I read this whole thing, and there’s some truth here and there throughout, along with quite a bit of stuff that appears to have been made up on the fly because it sounds good. The post by Northwoods Snowman comes closest to painting the whole picture, but doesn’t quite cover the wall on the first coat. So I’m going to take a shot at it.
First, I want to be clear with anyone who bothers to read this on some terms I’m going to use. First there is the center of gravity, or “CG”. That speaks to the point in a 3 dimensional object around which it will rotate if it were to be tossed into the air and spun in any direction, and the point at which it would be balanced end for end, side to side, top to bottom if it could be perched on that single point. It’s found up in the middle of a motorcycle somewhere, and reflects the weight distribution in any of those directions. The “X axis” is the fore and aft center line running through the CG, and when the bike rotates on this axis, it is said to “roll”. The “Y axis” is a vertical line through the CG, around which the bike “yaws”. The “Z axis” runs from left to right through the CG, and the bike “pitches” around this one.
Vehicles with two inline wheels steer, or turn, using a combination of dynamics that blend to varying degrees, very well or poorly, depending on the overall chassis setup. First, since it’s been brought up so many times, let’s examine counter steering. Counter steering can be used to initiate a turn by encouraging the motorcycle, bicycle, or scooter to bank, or lean into a corner. Essentially, it is a way to force the bike to roll on the X axis in spite of the angular momentum (gyro effect) of the wheels which resists its doing so. The concept is rather like that used to balance a hammer or broom on one’s finger tip with the heavy end up. Balance is maintained by shifting the low end of the mass in any direction on the X-Z plane, causing the object to roll or pitch in such a way as to remain upright and counteract the tendency to fall one way or other. When a bike running straight and upright is counter steered to the left, the contact points of the wheels are moved to the left under the CG, and the bike rolls, or falls, toward the right. Note that it is also true that one can simply shift the rider weight to the right over the CG and cause the bike to roll right in response to that without counter steering, so counter steering is not an essential element of turning, just a more effective way of initiating a turn more quickly and with less motion than shifting the rider weight.
Gyroscopic precession does not initiate or maintain a turn at all, and in fact works specifically against any rotation of the axle in any direction from its position on the Z axis. It is a highly stabilizing influence on the chassis, owing to the mass of the rotating tire and rim assembly. The strength of the effect depends the mass involved, and on the diametrical location of the rotating mass, the farther from the center, the more effective it is. It resists leaning the bike in either direction, and that is the main reason that counter steering is used to more forcibly cause the lean than by shifting the weight of the rider.
Someone mentioned bicycles with very small wheels being ridable, which they are, but to anyone who suggests that the gyro effect doesn’t stabilize the bike, I would say they should try to ride a Razor scooter on a pair of the training rollers posted earlier. Such small wheeled bikes are balanced almost entirely through counter steering in the manner of balancing a hammer, as discussed above.
As to the whole tire contact patch thing, there may be some effect in that, but before you buy into that being a primary causal effect, try rolling a coin across a flat surface and watch what happens when it leans one way or other; it turns in that direction, and the more it leans, the tighter the turn is. Why? Think of the diametrical edge of the coin, wheel, etc., not as a continuous line, but as a series of points along the circle. When upright on edge, it rolls straight because, as it rolls off of one point of contact and onto the next, the next point that rolls down onto the ground is coming from a point directly in front of the current contact point, and the next one after that is also directly in front, and so on. When the coin leans left, the next point approaching the ground as the coin rolls is located to the left of vertical, and the next is farther left, and the next farther left than that, so the coin turns in a circle to the left. As the coin leans farther, the circle tightens until it finally looses traction and centripetal force makes it slip and fall. All that with a sharp edge as a contact patch. Any wheel and tire assembly will naturally roll in a curved line in the direction it is leaned over from vertical for this reason alone. The size of the circle will depend on the radius of the tire.
So part of the reason a motorcycle banks into a turn is to cause the wheels to roll in the direction of the lean, but also, there is the matter of countering the centripetal force that would otherwise cause the bike to fall over to the right when turned left. The Y axis through the CG must be placed in a balanced condition against this force in order to counter it.
