Helmet Head

Are these fair estimates?

m = 200 lbs (bike + rider) = 100 kg
v = 6 mph = 10 kph = 2.7m/sec
r = 10 feet = 3m

F = 100kg ((2.7m/sec) ^ 2) / 3m

= 100kg (7.29 m^2/sec^2 / 3m
= ( 729 kg m^2/sec^2 ) / 3m
= 243 kg m/sec^2
= 243 N

So, if I got that right, you were up against about 200 Newtons, give or take.

MrCjolsen

I just will my bike to turn. And it does.

pj7

How's this?
When I was a kid I had a kite. One day I remember flying it so high that I could not see it anymore. My brother told me it must be 5 miles up in the air and we had better get it down before an airpane hit it.
Now, if I were to fly a kite today, I'd know it was not 5 miles high because I would not now, nor could I then, afford 30,000 feet of cord to fly it that high.
Still, I have fond memories of flying my little "hang glider man" kite five miles high.


I can't believe there is actually a discussion on how a bicycle turns. And I find it harder to believe that there is an argument over it wherein no one has taken the time to actually search the subject out.

LittleBigMan

I wanna see that guy hit some wet leaves...

Helmet Head

I'm pretty sure that's not true at all.

Right, which is why you (subtly) turn into the fall, over and over, continually, keeping the bike balanced. If you have a someone on the back of a tandem shift around, you don't shift your own weight to compensate, you steer to compensate.

What's really interesting are those side by side tandems where you sit on either side of a frame. The two riders might differ in weight substantially, but you still balance the bike easily, by steering.



I'm not following.


So do you think you counter-steer right first, then turn further left into the turn, or do you steer left into the turn first, and then counter-steer right "to balance the 'fall' of the rider above the bike"?

chipcom 

Let's throw a little more fuel on this thread. Turning while riding with no hands on the bars...what's happening here, weight-shift, leaning, turning the bars in the direction of the turn, counter-steering, all of the above, none of the above?

Brian Ratliff

Why would lean alone turn a bike? The wheels are still in line with each other, so they still track straight; to make a turn, one wheel will have to slip along the ground.

What are the forces on the bike? How does one initiate a turn and how does countersteering come into play.

To turn a bike, there are some forces involved. There is a normal force of the ground acting on the wheels. Presumably, the center of mass is offset from the wheels, providing a moment around the wheel's contact point to counteract the centrifugal force acting toward the outside of the turn at the center of mass. So, this means that to turn, the center of mass must be offset from the wheel's contact patch; whether this is done by leaning the bike is up to the rider. Because none of the wheels are slipping along the ground, the wheel has to be turned as well. If the wheel doesn't turn, then the bike just falls over due to the center of mass being offset from the wheel's contact patch.

So, we've established that the center of mass of the bike and rider system must be offset from the wheel's contact patch, and that the wheel needs to be turned. Now, to answer the question of countersteer, we need to investigate how we get the center of mass off from directly over the top of the wheel's contact patch.

First take the simple case of a bike which is arranged so the rider's back is completely vertical. I'll come back to the reasoning in a moment. It does no good to simply shift the rider's weight to initiate the lean, because all the rider has to push against is the bike, and the bike being an inverted pendulum means that just shifting your body weight while keeping the wheel straight just moves the bike and the body in opposite directions and the center of mass stays centered. Note that this is why a bike with a locked headset will remain straight, despite the best efforts of the rider. That it eventually falls over is just a tribute to the bike being an inverted pendulum; like balancing a ruler vertical in the air on your nose. Unless you are allowed to make adjustments with the headset/wheel, the bike will eventually fall over.

So, with a vertically oriented rider, the only way to cause the center of mass of the system to move off top dead center is to steer the bike out from under the rider, using inertial force to change the center of mass of the system. Hence, countersteering is necessary and the only way to make the bike lean, and so, turn.

Now, on a road bike, saddles have horns, and bicyclists lean forward. This adds up to a second way of moving the center of mass out from over the wheels. The cyclist can "scissor" his body in the horizontal plane using the saddle horn and the two wheels as a force to push against. This is what happens when you steer the bike using a twist of your hips. This does not require a counter steering action because it is unnecessary to steer the bike out from under the rider to initiate the offset in center of mass.

