On the topic of why it is easier to balance on a moving bicycle than it is a stationary one, the initial explanation consisted of gyroscopic stability created by the spinning wheels…and then your staffer Ms. Karen Lingel brought in the effect of “trail” as the real reason. Both are wrong. Balance and control must be dealt with separately. Let’s examine control first.
“Trail” plays no part in balance, but it has everything to do with controllability. Ms. Lingel cites the story of a bicycle with the steering gear located at the rear wheel. This is akin to placing the rudder of a boat beneath the bow, or the vertical stabilizer (tail or rudder) of an aircraft above the cockpit. Lots of boats and planes would become recyclable material in short order if this were the case. The Wright Brothers had control problems with their first Flyer because they placed the horizontal stabilizer (elevator) ahead of the main wing. This created a very narrow pitch “window” in which the aircraft could be safely operated. Several crashed before the engineers determined both the rudder and elevator needed to be at the rear, in trail of the main wing.
In steering systems, a nice-to-have engineering accoutrement is what’s known as “rake,” or negative angle in the steering axis. On an automobile, this rake is called “caster,” with the steering kingpins angled rearward at the top. This allows the weight of the car to assist with keeping the front wheels pointed forward. Bicycles and motorcycles are built the same way—the steering tube (where the front wheel fork spindle mates with the frame) has an angle built into it. Some forks even have downtubes that scoop forward to carry the front wheel axle ahead of the vertical steering axis. Some “chopper” motorcycles have such an exaggerated steering rake that they’re hard to control at low speeds. Rake is also the reason we now have sayings like, “Look, Ma! No hands!”
As for balance—whether the steering is at the front or the rear wheel—a person can balance just as long on a stationary bike. But why is it easier to balance on a moving bicycle than a stationary one? Because once a bike is underway, the rider is afforded the ability to move the bike laterally beneath his center of gravity. Standing still, there’s no risk of falling over the handlebars or off the back of the bike, thanks to the wide stance of the tires’ footprints. It’s a bipod—very stable front to back—except now the rider must find a way to keep from tipping right or left. Options include throwing the body left or right, or hopping the bike left or right to compensate for imbalance. Once underway though, the rider now has the necessary control. If the rider leans left or right, the handlebars can be moved to bring the bicycle under the rider’s center of gravity.
The same goes for unicycles. That’s why a unicyclist is always doing the front-to-back ocillating motion when stationary…but it’s not really stationary.
So the real answer to the question is because forward motion allows lateral movement of the bicycle beneath the rider to maintain center of gravity.
Another aspect of bicycle and motorcycle balance, known as counter-steer, comes into play at speeds somewhere near and above eight miles per hour. Below these speeds, the rider simply leans into the turn, and compensates for loss of balance by correcting with steering input to maintain balance through the turn. Above eight M.P.H., however, counter-steer is used to cause the vehicle to lean in the desired direction. For example, a rider is traveling at 20 M.P.H. and must make a quick left turn. The handlebars are quickly (and usually subconsciously) tugged slightly to the right; forward inertia causes the bike to go into a pitch to portside. Immediately the handlebars are brought left to regain balance and control through the turn.
With respect and admiration,
Geno Becker
