Is a bicycle wheel really heavier when it is spinning?

Question

I read an article that said if you really want to cut down a bikes weight to use light rims and light tires, because shen the wheel goes around it creates a ton of centrifugal force. But wouldn't this force push up just as hard as it pushed down?

Is it harder to move from side to side if it has more mass?

Answer 1

The wheel does not gain any measurable weight by spinning. Centrifugal force is, as you guessed, irrelevant. However, because the wheel is spinning in addition to having its center of mass moving with the bike, it has almost twice the kinetic energy *per unit mass* as the bike frame. That means it takes twice as much work per pound to get up to speed. Reducing the wheel weight by given amount, therefore, is more advantageous than reducing it elsewhere.

Answer 2

"heavier" implies weight. Weight is just the result of gravity acting on a mass. it has nothing to do with what that mass is doing (spinning or otherwise).

Having said that, there are other forces at work when a mass is moving relative to some reference frame. But they are not weight. Some of these other forces include: frictional force, centripetal force, and gyroscopic force.

Frictional force comes into play from where the hubs, and axles rub against the wheel and gear sprockets. it also comes into play through air drag, which is why racers lean way over when going fast...to cut the wind drag.

Finally, frictional forces come into play where the rubber hits the road. Although a racer needs traction to go foreward, that traction is not without its costs.

Friction forces always pull backward, away from the direction of the object moving over a surface. That is to say, we never get additional energy and propulsion from friction. Friction depends in part on the weight of the total bike and rider system. Thus, light frames and wheels tend to reduce the friction forces and increase the net forces forward in the direction of travel.

Centripetal force acts along the spokes toward the hubs. It gets stronger as the wheels go faster around. It also depends on the mass of the wheel in that for a constant angular velocity of a wheel of the same radius, centripetal force will be greater for the greater mass. Centripetal force is greater because it has to be equal and opposite to centrifugal force; otherwise, the wheel would fly apart.

As these force vectors act inward and outward from the wheel, they are not along the path of the bike. In fact, the net forces due to the wheels rotating have to sum to zero or those wheels would separate.

Now the last force...gyroscopic. Unfortunately, the physics that fully explains this force is way beyond introductory physics. But let's look at this guy qualitatively.

A rotating bike wheel is like a top when it is spinnning. That is, like a top, the bike wheel want to stay upright. The forces that keep that spinning top upright are gyroscopic forces. Well, the bike wheel has gyroscopic forces as well. And "up" for a bike wheel is actually out sideways from the bike. It's something like spinning a top on its side so its "up" is actually horizontal rather than vertical.

We can find out where "up" is to something spinning by the right-hand rule. Curve the fingers of your right hand to follow the direction of the spin. Stick out your thumb on the right hand...that's the direction of up for a spinning top or rotating wheel.

The real cool thing about gyroscopic forces is that when anything tries to change the orientation of the spinning object, the gyroscopic forces will resist that attempted change. They will try to push that spinning thing (top or wheel) back upright again. That's why a top stays up...every time it starts to tip over, the gyroscopic forces push it back upright again.

Same thing on a bike. As the bike starts to fall over, the gyroscopic forces on the two rotating whieels try to bring the bike (and wheels) back upright. And in this sense, the gyroscopic forces impart a sense of stability to the bike.

Well, greater mass wheels rotating at higher speeds than usual due to racing, tend to give a bike too much stability. They are hard to steer, not very agile. And that is not a good thing for a racer.

And you may hear the term "this bike steers heavy." It's not the weight due to gravity, it's the gyroscopic forces due to the more massive wheels going at faster than usual rotation that makes the bike steer heavy. This is why less massive wheels are better...more agile steering and better control.


Good question.

Answer 3

Of course light rims and tires cut weight, and give you a slight advantage. Spinning doesn't have anything to do with it. You're right, centrifugal force would pull in all directions and would not add weight.

I don't know if the gyroscopic effect mention in the earlier answer would have much effect on the bike. If it did, I would think that heavier rims and tires might add a little stability when riding at speed.

Answer 4

No, it is not heavier.

It does require more energy to accelerate.
Decelerating is also more difficult but that hardly matters because the energy doesn't come from you.

Yes, it is harder to move from side to side and to turn, however this is usually more of a benefit in stability than an energy cost.

The benefits of lighter rims and tires come from the overall weight reduction. Lighter weight means easier up hill climbs,[no you don't get much of the energy back on the way back down hill - as much as 80% is lost to air friction depending on how fast you are moving]. Lighter weight also means less energy loss to friction between: A. (tires and road) and B. (axles and wheel bearings).

Answer 5

I know this doesn't really answer the question but I had to share this. In my physics class my teacher brought in a heavy bike wheel (for more inertia). We would sit on a spinny chair (that is currently stationary) and hold the wheel horizontally (I'm holding on to the axis). someone would start it spinning really fast then we would tilt the wheel the other way until it is horizontal with the other side facing the ground. When you start to tilt it the spinny chair starts to turn because of rotational intertia. It was pretty cool.

Answer 6

If you are constantly accelerating and slowing down then lightweight will give you a small advantage. If you are in heavy stop and start traffic you will get an easier ride with light rims.