• Quick note - the problem with Youtube videos not embedding on the forum appears to have been fixed, thanks to ZiprHead. If you do still see problems let me know.

A bicycle physics question:

Just thinking

Philosopher
Just thinking
Joined
Jul 18, 2004
Messages
5,169
How is it that one can easily stay balanced on a bicycle even at very slow speeds, but once the bike comes to a full stop one usually falls to one side or the other? Even when coming to a stop, one can usually remain balanced until the very end of motion -- but then that's it, once the bike fully brakes.

Are the wheels supplying even at that extremely small rotation enough gyroscopic inertia to aid in balancing? -- or is it the result of something else?

I do have an alternate theory, but would like to hear from some others first.
 
Just thinking said:
How is it that one can easily stay balanced on a bicycle even at very slow speeds, but once the bike comes to a full stop one usually falls to one side or the other? Even when coming to a stop, one can usually remain balanced until the very end of motion -- but then that's it, once the bike fully brakes.

Are the wheels supplying even at that extremely small rotation enough gyroscopic inertia to aid in balancing? -- or is it the result of something else?

I do have an alternate theory, but would like to hear from some others first.
Click to expand...

I would call it angular momentum, not "gyroscopic inertia," but no.

It's that with some motion, a small turn of the handlebars can move the contact points under the center of mass.
 
epepke said:
I would call it angular momentum, not "gyroscopic inertia," but no.

It's that with some motion, a small turn of the handlebars can move the contact points under the center of mass.
Click to expand...

Yes, I had that same feeling that minute steering motions could allow the rider to turn into the fall, so-to-speak, as long as there was some forward motion. A way to prove this (over the angular momentum argument) might be to try and stay upright as the bike is pushed ever so slightly backwards. I suspect it won't be so easy.
 
Last edited:
As a BMX freestyler since time immemorial, theres also the issue of the caster angle or rake of the forks

Ride a flatland bike with a 90 degree steer tube angle (forks straight up and down, and ZERO forward trail in the dropouts) and even low speeds will be quite difficult to balance
 
A slight threadjack, but what the hell.... A cool thing re: bikes is "counter-steering". This is (or at least, used to be) a subject of some controversy in the motorcycle world. In brief, it turns out that in order to steer a bike, whether motorised or not, you actually have to turn the bars in the "wrong" direction - to turn left, you push the left bar forward. This sounds so mad that many people refuse to believe it, but if you go out and specifically try it, you'll find that it works, and by extension it therefore follows that that's how it *always* works.

When riding a motorcycle fast, it's important to use counter-steering in order to get the bike over onto its side as fast as possible. I used to get a kick out of it, until the accident....

Everyone knows how to ride a bike, so it's a good example of how counter-intuitive physics / the world can be.
 
My bicycling reference book, Bike Cult:

http://www.amazon.com/gp/product/15...f=pd_bbs_1/103-8445205-2939034?_encoding=UTF8

Has a couple of chapters on the bizzare physics displayed by two-wheeled vehicles.
A great deal of slow-speed stability is due to the characteristics of the rolling wheel, and it's tendency to self-correct when deflected. As well, the standard steering geometries caused by the caster effect and rake angle.

But human input, even in tiny amounts, plays it's role as well. Many the experienced cyclist can "trackstand" immovably for long periods of time, using inherent balance and tiny inputs of pedal pressure and fork angle.

There's an interesting article on attempts to make an "unrideable" bicycle, with extreme or bizzare steering geometry. Most attempts failed!
 
Crispy Duck said:
A slight threadjack, but what the hell.... A cool thing re: bikes is "counter-steering". This is (or at least, used to be) a subject of some controversy in the motorcycle world. In brief, it turns out that in order to steer a bike, whether motorised or not, you actually have to turn the bars in the "wrong" direction - to turn left, you push the left bar forward. This sounds so mad that many people refuse to believe it, but if you go out and specifically try it, you'll find that it works, and by extension it therefore follows that that's how it *always* works.
Click to expand...

I'm not sure why this is mad. I'd figured that out by the time I was eight or so. The countersteering sets the bike up for a recovery, so by the time you're steering, the contact line is on the other side of the center of mass from how you want to go, and that's just what you want.
 
Of course, going *against* countersteering can stick you like GLUE to the bike

Lots of videos on my website such as http://psychicflyingmonkey.com/ImpetusofCletus.html
Show all manner of physics in action. Some of it seems SO counterintuitive as to boggle the mind, but there you are

I always wondered just how much of how we thought of out riding was woo

For instance: Beating gravity-when you are coming down a ramp, or trying to stick on a wallride, it feels as if a clock starts between you and gravity, and as long as you can "stay ahead" of gravity, you will be allowed to stick to the surface. The whole time you can feel "gravity" knocking on your shoulder or tugging at your tires like an unseen foe, but when you beat him, you make it down just fine.

Grabbing your back brake in the air to drop your nose

Grabbing your front brake in the air to drop your tail

"Alley-oop"ing or turning the wrong way JUST before you spin the right way seems to give you MUCH more spin, like cracking a whip.

Or my all time favorite, probably woo, but has saved me so many times: If you are going to crash be sure to HURT the ramp or the jump as much as it tries to hurt you. I've cannonballed thru crashes that should certainly have broken limbs
 
I can see an effect from braking in mid-air. The spinning wheel has considerable momentum, and stopping it abruptly would transfer a fair bit of force to the frame. In the case of the rear wheel, this force would be in the direction of travel of the wheel, in this case "down".
 
Just thinking said:
Yes, I had that same feeling that minute steering motions could allow the rider to turn into the fall, so-to-speak, as long as there was some forward motion. A way to prove this (over the angular momentum argument) might be to try and stay upright as the bike is pushed ever so slightly backwards. I suspect it won't be so easy.
Click to expand...


This becomes the obvious answer if you take away one wheel and look at how a unicycle works. The rider is constantly moving the contact point back under the center of mass with small movements of the pedals, rather than the handlebars.
 
One more, there seems to be a gyroscopic effect of the spinning wheels to resist you from spinning in the air

Going into the air on a quarterpipe, you used to go straight up, then when you reached just slightly past the apex and started to just come back down, youd grab your brakes, spin 180 degrees and come back down the ramp straight opposite of the way you went up

Once the modern much better cantilever and u-brakes replaced the old sidepull brakes, the wheel stopped so fast that people complained of "getting loose" and over rotating, which you can imagine was rather damaging to the rider. I think this contributed to the more modern "carving" style of brakeless ariels where you covered the 180 over a LONG horizontal distance with a series of 45 degree turns, instead of going straight up and down
 
I got another one: Why does making more crosses on the spokes seem to make the wheel stronger? And why is radial lacing weak up and down and strong side to side vs cross strong up and down and weak side to side? or are they?
 
Hehe- you should go to http://www.bikeforums.net/ and search on these topics. The "discussions" tend to get rather long, involved, and heated.

There are advocates of radial lacing, "cross two" lacing, "cross three" lacing, etc. And of course, a variety of very snooty high-end wheels which feature only a relatively few exotic-material spokes laced radially.
 
flatlanders always want radials, while street riders seem to always want 3 cross or twisted

I ride ramps, always have a radial rear with a cross front, not by choice but it always happens, and to tell the truth, if Im going to taco a rim, it seems to care little how the spokes were put on
 
You must log in or register to reply here.

Back
Top Bottom