Now, low speed. Low speed handling can be done by mostly just steering the wheel. However, a very slight offset in the center of mass is necessary to balance the bike, but only a very slight offset because the centrifugal force is slight. Again, the same two methods of initiating the offset in center of mass are used. Most cyclists though, during slow speed handling, tend to do mostly the scissor method of torquing the body against the saddle horn.

In real life, it is a combination of these two methods that cyclists use to intiate a turn. A cyclist on an upright bike will have to countersteer more, and a cyclist on a road bike will make more use of his hips. Recumbents pretty much only steer using countersteering as there is no way to torque the body in the horizontal plane to offset the center of mass.

Now, for the disclaimer. This is an issue of fact. If anyone is curious, the answer can be solved using math - look it up in an engineering Dynamics textbook. The dynamics of a bicycle are actually pretty complex and are neither simple or obvious. To solve the system correctly and without massive simplifications (for instance, gyroscopic forces have a part to play in this that I conveniently left out, the rider has been reduced to a horizontal or vertical stick, and moments applied to the bicycle through the offset pedal have all been ignored), this is one of those problems which will have to be numerically modeled with some complexity.

Helmet Head

To go straight, I think it's weight-shift: if you sense you're starting to fall left, then shift right, causeing the front wheel to turn left, into the fall.

To turn right, I think it's -- in addition to the above -- shift weight right to cause counter-steer to the left, followed by almost immediate weight shift left causing steer to the (desired) right.

Bekologist

I've seen three and four year olds trying body english to turn bikes at the bike shop, as well as turning the handlebars. its pretty intuitive.

was this thread to prove Mr. heads' argumentative, I mean, sophistic, skills to the crowd once again?

I only actively "countersteer" while doing skid turns on snow or ice, and doubt mr. head's got that turn nailed. can you turn a bike totally using countersteer, mr. head?

It's easy with a LOT of practice, just like motorcycles at the icetrack. well, pretty close anyway (no power to the rear wheel while skid turning a bike) - but this isn't the motorcycle forums.

Eatadonut

Why have you completely ignored posts that have answered your question fully and completely, complete even with a deeper reasoning of why the answer is the way it is?

As a corollary, why do you continue to tell people they're right when they agree with you, when you start sentences with "I think". There are people here who know more about 2-wheeled machines than you ever will. I'll defer to them, and I'd suggest anyone reading this thread does the same.

Bekologist

turning a bike no-handed... I do that a LOT. every day on my commute, just to wow the crowd. Coming up from a highway underpass If I've got the light, swinging it round about 20 miles an hour, right out of the traffic lanes....

its body english, but the front wheel will DEFINETLY be moving in the direction you're turning.....

Helmet Head

Good luck getting Pete to agree.

By the way, I've almost entirely given up on the lean alone can turn a bike theory, your post is just another nail in that coffin. I need to do my own test. See if you can explain what pj7 reports about his unicycle with a trainer.

Brian: "Low speed handling can be done by mostly just steering the wheel. However, a very slight offset in the center of mass is necessary to balance the bike, but only a very slight offset because the centrifugal force is slight. "

Right, and it's probably not noticeable relative to the amount of steering already required just to keep the bike balanced to keep going straight. So the counter-steer to initiate the turn seems like noise.

Brian Ratliff

BTW, some side issues.

Turning without hands. The trail on the fork allows the wheel to steer into the lean. No trail, no steering; a bicycle without trail is nearly impossible to control. A big difference here is that, whereas before, it is only the center of mass which concerns the turn, now lean of the bicycle frame plays a part. Lean the frame, and the wheel will turn into the lean. BUT, to continue with the turn, the center of mass has to be displaced as before, so, the bike after initially leaning one way and turning that direction, will now proceed to reverse the lean due to countersteer and the resulting displacement of the center of mass of cyclist and rider and hence, reverse the turn, which will now be steady state. I don't think you can use the scissor method to turn a bike no handed unless one is very careful to gradually lean the bike frame into the turn as the center of mass is displaced; the wheel doesn't turn (no handed, remember) without leaning the frame.

