We still don't really know how bicycles work

http://www.newstatesman.com/2013/07/mysteries-bicycle

by ColinWright • 13 years ago

118 64 13 years ago

newstatesman.com

netrus• 13 years ago

I highly recommend the following TED talk on bike physics by Arend Schwab from Delft University of Technology:

http://www.youtube.com/watch?v=2Y4mbT3ozcA

It focuses on the question why bikes do not fall, even without a rider on them (spoiler: coriolis is insignificant).

hownottowrite• 13 years ago

http://ruina.tam.cornell.edu/research/topics/bicycle_mechani...

speeder3• 13 years ago

Now I read this, and the article, and now I am confused, because I was a staunch believer it was gyroscopic effect fault, now the thing can work without a gyroscope and what the hell?

Now /I/ want to know why the bike do not fall.

dljsjr• 13 years ago

Andy Ruina is a great guy to listen to about stuff like this. There's an annual conference for scientists in all fields (robotics, biology, cognitive science, etc.) that study walking locomotion called Dynamic Walking; our lab hosted it in Pensacola last summer and Andy gave a "Greybeard" talk about the mechanics of sailboats and sailing down wind faster than the wind that was really entertaining.

I'm not sure where his fascination with esoteric mechanics comes from, but he has a pretty clever and engaging manner of discussing the stuff.

Edit: Didn't realize that Ruina was a co-author on the paper referenced in the article. Even better. It all goes full-circle.

barrkel• 13 years ago

Well, we know enough to know how to increase stability - primarily by increasing trail and rake. Decreasing rake angle is the primary way of making a motorcycle turn faster, with the known risk that it makes it more likely to head shake or even tank-slap.

It's fairly clear that common two-wheelers are stabilized by the rake and trail inducing a counter-steering effect when pushed horizontally - try to shove over a bicycle and the front of the front wheel will turn in the direction of the push because the point of contact of the tyre with the ground is behind its axis of rotation (the amount of which is the trail). And if the bicycle is moving forwards, this turn of the wheel will cause a torquing effect to roll the bike in the opposite direction of the shove. So long as the correction is somewhat less than the overall shove, the system should be self-damping.

But if the dynamics were fully understood, we'd be less likely to get motorcycles with issues like the well-known Pan Weave[1] and other high speed stability issues. At the limit, things like aerodynamics, chassis rigidity etc. start coming into the equation.

[1] http://en.wikipedia.org/wiki/Honda_ST1300#Pan_weave

Sharlin• 13 years ago

For those, like myself, who weren't familiar with the terms "rake" and "trail": http://www.seegercycle.com/Rake-and-Trail.aspx

deveac• 13 years ago

Thanks Sharlin, that was not only instructive on the terms, but a great read as well. Now I understand precisely why my uncle can always ride is raked Harley with no hands far longer than I can on my stock bike.

Jimmy• 13 years ago

Decreasing rake angle is the primary way of making a motorcycle turn faster, with the known risk that it makes it more likely to head shake or even tank-slap.

This must be what non-programmers feel like when they hear programmers talk about code.

peterwwillis• 13 years ago

"Here you go. All that was needed was to parse the cat root slash dev etcetera file for eth 0 and pugle the forward identity-locking rehooliginator and symlink it to the libgc perl humongisooler module after a kernel decompile and basic repatch update. Nothing to it, just RTFM and you'll figure it out!"

http://www.theregister.co.uk/2002/10/02/mandrake_9_0_vs_mand...

VBprogrammer• 13 years ago

A slightly mind blowing fact is that if you ride a motorbike at reasonable speed in order to enter a turn to the right you actually turn the handle bars to the left. This causes the motorcycle to lean-over to the right and thus entering a turn in that direction (due to the horizontal component of the weight).

Terretta• 13 years ago

Thank you. We do know how they work, and that it's nothing to do with gyroscopes and everything to do with (what parent said, but simplifying a bit) front fork geometry.

Pretty much anyone can see this for themselves. Walk a bike with your hand holding the back of the seat, not the handlebars. Steer the front wheel by leaning the bike. Lean father to correct faster. If it's a cheap bike, let go and watch this happen on its own till it wobbles too far to counteract.

This is one of those idiotic tropes like bumblebees not being able to fly.

nemetroid• 13 years ago

There's a huge difference between "know how they work" and "know what influences their dynamics".

anonymous• 13 years ago

bumblebees not being able to fly

I'm pretty sure science still cannot explain how bumblebees would be able to fly a Boeing 747

lambda5• 13 years ago

If you read the actual paper referenced, we know some of how it works, but not all.

