The Counterintuitive Physics of Turning a Bike (2015) [video](youtube.com)
youtube.com
The Counterintuitive Physics of Turning a Bike (2015) [video]
https://www.youtube.com/watch?v=llRkf1fnNDM
11 comments
Even cars turning, quoted here as something easy, is deceptively complicated once you really start thinking about what the rubber actually does on the road surface. In the end, both an angular momentum and a centripetal force are generated, but it's very hard to figure out why those things happen from first principles.
Can you explain or provide a link? Thinking about it all seems pretty straightforward to me so I'm curious what I'm missing, what's the complicated part.
https://en.wikipedia.org/wiki/Ackermann_steering_geometry
https://www.sae.org/news/2018/10/2018-yamaha-niken-chassis-t...
I'm just a software engineer, not a mechanical engineer, so any explanation would fall short. However, I've been surrounded by world-class Mech E's for decades, so I can remember these conversations being out of my depth!
I've also done urban commutes on bicycle and motorcycle for decades, so I can say given my experience, this is much easier to feel than reason about. You can tell by watching a person ride at licensing whether a person has integrated this feeling or is still reasoning about it. Hope this helps.
https://www.sae.org/news/2018/10/2018-yamaha-niken-chassis-t...
I'm just a software engineer, not a mechanical engineer, so any explanation would fall short. However, I've been surrounded by world-class Mech E's for decades, so I can remember these conversations being out of my depth!
I've also done urban commutes on bicycle and motorcycle for decades, so I can say given my experience, this is much easier to feel than reason about. You can tell by watching a person ride at licensing whether a person has integrated this feeling or is still reasoning about it. Hope this helps.
The key to understanding car steering is a concept called "slip angle".
That is, the amount of "misalignment" between the wheel relative to the direction of motion.
Imagine a wheel on a treadmill, with the wheel angled one degree to the right, but held in place so it's kind of slipping slightly. This would have a slip angle of 1 degree.
The fundamental thing is that for a tyre, the lateral force generated by the tire is directly related to the slip angle (also the vertical load and the camber, but these can be removed for a simple model). Slip angle is the main thing.
If you know the relationship between slip angle and lateral force, you can model the movement of a car given the angles of the 4 wheels.
That is, the amount of "misalignment" between the wheel relative to the direction of motion.
Imagine a wheel on a treadmill, with the wheel angled one degree to the right, but held in place so it's kind of slipping slightly. This would have a slip angle of 1 degree.
The fundamental thing is that for a tyre, the lateral force generated by the tire is directly related to the slip angle (also the vertical load and the camber, but these can be removed for a simple model). Slip angle is the main thing.
If you know the relationship between slip angle and lateral force, you can model the movement of a car given the angles of the 4 wheels.
Rough approximation is reasonably straightforward, but it quickly gets complicated if you want more precision. At a high level cars want to roll because momentum is resisting a force being applied by the road. But, dig deeper and things get even more complicated.
An uneven amount of weight is distributed to each tire through a turn. Tire temperature, tread, pressure, etc impacts the surface in contact with the road. Even slicks don’t have a uniform pressure across the surface in contact with the road. etc etc.
An uneven amount of weight is distributed to each tire through a turn. Tire temperature, tread, pressure, etc impacts the surface in contact with the road. Even slicks don’t have a uniform pressure across the surface in contact with the road. etc etc.
These conversations often remind me of the power of the human mind to intuit but not understand something conceptually.
Someone like myself loves knowing the details of how stuff works, and even with a head full of fundamental knowledge, someone who just does it and feels it can drive better. You find it in biking too, tonnes of deliberation goes on online about suspension and ergonomics and technique etc, then someone who doesn't think about any of that blows past you on the trail.
Someone like myself loves knowing the details of how stuff works, and even with a head full of fundamental knowledge, someone who just does it and feels it can drive better. You find it in biking too, tonnes of deliberation goes on online about suspension and ergonomics and technique etc, then someone who doesn't think about any of that blows past you on the trail.
