Trail Geometry on a Bike

Trail on a Castor Wheel

Have you ever wondered how it’s possible to ride a bike with no hands, or why if you let go of your car steering wheel the car will straighten out on its own?  This is due to a concept called “trail.”  Different than the trails you walk on, this trail refers to the distance on the ground from the steering axis to the contact point of the front wheel.

The trail provide a force that restores the wheel to the forward position.  When the front wheel is turned, the point on the tire that touches the ground is pulled backwards by friction.  A component of this force creates a moment about the steering axis, and restores the wheel back to its straight forward position.  The larger the trail the bigger this restoring force.

Think of the a front wheel of a shopping cart, called a castor wheel.  Have you ever noticed how, no matter which way you push the cart, these wheels always point forwards.  On bicycles and cars the geometry that produces the trail is slightly different, but the concept is the same.

In general, a larger trail results in stabler steering.  On a bike this means it’s easier to ride with no hands without falling over.  However, a larger trail also means the steering is more sluggish.  Mountain bikes usually have a large trail because they need to be as stable as possible, so you don’t lose control when you hit that big rock in the path.  Street bikes on the other hand have smaller trails so that their steering is more responsive.

Images from:
http://www.bikemagic.com/news/images/geometry_trail_diagram.jpghttp://bp2.blogger.com/_SZx2oaxuJSA/RjsRkwZu2AI/AAAAAAAAAcY/j1XT_XWFr4U/s1600-h/TrailCastor.jpg

First of all, one must understand that a bicycle is actually an “inverted pendulum” (think of a pendulum, such as one in a clock, except upside-down). In other words, a bicycle will surely fall to one side if it is not balanced. We know this from everyday experience.

So, why is countersteering necessary? The concepts lie in basic mechanics. When riding a bike, turning the front wheel in one direction causes the bike to lean in the opposite direction due to a centripetal force. Therefore, the initial countersteering will cause the bike to lean into the direction of the desired turn. This lean is necessary because when executing the actual turn, there is a centripetal force that acts outward on the bike, and the force of gravity acting on the inward-leaning bike will cancel the outward centripetal force to keep the bike upright during the turn. If there is no inward lean during a turn, then the outward centripetal force will knock the bike over and cause it to fall down to the ground, resulting in a poor cycling experience for both the rider and the bicycle.

Despite being a necessity, countersteering isn’t something that is explicitly taught to a beginning bicyclist. It is something natural that is picked up when one attempts a turn on a bicycle for the first time. This makes countersteering an interesting and subtle phenomenon in bicycle dynamics.

Countersteering is important in bicycle dynamics.