Rotating Weight Doesn't Matter (GCN Content)

[dancinkozmo]

Originally Posted by bicycletricycle

if you think about it, nothing matters.

[vincenz]

Originally Posted by Mark McM

That was covered. The extra energy to accelerate the heavier wheels is not lost - it is returned by the bike decelerating slower when the rider reduces his power. Even if a rider varies their power during the climb, the net result will be the same.


Its not so obvious. "Getting away" means not just accelerating, but also maintaining a higher power to stay away. If you ignore all other drag, then when riding at steady state, all the power exerted by the riders is used to overcome gravity. Because they weigh the same, the power to overcome gravity is the same, so for the same power both riders go the same speed. If one rider's wheels weighed 400 grams more than the other rider's, the rider with the heavier wheels would only have a an inertia of about 0.5% more than the other's. If both rider accelerated at the same power, the rider with the lighter wheels might inch away from the other rider, but not very quickly. And once the acceleration was over, they'd be back going equal speeds again. Since one can only go full power for a short time, even after a long acceleration the rider on the lighter wheels might only pull ahead enough for the other rider to slip behind their wheel to draft.

Now, if the reason one rider's wheels were heavier is because they were more aerodynamic, the situation would likely be reversed. And because that rider would require less total power (gravity + air resistance) to go a give speed, that rider would be able to keep pulling away, at the same power output.

I think a big problem a lot people has is a misunderstanding of Newton's 3rd law of motion: F = ma. They assume that for some given force applied to the pedals, the lower the mass of the bike, the faster a rider will accelerate. But that's not the correct application of this formula. The 'F' in this equation isn't the force applied by the rider, it is the net force, after all the drag forces have been applied.

For example, say a rider has a total system mass of 200 lb (90 kg) and is traveling at 20 mph (9 m/sec), and at that speed there is 20 N of air resistance force and 2 N of rolling resistance force (total drag of 22 N). If the rider applies force on the pedals that produces 15 N of force at the wheel ground contact point, how fast do they accelerate? Do thy accelerate at 15 N/90 kg = 0.167 m/sec^2? No. In fact, they don't accelerate at all, because 15 N isn't even enough to overcome the drag forces. The net force is 15 N - 22 N = -7 N, so they slow down at -7 N/90 kg = 0.077 m/sec^2. In order to accelerate, they need to apply more than 22 N, and only the force in excess of 22 N causes an acceleration.

So let's consider the case where the rider applies 30 N, which is more than enough to cause an acceleration. Let's compare the case of the bike from above, and another identical bike, except that the 2nd bike has wheels that weigh 1 kg more, but reduce the aerodynamic drag from 20 N to 18 N (at a speed of 9 m/s = 20 mph), reducing its total drag to 20 N. Which accelerates faster?

The first bike has a mass of 90 kg and drag of 22 N, so its acceleration rate is (30 N - 22 N)/90 kg = 0.089 m/sec^2.

The second bike has a mass of 91 kg and a drag of 18 N, so its acceleration rate is (30 N - 20 N)/91 kg = 0.110 m/se^s.

Under the same power the heavier bike accelerates faster!

Fine, but we weren’t supposed to factor aero into it, only weight. The title should have “(Because aero)” appended to it if it came down to just about being aero then.

If like you said, the rider with the lighter wheels would inch away ever so slightly, that would matter if the scenario happened directly at the finish line, would it not?

Originally Posted by Monsieur Toast

Pretty straightforward video. Thanks for the link.

I had been under the vague impression that rotational mass was a big deal due to cycling forums and mags preaching the gospel of light wheels.

And then just like that, engineers and scientists swoop in and ruin all the fun (as usual) with the cold shower that is reality.

In the end, it's nice to know that weight is weight and if I can shave the same amount of weight from a cockpit setup for half the price of reducing said weight from my wheels, it'll make no difference except in my wallet.

Once you finish getting the light parts from everywhere except wheels, you’ll have nowhere else to upgrade but the wheels. And I don’t know about you, but if you gave me a choice between two super bikes at the exact same weight and aero except for a big difference in wheel weight, I would pick the one with the lighter wheels every time because that directly translates to how fun a bike feels, which is priceless.

[Spoker]

Originally Posted by pasadena

fill your rims with lead buckshot. See how you go.

Italian track team had rims weighted with lead long time ago. Helped with inertia. Don't think anybody is doing it now, but for "the hour" it still may help.

