PmWiki | Physics / Physics

Having studied as a physicist for a time in my undergrad career before ultimately switching to C.S., Physics is near and dear to my heart. Unfortunately motorcycle physics is a painfully misunderstood subject. This is especially evident in literature. I believe that it is an understandable topic. Rather than providing a full bottom-up explanation, I am simply going to create an 'addendum' of corrections that I think need to be added to common references.

Most of these are highly controversial topics. Many of my addendum are a bit heated given the vehemence of the opposing views. The important thing to remember is that motorcycles are very complicated. Many things happen as primary, and secondary effects. Identifying causes is very difficult. To some extent this is an open problem. Some things are known, however, and it is these that I hope to clarify.

RRR

To start with I would like to define a few terms.

Stability (From Dictionary.com) 3. Continuance without change; permanence.

Control (From Dictionary.com) 1. to exercise restraint or direction over; dominate; command.

Let us think about the relationship of these 2 features of dynamic systems under 3 conditions.

In the case where the vehicle is highly unstable, control is clearly at a minimum. It is quite difficult to control a vehicle that is almost never doing the same thing from one moment to another. Imagine trying to balance a basketball on the very tip of a small pin. It is easy to move the pin around, but the ball's motion is unpredictable.

In the other extreme, let us imagine an incredibly stable vehicle. Here control is again at a minimum. We are now balancing a basketball on the very tip of a broad table. Huge inputs on our part have almost no effect on the position of the basketball. It remains "without change" whether we like it or not.

The case we all want our motorcycle to be in is that of a medium stability and maximum controllability. If you want fast response like a motoGP bike, you sacrifice some stability to increase controllability. If you want a bike that will continue without much input, you sacrifice a bit of controllability to increase stability. All vehicles strike this balance in different ways.

This is important to remember in the face of people who don't understand this and will try to convince you to put your bike into very dangerous conditions.

I bring this up primarily because of: http://www.reverserotatingrotors.com/

This is a product which has a planetary gear which causes a large weight to rotate in the opposite direction at the center of the axel of the front wheel. This cancels much of the gyroscopic momentum of the front wheel and is intended to remove the gyroscopic precession of the front wheel. This, in isolation, is true. Unfortunately the site goes on to make some very bold and contradictory claims. For example:

Go to http://www.reverserotatingrotors.com/whyitworks.html

"Motorcycles are never laterally stable. Let me repeat that. Motorcycles are NEVER laterally stable! A motorcycle is a motorized bicycle, if it is laterally stable it ceases to be a bicycle. If a motorcycle were laterally stable it would not counter steer it would turn like a car. No gyroscopic precession of any source on a bike resists lean angle change as a primary effect."

"What is perceived as stability is really the gyroscopic precession of the front wheel resisting the rider's efforts."

"When you are going in a straight line the increase in steering effort that comes with speed does give the perception of gyroscopic stability. That resistance to change that you are feeling is the precession of the front wheel. It is not holding the bike up, lateral acceleration does that."

"This is why the front wheel is the source of dynamic resistance (it is generated only while lean angle is changing). Let me say it another way, all of the gyroscopic precession of the front wheel is transferred directly to the hands of the rider. If that resistance is canceled out then a motorcycle will steer almost effortlessly."

What is said here is right for the wrong reasons and wrong for plenty of reasons... To start with, let us just look at very simple, non-motorcycle specific definitions.

"That resistance to change that you are feeling is the precession of the front" - Doesn't this sound an awful lot like the definition of stability? Take this in a very simple case of isolation. Sit on your motorcycle, kick stand up with both feet on the ground. Then, put your feet on the riding pegs and don't move. See how long you can sit there before you and your bike fall to the ground. Now do the same experiment while moving forward. If no lateral stability existed in the motorcycle at speed, it would fall just exactly like it does while you are stationary.

The 'transfered directly to the riders hand' bit is really where I grow most frustrated. Motorcycles steer via changes in steering geometry. If the handle bars cannot turn, neither can the bike. Lean does not turn the bike. This is covered below in another section. Taking that as truth for now, if the bars are what cause turning, and the gyroscopic forces are resisting the bars' turning, the gyroscopic forces are resisting the bike's turning. They are directly causing stability.

