The bicycle/motorcycle isn’t a perfectly smooth body on a frictionless surface.
I was just thinking: Given the large number of people who doubt the veracity of the Moon landings despite overwhelming evidence to the contrary, what chance does a subtle effect like counter-steering have? For the record I believe that counter-steering is a thing, and that it’s impossible to initiate a turn on a bike without it. A turn without counter-steering is known as a “crash”. I had been riding bikes for 25 years before I even came across the concept of counter-steering, and I’m not even consciously aware of doing it now when I’m cycling.
I think we may occasionally be talking at cross-purposes here - I’m fundamentally not disagreeing with what you’re saying, but it’s difficult to discuss the finer points of cornering forces without vector diagrams. Leaving out counter-steering (which is a thing) for a moment, the force I’m most interested in is the resultant vector that describes a line between the combined bike/rider’s centre of gravity and the tyre’s (let’s make it a monocycle!) contact patch. In a straight line this is simply mass X gravity. In a leaned turn it’s the resultant vector between mg and the centripetal force. There’s also a reaction force acting on the tyre at 90 degrees to gravity, which is the friction. I’m saying there’s an increase in that friction when cornering, and an increase in the force vector between tyre contact patch and bike+rider COG. This will make the bike’s suspension compress when the bike is cornering. I’m fairly sure those are safe assertions.
I’ve tried to google a cite with some nice vector diagrams that explains the physics of cornering forces, but though there are plenty out there, no two are the same! Some talk in terms of rotational torque, some call the reaction force centrifugal force, and one even talks in terms of a “fake” force. It are compliacated. For some reason, all the threads discussing the topic descend into chaos and anarchy.
Lastly, thank-you for your patience and explanations Machine Elf, much appreciated.
By the way, I may be arguing with you a bit, but I’m not entirely convinced you are wrong. I’m trying to understand the physics here.
I know from practical experience that if you lean on a bicycle or motorcycle quickly, it will do exactly what you say. The bike will simply lean in the other direction. But if you lean more slowly, you’ll turn the bike without any perceptible countersteer.
Of course a key word in that sentence is “perceptible”.
But it seems to me that you should be able to lean and change your center of gravity. Yes, there’s Newton and all that, but we’re not on a frictionless surface here and you should be able to transmit the force to the ground laterally and have it get lost as heat (rubber deformation and what-not).
If it’s not possible, I’d like to know why not.
<quote snipped and bolding mine>
I doubt that your MSF teachers said that, because the correct mantra is “push left to go left; push right to go right.”
I think you just mis-typed that.
Neither is the ice skate; it’s got plenty of lateral grip, just like the bike.
I’m wondering if we may all be talking past each other a little bit, so I will cleanly and explicitly sum up my claims here.
Assuming a bike (bicycle or motorcycle) with rider, traveling in a straight line, with the bike’s and rider’s centers of mass both directly above the contact patch line, my theory-based claims are as follows:
[ul]1A. If an externally applied force (e.g. a breeze or a bump in the road) causes the bike/rider to begin to fall to the left, the bike’s self-stabilizing steering geometry will cause the handlebars to turn to the left (counterclockwise as viewed from above), moving the contact patches back under the combined center of mass and restoring stable straight-line travel.
[li]2A. If the rider wishes to deliberately commence a left turn with direct inputs to the handlebars, he must first turn the bars to the right to steer the contact patches out to the right (this is the countersteer). Once the rider/bike has begun to lean to the left, he can turn the handlebars to the left to start traveling on a left-curving path.[/li][li]3A. If a rider wishes to deliberately commence a left turn without touching the handlebars, he must first push his body to the left, which will lean the bike to the right - at which point the self-stabilizing steering geometry will cause the handlebars to turn to the right (this is the countersteer). As the contact patches move out to the right of the combined center of mass, the bike comes back to vertical and then leans over to the left, at which point the self-stabilizing steering geometry causes the handlebars to turn to the left, and the bike/rider starts traveling on a left-curving path. [/li][li]4A. It is not possible for the rider to push his body to the left without concomitantly causing the bike to lean to the right. If the hands are off of the handlebars, the bike will then countersteer itself; if the countersteer is inhibited by mechanical means, the bike/rider will continue to travel in a straight line - with the rider hanging off to the left, and the bike leaning to the right.[/ul][/li]
And now my practical claims based on the real-world experience of operating a pedal bicycle and a motorcycle:
[ul]1B. Steering a bicycle with body english alone works pretty good, for typical bicycle weights (~20-25 pounds), typical bicycle speeds (15-20 MPH), and typical non-emergency bicycle maneuvering requirements.
