A bicycle with compliant training wheels, half way between a
bicycle and a tricycle, is uncontrollable
ANDY RUINA, Cornell University,
Mechanical Engineering
We have built and tested a vehicle that can balance and
steer like a bicycle, a tricycle, or anything in between. A bricycle is essentially a
bicycle with springy training wheels. The sti?ness of the training wheel suspension
can be varied from in?nite, when the bricycle is a tricycle, to zero, when it is a bicycle.
One might expect a smooth transition from tricycle to bicycle as the sti?ness is
varied, in terms of handling, balance and feel. But the situation is more complicated.
Rather, the controllability of a bicycle depends on gravity. Without gravity, lean
and direction cannot be controlled independently. Springy training wheels e?ectively
reduce or even negate gravity. Indeed, experiments with the bricycle show problems
when the total e?ective gravity is about zero. People can then still balance easily
but can no longer turn the brike. The theory and experiment show a qualitative
di?erence between bicycles and tricycles. A di?erence that cannot be met halfway.
> Since the wheel doesn't roll side to side, I imagine it's much the same as balancing a bicycle.
A bicycle moves. A stationary bicycle is impossible to balance (for most people anyway). I don't see how this would be any different. As soon as you stop you fall off?
If it were me I'd add two small wheels (training wheels? :) on springs + shocks on either side. (Make them fold in to maintain portability.) You'd have to make the spring force tunable for different people, but that should be possible by adjusting a screw to change the effective length of the spring.
Interesting video. I'm somewhat surprised no one has been hurt trying it out at his lectures, though. In some of the clips, the challengers seemed close enough to the edge of the raised stage that with a little bad luck it looked like they could have rolled off the stage or fallen sideways off it.
There are a couple followups that would be interesting.
1. Try this on a tricycle. On a tricycle you steer in the turn direction and do not need to lean, as opposed to a bike where your counter-steer and lean. It would be interesting to see if that less complicated steering interaction would make it easier or quicker to adapt.
2. Try on a bicycle with training wheels, adjusted so you can still lean but aren't going to actually fall over. (This is important because if you can't lean the training wheels have essentially made the bike into a trike and so the experiment has been reduced to #1).
The idea here is that in most of his videos people were failing very fast. They might not be getting a long enough ride each time to provide enough examples of control actions and responses for their brain to learn much.
Perhaps the training wheels would provide longer trials, giving the brain a lot more to work with.
You also need some way to counter the centripetal acceleration. The training wheels do that in the tricycle, and leaning does it in a bicycle, but you need gravity for leaning, so in zero g with magnetic wheels&track you could not steer a bike the normal way, and perhaps not at all, but you could steer a tricycle.
It's worse than that even... Historically, (before balance bikes) a lot of kids would progress from having training wheels all the way down, to two-wheeled dynamics. The problem is, having training wheels all-the-way-down means you have a tricycle, which doesn't have countersteering dynamics. So as soon as you pull the wheels up, they have to totally forget what they've learned.
A basic principle of bicycle framebuilding is that if you think you understand bicycle dynamics, you don't. Frame geometries that should ride perfectly develop terrifying handling problems in the metal. Frame designs that are unridable in theory turn out to be relatively practical - perhaps the oddest example of this is the Python recumbent, which steers in the middle, drives at the front and has no handlebars[1]. Bicycles that are self-stable and balance on their own aren't necessarily good to ride and vice versa.
Making sense of bicycle dynamics is particularly bewildering because they are such a natural and direct extension of the rider. Learning to ride a bicycle is a completely subconscious process and what your body is doing runs counter to what your brain thinks is happening. Most people who ride bicycles believe that they steer in the direction of a turn, when in fact the opposite is the case. A child who learns to ride a bicycle with training wheels actually takes longer to learn to ride on two wheels because of this - they have to unlearn steering before they can learn to countersteer.
I tried that, and the kid will then just ride the bike with it leaning all the way to one side till the wheel hits the ground.
I'd really like to see training wheels with adjustable spring tension. Gradually make the spring weaker so the training wheels have less effect every couple weeks, until they aren't really doing anything at all.
The biggest problem with turning in zero gravity is there's no other force than forward momentum.
The only thing that keeps bicycles upright in gravity is forward momentum. If a bicycle in gravity with no rider tried to turn right on its own, it would fall over. Only if the road curves naturally to the right will it follow and not fall over.
Imagine no bike. You're floating forward in zero-G. Now you want to go right. You turn to the right.... but you're still going forward. It's like that, but on the bike. Nothing is keeping you stuck to the ground, so no matter how you try to move the bike, it will want to keep going in the original direction of momentum, and you will just end up tumbling over if you try to turn the wheel or lean anywhere.