The unicycle turns when it is leaned because you are not only leaning the machine. You are also using your hips to torque the wheel around it's single contact point. For those of you still convinced that lean is the only thing necessary, i.e. sufficient, to turn a bike, take your bike out to a sloped surface and ride a straight line on a countour of the slope (across the face of the slope). This is technically leaning the bike (with relation to the ground that is), but the line is straight and the wheels aren't slipping, which means that lean by itself is insufficient to turn a bicycle. The bike turns when it is leaned simply because the trail on the front wheel forces the wheel itself to turn into the lean. This is the only reason why bicycles are controllable in the first place; for every little lean of the frame initiated by a shift of the rider's weight or the road contour, the bike attenuates this lean by steering back under the rider.

Brian Ratliff

Remember though, one can torque in the horizontal plane to initiate a lean as well, so there is not necessarily any countersteering done at low speed. In fact, because countersteering is a gross effect and twisting the hips is more subtle and, thus, more controllable, most low speed turning is probably accomplished using hip twist, i.e. the scissor method of offsetting the center of mass from over the wheel's contact patch.

pj7

Yeah, you're probably right.
I forgot what forum I was in for a second.

Helmet Head

I did not intentionally ignore any posts. Any one(s) you have in mind?

I try to say "right" only to stuff which I have a lot of confidence in.

How do you know that?

I'm just the moderator of the discussion, getting it started and hopefully keeping it going.

But, in the end, I hope to be able to have a way to describe all this that everyone agrees with.

Helmet Head

Can you describe this in more detail? I don't get it.

chipcom 

The correct answer to this...and every other turning scenario is ALL OF THE ABOVE. Every manuever on a bike is a combination of a weight shifts, leans (which could be considered the same thing) steers and counter steers. There is not just one 'technique' it's a combination of many interactions between your body, the bike, the terrain and physics. Watch some track cycling, or better yet, do some trackstanding, and you'll see...no rather you'll feel what I mean.

chipcom 

We should really confuse the issue and throw steering while cycling backwards (maybe even with no hands) into the mix.

John Forester

Oh, my, what a naive discussion in so many ways. People talking about shifting body weight in a lateral direction. I presume this means moving the center of mass laterally. OK, how do you develop a force that will do that? When moving fast enough, one might be able to do so by rotating one's shoulders to develop a small lateral aerodynamic force, but at normal cycling speeds trying to push on the air is fruitless.

pj7

WOW, all I can say is wow.
I just got off my lunch break, which I totally blew by performing a few tests.
FIRST, to the person or persons that I was saying "you are wrong" to in not so many words, I'm sorry. And here is why.
I took some spotting fluid and made a puddle on the floor. Then I took the unicycle with the trainer wheel and rode thru it. After a few passes I took a close look at the tracks. What I saw was vry interesting. Leaning the unicycle was not causing the turn but rather aiding in me keeping my balance. There were marks on the floor that showed me that the big wheel actually twitched (turned) and that is what caused me to turn. At that moment the training wheel actually lifted off the ground. So what was causing me to turn was a twitch in the wheel, not the lean.
I then went outside and hopped on my bike. I had a few of the guys I work with help me on this one. We used a tube clamp to lock my steereer in place. I would ride and lean... would I turn? No. I feel flat on my ass the first time. After much cursing by me and laughing by my friends we tried it again. Still, I did not turn. They even held me up to help me keep my balance and still I could not turn. We took the clamp off and tada! I was able to turn. The leaning aided in me keeping my balance, the more I leaned, the sharper and faster I could turn.
We then brought the bike inside to use the spotting fluid. There were tale tale marks in my tracks that showed me that as I was turning, my wheel was changing direction. I did not know I was moving my handlebars but it is obvious that I was.
I'm not going to go into much more detail in this post because I have a mess to clean up now but will answer any questions that you all have to ask of me tonight.

Funny how this has become such an interesting subject.

So to sum it up. Leaning alone did nothing in "causing" me to turn. It only helped "after" I turned my handlebars.

Eatadonut

If we bring trials, mtb, or anything that involves skidding tires into this conversation, it'll go nowhere.

...nevermind.

Helmet Head

First, we were talking about very slow speeds.

If it's not possible to steer a bike by shifting body weight, then how does one control and turn a bike when riding with no hands?

Brian Ratliff

Forgive the sketch and handwriting quality...

pj7

The test I performed just 30 minutes ago I did this.
When I ride with no hands and I lean, it causes the steerer to turn my handlebars, causing my wheel to turn in the direction I would lean.