Rotating Weight Doesn't Matter (GCN Content)

[charliedid]

Originally Posted by bicycletricycle

if you think about it, nothing matters.

I thought everything mattered?

[pasadena]

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

[ddtn]

Originally Posted by vincenz

I can think of a scenario like a very steep section of a long climb where two riders are playing cat and mouse on two bikes of the same total system weight, one with super lightweight wheels and the other with a heavy set. Aero aside, I think it’s obviously which one can out-accelerate the other and get away.

The rider with the better/fresher legs will out-accelerate the other and get away.

The difference of a few hundred grams in wheels will be so insignificant in that scenario. I cannot imagine losing a climb and then claiming it was down to the wheelset. If the difference is more than a few hundred grams, then it's the rider's fault for choosing the wrong wheels to climb.

[vincenz]

Originally Posted by ddtn

The rider with the better/fresher legs will out-accelerate the other and get away.

The difference of a few hundred grams in wheels will be so insignificant in that scenario. I cannot imagine losing a climb and then claiming it was down to the wheelset. If the difference is more than a few hundred grams, then it's the rider's fault for choosing the wrong wheels to climb.

The point is if you’re going to make an lab experiment out of it, there’s more than one side to the coin.

Obviously in that scenario everything else is the same, rider energy, aero, etc.

Numbers are fine, but point is also real world isn’t a lab.

[FlashUNC]

Originally Posted by vincenz

I wasn’t even factoring in aerodynamics as this isn’t about that. The ~4s difference the guy quoted in the vid of two riders going uphill on a light vs heavy wheelset is only if they were at constant power the whole way up. No one rides, and especially no one races like that..

“Mostly insignificant” doesn’t cover all cases. I can think of a scenario like a very steep section of a long climb where two riders are playing cat and mouse on two bikes of the same total system weight, one with super lightweight wheels and the other with a heavy set. Aero aside, I think it’s obviously which one can out-accelerate the other and get away.

FWIW they also deal with acceleration use case on the criterium portion, and argues there is no appreciable difference, but rather tradeoffs. Lighter wheels will spin up quicker, but also spin down more quickly.

[vincenz]

Originally Posted by FlashUNC

FWIW they also deal with acceleration use case on the criterium portion, and argues there is no appreciable difference, but rather tradeoffs. Lighter wheels will spin up quicker, but also spin down more quickly.

Yeh I can understand that as I have a sub 1000g wheelset and a 1400g wheelset and I can feel that for myself. But for example on a very steep short section of a climb where there’s no aero or draft advantage, it could “matter” very much so.

[tomato coupe]

Originally Posted by bicycletricycle

if you think about it, nothing matters.

Originally Posted by charliedid

I thought everything mattered?

Nothing matters ... until it does.

[reuben]

Originally Posted by bicycletricycle

if you think about it, nothing matters.

Everything counts (in large amounts).

[Mark McM]

Originally Posted by vincenz

I wasn’t even factoring in aerodynamics as this isn’t about that. The ~4s difference the guy quoted in the vid of two riders going uphill on a light vs heavy wheelset is only if they were at constant power the whole way up. No one rides, and especially no one races like that..

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.

Originally Posted by vincenz

“Mostly insignificant” doesn’t cover all cases. I can think of a scenario like a very steep section of a long climb where two riders are playing cat and mouse on two bikes of the same total system weight, one with super lightweight wheels and the other with a heavy set. Aero aside, I think it’s obviously which one can out-accelerate the other and get away.

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!

[ultraman6970]

So all of these means that I can get the bikes of a collegiate bike from the 70s and put them in my bikes and i wont feel the difference?

[martl]

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.

That is entirely not the point of *rotating weight*. *general* bike weight does matter. Rotating weight of the wheel doesn't.

Easily reproduced by setting the lightest wheel into rotation (like on a bike in a bike stand) and breaking it down to 0 by holding a finger against the tire. Repeat with you heaviest wheel. (The fact that this is possible without even getting a red spot on your skin is telling enough)

The ammount your finger warms up more, that is about 50% of the energy sored in the rotating heavy wheel. It will push you forward about 1/10th of a millimeter.

Another proof would be the good old "slip from a roller trainer" test. No, it wont catapult you into the next doorframe. You will stop dead and fall over.

[eddief]

You know 40 miles, 2K climbing, 14 mph average = me. I swear my newest DT Swiss 36 mm, 1550 gram wheelset puts a smile on my face every time I ride. Granted, I am not trying to win a race, only trying to have great fun. And...I like to stand up and do sprinty things a lot. On undulating, flat, and climbs, I just like to step on it. These wheels do seem to get up and go in a way more enjoyable way than the 1750 gram set they replaced. Am I fooling myself?

[bostondrunk]

Originally Posted by ultraman6970

So all of these means that I can get the bikes of a collegiate bike from the 70s and put them in my bikes and i wont feel the difference?

Joking aside, you'd -feel- a difference, but any difference in speed would be mostly due to presumably more aero wheels on the newer bike.

With regards to what you feel...a lighter bike, whether it be lighter wheels, lighter frame, or both, will feel a little different underneath you. But its not moving forward any really amount faster.

Or you can not believe any of this and just say physics doesn't apply to you I guess.

[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.

[martl]

Originally Posted by eddief

You know 40 miles, 2K climbing, 14 mph average = me. I swear my newest DT Swiss 36 mm, 1550 gram wheelset puts a smile on my face every time I ride. Granted, I am not trying to win a race, only trying to have great fun. And...I like to stand up and do sprinty things a lot. On undulating, flat, and climbs, I just like to step on it. These wheels do seem to get up and go in a way more enjoyable way than the 1750 gram set they replaced. Am I fooling myself?

A better/lighter/stiffer/more aero wheelset will feel better for a number of reasons, but rotational inertia isn't one of them.