Another way to look at it, just like in the video: if you are tilted, you keep going forward, while tilted. You would literally need some force to pull you in a direction other than forward in order to turn without tumbling toward your original direction.
With a tricycle, when you turn the wheel, the inside wheel is essentially anchored to the ground where it is, and the outside wheel follows the only path that it can, since it can no longer continue going forward. If you were going fast enough during this turn, the whole thing would tip over, similar to how cars in gravity will flip over when they try to turn too fast. Momentum just carries them forward.
I think that's not quite accurate. I remembere there were some experiments where wheel had a counterspinning weight attached to nullify its momentum and the bicycle still mostly worked as usual.
I duplicated the paper to see how the controllers performed on arbitrary bike proportions and rough terrain, its neat that so little can balance a bike
It's true that increasing trail and rake both increase stability. But curiously, a bicycle can be made stable with both negative rake and negative trail.
An interesting take on this, with both mathematical modeling and real prototypes, can be found at [1]. Note also that gyroscopic stabilization is not necessary: in the prototypes, a counter-rotating extra wheel cancels out the angular momentum of the front wheel.
According to the authors, it's not yet even proven that a stable bicycle must turn towards a fall. Almost the only sure thing, so far, is that "at least one factor coupling lean to steer must be present". We know a lot of sufficient conditions for stability, but not what is necessary.
The conclusion of the paper: "As a rule, we have found that almost any self-stable bicycle can be made unstable by misadjusting only the trail, or only the front wheel gyro, or only the front-assembly center-of-mass position. Conversely, many unstable bicycles can be made stable by appropriately adjusting any one of these three design variables, sometimes in an unusual way. These results hint that the evolutionary, and generally incremental, process that has led to common present bicycle designs might not yet have explored potentially useful regions in design space."
EDIT: here's a not-paywalled version of the paper linked in the submitted article.
Yup. There's probably a lot of little such things where the model deviates from reality. Might be a way to exploit the friction in the various hinges, etc. It may be easier to turn on a low-friction surface where the ground friction doesn't overwhelm these effects, etc. Fun to play with, but the end result is still that even if the real world doesn't give us a "truly" unsteerable bike, I am still surprised that this curve between "steerable bike" and "steerable trike" passes near "zero steerability" at all. I would not have guessed that.
How does the kid steer? Bikes need the front wheel to be pretty close to perpendicular with the ground. Tilted over, you'd just go in a tight circle forever, and trying to turn would lift you off the training wheel.
What he demonstrated first of all is that you simply can't ride a bike that prevents the steer from turning in one direction. A bike continously falls to one side or the other, which gets corrected by steering in that direction to put the contact area back under the center of mass. This happens not necessarily through the rider's (conscious) action, even a bike with no rider does it to some extent, but in all cases it requires the ability to turn the wheel assembly freely around the steering axis.
IMO that means the bike in the video demonstrates failure to keep basic balance even before it gets a chance to demonstrate failure to turn properly.
The opening of the video describes the difficulty (impossibility?) of balancing a bike with the handlebars locked.
I've had frustrating arguments with engineers on the degree to which the gyroscopic effect of the bicycle wheels are what keep the bike upright. They seemed to think it's the spinning wheels that allow you to balance. (I suppose in their minds this would explain why it is difficult to balance when rolling very slowly.)
Give the bike a brisk shove, I argue, with no one on it and see how far it balances. In my experience, a bicycle balances for about as long as it takes to fall over.
I see the rider+bike as an inverted pendulum ... or like someone balancing an inverted broom on the tip of their finger if you will. Steering is the rather counter-intuitive means by which one moves the contact point with the ground left and right under the larger mass (the rider) to balance it.
Maybe the opening to the video or the Arduino-controlled contraption (lacking gyroscopic wheels) will get more people to re-think this.
A bicycle with compliant training wheels, half way between a bicycle and a tricycle, is uncontrollable
ANDY RUINA, Cornell University, Mechanical Engineering
We have built and tested a vehicle that can balance and steer like a bicycle, a tricycle, or anything in between. A bricycle is essentially a bicycle with springy training wheels. The sti?ness of the training wheel suspension can be varied from in?nite, when the bricycle is a tricycle, to zero, when it is a bicycle. One might expect a smooth transition from tricycle to bicycle as the sti?ness is varied, in terms of handling, balance and feel. But the situation is more complicated. Rather, the controllability of a bicycle depends on gravity. Without gravity, lean and direction cannot be controlled independently. Springy training wheels e?ectively reduce or even negate gravity. Indeed, experiments with the bricycle show problems when the total e?ective gravity is about zero. People can then still balance easily but can no longer turn the brike. The theory and experiment show a qualitative di?erence between bicycles and tricycles. A di?erence that cannot be met halfway.
Related: http://ruina.tam.cornell.edu/research/topics/bicycle_mechani...
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