Two of the proposed theories (that it has to do with gyroscopic effects, and that it has to do with trail), have been disproven by creating a self-stable bike with no gyroscopic effects and (slightly) negative trail. The paper introduces one additional factor, the difference in center of mass between the steering assembly and the rigid body of the bike; the steering assembly having a lower center of mass causes it to fall faster, providing the necessary corrective steering to achieve self-stability.

So, there are several factors we know about, which can increase stability. We know how to locally optimize stability for certain designs. But we don't yet have a full set of necessary and sufficient conditions for a bike to be self-stable. We haven't even proven, analytically, the intuitive notion that a bicycle must lean toward a fall, though given our intuition it is believed to be true.

Here are the two necessary conditions that the paper provides:

To hold a self-stable bicycle in a right steady turn requires a left torque on the handlebars. Equivalently, if the hands are suddenly released from holding a self-stable bicycle in a steady turn to the right, the immediate first motion of the handlebars will be a turn further to the right. This is a rigorous version of the more general, as-yet-unproved claim that a stable bicycle must turn toward a fall.
Another simple necessary condition for self-stability is that at least one factor coupling lean to steer must be present [at least one of Mδϕ, Cδϕ, or Kδϕ must be nonzero (SOM text S3)]. These coupling terms arise from combinations of trail, spin momentum, steer axis tilt, and center of mass locations and products of inertia of the front and rear assemblies.

That's what is meant when people say "we don't know how bicycles work." We do know some of how they work; we know that the designs that we create steer into a fall, and do so in such a way that damps the wobbles and eventually goes straight again. And we do know some necessary conditions for a self-stable bicycle, like a requirement that something that couples lean and steering must be present; but we don't know if the steering into the fall is absolutely necessary, or if you could build a bike that managed to achieve self-stability without it.

So, I would say that a more accurate summary is "we know how current bicycle designs work, but we don't know exactly what aspects of them are necessary, or how to completely characterize the sets of designs that work or don't work." But that's a bit more of a mouthful than "we don't know how bicycles work", so that's what gets repeated.

vortmeester• 13 years ago

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.

[1] PDF link: https://scholar.vt.edu/access/content/group/141943de-4222-46...

zeidrich• 13 years ago

We know quite a lot about how bikes behave, we've been building them for decades, centuries.

While we might not grok all of the specifics behind the physical dynamics, consider this: We also don't know the specifics of why gravity works. We know enough about it to make practical use of it, it keeps us on the ground and we fall at about 9.81m/s^2, but how we are affected by gravity over vast distances is still a mystery. Why we are attracted.

If we can get along just fine without being blown away by the fact that we don't even know how we stay on the ground, I don't think it's that amazing that we struggle with the dynamics of a particular system of locomotion. We can make it work well enough.

aaron695• 13 years ago

Except we know exactly how gravity works. If you google it all the exact equations are there.

As you say exactly why is still a unknown but then so is everything if you keep reducing the problem.

brudgers• 13 years ago

"Well, we know enough to know how to increase stability - primarily by increasing trail and rake."

..or, absent ideological zeal, the number of wheels.

mjn• 13 years ago

If you want stability above all else, sure; tricycles exist and are more stable than bicycles, and you can go up to 4 wheels, too. They have other downsides, however. More friction, more width needed to both travel and park, and actually worse stability when cornering at speed (they can't lean, so are at risk of flipping in a turn).

Cargo bikes often have more than two wheels, but people who don't need to carry significant weight in a basket tend to prefer bicycles as the more practical option. Parents in Copenhagen do often choose the tricycle-with-basket at a moderate speed, as a safer option than putting a kid on the back of a bicycle. But that only works when you have wide bicycle lanes.

PhasmaFelis2• 13 years ago

Ideological what, now? Bicycles, you may have noticed, are muscle-powered. adding a few dozen pounds' worth of wheels and frame is going to seriously impact acceleration, hill-climbing, and rider endurance, and the increased width will have major effects on hazard avoidance, navigation in narrow spaces, turning radius, and general maneuverability.

I've ridden adult-sized trikes, and they are fun and relaxing, but they're much less suitable for most serious transport/travel applications.

SeanLuke• 13 years ago

primarily by increasing trail and rake

This is oddly redundant, kind of like saying "primarily by using computers and laptops".

Trail is the tendency of the front wheel of a bicycle or motorcycle to act like a caster. If you change the bike geometry so as to increase trail, the bike will increase its resistance to being turned (steered). This makes the bike easier to handle, particularly at high speeds, and makes it much easier to ride no-hands. But it's not all good: it also makes it very hard to control the bike when the wheel has a lot of load on it (like front panniers) and makes the bike less manipulable.

