How does a bike steer?
 

Hi All!
I'm new here, but here with a question.

I'm 62 with some time on my hands. I rode bikes 'till I got out of school, then Harley's, now I'm thinking about building a recumbent something. ??

As I was studying my project and thinking about the steering, something came to my mind. When I was riding motorcycles, I found that steering works the opposite of what most think. In other words, if you wanted to turn to the right on a cycle, you would "push" on the right bar or "pull" on the left bar. Most people will tell me I'm nuts, but most have never tried it. (I never did on a bicycle, but I would expect it to be the same as a cycle. ???) This isn't something that most think about, as it kinda happens naturally. Did you ever get dangerously close to the burm of the road with a two wheeler? Probably, you ran off the edge, as you tried to steer away from the edge. Pulling on the bar closest to the edge is really "unnatural"!

Tomorrow, I'll try it, but was curious if anybody "knows for sure" and also knows why this happens.

Regards, CATZ

I use that technique to play. You don't need to push though. Just pull back on the opposite side you wish to turn. The steering will correct itself quickly and you will make a very sharp turn.

Just like your motorcycle, you can either steer by leaning, or by countersteering, or a little of both. There are schools of thought on this. For most folks, it never matters. For hardcore racers, either on the street or on the dirt, going around corners fast can be a race-winner.
Counter-steering keeps the bike a little more "under" you, and may be safer under certain low-traction conditions.
Out on the dirt, this may keep you from "high siding" in a hard turn, as the bike will slide out rather than tossing you into the puckerbushes.

This is my favorite question.

The key to a bike steering is the "trail" of the steering, the fact that the contact point of the front tire is not directly in line with the axis of the handlebars. When you move the handlebars to the right, the contact point moves to the right. Moving the contact point to the right without moving the center of gravity causes you to lean left -- just like if you moved your feet right without moving your hips you would fall left. On a moving vehicle, falling to the side causes you to turn in that direction.

This is also how you can use steering to balance. If you start to fall one way, you steer in the direction of the fall. This moves your contact point under your center of gravity, and you stop falling. This effect is really noticeable at low speeds, such as going up a steep hill.

Typically a cyclist uses lean to turn and uses the handlebars to fine tune his position.

A bike without trail would be impossible to balance or steer.

Well, that seems to make sense.

Now, let's muddy the waters a bit more. What happens on a trike? I believe on a rigid trike, you would have to "steer" it .. in the direction of the turn. (much like a childs tricycle)

Now, if we were to allow the front wheel and rider to lean, independent of the rear wheels (articulate, I think), would the steering be like a conventional bike or a tricycle?

You have got it right in the second paragraph. The "trail" is the key to bike stability and not to steering. When you turn the bars to the right the contact patch will move to the right regardless of how much trail there is, because the wheel will roll in that direction. Since the contact patch is below the c of g this will cause the bike to lean to the left. A leaning wheel will turn in the direction of the lean. In a turn the trail will exert a force tending to centralize the bars, which gives the stability.

To recap the above, countersteering and leaning are essentially the same thing. The people that think they're leaning to turn are using countersteering to get there, they just don't know it. Search the web for countersteering and you'll find tons of stuff on how this works via the contact patch, gyroscopic precession & more. If you're into physics its pretty interesting.

You got it. If the trike is rigid, it'll steer like a car - i.e. you point the steering tire(s) in the direction you want to go. This is typically a pretty unstable setup. If you can lean the turning wheel(s) you'll get better performance, particularly if you can lean the payload too (i.e. rider) - its helpful to move the c.g. towards the inside of the turn to reduce the risk of tipping. Same reason you lean on a two-wheeler - beyond a certain speed, if you don't move your c.g., you're not going to stay on the bike.

Thanks for starting a fascinating thread, Catz. Countersteering and contact patchs! And I thought steering a bike was something I learned 40 years ago.

Now for a new term, the Sticky Edge - the attraction I've felt for the edge of the road when I've come close to going over. I'm there thinking: I'm going to go, one wrong move and I'm going to go. I remember that I didn't want to steer away too quickly because I could 'feel' that if I tried that then the bike would definitely go into the dirt. It just felt unsafe to try that. And I didn't dare steer towards the dirt, because then (I thought) that's where I'll end up if I do. That was my thinking anyway, not knowing anything about countersteering.

So, to get away from the edge I would try to steer perfectly straight while leaning away from the dirt. And it really was confusing, because to keep the bike upright I'd have to steer slightly towards the dirt. And I didn't want to do that but I had to. So I'd be trying to lean one way, steering the other and wondering why I was going in a straight line and not able to get away from the edge. I can remember the 'sticky' feeling (sometimes it lasted for several seconds): how come I'm staying so close to the edge and not going one way or the other? Until a small perturbation decided for me and I'd be clear of the edge, either in the gravel or on the road. Phew! Anything but that sticky edge.

I'm off to check out the physics of the thing.

Here is a fun experiment you can all try!

Take a bike wheel, and slip something through or onto the axle so you can get a grip on both sides of it. You may be able to get enouch purchase by holding onto the quick release skewer.

