I still haven’t found a good explanation for precession. I didn’t understand it when my physics class covered gyroscopic motion, and I still don’t understand it. It seems to cause the pedal to rotate in a direction opposite from what would be expected due to the friction in the bearings.

I was trying to figure out how best to describe it and gave up. I could demsonstrate, but it would be a long explanation that most likely would be incoherent at it’s conclusion. I did find a wiki article about mechanical precession that explains it pretty well.

the try all tires work both ways. infact they work better for trials backwards becouse of how the tread pattern is. though they roll smoother when they are forward.

and everybody knows that the crank that says R goes on the right and the crank that says L goes on the left.

Pedals are affected by mechanical precession which is actually a different process than gyroscopic precession. The pencil example given in the previous link is the easiest way to demonstrate mechanical precession. Essentially when a cylindrical object is placed in a socket (such as the pedal axle into the crank) and a force is applied in one direction (weight on the pedal) the friction on the compressed part of the cylinder is greater than the opposite side (even in tight pedals because metal deforms slightly under load). Hence, when the the direction of pressure rotates (or the crank rotates in relation to the downward force) the friction at the compressed side is greater and causes the cylindrical object to roll within it’s socket. Similar to how a wheel moving forward is actually rotating the opposite direction at it’s bottom point.

Gyroscopic precession AKA torque-induced precession is totally different. It is affected by conservation of momentum. Essentially when a spinning top is pushed over or starts to fall due to gravity, that side of the top is accelerated downwards, while the opposite side tilts upwards. Since the top is spinning those particles which have been accelerated are also turning around the top’s axis. As they rotate they carry the downward or upward acceleration with them. This causes the tilting introduced to follow the direction of spin in the top. The upward and downward accelerations rotate around and dampen each other hence making the top stay up longer than if it weren’t spinning.

I’m afraid they’re right mate :o
The right-handed thread needs to be on the right, left-handed on the left. The reason the pedals undo is not because of the friction in the bearings unscrewing the pedal axle (your theory would only stand up if your pedals are utterly seized). As you wiggle the axle round in one direction (by pedalling), it actually wants to turn the other way. Weird but true. You can try it by putting a rod (pen or something) through a hole and wiggling the end around. Because the weight is on the bottom of the outside of the hole (would be the thread) and the top of the inside of the hole (thread) it makes the rod (pedal axle) rotate in the opposite way from the wiggle.

Rob

Edit: Just noticed that was pretty much said in the previous post.

sparky marky has the right idea somewhere in his head…
but has the wheel flipped backwards, when you pedal forwards, its very much like the pedal spinning opposite the wheel, as in if you are above the wheel, it is moving forward, you would have to spin the pedal toward you to draw correct rotation. that is if the pedal is spinning, at the instant it is vertical, the top is going away from the direction of motion.

PUT YOUR Left Pedal on the LEFT and R pedal on the RIGHT.
i ruined like 4 sets of cranks because i put the seat on backwards. who cares about wheel direction, its probably backwards so that when you land a rolling hop, you don’t rip as many knobbies off, or slip as much

Sorry bud, you’re still wrong. Left crank, left side of uni, right crank right side of uni.

And to clarify for Liddle-Peter, if you would like, you can remove the crankarms and place them on the opposite sides of the wheel. The hub does not have right or left sides, so it doesn’t matter which crankarm is on which side