The same with music. You can debate the finer points of music theory all day long, but some kid will just walk up, pick up an instrument and start playing. They may not be able to use the right terminology for what they do but there is a fair chance they'll play a lot better than those that do know all the right terminology. In part this is because the one is 'just' knowledge, the other is endless practice to get it right. They are two different skills and really gifted individuals are good at both.
The front and rear wheels have different angles of attack on the vector of the car, so they have slightly different friction vectors.
The result can be oversteer (when the back slide more) or understeer (back slides less).
And it can change depending on road conditions, tire inflation, velocity, acceleration/deceleration. And of course changes radically from vehicle to vehicle.
Covered in depth in the movie Cars "You need to turn left to turn right". :-)
Of course the same thing applies to two wheeled vehicles also - see trail braking.
The result can be oversteer (when the back slide more) or understeer (back slides less).
And it can change depending on road conditions, tire inflation, velocity, acceleration/deceleration. And of course changes radically from vehicle to vehicle.
Covered in depth in the movie Cars "You need to turn left to turn right". :-)
Of course the same thing applies to two wheeled vehicles also - see trail braking.
related content: Would it even be possible to turn left if steering right would be blocked?
"Most People Don't Know How Bikes Work"
https://www.youtube.com/watch?v=9cNmUNHSBac
I've watched literal hours of YouTube videos that explain the physics of steering in the context of motorcycles but none debunk all the misunderstood concepts as clearly as this one. Take my upvote.
(And every time this subject comes up somewhere I am thoroughly amused at all the people who are confidently wrong)
(And every time this subject comes up somewhere I am thoroughly amused at all the people who are confidently wrong)
I posted the OP video in a different thread, with this fun fact:
> Another cool thing is how the steering of a bicycle works. You don't really turn the way you want to, you actually turn the other way first to initiate so the bike moves from under you, and you then lean the other way to actually turn. This is why, if you're for instance biking close to a curb and want to get away from it, you really can't and it feels like the curb is "sucking" you closer and closer. Since it "feels" wrong to first turn towards it, but without doing that you actually can't get away from it.
So kinda the same: If a curb or something on your right either blocks you from turning that way, or you "block it in your head" so you refuse to turn the handlebars to the right, you wont be able to actually turn left and get away from whatever is next to you.
https://news.ycombinator.com/item?id=35007628
> Another cool thing is how the steering of a bicycle works. You don't really turn the way you want to, you actually turn the other way first to initiate so the bike moves from under you, and you then lean the other way to actually turn. This is why, if you're for instance biking close to a curb and want to get away from it, you really can't and it feels like the curb is "sucking" you closer and closer. Since it "feels" wrong to first turn towards it, but without doing that you actually can't get away from it.
So kinda the same: If a curb or something on your right either blocks you from turning that way, or you "block it in your head" so you refuse to turn the handlebars to the right, you wont be able to actually turn left and get away from whatever is next to you.
https://news.ycombinator.com/item?id=35007628
I occasionally transport long wood sticks on my bike and when I tie it to the frame so that it sticks in front (3m long, it sticks about 1m in front of the bike) no matter how tight I tie it, it always feels loose. There is this delayed movement of the stick in front that gradually catches up. It's really weird sensation, especially when turning.
Bike-based jousting? Interesting concept!
When I was in high school, I rode my bike everywhere. I was mystified by this. I eventually did figure it out by carefully watching what I was doing in a turn. Weird that my body knew it, but my brain didn't!
Except at very slow speeds (walking pace) where I think there's a different mode of action: the bike is no longer self stabilizing and is staying upright more because of your own balance skill. At this point if you turn the bike left it does go left.
...I think. I've been wrong about bike physics before, so I could be again.
...I think. I've been wrong about bike physics before, so I could be again.
Motorcyclist here. The physics are the same for bicycles and motorcycles, and yes, you have it correct.
