The Physics of Rollers
 

On a typical roller setup (a frame, 3 drums and a belt)...

1) What is the function of the belt between the front and middle drum?

2) How would removing the belt affect riding them?

3) How would removing the belt and middle drum affect riding them?

4) How would removing the middle drum but running a belt between the front and rear drums affect riding them?

5) Why do smaller diameter drums provide more resistance than larger ones?

6) Should I have posted this in the mechanics forum? :D

heh.

To balance a bike you have to be able to steer.

If the front wheel is spinning, you can steer. If it is not, you cannot steer.

There are two drums in the back to keep the bike from hopping over the front or rear of the rollers.

If you take off the belt, you won't be able to ride them.

If you have just two drums, you'll probably roll off the drums before you can pedal.

Smaller rollers deform the tire more and increase rolling resistance.

cdr

WARNING: NOT A PHYSICIST

1) keeps your front wheel turning so you can balance by turning handlebars like you do on the road.

2) you wouldn't ride them ^^^

3) not only would you not be able to balance, but you most likely couldn't keep the bike on the thing for even just long enough for a hilarious fail

4) without adjusting the wheelbase, you'd roll right off the front

5) smaller drums cause the wheel to spin more? i don't know how to say that correctly.

6) who knows. my guess is that most people in the mechanics forum are bike mechanics, not roller mechanics, but still mostly mechanics. at least on here you'll have a decent chance of getting a credible response if a physicist replies.

AGAIN, I don't claim to know anything about physics. I have taken one 100-level course. That is my only qualification. Don't listen to me.

beat me to it.

Not quite. On a bike, you don't actually balance yourself by turning the handlebars - rather, it's about where your weight is. So on rollers (just as on the real road), you maintain balance by shifting your weight. Now I don't want to get into angular velocity and gyroscopic properties and such, but basically your front wheel has to spin for you to be able to control the bike and keep it upright on the rollers.

That being said, try things out and let us know how it goes! (don't forget the video camera...and some health insurance:D)

So you're saying if you had a bike with the headset welded to prevent the handlebars from turning, it would be rideable?

absolutely would, with limited maneuverability. ever ride with no hands on the bars? you are able to stay up because of the forces at work on the bike and your body, as influenced by your shifting of weight. when going in a straight line at speed on a bike, steering does very little to keep you upright.

Totally incorrect!

If you ride with no hands, the wheel is still free to steer itself. The reason a bicycle is naturally stable is that lateral forces on the bike automatically steer the front wheel in an appropriate direction to keep the bike upright.

You can still initiate turns, etc by weight shifting while riding no-hands.

Bicycle stability: http://socrates.berkeley.edu/~fajans.../bicycles.html

I was going to find some experiments where folks eliminated gyroscopic effects and such as the key to riding a bike. It's the ability to steer. You slightly fall to the right, the wheel turns (whether you lean it or turn it or what), and the bike is now steering right a touch. You lean the other way and the same thing happens, only to the left.

You have to be able to steer to ride a bike. Non-cyclists rode 1 or 2" wheeled bikes, bikes with super long or super short wheel bases, no trail or reverse trail, etc., but once they locked the bars, boom, they fell over.

I did the "heh" because I thought, oh, obviously the person hasn't tried rollers. Or broken a band and thought "well, why not?"

It's like toe clips. If the toe clips don't support your foot (the strap did, when you pulled up), then why can't you eliminate toe clips?

Seems like a reasonable questions. So of course I tried that. Didn't work. You'd pull up, the strap would pull the pedal vertical, and your foot would be resting on the back of the pedal. It would also get strangled by the strap. Hence the non-clip strap mounts to the front and back of the pedal - the clip keeps the pedal level.

cdr

I just read an interesting few articles on bike stability, interesting stuff. learn something new everyday i suppose. looks like i'm done for the day.:thumb:

incorrect.

You cannot shift your weight with respect to the contact patch on the road without turning the handlebars. Newton's third law, I believe, the one about action and reaction. You are an inverted pendulum on your bike, and you stay upright in the same way as you do when you balance a stick upright on your finger: by moving the base around to keep it under the CG.