[reuben]

Originally Posted by Mark McM

That was covered. The extra energy to accelerate the heavier wheels is not lost - it is returned by the bike decelerating slower when the rider reduces his power. Even if a rider varies their power during the climb, the net result will be the same.



Its not so obvious. "Getting away" means not just accelerating, but also maintaining a higher power to stay away. If you ignore all other drag, then when riding at steady state, all the power exerted by the riders is used to overcome gravity. Because they weigh the same, the power to overcome gravity is the same, so for the same power both riders go the same speed. If one rider's wheels weighed 400 grams more than the other rider's, the rider with the heavier wheels would only have a an inertia of about 0.5% more than the other's. If both rider accelerated at the same power, the rider with the lighter wheels might inch away from the other rider, but not very quickly. And once the acceleration was over, they'd be back going equal speeds again. Since one can only go full power for a short time, even after a long acceleration the rider on the lighter wheels might only pull ahead enough for the other rider to slip behind their wheel to draft.

Now, if the reason one rider's wheels were heavier is because they were more aerodynamic, the situation would likely be reversed. And because that rider would require less total power (gravity + air resistance) to go a give speed, that rider would be able to keep pulling away, at the same power output.

I think a big problem a lot people has is a misunderstanding of Newton's 3rd law of motion: F = ma. They assume that for some given force applied to the pedals, the lower the mass of the bike, the faster a rider will accelerate. But that's not the correct application of this formula. The 'F' in this equation isn't the force applied by the rider, it is the net force, after all the drag forces have been applied.

For example, say a rider has a total system mass of 200 lb (90 kg) and is traveling at 20 mph (9 m/sec), and at that speed there is 20 N of air resistance force and 2 N of rolling resistance force (total drag of 22 N). If the rider applies force on the pedals that produces 15 N of force at the wheel ground contact point, how fast do they accelerate? Do thy accelerate at 15 N/90 kg = 0.167 m/sec^2? No. In fact, they don't accelerate at all, because 15 N isn't even enough to overcome the drag forces. The net force is 15 N - 22 N = -7 N, so they slow down at -7 N/90 kg = 0.077 m/sec^2. In order to accelerate, they need to apply more than 22 N, and only the force in excess of 22 N causes an acceleration.

So let's consider the case where the rider applies 30 N, which is more than enough to cause an acceleration. Let's compare the case of the bike from above, and another identical bike, except that the 2nd bike has wheels that weigh 1 kg more, but reduce the aerodynamic drag from 20 N to 18 N (at a speed of 9 m/s = 20 mph), reducing its total drag to 20 N. Which accelerates faster?

The first bike has a mass of 90 kg and drag of 22 N, so its acceleration rate is (30 N - 22 N)/90 kg = 0.089 m/sec^2.

The second bike has a mass of 91 kg and a drag of 18 N, so its acceleration rate is (30 N - 20 N)/91 kg = 0.110 m/se^s.

Under the same power the heavier bike accelerates faster!

Well, yeah. You slipped in 2N less air resistance. Now run the numbers with the same resistance.

[charliedid]

If wheels that spin up fast make me feel faster will I go faster?

I'm inclined to say yes.

[Dino Suegiù]

Originally Posted by charliedid

If wheels that spin up fast make me feel faster will I go faster?

I'm inclined to say yes.

I'm no scientist, but I'm not so sure that I truly believe that you truly believe that. The underlined parameters seem very vague.

Perhaps please measure your inclination, extremely precisely, and then post again? Thank you.

[makoti]

Originally Posted by bicycletricycle

if you think about it, nothing matters.

Anyone can see, nothing really matters...to me

[Mark McM]

Originally Posted by reuben

Well, yeah. You slipped in 2N less air resistance. Now run the numbers with the same resistance.

You missed the point (perhaps on purpose). The exercise was to show that lighter bikes won't always accelerate faster. If the difference in air resistance was omitted, then you get back to simplistic question of whether lighter bikes perform better, all else being equal, which we all agree was true. That would be true whether the weight difference was rotational or non-rotational.

But if we want to see what happens if there was no difference in air resistance, then maybe we should use a more realistic difference in weight. The 1 kg (2.2 lb) difference in the exercise is a huge difference in wheel weight. When we talk about the differences between "light" and "heavy" wheels, we're usually talking about differences of only 200 grams or so. So let's see what happens in this case.

