Why does the weight of the bicycle wheel matter more that the frame?

Question

A friend told me that the mass of a wheel contributes more to the overall intertia of a bicycle than the frame because it is spinning. I have a degree in Physics and I can't make any sense of it. Could someone enlighten me?

Answer 1

I'll try to keep this as simple as possible so please ignore flywheel effect and weight distribution,etc.

Force = Mass x acceleration

If you imagine lifting your bike off the ground. You apply a force to the wheels they begin to accelerate. (The bike is still stationary).

To get the bike accelerating horizontally you need to apply another force.

Roughly the weight of the wheels is counted twice.

Of course this is only of assistance while accelerating. When the bike reaches a "steady" speed acceleration equals zero so any advantage reduces to zero.(only a slightly reduce rolling resistance remains.)

Bike riders know that cycling is rarely done at a constant speed. And any advantage as small as that may be, can mean the difference between winning and losing. To demonstrate losing one yard in every thousand can mean losing the peloton. (0.1%)

Most lighter wheels also have aerodynamic advantages. This accounts for most of the inflated testimonials. At 32km/h (20 mph) wind resistance typically accounts for 80% of effort.(Solo)

Answer 2

Scott might have a point in that maybe it's perception, but I don't agree completely with him.

As long as you don't have to lift the bike for long period of time, lets say that what is more important to the motor (you) is how to make your bike moves from point a to point b.

When you spin your cranks, you try to rotate your tires to provide momentum and power, and hence, move the bike. If you have more weight at the wheels, more specifically, at the tire/rim section, it will cost you more to get them rotating than if they are lighter.

Also, tire selection is very important, since it is very important in the weight. A good wheelset (without tires and cassette), might go for around 2,000 to 2,100 grams a pair. Some tires are 500 grams each, some are 800 grams, and some heavier ones can go above 1,000 grams eachs. So, lets say that your current tires weight 800 grams each, which would make it 1,600 grams of rotational weight. Which, in part, weights about 76% percent of the wheels.

You could probably change them for another pair of tires that weight 600 gr each, so you have 1,200 grams for 80 bucks. You save like 400 grams for not much money, and it's more in the part that weights the most, or cost you to move the most.

Answer 3

That's one of the endless debates among bike geeks. There seems to be reality to it (perception-based) so it stays alive.

The most common reason stated is that for every acceleration of the whole bike/rider system, the wheels accelerate 2x as much, because at the top of the revolution the wheel is going 2x the bike speed.

I'm not aware of any controlled studies proving this either way, it's just one of those conventional wisdom things.

Now, in general, lighter is better on a bike, and wheels are one of the heaviest components of a bike, so it's worthwhile to lighten them (more mass to lighten, bigger opportunity).

Another factor is aerodynamics - frequently, a lighter wheelset will have fewer spokes or better overall aerodynamics, so of course it will be "faster" than the previous (heavey) wheelset... but it may just be wind drag, not weight.

Arguments like this tend to ignore real-world factors like the 20 pounds of belly fat on most riders.

My opinion: ride more if you want to get better. Lose the pounds from the belly, not the bike (and wallet).

Answer 4

Great questions and great answers!

I imagine it is something like the forces that apply to a Olympic hammer throw. The hammer weights the same when hanging as when spinning, but the force exercise by the spinning hammer increases at a faster rate that does the speed at which it is spinning.
Substitute the hammer for a wheel and you will see why a heavier wheel will need more force to be moved around.

Answer 5

Good Question!

Here is the answer. go get a bike with heavy tires and tubes ride it twenty miles and document how you feel.

The next day take the same bike swap the tires and tubes for something lighter and put the weight that you took off the tires and redistribute it to the frame. make sure that the widths of both tires are the same. Ride that bike and see how you feel after words. Trust me you would rather take the weight off the wheels.

Answer 6

Hi! its pretty simple really.

If you have a heavier wheel it may roll down a hill quicker than a lighter wheel but on normal road conditions u r usually dont get the help of gravity and so u the pedaller are havin to work against friction alone without the help of body + bike weight.
so if u r doin nrml road travel less weight better. if u r goin down a hill heavier wheels may the advantage.
You can see this in real life mountain biking is all about going down well mountains and those bikes are like tanks compared to ur road bike which are like anorexic compared to mntn bike

Answer 7

It takes more energy to accelerate, decelerate and move a rotating mass than a static one, and the farther away from the center a mass is the more energy it takes. Lighter wheels allow the bike to handle much better overall, and are one of the single best upgrades you can do.

If you have a degree in physics you would understand this, so I officially call BULL$HIT!

Answer 8

Your friend is correct.

It is a relatively simple physics problem... rotating mass vs. fixed mass, inertia.

As the wheel turns, it's mass isn't only directed towards the back of the bike, but also towards the front, so you need to overcome the front motion just to get the bike moving. Less weight means less energy to keep it moving.

Answer 9

lighter wheels spin faster and as the wieght of the frame is pushin it will slow it down .The lighter the wheel the faster the bike

Answer 10

Paint your own toe-nails as opposed to sitting for a pedicure.