If you ride a bike with a sticky headset, it literally feels like there is a giant hand trying to push you over (from my own experience). It's impossible to ride a bike with a welded headset. And it's been shown experimentally (to back up the theory) with a special bike where the gyroscopic action of the wheels are canceled by counter-rotating disks that gyroscopic forces play little or no part in keeping a bicycle upright.

EDIT: the points have already been made by CDR...

after contemplating how rollers work, and how a track stand works, one should be able to recognize that gyroscopic forces have almost nothing to do with keeping a bike upright. If you could make a bicycle with zero diameter wheels, it would still be ridable. Try riding a bike on rollers when someone is holding your handlebars. It doesn't work. You have to be able to keep the center of gravity balanced with the reaction forces at the wheels. A bike with a very stiff headset is also incredibly difficult to ride.

If you do this, be sure to videotape it and post it on youtube. Should be entertaining. :p

" ... So you're saying if you had a bike with the headset welded to prevent the handlebars from turning, it would be rideable?..."

Good Luck with that ..... :lol:

One roller per wheel ?? I'll leave that to Chinese acrobats

I've seen similar things:

There was an article in Physics Toady in the early 70's or something that was reprinted a few years ago where someone modified a bunch of essentially regular bikes to make them have zero angular momentum (basically plop another wheel on top of each wheel to automatically spin at the same rate in the opposite direction) and they're still rideable. I've seen stayer's bikes with the fork turned completely around and they ride them at 45-60 mph behind motorcycles... but lock the bars and it's unrideable.

I meant one roller per wheel, with the rear drum behind the rear wheel and the front drum ahead of the front wheel. Can you visualize that? Why wouldn't it work?

probably, yes. not rideable at slow speeds though

If you had one drum in front of the front wheel, and another behind the rear wheel, and they were connected with a belt, the wheels (on the bike) would spin in opposite directions. That would make balancing very tricky.

It wouldn't be stable - the front wheel would ride slightly up the front roller and the rear wheel would fall to the ground.

EDIT - if you had the rollers far enough apart, it might be stable, but then the bike would be sitting pretty low between the rollers and the force on the rollers would be very high.

Lol, no, not unless you crossed the belt. Think about it.

Like this.

I put some bearings in backwards once on a headset. It was tight and thought it would loosen up on my ride. When I rode it, I definitely felt like a giant was pushing me over. Actually it is a very "weird equilibrium off" type feeling. There is no way in hell you could stay up right on a bike with a welded headset.

These sure are some goofy questions. What is the reasoning?

Ever see a bike on a tight rope?

I think they are interesting. We learn by asking "why is it done this way?"

Here's another question: what if you DID cross the belt over on a standard set of rollers, so the front roller spun backwards. Would the rollers be unridable? Or would the rider just have to remember to un-countersteer?

1+ Your contact point with the front wheel is in front of the steering axis. You couldn't keep it upright because it is unstable; instead of the front wheel wanting to return to be in-line, it will want to flip the bars around backwards. If you crossed the belt, the wheel would spin backwards and you'd have to steer opposite of normal, but it might work, except that...

...with three rollers, your bike is constrained in a plane parallel to the ground, but with only two rollers, it's not and is free to ride up over the front roller or back over the rear. It just won't work for several reasons.

On a normal set of rollers, the three rollers constrains the bike in a plane parallel to the ground and the contact point of the front wheel is the same as it is on the road. The front roller is directly under the axle of the front wheel, which makes steering feel normal. If you move the roller forward, the trail of the wheel is effectively shortened, and the steering becomes more twitchy.

It would be unstable unless you simultaneously moved the front roller so it's contact point is forward of the steering axis. That would make it stable; then you could steer it and keep it upright, but the steering input would be directly opposite of normal.

In addition to the steering axis issue Brian mentions, "remembering to countersteer" is a lot harder than it sounds. I read another article about a guy who put a set of gears in a headset so that turning the bars left turned the wheel to the right. He had a street carnival exhibit and offered prizes to people who could ride it. None of the riders could complete the distance because it was just so counterintuitive, except for one guy who rode the bike no hands.

quoted for awesomeness. Fajans is a nice guy and his link = /thread