Because air resistance increases non-linearly with speed, doing the true calculation would require non-linear differential equations. But for simplicity, we can get a ballpark figure by holding the drag constant during the acceleration. The distance traveled D from an intial speed V0 over an amount of time T is:

D = V0*T + (A*T^2)/2

Acceleration is F/M (force over mass), so the equation becomes

D = V0*T + (F/M)*(T^2)/2

Over a 30 second acceleration with the lighter wheels:

D = (9m/s)*(30 s) + (30 N/90 kg) * ( (30 sec)^2)/2 = 420 m

With the heavier wheels over the same 30 second acceleration

D = (9m/s)*(30 s) + (30 N/90.2 kg) * ( (30 sec)^2)/2 = 419.67 m

So, the lighter wheels come out 1/3 m ahead after 30 seconds of acceleration. Given all the other variables that affect bicycle performance, that's pretty small. Other factors will be more important. (Note: The video didn't say changes in rotational inertial will produce no difference at all, it said they will be relatively insignificant).

So, if a rider has a choice between two wheel sets, where one is has less air drag but more rotational inertia, the choice is clear - the aero set will be better in just about all cases (including climbing and accelerations).


Edit: After pondering a bit more, I realize the simplifications in the calculations above exaggerate the difference in distance traveled. In reality, air resistance will increase as the bike speeds up, and pedal force delivered to the road (for a constant power output) will decrease as the bike speeds up, so the actual acceleration rate will decrease as the bicycles go faster. So in this example, the actual difference in distance traveled will be a bit less than 1/3 me.

[charliedid]

Originally Posted by Dino Suegiù

I'm no scientist, but I'm not so sure that I truly believe that you truly believe that. The underlined parameters seem very vague.

Perhaps please measure your inclination, extremely precisely, and then post again? Thank you.

I'm waiting for my inclination caliper to be calibrated. Should be early next week! Then we will have answers, real answers.

Until then I'm sticking to my guns.

[maxim809]

The key takeaways are simple. This video is literally just a lesson on terminology.

1. Rotational weight is different from weight.

2. Some/Many of us have been erroneously been calling the effects we feel of heavier/lighter wheels due to "rotational" weight, when we should just call it "weight".

Every time this topic comes up, I really view it as the terminology police/grammar Nazi coming in to clean up some misnomers. And some people get prickly about it and miss the point.

There ARE perceivable and performance differences between wheels of different weight. Period. It's just not due to rotational weight, for the speeds and power put out by a human on a pedal bike.

[54ny77]

It matters.

[Clancy]

Originally Posted by charliedid

If wheels that spin up fast make me feel faster will I go faster?

I'm inclined to say yes.

Exactly!

It’s what they’re counting on

[bicycletricycle]

Originally Posted by charliedid

I thought everything mattered?

Same thing

[ergott]

Originally Posted by vincenz

Clickbait title.

If weight didn’t matter, why would manufacturers go to carbon instead of sticking with alu or steel? Make the wheels as heavy as possible for maximum aero and for durability.

Of course it matters. We don’t ride in a lab. We don’t pedal at one power consistently. Real riding and racing happens dynamically. If you have a 5kg bike with the lightest wheels possible and a 5.5kg bike with heavier wheels, the lighter one will be faster up the hill, everything else remaining equal. How would it not matter in that case? The guy in the video trying to sell his wheels confirmed as much. Weight is weight.

Let me put this in as simple terms as I can.

If you take two identical bikes and in one of them you add 400g of lead inside the rims and the other you add 400g of lead inside the frame the result will not significantly show up in any scenario. For every acceleration applied to the pedals from the rider there will be an equal amount of resistance to deceleration whether it's in the wheels or the frame.

That's the crux of the video, not whether a lighter combination of bicycle/wheels/rider is faster.

[eddief]

flat, downhill, climbing? zero or significant?

Originally Posted by ergott

Let me put this in as simple terms as I can.

If you take two identical bikes and in one of them you add 400g of lead inside the rims and the other you add 400g of lead inside the frame the result will not significantly show up in any scenario. For every acceleration applied to the pedals from the rider there will be an equal amount of resistance to deceleration whether it's in the wheels or the frame.

That's the crux of the video, not whether a lighter combination of bicycle/wheels/rider is faster.