# Thread: What keeps a bicycle balanced?

1. Hi all,

My first post on these forums... I apolgise if this has already been discussed before, maybe I missed it, but I didn't get anything in the search results.

With the disclaimer out of the way, now with the question: "What keeps a bicycle balanced when it is in motion?"

Look forward to some great answers. Thanks!

Nentut.

2.

3. Its easy man. just keep practicing and you'll get the hang of it

jk..but id imagine the guy on the bike keeps it balanced. sure gravity lends a hand.

on a side note: i remember my 4th grade teacher telling me that once a year you can stand an egg up on flat surface all by itself. i've only been able to do this one time. but i forgot what day of the year it was. i have since tried and can't get the egg to stay up..

4. If its the guy on the bike that keeps the bike balanced, then what happens when the bike is at rest? Why is it a hell of a lot more difficult (if not impossible)?

Netnut.

5. Originally Posted by netnut
If its the guy on the bike that keeps the bike balanced, then what happens when the bike is at rest? Why is it a hell of a lot more difficult (if not impossible)?

Netnut.
perhaps it has something to do with the center of mass while in motion as opposed to the center of mass while not in motion.

i myself don't know much about center of mass so its just a thought.

6. The wheels act as gyroscopes to keep the bike balanced in an upright position. The faster they spin, the more stable the bike is. The wheels resist changes in their orientation due to their angular momentum.

7. Here is something that I found DarcgreY: http://www.sheldonbrown.com/brandt/gyro.html

It completely opposes the idea of gyroscopic forces lending a hand in balancing the bicycle.

Netnut.[/url]

8. It's the guy on the bike that keeps it balanced, if while riding, the bicycle starts to lean to the left then the rider automatically turns in that direction, the body then tries to go straight on which in itself moves the centre of mass back over the centre of the two wheels and thus the rider retains an upright position.

As for gyro stability, that is true on high speed racing motor cycles due to the much higher wheel mass and speed of rotation, it accounts for why bikes can continue upright for some time after the rider has become unseated - and helps with some riding tricks. I don't think the mass or speed of bicycle wheels would have much effect during normal cycling.

9. You kinda get better at searching, netnut. I whacked bicycle and stability into Google and whaddya know:

http://www.dclxvi.org/chunk/tech/trail/

10. THanks farsight, It's nice to see at least one bit of my physics was spot on!

11. Originally Posted by netnut
Here is something that I found DarcgreY: http://www.sheldonbrown.com/brandt/gyro.html

It completely opposes the idea of gyroscopic forces lending a hand in balancing the bicycle.

Netnut.[/url]
You didn't ask what keeps the bicycle upright, you asked what keeps it balanced.

Clearly the rider plays an important part in balancing a bicycle in an upright position, especially at lower speeds. The faster the wheels spins the more balanced the bicycle is in whatever orientation it's in, due to the angular momentum of the wheels.

12. Originally Posted by Megabrain
It's the guy on the bike that keeps it balanced, if while riding, the bicycle starts to lean to the left then the rider automatically turns in that direction, the body then tries to go straight on which in itself moves the centre of mass back over the centre of the two wheels and thus the rider retains an upright position.

As for gyro stability, that is true on high speed racing motor cycles due to the much higher wheel mass and speed of rotation, it accounts for why bikes can continue upright for some time after the rider has become unseated - and helps with some riding tricks. I don't think the mass or speed of bicycle wheels would have much effect during normal cycling.
Motorcycle wheels do exert more force than bicycle wheels but they're also working to stabilize a much greater mass. A bicyle will also continue on in a straight path without a rider. If you doubt the ability of bicycle wheels to resist a change in orientation do a simple experiment. Dismount a bicycle wheel and holding on to the axle spin it. The faster it spins the harder it is to change it's orientation. It's surprising how much force is exerted.

13. Thanks all, I think I got my answer.

Netnut.

14. Originally Posted by Megabrain
It's the guy on the bike that keeps it balanced, if while riding, the bicycle starts to lean to the left then the rider automatically turns in that direction, the body then tries to go straight on which in itself moves the centre of mass back over the centre of the two wheels and thus the rider retains an upright position.

As for gyro stability, that is true on high speed racing motor cycles due to the much higher wheel mass and speed of rotation, it accounts for why bikes can continue upright for some time after the rider has become unseated - and helps with some riding tricks. I don't think the mass or speed of bicycle wheels would have much effect during normal cycling.
Try spinning a cycle wheel [ out of the cycle] while holding onto the axle
not fast, say 70- 80 RPM, then try to move axle, you will then see how much force there is.

15. I suggest you guys do the maths, my post was correct, and not in any way flawed.

16. I think it does help a bit, Megabrain. This is something of a disputed area:

http://www.exploratorium.edu/cycling/brakes2.html

My own feelings are that a large-wheeled bicycle is easier to ride than a small-wheeled bicycle because of gyroscopic stability, but in either case if you haven't "learnt to ride a bike" by steering into a lean, you're stuffed. And it's the latter that's more important.

17. I can see that holding a wheel by the axle and spinning it then trying to rotate the axle is against the force we've all done that, but look at the ratio of the forces, the radius of the wheel is about 10-12 times larger that the radius of the hand points, when the wheel is installed the forces are reversed the COG is about 2.5 times further from the rim. Look at how much your bodyweight provides leverage, of your entire bodyweight against what the tips of two fingers can do. If you want to see how little the gyro effect has on a bike, turn it upside down, crank the pedals as fast as your can then using a spring guage pull the bicylce over, repeat with the wheel stationary, there is a slight difference but as I initially indicated it is marginal. As for the Motorcycle, the ratio of the wheel mass to the mass of bike plus rider is far far higher than a bicycle. Also the gyro effect is most effective where the centre of rotation is through the center of the rotating mass, when a bicycle falls over sideways, the rotation point is at ground level and not through the centre of the wheelshaft.

18. I doesn't take much force to balance a bicycle if it's in an approximately upright position to start with. It doesn't matter where the wheels are postioned on a bicycle, if they're spinning they act as gyroscopes and the faster they spin the more effect they have.

So starting off the rider is doing most of the work in balancing, but as his speed increases so does the inherent stability of the bike.

19. Yes but as I have now indicated twice, by a very tiny amount. Calculate the angular energy of a lightweight aluminum wheel spinning at 70-80 RPM and you'll get about 300gms at it's circumference, now stick an 80 kilogram rider on it who will give about 170Kgms of leverage and you have the answer.... - it is as near as dammit insignificant, whilst the bike is upright and riderless and requires no more than about 160gms to correct it, it will stay upright, once it leans more than about 5 degrees ... crash! at best it contributes about 1/500th of the force required to stabilise when ridden. Since the rider controls the bike anyway, the turning force given to the handlebars by the bikes gyro effect will not move the riders hands and therefore take even less of a part. I doubt that the gyro force could even correct for a small breeze.

As I said, turn a bike upsidedown, crank it and measure the force to tip it over - I could not detect any repeatable difference when I tried it earlier.

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