Bicycle Physics
 

hello,
notice when you take a wheel of a bicycle and spin it really fast in your hands like a gyro and then suddenly bring the wheel upwards or downwards, the spin of the wheel either slows down (even stops) or spins faster during the acceleration of the wheel upwards or downwards. why is this? hope this makes sense.

I'm not completely sure, but it probably has to do with the force vectors up or down either complementing or opposing the rotation of the wheel. That's a poor description, but I'd almost have to draw a picture to explain what I mean.

It's magic.

At least gravity has little or no effect on this as on my last shuttle mission we conducted this very experiment to see why. I still have no answer.

No joke. He posted a link a while bike where he experimentally proved in space that a fixed gear was mechanically superior to multiple gears. Something to do with a higher percentage of the force applied at the cranks being converted to work at the rear wheel than you get with a geared set up.

The principals of rotational/angular momentum and inertia.

Same reason you get more stable with speed (up to speed shimmys from an inbalance. There is a difference between linear physics and rotational. If our friend is an astronaut, they would have studied this in a pre-requisite for getting into the program, or at least in high-school physics. Earth's gravity plays no role.

This is the same principal that lets a top spin on a small point and not fall over. It is not magic, or defying gravity. Its rotational force and momentum are just stronger than the force of gravity. When, do to friction, the angular momentum is not stronger, then it falls.

The wheel has angular momentum and it is based on an axis. Moving it upright changes that axis, and thus it is a "new set" of momentum and intertia, just in rotational way. Same story of when you make a turn, you are not going as fast, but still cary some speed.

Though small turns aren't too noticable, this is why a lot of turns takes more energy than just the extra distance traveled.

This is why wheel weight at the rim is more important to racers than the weight of their seat post.

I was even told by a physicist that angular momentum is what allows a nicely thrown football to travel farther than is should (based on linear physics alone). To that, I don't know...

oh yes, say if it slows down if i move the wheel up and why i exchange position of my hands on the wheel so my right hand is touching the other side and my left the other and then i move the wheel up it speeds up in rotation. maybe it gotta do something with the way the spokes are placed? i doublt that.

are you moving the wheel along the axis of rotation or is the line of movement perpendicular to the axis?

perpendicular to the axis of rotation. basicaly pretending your a bicycle fork with your arms and moving the wheel upward in the direction of your head.

Try this :
Sit on a chair that can rotate around, then hold a wheel in your hands, give it a spin and try moving it. I heard you will spin around in your chair.

What does it mean really?

OK, I just tried this and cannot duplicate it. Also tried it w/ a gyroscope w/ the same result.

As far as I know, such a motion could never in itself apply any tangential torque to the wheel (or rather any *net torque* which would affect the wheel's spin-- the mass of the rim does provide a torque becasue of its inertia but, this will always be balanced equally by the opposite side of the wheel).

Is the wheel imbalanced or are the bearings loose/worn?

The change in axis thing sounds convincing, but I think I'll try to explain my earlier hypothesis a little more clearly.

Essentially, I'm saying that the force generated by inertia or acceleration(depending on the angle of movement) add to or detract from the rotational speed of the wheel. Think of it this way: if you hold a non-rotating wheel and move it up or down, it will eventually start to spin, albeit not very fast. I suspect that's the same force at work in the question.

yah i think that is the same force.

no, its a praticaly a new bike. its a trek 520 ehehe.

This is not a correct picture. Conservation of Angular Momentum requires torques (ie, tangential forces *around* the axis of rotation) to be introduced to alter rotational motion. Linear motion of the axis (in any direction) has NO effect on the spin.

If the wheel starts to spin in this case it's because of imperceptible rotational motions you are unintentionally introducing (which can add up)

.......or most likely perhaps a not perfectly balanced wheel...

This is most definitely true-I wonder if they still do that in freshman physics at Purdue.