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Gruntled
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Discussion Starter · #1 ·
The importance of rotating weight (or, more accurately, polar moment of inertia) seems to be a persistent myth in cycling. I know it’s been debunked more scientifically at the analytic cycling web site, but for those of you who haven’t seen that or aren’t convinced by it, I propose the following experiment:

Step 1:
Put your bike in a workstand. Shift into 53 x 15. Now turn the cranks by hand until you reach 100 rpm. You can start in a lower gear if you want and shift up, just make sure neither the rear wheel nor the cranks are moving when you start. Time how long it takes to get to 100 RPM in the 53 x 15, and notice how much effort it takes. Not much, right? You did it with one hand and aren’t even breathing hard. You rear wheel is now spinning at 353 RPM, equivalent to about 28.4 MPH.

Step 2:
Take your bike outside and accelerate from a dead stop to 100 RPM in the 53 x 15 (again, about 28 MPH). Try to do it just as quickly, i.e. in the same amount of time, as you spun up the cranks in step 1. That was a lot harder, right? If you could do it at all, chances are it took all the power you could produce with both your legs, and your heart rate and breathing are now very rapid.

You accelerated your cranks, pedals, and rear wheel to the exact same rotational speed in steps 1 and 2. (OK, the front wheel wasn’t spinning in step 1, but you get the point. I hope.) The difference between the two efforts is the force required to accelerate the total mass of your body and bike, plus overcomong air resistance. Overcoming the polar moment of inertia of your wheels is trivial by comparison.

I have no axe to grind with this, and if you still want to buy a lighter wheelset, go right ahead. I just think we will be smarter consumers and better riders if we stop believing in fairy tales and start applying science to our training and equipment.
 

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Jim Nazium said:
I have no axe to grind with this, and if you still want to buy a lighter wheelset, go right ahead. I just think we will be smarter consumers and better riders if we stop believing in fairy tales and start applying science to our training and equipment.
You don't even have to get all scientific and mathematical to figure this out. An alternate example is equally convincing. Stick a wheel in a truing stand and give it a good spin by hand, not all that hard to get it going as fast as you ride.

But to keep it spinning, you can give it a tiny flick of your finger ever 30 seconds or so with almost no force and it will keep spinning. And the kicker is your flicking is maintaining the rotational inertia as well as overcoming the bearing friction and the air resistance. (Yes, there is air resistance to a spinning wheel, it is tiny compared to air resistance riding forward)
 

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Not quite sure what you mean by "maintaining rotational inertia". In the absence of friction, the wheel would spin forever once you get it going.

In an hour-record bike, speed is constant once you get going, so inertia is pretty negligible. Some riders seem to like high inertia in those events because they prefer the feel...
 

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Resident Dutchbag
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Yep, I figgered that one out when I put my bike in a trainer and put my foot down without the tire touching the roller. Takes about as much effort as flushing a toilet.
 

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rruff said:
Not quite sure what you mean by "maintaining rotational inertia". In the absence of friction, the wheel would spin forever once you get it going.
That would be perpetual motion. There's a law against that. :cryin:

Actually, there is some "gravitational friction" associated with the spinning wheel unless you were to perfectly balance it. Eventually, it would settle with probably the valve stem or the joint at the bottom. But thermodynamics says we'll lose a tiny amount of the energy on each rotation because of this and it will eventually stop.

That's the oft forgotten part of "objects in motion stay in motion... unless acted on by an outside force." Same reason the solar system will eventually collapse.
 

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Jim Nazium said:
The difference between the two efforts is the force required to accelerate the total mass of your body and bike, plus overcoming air resistance.
So weight still matters, and, all else equal, lighter is better. The three largest/heaviest parts of a bike are frame, wheels and fork. The lightest wheels are around 800grams, which is ~600 grams lighter than other very nice wheels. So there is relatively substantial savings to be had with wheels.

And in case you want to get all mathematical about it heres the equation for acceleration of a bike that backs up your logic (credit to Mark McM at Weight Weenies and Forrest Root):
 

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Now I dont know much about science but the way I see it when riding you are simply turning your back wheel, nothing more. the rest of the bike is attatched at the dropouts and the front end is propped up by another wheel that is simply along for the ride. So logic would tell me that a lighter rear wheel would accelerate faster since you are reducing your load. Kind of like if im doing the chest press at the gym on a machine. If I reduce the weight attatched to the cable I will be able to "accelerate" my reps faster. Now I do see how once the bike is up to speed on a flat or decline the heavier wheel should have more inertia since it weighs more and will maintain speed on its own easier than a light one. Kind of like how you can throw a baseball farther than a whiffle ball. Although Im sure that the initial acceleration of the whiffleball might be a little quicker. Im not saying that im right about all this, just telling you my thoughts and wondering if you can either support them or explain why Im wrong so that I can understand.
 

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B15serv said:
So logic would tell me that a lighter rear wheel would accelerate faster since you are reducing your load.
A fact no one disputes, but tell me how much faster. Reduce wheel weight 50% and how much faster will you accelerate? 50% , 5%, 0.05%, 0.000000005%?* That's where logic fails and you need some science.

