How We Ride

One Man's Take on Stabilizing the Unstable

Part Five: How We Ride

To return to what we started on page one, there are two ways to balance a bike on the roll axis. Method one is moving the support points. This is done in motion by turning, turn left and the support points move left. Simple enough. Method two is shifting the center of mass. This method is all about bicycle geometry, which you must know a lot about by now if you paid attention.

Most people know it's easier to balance your bike when moving rather than at a stop, and the faster the better. Well, up to a point, hence the term "breakneck speed." It's forward momentum and some bike geometry doing the trick. That and steering, which is useful to direct the bike where you're headed as well as not falling before you get there.

All the same, we don't actually rotate our handlebars that much normally. Bikes have a short wheelbase and are pretty nimble. Cruising around your average street corner takes a 10° or less rotation. Cutting the corners on the sidewalk is about an 18 foot diameter turn. A 30° steering rotation is about an 11 foot diameter turn. In normal riding we rarely turn the wheel more than 25° at slow speed. And we rotate the handlebars less and take wider turns the faster we go.

Going straight

Standing still you have no forward momentum to help you counter falling from gravity. You could put a foot on the ground, but that's cheating and dangerous when moving. On the move every time the bike turns forward momentum creates a centrifugal effect pushing toward the outside of the turn. At slow speed you start to get a little centrifugal effect when you turn. You get more the faster you go and the tighter you turn.

If you lean and turn the front wheel, geometric yaw means you roll and turn the same way. If you don't lean, you counter-steer and balance by rolling the bike with centrifugal effect. When riding straight the front end moves back and forth a couple degrees or less as you travel along keeping your balance.

In the end, you balance while riding straight by not going straight. Instead you balance by steering and turning a tiny bit one way or the other, which is straight enough because most roads and paths are wider than a bike's wheels. That's why if the front wheel gets in a rut, like trolley tracks, you fall. (I speak from experience there.)

Whether we counter-steer or use geometric yaw more to go straight, I couldn't say. Both methods are quite subtle upright at small steering rotations. Both must apply, likely one more than the other. Could be GY as a counter to centrifugal effect helps smooth out the counter-steering while going straight.

Turning

When you begin a turn you start at a zero degree rotation and go to, say 20 for example. This doesn't happen instantaneously, it takes some time, though not much. I haven't timed it, but it seems less than a second to turn the handlebars into a turn. During that the bike is tracking so the total turn would be a parabola of decreasing turn radii and increasing turn angles. It is in the early stages as you start to turn from 0° to 1° to 2° to 3°... etc. that if you lean GY dominates until the centrifugal effect increases and stabilizes the roll.

Right: Turning using geometric yaw. In this picture rider upright rides straight on purple path (A). Rider leans left tilting bike right just before turn (B). Handlebar rotation starts at red path, where turn begins. As rider rotates front end geometric yaw and gravity roll the bike into turn (C). At point C the steering rotation is half done at 10 degrees. Steering rotation of the full 20° ends and is held along green path. Bike continues at this radius with a 20° rotation (D).

Though I show it longer than it likely would be, the red path is the parabolic section where the steering rotation and geometric yaw happens. Or to return to the hinge analogy where the hinge is unfolding inward so gravity rolls the bike. Along the red path centrifugal effect is steadily increasing while geometric yaw starts strongest and increases at a diminishing rate. The imbalance also decreases as the bike rolls inward. At D gravity and centrifugal effect are in balance and so is rider going through turn.

You wind up with two opposing forces which would roll the bike in opposite ways. Centrifugal effect growing steadily, GY growing at a diminishing rate and eventually reversing. Add to that how you lean effects how much imbalance GY creates by pitching the front wheel. Don't forget gravity working at the same rate all the time. Playing these forces off each other makes a bike manageable on the inherently unstable roll axis.

