hit "Calculate".
Because we're aiming for a percentage, it doesn't matter if you measure in inches, centimeters or megafonzies. What is important is that you use the same unit of measure for each figure.
Sag: 0
Stiction: 0
The formulas for calculating sag, percentage and stiction are as follows:
Sag = FE - ( ( SC + SE ) / 2 )
Percent = (Sag / TT) * 100
Stiction = SE - SC
For street bikes, the ideal sag is between 1/4 (.25 or 25%) for firmer/sportier handling and 1/3 (.33 or 33%) for a cushier ride.
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. Write an article!
