3 ms·

You have any sources for this? Not trying to call you out but I am generally curious. We know on a car that rotational mass makes a huge difference but a car is

You have any sources for this? Not trying to call you out but I am generally curious. We know on a car that rotational mass makes a huge difference but a car is continually accelerating and has a constant type of locomotion that is very different than a person riding a bike. In addition the rotational mass of a bicycle wheel is much, much larger diameter comparatively to the total weight of the vehicle (compared to a car) and provides a gyroscopic effect that is critical to the way a bicycle rides and stays upright. I would expect if we could make impossible light bicycle rims and tires the bike would have trouble balancing and feel like shit and also decelerate much faster. But surely there is some middle ground that is ideal?

wiredfool 4y ago
It's first year kinematics. The (first order) math is pretty easy to work out. 1/2 I w^2 is the rotational potential energy, Assuming all the mass of the wheel is at the rim (worst case) I = mr^2, w=v/r, so that works out to 1/2 mv^2, which is the same as the translational kinetic energy. So the 'energy penalty' of rotating mass is 2x. So, a 10kg bike with 2kg of wheels and 100kg of rider is approximately equal a non-rotating mass of 112kg. However, that rotating mass gives you 2% more kinetic energy to trade for wind drag, gravitational potential, or other friction. So, not much. A bike's does not require gyroscopic effects to stay up -- there was an experiment with counter rotating wheels (to cancel the gyroscopic moment) that was ridable. The actual stability depends on the geometric and pneumatic trail, flop, weight distribution on the steering axis, and a few other things. There is a middle ground that people have converged to -- Somewhere in the range of around 600mm bead seat diameter and an inch or two of tire. (26"->700c, 25->50mm width covers most of the biking world.)
- Melatonic 4y ago
  Sounds like those crazy 650B people then have been right all along!
ActorNightly 4y ago
You can test this yourself. Flip your bike upside down, shift it into a gear that you use to accelerate, take a torque wrench with hex socket and put it in the left pedal socket on the backside of the crank like you would when installing it. Then test the torque required to get the wheel moving. You would find that these numbers are minimal. Also gyroscopic effect has very little to do with the bike staying upright. The reason a bike stays upright is because of the trail interaction and camber thrust. When the bike leans left, camber thrust of the tire makes it turn left, which moves bottom of the bike to the left, correcting the initial lean, while the geometric trail prevents the tire from turning left completely.
- Melatonic 4y ago
  Interesting!

wiredfool 4y ago

It's first year kinematics. The (first order) math is pretty easy to work out. 1/2 I w^2 is the rotational potential energy, Assuming all the mass of the wheel is at the rim (worst case) I = mr^2, w=v/r, so that works out to 1/2 mv^2, which is the same as the translational kinetic energy. So the 'energy penalty' of rotating mass is 2x. So, a 10kg bike with 2kg of wheels and 100kg of rider is approximately equal a non-rotating mass of 112kg. However, that rotating mass gives you 2% more kinetic energy to trade for wind drag, gravitational potential, or other friction. So, not much. A bike's does not require gyroscopic effects to stay up -- there was an experiment with counter rotating wheels (to cancel the gyroscopic moment) that was ridable. The actual stability depends on the geometric and pneumatic trail, flop, weight distribution on the steering axis, and a few other things. There is a middle ground that people have converged to -- Somewhere in the range of around 600mm bead seat diameter and an inch or two of tire. (26"->700c, 25->50mm width covers most of the biking world.)

Melatonic 4y ago
Sounds like those crazy 650B people then have been right all along!

ActorNightly 4y ago

You can test this yourself. Flip your bike upside down, shift it into a gear that you use to accelerate, take a torque wrench with hex socket and put it in the left pedal socket on the backside of the crank like you would when installing it. Then test the torque required to get the wheel moving. You would find that these numbers are minimal. Also gyroscopic effect has very little to do with the bike staying upright. The reason a bike stays upright is because of the trail interaction and camber thrust. When the bike leans left, camber thrust of the tire makes it turn left, which moves bottom of the bike to the left, correcting the initial lean, while the geometric trail prevents the tire from turning left completely.

Melatonic 4y ago
Interesting!