You can change the trail by changing the angle of the steering column (steeper angle, lower trail), changing the size of the wheels (smaller wheels, lower trail), or adding "rake", which is the forward swoop that many bikes have in their front fork (more rake, lower trail). Though some think rake is to provide a bit of bounce or suspension, it's really not. It's a device used to add trail.

whiddershins• 13 years ago

Nor do we know how airplanes work. Or whips.

speeder3• 13 years ago

What parts we don't know about these?

rayiner• 13 years ago

I don't know about "don't understand" but did you know that there is a lot more to why a wing creates lift than the "pressure differential" explanation commonly put forward: http://en.wikipedia.org/wiki/Lift_(force). In particular, the typical assertion that the air at the top has to speed up so it arrives at the trailing edge at the same time as the air from the bottom is false.

sp332• 13 years ago

Well, there's a common incorrect explanation: Those who believe the "popular" explanation are wrongly insisting that any parcels of air divided by the wing's leading edge must meet again at the trailing edge. This is incorrect. It doesn't occur: experiments easily show that the air above a wing far outraces the air below, and parcels never meet again. But the real answer is known: http://amasci.com/wing/airfoil.html

psidebot• 13 years ago

The operation of airfoils is non-intuitive and many (event most pilots) hold misconceptions about how they work. That does not mean that the physics is not well understood by those who design them.

There maybe a few gaps (particularly for supersonic flight) still being explored, but in general it's inaccurate to claim that we don't know how airplanes work.

schoper• 13 years ago

We do know, here is the paper that the journalist refers to:

http://www.sciencemag.org/content/332/6027/339

And here is a great video by the author:

http://www.youtube.com/watch?v=YdtE3aIUhbU

Short answer: "a bicycle should turn into a fall."

tudorconstantin• 13 years ago

"Check mate, scientists" :))

JoeAltmaier• 13 years ago

Urban legend. Bicycle physics is well-understood. My old job, we even made a fuzzy logic model of motorcycles/riders for Harley Davidson. No mystery here, move on.

larrydag• 13 years ago

We studied the physics of bicycles (angular momentum if I recall) in engineering school.

crusso• 13 years ago

Isn't it amazing how confident people are that their own ignorance is everyone's ignorance?

It's so much worse when the ignorance belongs to a "journalist" who really just needed to do a little research before promoting such a poor theme.

Yes, we know how bicycles works. No, it wouldn't be that big of a deal if nobody was working on solving the mysteries of the bicycle in favor of really hard problems like "dark matter".

asynchronous13• 13 years ago

The gyroscopic theory has been proven wrong, yet many people continue to believe it. But just because many people are wrong, does not mean that no one knows.

A bicycle in motion adjusts its center of gravity to remain upright. It's very similar to the inverted pendulum problem.

Look at a bicycle directly from behind with the wheels exactly lined up. Now imagine that you could frictionlessly slide the two tire patches left and right. The similarities to the inverted pendulum become more clear.

Of course, it is more complicated than the classic inverted pendulum. Instead of one point of contact under the mass, there are two. And the two points of contact (i.e. the wheels) have their own complex dynamics.

Having a rake angle on the front wheel makes a bicycle self correcting (if the c.g. is on the right side of where the wheels contact the ground, then a right turn is induced in the front wheel by the rank angle)

There are two major forces that must be in balance to turn a bicycle - the side force from being off center with respect to c.g., and the centripital force in the turn.

Ever watch a cyclist train on rollers? That's much closer to an inverted pendulum. And since there is no forward momentum, there is no centripital force, which makes it more difficult to remain upright on rollers than on pavement.

raverbashing• 13 years ago

I'm not seeing the relationship between a (running) bike and an inverted pendulum. (or better, I don't see how it explains, since an inverted pendulum is unstable)

hillbillyjack• 13 years ago

The bicycle is an inverted pendulum (and is unstable when still) but similar to an inverted pendulum when it is stabilized through rocking back and forth the bicycle achieves this stability in motion.

U2EF1• 13 years ago

Even better, you can demonstrate the effect pretty easily. Jog alongside a bike and let it go. If it's fast enough, it will stay upright. You can even try to kick it over and the front wheel will turn towards the direction the bike is falling, "catching" the bike and converting the fall into a turn.

Retric• 13 years ago

The gyroscopic theory is incomplete but at 25+ MPH there are significant gyroscopic forces.