Hold the wheel in your hands, as though your arms are the fork. Get the wheel spinning forward as fast as you can (don't get your hands caught in the spokes, or tag your face with the tire!)

Now that your arms are extended, and you have a wheel turning forward in your grasp, try countersteering, by holding on tight (again, like a fork), and then pushing forward with one hand. You should feel the wheel dive to the side, in the same direction you pushed. A push with the left hand will yield a lean to the left, and a push to the right will yield a lean to the right. Thats called gyroscopic precession! If the wheel is moving fast enough, you'll actually have to fight back pretty hard to keep it from slamming into your arm! There is a lot of leaning force available there.

There are more aspects of turning than just precession, including trail, and other geometric concepts, but the idea of a countersteer is to induce a controlled lean by forcing the gyroscope of your wheel to precess in the direction of the push. This lean then works with the trail of the bike to cause you to turn.

The great thing about countersteer is that because you are forcing the gyroscope to lean on its own instead of having to lean it by brute force, it lets you manipulate a gyroscope much more powerful than yourself. Thats why 96 pound people can race 540 pound motorcycles. They don't have the weight to lean the bike at 145mph, but they can countersteer, and let the bike lean itself!

Steering of a two wheeled device is facinating physics, that actually took a lot of development. If you want to ride a hi-wheeler 'pennyfarthing' bike, you actually have to unlearn most of your bike instincts, since it uses very different geometery and steering concepts.

peace,
sam

Even among experts the effect of gyroscopic precession on bicycles is a controversial topic. At typical bike speeds, and with the weight of typical bike wheels, the effect is nominal.

The pioneering research in the field was done by David Jones, who published an article called " The Stability of the Bicycle" in Physics Today in 1970.
(See http://ist-socrates.berkeley.edu/~fa...onesBikeBW.pdf)

He did a series of experiments where he attempted to create unridable bicycles by cancelling out the forces believed to contribute to stability. In one of his experiments he cancelled out the gyroscopic forces by mounting a wheel of equivalent weight on the front wheel, rigged so it would turn opposite the front wheel. He found this modification had no impact on stability, and he was able to ride this bike no-hands.

What's interesting is that at higher speeds, gyroscopic forces become significant. I've definitely noticed that going down big hills the steering becomes a lot heavier, and gyroscopic forces are a significant contribution to the stability of motorcycles. What's even more interesting is the way that these forces work together so that a bike handles pretty much the same way at 40 mph as it does at 10 mph. One of the things that Jones points out is that a bicycle has to have a very specific geometry in order to balance, and that every bike built in the past 100+ years has had steering geometry within a very narrow range of angles.

Well, I agree that the geometry is more important in general to stability, because of the self stabilizing nature of a wheel with front trail, where it's lowest center of gravity is found at the straight fork position. It is indeed a myth that gyroscopes stabilize bikes (gyroscopes don't stabilize Segway's, either, at least not directly). However, the current discussion is in relation to countersteer, which does have a tie to precession, even on a bicycle.

The two models for countersteering (not general stability) are outtracking, which suggests that as you press the left-hand bar, the contact patch moves to the left, which now places your CG to the right of the contract line, hence, you begin to fall to the right, initiating a lean. But as that happens, traditional stability geometry kicks in, the wheel attempts to return to its position under the CG, and you turn right. Centripetal forces keep you generally upright during this whole procedure.

The other model is the precession model, which suggests that as you press on the left-hand bar, gyroscopic precession changes the direction of the force, IE, it translates a horizontal rotation into a tilt, which inititates a lean. As the wheel attempts to move back under the CG, the entire machine turns to the right.

Both explanations rely heavily on the geometry of the bicycle, which is very important. I believe that, having ridden bicycles and motorcycles of all sizes and shapes, that it is really the harmonious interaction of both outtracking and precession that result in countersteer. If you get a wheel spinning in your hand and countersteer with it, you really must fight the thing, precession is very efficient way of translating the direction of force, and countersteering a quickly moving vehicle requires a fair quantity of force, more than enough to 'push' the bike into a lean. Imagine someone in a car moving alongside you, and pushing your top tube to the right, this would initiate a turn. The amount of force they'd have to exert on your top tube would be very similar to the amount of force you'd need to exert on your handle bar to inititate a lean through precession.

Interestingly, in the paper you cite, the concept of the chopper is not well discussed. I've seen choppers with many many feet of trail, that remain rideable, though their handling is indeed quite questionable. He accidentally creates a chopper on his own, though he clearly didn't get the cultural implications. ;)

As an experiment, I'd like to build a bike with no trail, and play around with countersteer. I've ridden a few tall bikes with incredibly little trail, and they do indeed handle in a very squirrely manner, but it is still possible to countersteer them. Maybe that should be one of my next mutants... The quest for the unridable bike often creates some of the most amazing mutant bikes, like Chunk 666's springy bike, which is a bike completely cut in two at the top tube and down tube, and reattached with about 6 inches of heavy steel spring, so they are only loosely coupled to each other. Apparently it is ridable, just incredibly difficult. I'm amazed at what the human body can convince to stay stable. :)

peace,
sam