[li]2B. Understanding and deliberately applying countersteering is necessary to meet atypical/emergency bicycle maneuvering requirements.[/li][li]3B. Steering a motorcycle with body english alone has very limited utility for typical motorcycle weights (300-1000 pounds) and typical motorcycle speeds (40-80 MPH) and typical motorcycle maneuvering requirements. [/li][li]4B. Different motorcycles have different steering geometries and different propensities for self-stabilization. When it comes to inducing a turn with body english, cruisers are slightly more responsive than sportbikes. Conversely, the greater self-stabilizing tendency of cruiser bikes means that they tend to resist a little more when you try to make them turn with handlebar inputs.[/li][li]5B. You will not be able to perform the emergency swerve maneuver taught during the Motorcycle Safety Foundation’s beginner course (and included, for example, as part of the Washington state rider skils test) unless you understand countersteering and deliberately apply it.[/li][li]6B. An extension of 5B is that failure to understand/apply countersteering will limit a rider’s ability to safely avoid obstacles that suddenly appear in your path out in the real world.[/ul][/li]
Yes, ‘perceptible’ is absolutely key. Consider this plot I previously linked to, from the Wikipedia page on countersteering. This is for a bicycle at 13 MPH. Check out the input steer torque: just a quarter of a Newton-meter, or about 2 pound-inches. If your bicycle’s handlebars are 16 inches grip-to-grip, then we’re talking about just 2 ounces of forward push on the left grip and two ounces of rearward pull on the right grip. The result? The handlebars deviate from straight ahead by a mere 0.5 degrees - and the bike promptly leans over to the left. If you don’t know about countersteering, you’re not going to notice such subtle phenomena without instrumentation.
If the rider/bike is vertical on a frictionless surface, AND the rider’s center of mass is at the same height as the bike’s, then the rider could translate his own mass out to the left, and it would translate the bike’s mass to the right without causing the bike to roll to the right. This is because there would be no moment applied to the bike at all; the rider’s body would just be pushing straight at the bike’s center of mass, .
For a bike/rider on a real ice-free road with grippy tires, translating the rider’s center of mass to the left causes the bike’s center of mass to translate to the right. This has two effects:
[ul][li]If the rider’s center of mass is higher than that of the bike, then the force applied by the rider to the bike is offset from the bike’s center of mass, and so it will, by itself, create a moment that causes the bike to also roll to the right. [/li][*]As the bike tries to translate to the right, the grip of the contact patches causes the road to exert a leftward reaction force at the bottom of the tires, resulting in an additional moment that causes the bike to roll to the right. This will be true if the rider’s center of mass is at any height above the contact patches (because this roll moment is the result of the rider’s own translational force on the bike plus the reaction force from the contact patches).[/ul]
Agree with everything…
But
… When you are dealing with heavier motorcycles, the weight of the bike & rider is less important than the geometry and length of the motorcycle.
Weight shifting has a much faster on the shorter crotch rocket motorcycle reaction than it does on a longer cruiser in my experience.
In other words, the physics as explained above is all correct but things like geometry * wheel base distance, wheel size, diameter and tire width, and the bigger weight of the cruiser make it more sluggish in the real world…
The other thing is the side gust of wind. Riding with no hands, I have never had the bike straighten itself out. I always have to make a deliberate recovery. Riding with no hands a very gentle lateral impetus can be corrected with an unconscious weight shift which might seem that the motorcycle did it on it’s own but if you have to put a hand on the handlebar to keep tracing straight, then is is obvious the bike can not recover unless you are on a huge paved area where you weight shifting, if quick enough and your % of total weight is big enough to do it at all. I have never been on a bridge or passed a big truck where I could not need to go for the handle bar.
Now I have been on some really strange choppers that really do not want to turn at speed due to rake and trail and even those need a hand in side gusts. The little rake there is on a bicycle and the shortness of their wheel base = a large part of their twitchy description as compared to the larger, longer motorcycle.
Real world conditions are not relevant to theory discussions I know but if this were true about side gusts as stated, an un-ridden cycle ( bike or motor ) would not fall over from a side gust until it got too slow. So what am I missing about what you wrote?
Does there have to be a center of gravity above the point that is the ‘no rider’ center of gravity? Would the combined weight that produced a center of gravity below the height of the lowest wheel axle height make it do that kind of self righting with no steering input?
A bicycle is light enough that you can pick it up quickly enough using weight and countersteering. A bicycle also doesn’t have enough pedal/ground clearance to allow you to power out of a high lean angle, you need to pick the bike up enough so you won’t hit the ground, then pedal.
I stumbled on this video today and thought of this thread. This guy has a bike (made by some welder friends) where the handlebars work opposite to a normal bike. His video talks more about how cognitively difficult it is override your ingrained sense of how to ride a bike, rather than the physics of the thing itself with respect to countersteering, but it’s still an interesting watch and I was thinking about this thread and how a contributing factor to the difficulty might be the conscious effort to try to “steer” one direction or the other might override the subtle countersteering that people do without thinking about it.
The Backwards Bicycle:
Seems like a bike is something that we figure out on a very physical level, more intuition than rational thought about “turn the wheel this way, go that way.”
Hint: check post #27.