I suggest this is how most everybody turns and balances a bike when "cruising." Try it for yourself. Get going straight at a nice comfortable speed. Then lean the way you want to turn, give your front end a small rotation. Violla! The bike will turn and roll automatically. Now lean and rotate the other way, the bike straightens out and rolls back upright.

All that said, you can also turn by counter-steering as explained near the end of Part One which is duplicated below.

Below: Counter-steering redux. On the left rider rides straight, momentum (A) is straight ahead, and gravity (B) is straight down. At start of turn in the middle, right front end rotation puts a centripetal force (C) on contact patch to the right. Combined with momentum this creates a centrifugal effect (D) to the left, which combined with gravity rolls the bike left. To continue turn front end rotates left and bike starts going left. This reverses the centrifugal effect to the right which is countered by gravity as bike is already rolled left so you maintain angle through the turn.

Being mechanically set geometric yaw would only roll you so much and so is progressively less effective the faster you go. Counter-steering gets faster results and can get you into rolls and turns GY simply can't manage. That's why racers counter-steer all the time. It is also useful in an emergency for your non-racer out on the road trying to avoid some careless driver who seems intent on using their car as a weapon against innocent bikers.

Stability, It's the Rider

As to stability at speed, it takes less turning the faster you go to create centrifugal effect to stop falling over. Less turning means a straighter path with less wobble which feels just like more stability. Pretty simple, really. Though I would say the bike isn't really more stable, it's just easier for the rider to stabilize.

When you add it all up, the real key is the rider. People have the strength and agility to ride bikes of all kinds and keep them balanced with a little practice. How to keep all the various forces of gravity, centrifugal effect, GY and the rest at the right rate is what you learn when you learn to ride a bike. That, and it's better to crash on the grass than on the pavement.

Still, a lot of folks are enamoured of the exotic sounding idea trail or wheels as gyroscopes helping you turn and maintain stability and balance. Unfortunately it doesn't work that way. Trail and the gyroscopic effect of the wheels is generally over-rated. Bicycle stability at speed comes from forward momentum, you don't need gyroscopes. Though if you haven't learned the proper techniques you may need training wheels.

Summing up at Last

There you have it. You balance a bike by steering and leaning, even though the leaning works indirectly by changing the stance of the bike. You can turn right and roll right because of the steering tube angle producing geometric yaw. You can turn left and roll right by counter-steering. It all works because we apply forces from within the system on the stable pitch and yaw axes to control the bike on the unstable roll axis.

That's about the best I can do by way of explaining it. Whether it makes sense I can only hope. No matter because you don't need to know how it works to ride a bike. It's probably better if you don't think about any of it because it'll be easier if you just do whatever you're doing now it'll all work itself out. I mean, you see people riding bikes all over the place without killing themselves. Even little kids manage it. It ain't rocket science.

This may not strike the reader as a full, scientific explanation, what with the total lack of calculus and formulas. I suppose it isn't, but I can only do so much. I'm not Sir Isaac Newton, nor have I been knighted by any queen of any sort as far as I can remember. My explanation of all things bicycle is what I'd call the reader-friendly version. What unfriendly readers might call it I probably shouldn't repeat.

Though there still are many things I haven't touched on bicycle-wise, here is where it ends. As for me, I'm moving on to other vehicular puzzles. Such as, how does Fred Flintstone turn his car when it's just a pair of oversized, stone rolling pins in a log frame with open ended axle slots? Wouldn't that thing just fall apart? Does he steer it with his feet? Curious, no?

Part One: Bike Basics
Part Two: Observations & Tests
Part Three: The Front End
Part Four: More Bike Geometry
Part Five: How We Ride

Additions to the Article

Part Six: Wheels as Gyroscopes

Part Seven: The Front End Cam Action

Part Eight: Additional Thoughts on GY

Part Nine: Interpretations of the Riding Tests

The Wheels That Don't Turn How to Turn a Bicycle (Without Gyroscopes or Cones)

Read about bikes with self-cancelling gyroscope wheels at Lose the Training Wheels