(c) Copyright 2013 David Dilworth
Gyroscopic effects are one of the few physics principles that are not intuitive to me. I’ve studied them and used them for decades, yet when I spin a bicycle wheel holding it by the axle I’m still never quite certain which way the gyroscopic effects will torque the wheel – until I test it with my own hands and see which ways things move.
This is a brief overview of the three separate gyroscopic effects that affect bikes (that I wrote in part to help me remember). It does not deal with other very important bike dynamics including the several Polar Moments of Intertia (roll, yaw, pitch) that come into play when a bike is conducting a turn.
I’ve used an example of a bike turning left so this should make more sense than a page of equations or having you wade chest-deep through ambidextrous physics jargon. If I get enough encouragement I may try to make some diagrams for us all.
1. HandleBar / Gyroscopic Steer-Lean Effect: gyroscopic effect created by a steering maneuver when the front wheel is rotating.
We’ll start with some basic bike dynamics and examine the gyroscopic effects independently of each other.
a. To initiate a left turn a motorcycle’s handlebars are turned away from the turn to the right, slightly. This leans the bike to the left.
b. When the desired angle of lean is reached, the handlebars are brought back to neutral then slightly turned left into the corner.
Turning the handlebars away from the turn, to the right, creates a gyroscopic effect of the spinning front wheel trying to lean left. (That’s good, the gyroscopic effect is helping us get what we want.)
When the handlebar is brought back to neutral, the effect is stop the leaning gyroscopic torque; to slow the bike from leaning further. When the handlebar is turned to the left, the effect is to start a gyroscopic torque that tries to unlean the bike; to straighten it up;
to push against the bike leaning any further.
This is good, these gyroscopic forces are working in exactly the directions we want.
Note 1: The faster the front wheel is spinning, the more gyroscopic force is generated requiring more torque to move the handlebars, to lean the bike over and also to reverse its lean. A simple thoughtful review of this easily debunks the myth that on a stable bike “bikes do not become more stable the faster they go.” (That linked website has generally very good information, its just that single claim which is incorrect.)
Note 2: This gyroscopic effect is generated by the front wheel alone. The rear wheel provides zero gyroscopic effect from steering movements. The rear wheel’s gyroscopic effect from leaning is a wholly independent phenomena we’ll discuss here next.
2. Gyroscopic Roll-Steer Effect: The gyroscopic effect generated by rotation of both wheels, while the motorcycle is purely leaning (rolling from side-to-side) to the left independent of any handlebar movement. Imagine riding and the handlebars are suddenly frozen in place; locked to the frame. The bike would start leaning over (with negligible turning from Gyro effect #1 above) and generating this effect.
While a bike is rolling down to its desired lean angle for a left turn, if the back wheel were loosely constrained in the frame and allowed to steer right or left, this effect would cause the back wheel to steer into the turn to the left. Because the back wheel is constrained by the frame, the actual effect is to put a torque on the frame which then very slightly presses the front tyre into the pavement, but only for the brief time when the bike is rolling into a turn. As soon as a steady state lean is achieved, this effect disappears.
The front wheel gyroscopic effect from this occurs, but its force is so small it should be ignored as the handlebar turning is by far the most powerful operator. In any case it also steers the (originally vertical) wheel to the left; into the corner slightly helping the rider turn the front wheel to halt further leaning.
3. Gyroscopic Bike Yaw-Righting Effect: The gyroscopic effect generated by rotation of the wheels in a constant turn – as when doing endless circles on a skid pad.
This is the only gyroscopic effect that fights a rider in a turn. In a steady state turn both bike wheels try to straighten up vertically and then lean against the turn. To counteract this effect while turning left, the rider must turn the handlebars slightly less than the amount of left turn desired to force the bike to stay leaned over to the amount needed.
Note 2: This last effect has nothing to do with, is independent of, correcting a slide by turning the handlebars towards a sliding rear tyre.
Let me know if this article is helpful for you.
Additional reading :