* The answer is about 0.4% http://www.biketechreview.com/archive/wheel_theory.htm
 

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B15serv said:
Im not saying that im right about all this, just telling you my thoughts and wondering if you can either support them or explain why Im wrong so that I can understand.
Unless your tires are slipping the front wheel accelerates just the same as the rear. Also, *you* are being accelerated... and you weigh a lot more than the rest of the system.

As for high inertia being good for maintaining speed, this is only true if you coast. If you are pedaling and moving at a fairly consistent speed, inertia becomes irrelevant... you need to apply power to overcome frictional forces (and gravity on a hill) and that is it.
 

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Gruntled
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Discussion Starter · #11 ·
threesportsinone said:
So weight still matters, and, all else equal, lighter is better. The three largest/heaviest parts of a bike are frame, wheels and fork. The lightest wheels are around 800grams, which is ~600 grams lighter than other very nice wheels. So there is relatively substantial savings to be had with wheels.
Yes, but the point I was trying to make is that weight is weight, whether it's rotating or not.

Also, just for perspective, I weigh 85 kg, my bike weighs 7.6 kg, and there's probably another 1 kg of shoes, helmet, and clothes. Total mass = 94 kg. 600 grams is 0.64% of that total.
 

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Number 2 on the course.
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I think you are beating a strawman here to a certain extent. Any reasonable person knows that these variations are relative, since even a 2000g wheelset is feather light compared to an adult rider.

Also, your experiment seems a little bunk to me. In step on the wheel is completely free of any friction caused by the ground (and amplifed by the rider's weight). A fair comparison to the spinning wheel in the workstand would be riding the bike on a trainer with the drum completely disengaged. Or you could start by turning the crank by hand while the drum is engaged and an appropriate resistance setting has been selected.
 

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Unsafe at Any Speed
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It is absolutely correct that the effort required to overcome rotational inertia of a wheelset during acceleration, is miniscule compared to that of accelerating the bike with wheels and everything plus yourself.

But there is more! The polar moment of inertia of the front wheel determines the magnitude of the gyroscopic moment of inertia, which creates a torque attempting to tilt the bike when you steer the rotating wheel.

Bluntly put, this is what resists your steering of the fork. It makes the bike feel heavy and sluggish in steering. Lift the front wheel and steer the fork. Then spin the wheel very hard and try again.

Some people have no issue with this. They like the blunt, heavy solid, stable feel. Myself, I like the light, responsive, alive feel. I don't care about 1 minute this or that way on a 30 mile ride. I do however care about how I feel after the ride. And after a ride on light wheels with an alive, responsive feel, I feel more satisfied and less fatiqued. I feel I have experienced a better synergy of man and machine. Much of it is of course psychological but what is wrong with that.

Try a heavy set and a light set on your bike. And by light I mean one with light rim. Not heavy rim and light hub. Some low spoke count wheelsets boast low mass on paper, due to ridiculously light hubs but still with too-heavy rims.

Try them and ride. If you can't feel the difference, count yourself fortunate and enjioy heavy wheels. And pity the poor lost souls like myself who struggle to get the wheels we want.

Heck, I am trying to sell my low-cost heavy prebuilt set. No takers. Why not? Could it be that more people want low rotating mass? Or is it simply the high pulished mass of this wheelset makes them unfashionable?

I have another (old, on another bike) set, 32 spoked, weighing only 100 grams less than my prebuilts. But the rims are meaningfully lighter. And on the road, they 'feel' another 200 grams lighter. Rotating mass baloney, right ??
 

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Unsafe at Any Speed
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"
Actually, there is some "gravitational friction" associated with the spinning wheel unless you were to perfectly balance it. Eventually, it would settle with probably the valve stem or the joint at the bottom. But thermodynamics says we'll lose a tiny amount of the energy on each rotation because of this and it will eventually stop.
"


NO! The unbalanced wheel will speed up as the heavy part goes down, lose speed as it comes up. With no friction, this sinusoidal varying speed cycle will be maintained indefinitely.

Gravity will rob it of no net energy. What it costs in speed one way, it will give back the other way.

In other words: rotational kinetic energy plus gravitational potential energy is a total at all times if no energy is lost to friction. With the weight on top : gravitational potential energy is at maximum thus rotational kinetic energy is minimum hence wheel turns slowly. And vice versa with the weighted part at the bottom of its path.
 

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Gruntled
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Discussion Starter · #19 · (Edited)
Pieter said:
... weighing only 100 grams less than my prebuilts. But the rims are meaningfully lighter. And on the road, they 'feel' another 200 grams lighter.
So you've lightened your bike by the equivalent of 1.5 Powerbars ... but it feels like three Powerbars!

The myth lives on. Oh well, I tried.
 

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there's a difference between debunking it as a myth (which it's not) and doing some calculations to show it's a small factor overall. The study Francois cites seems to have an appropriate model behind it as well as an emprical test of many wheels. The result is a 'subtle' difference in terms of about 6 watts difference needed to accelerate a heavy wheel up to speed.

Some of the numerical simulations out there (like analytic cycling) that are used to look at issues like the effects of reduced weight on climbs are built on dubious assumptions (like constant power). Anyone who uses a power meter knows power is 'stochastic' as Coggan likes to say (at least extremely variable). So, while the power savings of an uber light wheelset in terms of rotational intertia may seem small, over the course of a long race, it could become significant (in the same way that an elite cyclist is worried about a pound or two of bodyweight).
 
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