Also, while a riderless bike is fairly stable, a rider overpowers the autocrorrective nature when they hold the handlebars. So, really bikes are stable in large part because the rider balances for the bike as seen by your ability to keep a bike upright without moving and it becomes far easer to do so at as you speed up.

lambda5• 13 years ago

Having a rake angle on the front wheel makes a bicycle self correcting (if the c.g. is on the right side of where the wheels contact the ground, then a right turn is induced in the front wheel by the rank angle)

No. If you actually read the paper linked (and the Supplementary Online Materials, which contains a lot of the actual information), it is shown that rake angle is not necessary for the bike to be self correcting. You can build a bike with a negative or zero rake angle that is still self-stable (at least, according to the bicycle dynamics modelling software they were using, JBike6; they didn't actually build this particular bike, but did build one that had small negative trail and no gyroscopic effects that was still stable).

As they demonstrate in the paper, none of rake angle, trail, or gyroscopic forces are either necessary or sufficient for self-stability. All of them can influence stability, so saying the gyroscopic theory has been proven wrong is not entirely right either; it is a part of the dynamics that adds stability, it is simply not necessary or sufficient on its own. In fact, the paper shows that on the "benchmark bicycle", removing the gyroscopic force makes it unstable, so on that particular design, the gyroscopic force is necessary for its stability (thus explaining why it was believed for so long that gyroscopic force is what provided stability).

What we know is that gyroscopic forces, trail, rake angle, and distribution of the center of mass of the fork and body of the bicycle all influence stability (in particular, the center of mass of the body of the bike being substantially higher than that of the fork); none of them alone are sufficient to provide stability, and likewise none of them alone are necessary as we can build bikes without them that are still self-stable.

emmelaich• 13 years ago

  > > Having a rake angle on the front wheel makes a bicycle self correcting ..
  > No. If you actually read the paper linked ...

Please don't start replies with a "no", especially when you don't disagree! You reply that it "is not necessary" which does not negate your interlocutor's point!

(probably going to regret going meta, but the initial 'no' in forums and irc really bothers me)

aray• 13 years ago

[1] Explanation of the research in question behind this article, as well as a demonstration of the bike they based the research on.

They very clearly describe (at the end of the video) that even with gyroscopic and tracking forces removed, the two most important factors are where the center of gravity is, and the tendency to turn into a fall (arguably just the second, but it is caused by the location of the center of gravity with regards to the pivot).

[1] http://www.youtube.com/watch?v=YdtE3aIUhbU

Edit: more info on the TMS (two-mass skate) bike at its wikipedia page: http://en.wikipedia.org/wiki/Two-mass-skate_bicycle

mhb• 13 years ago

How does he not even have a picture of the "Anaconda"? OK, here's a paper about it:

http://bicycle.tudelft.nl/bmd2010/CDProceedingsBMD2010/paper...

jccalhoun• 13 years ago

magnets

jokoon• 13 years ago

isn't it because the front wheel change the contact point with the ground when you turn ?

when you both turn and inclinate the bike, the contact point is more on the side which you turn, balancing the bike.

PhasmaFelis2• 13 years ago

Facepalm.

I look forward to New Statesman's thrilling coverage of how bumblebees can't actually fly, and nobody knows why duck quacks don't echo.

arh68• 13 years ago

If Feynman were still around, he'd set things straight. I don't think a bicycle staying upright is much different from a train keeping course [1], but I can't describe why. Instead of the rail veering off and the wheel adapting, it's like the bike does what it wants and the ground shifts beneath it.

If you've read about Einstein's pail-of-milk-on-a-lazy-susan, it's a similarly unintuitive frame of reference. Also, if the experts haven't figured it out: I don't know what I'm talking about.

I find it fascinating that we as a species can observe, define, and exploit Maxwell's law, neutrino physics, etc, but we can't clearly explain bicycles. Or why wings provide lift.

EDIT: forgot the Feynman video

[1] http://www.youtube.com/watch?v=y7h4OtFDnYE

mkoryak• 13 years ago

I had an idea to build a tall bike which contained a large and heavy wheel in the center of the frame, above the 2 wheels but below the rider. This wheel would be made to spin somehow and act as a gyroscope to help the bike stay upright when stopped.

This seems to suggest that my plan would never work. If gyroscopes don't stabilize, then why are they used on monorail trains etc?

joshrice• 13 years ago

If gyroscopes don't stabilize, then why are they used on monorail trains etc?

They do, it's just the bicycle's wheels aren't heavy enough to provide enough force to really make that much of a difference.

e3pi• 13 years ago

...and, like a horse, I still can't get on these newfangled things from the starboard side.

We still don't really know how bicycles work

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