Flywheel Bikes: The Solution to City Travel?

Electric bikes are inherently dangerous. We’ve all seen people fly around on dangerously modified electric bikes and scooters, not to mention the risk of those lithium batteries spontaneously combusting, but no one wants to pedal themselves up hills either. I have the solution. Using basic conservation of momentum, I can allow no precious kinetic energy to be wasted when a bike brakes or has to slow down, seems like magic? No! All we have to do is attach a flywheel!

Instead of braking by using brushes against the wheels or discs that slow the bike down by generating a lot of heat and often some noise as well, I propose that we can attach a heavy disc to the frame of the bike connected by a chain to the rear wheel, with a clutch like system we can  move the rotational energy of our bike wheel to this flywheel that spins between the riders legs this will rapidly slow the bike down as the linear momentum is converted to angular momentum contained within the system. Then, when the light has turned green or the traffic clears, we reconnect the spinning flywheel so the rotational energy is transferred back, accelerating the bicycle!

If we assume a bicycle + a person weighs about 80kg including the flywheel and we want to accelerate to about 15km/h, 0r ~ 5m/s for simplicity, how fast does this wheel have to be, and how heavy? We can find the moment of inertia of a disc I = 1/2 m*r^2, a measure of how hard it is to start or stop the wheel from spinning, and use it to find the kinetic energy K = 1/2 I w^2, where w is the angular velocity of the wheel and let this equal our desired kinetic energy of Kf = 1/2 (80)(5) = 200J we need to find a balance of m, r, and w=v/r, that are reasonable to put on a bike. A hard limit for m will be 10kg, because any more will be too hard to carry around, and the radius is limited at about 15 cm due to the size of the bicycle, so at this upper limit, the velocity of the wheel needed to get the desired energy is…. 59.62rad/s or around 9.5 revolutions per second! The velocity needed can be brought down by designing the flywheel to have more mass further away from the center to increase the moment of inertia without having to increase the mass, but these calculations give a good idea of the forces in play.

Alas, before we all start attaching massive spinning metal wheels between our legs, there are some minor drawbacks to this contraption. The huge amount of angular momentum in the system when the flywheel is spun up means the rider will experience significant gyroscopic forces. In the same way that a spinning top wants to stay upright, this flywheel really wont want to change its axis of rotation with 200 Joules of energy going about it, and it will be very difficult to get the bike to turn without some serious lean angles so if you’re braking with this thing, make sure it’s in a straight line.

Volvo has kindly supplied me with a diagram of the more complicated flywheel they use in their cars to help illustrate my idea

Apologies for the low quality, but the general idea is that the large cylinder on the right takes rotational energy from the driveshaft and stores it to be released later, meaning that in theory, the only loss in energy comes from wind resistance, rolling resistance, and friction in the system.

No doubt we will see hundreds of flywheel bikes on the road in the coming years, what’s not to love about ending the dreadful waste of energy that is conventional braking.