Then there is the complex matter of “rake”, more accurately called steering axis, or steering head angle, and how it interacts with trail. As the previously posted link showed, trail is the distance between a line drawn through the steering axis and the point at which the tire touches the ground. Normally (and hopefully) this point is behind the intersection of the steering axis and the ground. Trail is the cause of most of the self centering force on the steering while riding in a straight line, caused by the drag of rolling friction pulling back along the X axis at the contact point, but it also has a very pronounced effect on the chassis behavior when the vehicle is banked or rolled from vertical, and this effect works in combination with the steering axis angle to give the bike much of its cornering and steering behavior.
Visualizing how all this ties together is aided by getting your hands on a bicycle, or a motorcycle light enough to be leaned over quite a way without touching the handlebars. If you hold a bike by its seat and lean it to the left, you see that the wheel will “fall in” to point to the left of the X axis. This happens because the force of gravity, which was pushing up in line with the Y axis, is now working at angle to that axis and is able to rotate the steering assembly by pushing against what amounts to an effective lever, called a force arm, that is the length of the trail measurement, turning the wheel toward the direction of lean. In the first few degrees of lean, the “fallover” of the front wheel in this static, non-rolling condition is more pronounced, and the wheel of the bicycle actually begins to turn back more in line with the chassis as it is leaned farther. This happens for two reasons. The first is that the bicycle is standing still, and there is no “caster” or self centering effect on the steering because there is no drag pulling back on the tire from the contact point. Remember this point as we continue.
The second reason the wheel begins to turn back toward straight as lean is increased, and the reason it doesn’t fall all the way 90 degrees to the side in the first place, is because the arc of the leading edge of the tire comes in contact with the ground. What happens here is that the contact patch of the tire has moved from a point behind the steering axis (trail measurement) to a point just about even with it, neutralizing the force of gravity that is trying to turn the wheel in by pushing up and over on the force arm created by trail. In motion, this effect can be problematic. Lots of us have had the experience of a bicycle front wheel suddenly turning sharply inward, or “tucking under” as a result of the wheel falling in too far, and the contact patch getting ahead of the steering axis, and then being launched over the front as we “ride over” the front wheel. At low speeds and with steep head angles, this can happen.
Head angle plays into this such that shallower head angles (more “rake”) reduce the tendency to tuck, and steeper ones encourage it. Picture a really extreme chopper, say with a nearly 90 degree steering axis. With such a setup, steering depends a great deal on the natural circular tracking of a leaned over wheel, and a lot more rotation of the fork in the steering head is necessary to produce the same amount of steering. There is very little tendency to tuck, but a very pronounced tendency to push the front end, or understeer. Conversely, with a steep head angle, the front tire can move the tire contact patch ahead of the steering axis very early in the lean-in maneuver, showing a severe tendency to tuck, but at the same time very little fork rotation is needed to steer the machine, and it will seem very precise and responsive to small steering inputs.
The balance that has to be struck at speed is between the caster, or centering effect of trail, and the tendency for the front wheel to turn in. Head angle has to be chosen based on the amount of stability required versus how responsive the steering should be. If too shallow, the bike will turn in a vague and sluggish manner and tend to push the front, but it will be less prone to speed wobbles. If too steep, it will be skittish and unstable at speed, and prone to wobble, but will turn seemingly with just a thought. Once the head angle is chosen to match the intended use of the bike and other factors, trail is chosen to compliment and modify the turn in behaviors.
So, a turn at speed is initiated most effectively by counter steering to start the bike leaning, at which point the contact patches of both the front and rear tires shift to the side of the tire center lines in and amount governed by the tire size and shape, and the combined influences of the tire’s circular form, steering head angle, and trail produce a more or less balanced natural amount of fork rotation, while centripetal force holds the bike from falling to the inside of the corner. How this complex interaction actually works will affect whether you find yourself applying force to the bars to steer the bike into the corner more, or pulling back toward straight as the corner proceeds.
When it all works right, it feels easy and natural. But it’s definitely not simple.
