How to Calculate Swingarm Leverage Ratio

by Wilhelm Schnotz; Updated September 26, 2017

Items you will need

  • Plastic wire tie

  • Crescent wrench

  • Tape measure

  • Pencil and paper

  • Calculator

Because off-road motorcycles, particularly those used in motocross events, must be equipped with shock absorbers able to handle large, jarring bumps without losing traction, motorcycle engineers developed swingarm shocks, a system that places the rear axle on a lever that’s attached to a shock absorber. Using the mechanical advantage of leveraged forces, this allows shocks to absorb bumps much larger than the length of their springs. On a swingarm with a 2:1 leverage ratio, the rear axle moves 2 inches for every inch of compression the spring absorbs. The leverage ratio merely represents a measurement of the difference between the axle and the shock’s movement.

Place a plastic wire tie next to the rear shock’s head when the motorcycle is parked. Secure the tie tightly enough so it won’t slide on its own and must be moved by hand or by the movement of the shock.

Drive the motorcycle over terrain rough enough to exceed the swingarm’s capacity, where the motorcycle “bottoms out” on its rear axle. Motocross bikes may need a particularly rough terrain – most likely with jumps – to exceed the capacity of their rear shock absorber.

Measure the distance between the top of the shock absorber’s head and the bottom of the plastic tie. Record this measurement as R2.

Remove the nuts that attach the rear shock absorber to the final drive using a crescent wrench. Remove the axle and final drive from the shock absorber. Using your hand or a block, maintain the rear axle’s position when you remove the shock.

Measure the distance from the bottom of the axle to the ground, and record it as measurement A1. Lift the axle to its highest point in suspension, recording it as A2. Calculate the axle’s travel as A2 - A1. Record this number as R1.

Reaffix the shock absorber to the final drive using a crescent wrench and fastening hardware you removed from its moorings.

Calculate the swingarm’s leverage ratio by dividing R1 by R2. Use this figure, R3, as the basis for ratio, R3:1.

Tip

  • Many owner's manuals or service manuals list the swingarm's leverage ratio as part of its technical documentation.

About the Author

Wilhelm Schnotz has worked as a freelance writer since 1998, covering arts and entertainment, culture and financial stories for a variety of consumer publications. His work has appeared in dozens of print titles, including "TV Guide" and "The Dallas Observer." Schnotz holds a Bachelor of Arts in journalism from Colorado State University.