Part of the Motorcycle Safey Foundation (MSF) curriculum since at least the 1990s: "push left go left, push right go right"
Perhaps just me, but this "push left/right" advice messed up my understanding of counter-steer when I was learning and I found myself being unable to turn effectively. I would push the handlebars down or slightly forward and be able to turn but innefectivly.
Far better in my opinion is advice to simply "turn left briefly to go right". As in actually turn the wheel left, then let the bike balance by going right. Moto control explains it effectively: https://www.youtube.com/watch?v=ZLqyN5yy6I8
Far better in my opinion is advice to simply "turn left briefly to go right". As in actually turn the wheel left, then let the bike balance by going right. Moto control explains it effectively: https://www.youtube.com/watch?v=ZLqyN5yy6I8
Depends on your velocity. Push steering works great when riding through a corner at speed. When turning from a stop you need to momentarily turn the opposite direction to initiate the lean.
Also a core part of Keith Code’s teaching since I think the 70s. Highly recommend his school for any motorcycle rider. https://superbikeschool.com/the-curriculum/
Also from Keith Code, replete with terrible acting, the best instructional video a twist of the wrist: https://www.youtube.com/watch?v=DK9AoAAy7uM
And not understanding that can have fatal consequences: https://youtu.be/VVE79XT8-Mg
What's not explained in the text on that video is that braking while leaned over causes a motorbike to steer itself upright.
You can't go around a corner on a bicycle or motorbike at more than about 5mph without counter-steering. You have to tilt the bike into the turn or it will just fall over. You tilt by briefly steering in the opposite direction, which rolls the bike in reaction. This is counter-steering.
Braking while leaned over applies a torque to the steering column towards the side which is closer to the road, i.e. steering into the corner. This torque is due to the width of the front tyre; the contact patch is at an offset to the steering axis. This torque counter-steers the bike upright.
Braking also changes the geometry on most bikes as the forks compress and the bike dives at the front, steepening the steering angle, which makes steering more responsive, so the upright counter-steer is more pronounced.
There's also target fixation. You tend to steer where you look, and if there's an enormous hazard coming at you, you risk steering right into it. Look for the escape routes.
There's also a ground rush panic effect. When something is coming at you really quickly, your instinct is to tense up and brace for impact. When landing a parachute, you're trained to look at the horizon and depend on peripheral vision to feel the ground coming up to meet you, so you flare at the right time. On a bike, it ties in with target fixation; don't look at the fast thing coming at you, look at the escape routes.
You can't go around a corner on a bicycle or motorbike at more than about 5mph without counter-steering. You have to tilt the bike into the turn or it will just fall over. You tilt by briefly steering in the opposite direction, which rolls the bike in reaction. This is counter-steering.
Braking while leaned over applies a torque to the steering column towards the side which is closer to the road, i.e. steering into the corner. This torque is due to the width of the front tyre; the contact patch is at an offset to the steering axis. This torque counter-steers the bike upright.
Braking also changes the geometry on most bikes as the forks compress and the bike dives at the front, steepening the steering angle, which makes steering more responsive, so the upright counter-steer is more pronounced.
There's also target fixation. You tend to steer where you look, and if there's an enormous hazard coming at you, you risk steering right into it. Look for the escape routes.
There's also a ground rush panic effect. When something is coming at you really quickly, your instinct is to tense up and brace for impact. When landing a parachute, you're trained to look at the horizon and depend on peripheral vision to feel the ground coming up to meet you, so you flare at the right time. On a bike, it ties in with target fixation; don't look at the fast thing coming at you, look at the escape routes.
Motorcycles and bikes are not exactly the same in this respect. You can steer a bicycle without counter steer by shifting your weight to the left and right. Its possible to go around corners without touching the handlebars like this.
The fact that you can’t really make a motorcycle move by shifting your weight is probably why it is so important to teach countersteer- so people know what do to when they need to swerve quickly. Another counterinuitive thing (for me) is that motorcycle steering gets much heavier at high speed while bicycle steering is virtually effortless at most normal bicycle speeds.