Motorcycles are increasingly stable at speed. If you do not put input into them, they will continue, permanently, on their own, in a straight line. Any round rotating object will do so. Take this video:

http://www.youtube.com/watch?v=7wTXPAOo61c

The intro is a bit irrelevant so skip to where they are actually rolling the tires down the hill. Notice the wobble and change of direction early. As the tire picks up speed, the rotational momentum resists changes in direction so much to the point that even while flying through the air the tires continue unchanged. You cannot attribute the stability of the tire in the air to the flatness of the tires bottom. It's shape is totally irrelevant. Yet it is so stable that it touches down again moving so similar to it's original motion that it can continue to roll. If the tire were laterally unstable there would be absolutely no reason for it to touch down again after having flown through the air and continue to roll unabated. You will also notice that, despite having the widest footprint, the f1 tire never really stabilizes and continues to wobble pretty severely throughout the track. This tire has the lowest diameter/weight ratio of all of the tires.

Yes, motorcycles are more complex than tires, but they exhibit very similar stability. Watch Valentino Rossi ride around the track standing on the bike and tell me that it is laterally unstable. He is providing no input to the front tire directly. Only small inputs are possible with ones feet alone. Don't believe this claim? Redo the stationary bike experiment only standing on the pegs and see how well you can balance your bike. If it isn't moving, it will fall.

Do not confuse stability with controllability. The fact that large forces are required to change the bike's direction is precisely the result of stability. Gyroscopic precession may not be responsible for this stability, but angular momentum is.

From Wikipedia:

Angular momentum is important in physics because it is a conserved quantity: a system's angular momentum stays constant unless an external torque acts on it.

Counter-rotating masses do not just remove precession, they also remove (counter) momentum. As such, the bike becomes LESS STABLE. If you were to apply this same counter-rotating system to both the front AND rear wheels, the bike would perform as though it were stationary.

The end result of this... The counter-rotating-rotor system does have merits. Reducing counter-precession as a result of lean angle changes does help CONTROLLABILITY. It decreases stability in order to increase controllability. The 'tank-slappers' that the author went on about so vehemently are an example of an instability with respect to straight line motion, but are actually a good example of dynamic stability. Once the bike enters a tank slapper (as a result of rider input) It will tend to remain in one as the precession of the front wheel tries to counter lean angle changes. It isn't desirable, but if it continues on it's own, it is a stable system just as spins are stable systems in aircraft. Other conflicting elements like geometry changes, rider position changes, rider input, etc all conspire to cause the tank-slapper to become progressively worse in most cases.

I don't disagree that less precession won't help the controllability of a motorcycle. I just strongly disagree with the claims being made. The product is being over-sold.

Still don't believe that angular momentum provides any stability to a rotating object?

http://www.youtube.com/watch?v=545GwnupKAE

http://www.youtube.com/watch?v=pF_SUvPAOSs&feature=related

The final blow, in my opinion, is Keith Code's no B.S. motorcycle. http://www.youtube.com/watch?v=3nRUeEkS644

A second set of handlebars is fitted to the bike which are solidly attached to the frame rather than the steering geometry. The rider is allowed to move about the bike and use any body lean techniques to try to steer the bike. You will notice in the video that putting abrupt force on the bike using ones own mass as a momentum generation does cause minor changes in the bike. This is due to the force being applied between two ground contact points with a hinge in between. The mild cornering is a result of this change in steering induce by an abrupt force applied to the bike. All of which are eventually countered without the rider putting inputs into the steering.

Counter steering is not What turns a bike, it is how You turn a bike

To understand this, it is best to stand next to your bike, takes it's handle bars into your hand, and walk it around a flat surface like a parking lot. Hopefully you can do this with a light weight bike like a bicycle as it will require you to support the weight of the vehicle in mild leans. Do not do this with a heavy bike.

Starting at a standstill with the bike upright, walk forward at a slow pace. You will have to support the bike with your hip as you will be going too slowly for any forces to support the bike. Now, turn the handlebars to the left. The bike turns to the left. Turn the bars to the right, the bike turns to the right. (Remember to keep the bike upright with your hip)

In isolation, steering on a bike does exactly what one would naively expect. If you turn the bars to the right, the bike turns to the right.