The fact that you can’t really make a motorcycle move by shifting your weight is probably why it is so important to teach countersteer- so people know what do to when they need to swerve quickly. Another counterinuitive thing (for me) is that motorcycle steering gets much heavier at high speed while bicycle steering is virtually effortless at most normal bicycle speeds.
I always thought the connection between turning a plane and a bike were somewhat interesting. With a plane, if you rudder left, it causes some right roll. And vice versa. If you roll left, you have to apply left rudder to counteract right raw.
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you can also move your body to create the angle and then slightly turn to the desired direction
Yes; on bicycle it is certainly possible to ride and maneuver at speed without touching the handlebar, weight shift only. (Or at least it was possible 25 years ago when I was more nimble :-)
There is a trick to this: grasp the seat firmly with your thighs and think about trying to turn the bike by pushing sideways on the seat with the insides of your thighs.
A motorcycle is too heavy to do this so you you counter-steer to overcome the gyroscopic stability and change the angle of the bike. Essentially you are attempting to destabilize the bike, in a controlled manner.
The same physics apply to a bicycle.
The same physics apply to a bicycle.
FWIW I think that's only mostly true. I'm told there are some situations where you really do just shove the bike over into a lean. Of course even if that's true, it's very much a case of (as I like to put it) the exception that highlights the common case. :)
It's possible on a scooter, which has a lower speed, low centre of gravity (so you have more relative leverage) and small wheels (i.e. the amount of spinning mass is less, and closer to the axis of spin). On a big bike with big wheels at high speed, you can only make very gentle adjustments in the lean angle.
As a motorcycle rider I’m not sure what those situations are. Maybe dirt bikes work that way? My street bike is very light by motorcycle standards and it still weighs 400lb wet. It’s not something you can really push around, even at low speeds. Body positioning is critical but that’s different.
I think it was one of David Hough's books, talking about riding on gravel or something relatively niche like that.
Makes sense, I don’t ride offroad.
Swerving around an obstacle in the road you might do this
Not what I had in mind to say the least. That's definitely one of the premier cases where you either countersteer (hard) or crash.
Not if you want to avoid the obstacle.
It is really counter intuitive. Next time you are riding down a hill, dont lean or anything, and slightly push the left handlebar forward. Without leaning, you will turn left.
This is also the basis of an emergency swerve on a motorbike - if you need to turn left suddenly you push the left handlebar hard.
Countersteering is the basis of all steering on a motorbike.
What happens if you have a sidecar?
Typical sidecars pivot to allow a motorcycle to lean normally, so it's still countersteering. Rigidly-attached sidecars are used for some kinds of racing; those fundamentally steer like cars, though the "passenger" typically shifts their weight quite a bit to optimize handling.
If it's attached firmly, you are now basically in a 3 wheel car and steer accordingly.
Isn't it illegal to indicate that you turn right, but then still go left first?
It's much more subtle than that. Countersteering merely nudges the bike a tiny bit, basically throwing it out of balance so it leans into the turn. The reason why it's mentioned as being counterintuitive is that it's even too slight for someone to notice if they're watching you ride. And people who have been riding for a long time may be doing it without realizing what's happening in detail.
In fact I suspect that you also countersteer to ride straight, i.e., making the slight corrections that keep you on a straight course.
In fact I suspect that you also countersteer to ride straight, i.e., making the slight corrections that keep you on a straight course.
There’s no realistic way to drive straight (unstable equilibrium), so “straight” is really waving around it constantly. People who ride on easy mode (i.e. no hard tricks or turns) just learn to wait for a correct phase to start turning without realizing it.
You don’t have to steer anywhere to start a turn, in a sense. You’re always turning and only have to stop countersteering at the right moment.
You don’t have to steer anywhere to start a turn, in a sense. You’re always turning and only have to stop countersteering at the right moment.