When you are on the bike, traveling down the road, you notice a very different phenomena. When you turn the bars to the right, the bike goes left. This is what they teach you in the MSF course. So what is really going on?

When we talk about this we need a very clear sense of cause and effect. A lot of things are happening and nearly the same time so this is complicated. The first thing people tend to think is that precession is some how causing the bike to turn. This is not correct. The only thing causing the bike to change direction is the change in geometry of the two contact patches on the ground and the rolling of the two tires. This is why you can successfully walk a bike around a parking with the bike upright on your hip. You are only changing the steering geometry, and the bike is turning.

Counter steering does not cause the bike to turn. Well... it does, but it causes it to turn in the direction that you turn the bar. The primary effect of turning the bars to the right, is that the bike turns to the right. But it isn't really the whole bike. It is the steering geometry. The tires turn to the right. As the tires move to the right, they move out from underneath the center of gravity of the bike. As a result the bike leans the opposite direction of the turn.

So, the lean causes the turn right? No. If you steer right, and the bike leans left, and then you do not do anything else to the bars, the bike will not turn, it will simply fall over. Counter steering requires two opposite inputs. You can see this in any footage of a bike turning. The bike steers one way, leans the other, and then steers in the direction of the lean.

The lean does not turn the bike. It simply puts the center of gravity into a position so that forces acting through it travel down through the tires and into the ground rather than perpendicular to the ground. The lean is caused by the counter steer, and is sustained by the bike's turning. It is not the primary cause of anything.

Back to the parking lot. With bike on hip, walk in a straight line and slowly allow the bike to lean on your (again, light bike please). As you do so, make sure that the steering remains perfectly straight. You can lean the bike as far as you want and continue walking in a straight line so long as the steering geometry does not change. If you are using a sport bike or a bike with highly parabolic tires, you will notice you actually have to sustain a little bit of counter steer (steering away from the lean) to keep the bike going in a straight line. This is because the tire's shaping is changing the steering geometry as you lean further. Again, the primary cause of the bike's turning is the configuration of the two contact patches, not the lean.

If you build a motorcycle with no steering geometry, it will not turn at any speed.

In summary, the reason you counter steer is to lean the bike over so that the lean will counter the centripetal acceleration of the bike's cornering. You can do this yourself if you have 2 feet on the ground and you are walking the bike around. As a result, no lean is needed. Since you don't have feet on the ground when riding, you have to counter steer to knock the bike over into a position which will eventually sustain an equilibrium of centripetal acceleration and gravitational pull. Both secondary to the bike's going around in a circle.

Acceleration Rider Position

This little blurb is the result of a book I bought. Page 31 of Performance Riding Techniques states "Getting your weight over the front of the bike under acceleration makes it harder for the power and torque of the engine to lift the front wheel. This means more of the power will go into acceleration, rather than lift".

The first sentence is true. The second sentence is garbage. Isolating just the engine, frame, and tire, we have a very simple system to look at. As the engine spins the tire it delivers torque which is countered by the traction of the tire on the ground and creates counter torque of equal amount in the opposite direction. Since the bike is essentially on a lever with the fulcrum at the center of the rear axel, the bike's weight has a power leverage over this torque. As the torque increases, the force placed on the bikes frame in an upward direction increases. There is an equilibrium where this force is just countering the force of gravity and the bike's front tire is just skipping across the ground. More torque will cause the bike to lift. Moving your weight forward, increases the downward force on the front of the bike, but this has NO effect on the amount of upward force being applied by the engine. It can't cause "more power to go into acceleration". You have no control over where the engine's power goes. It does, however, allow you to stuff more throttle into it before lifting. This means you can accelerate harder without lifting. More throttle means more lift, not less. You are simply trying to keep the front end down for controllability and aerodynamics reasons. The bike isn't performing well if you are straight up on the rear tire. So you try to prevent this from happening while simultaneously applying as much power as possible. Regardless of what you do with your body, the amount of lift, that is the upward force of the engine, is directly and only controlled by the throttle.