[handsomerob]

After reading the very heated clinchers vs. tubulars debate with a sub debate on rotational weight perceptions and realities I came up with a couple of my own ideas. I am very good at math but I am not a physics major, so I would welcome any comments.

Wouldn't riders who do not spin very well (like myself) benefit more from a reduced rotational weight due to the micro acceleration differences derived from the alternating 2 major pedal strokes of your revolution? In essense having two accelerating and two decelerating periods with every revolution of your crank. Although your actual velocity might not vary more than a few tenths or hundreths wouldn't more than 1/2 of your pedal stroke be spent on true acceleration.

I have wrapped my head around the idea that at a "constant" speed, rotational weight makes very minimal difference, but who is pedalling at a truely constant speed without acceleration? As far as "engine" smoothness goes, I would think a dual piston engine (insert rider here) would not be a very constant propelling force. Combine that with the increased inertia? required to complete a revolution on a much less smooth uphill climb, where many riders have to get out of the saddle and 1 2 mash their way to the crest. I wonder how long it would take to develop a good smooth spin technique while out of the saddle.

edited to add.....I would think the periods of acceleration and deceleration climbing a hill would be much more pronounced with less (shorter) "maintainence" energy. For instance... and this is anything but scentific....

Let's say you are pedalling a slow (but mathmatically easy) cadence of 60 with each full revolution of your crank taking 1 full second.

Say the downstoke of your drive side crank arm takes 4/10 a second as does the downstroke of your non-drive side crank arm.

The "lull" or transitional period where the stroke is not as smooth (which I am sure would not be near as pronounced with a pro rider) is a tenth of a second on each side.

So here we are with 8/10 of a second spent on downstrokes of each piston and 2/10 of a second on the "transitional" time.

Assume the 3 of the 4 tenths on each downstroke is spent accelerating and 1 of the 4 tenths on the downstroke and the tenth of transitional time is spent decelerating. (there would be two much smaller fractions of a second where there would be neither acceleration or deceleration)

Wouldn't that mean we have spent 60% of our revolution as acceleration and 40% as deceleration? Surely at least on a micro level?

Does this then mean that rotational weight could be more important than formulas that use "constant" speed prove?

 

[alienator]

After reading the very heated clinchers vs. tubulars debate with a sub debate on rotational weight perceptions and realities I came up with a couple of my own ideas. I am very good at math but I am not a physics major, so I would welcome any comments.

Wouldn't riders who do not spin very well (like myself) benefit more from a reduced rotational weight due to the micro acceleration differences derived from the alternating 2 major pedal strokes of your revolution? In essense having two accelerating and two decelerating periods with every revolution of your crank. Although your actual velocity might not vary more than a few tenths or hundreths wouldn't more than 1/2 of your pedal stroke be spent on true acceleration.

I have wrapped my head around the idea that at a "constant" speed, rotational weight makes very minimal difference, but who is pedalling at a truely constant speed without acceleration? As far as "engine" smoothness goes, I would think a dual piston engine (insert rider here) would not be a very constant propelling force. Combine that with the increased inertia? required to complete a revolution on a much less smooth uphill climb, where many riders have to get out of the saddle and 1 2 mash their way to the crest. I wonder how long it would take to develop a good smooth spin technique while out of the saddle.

edited to add.....I would think the periods of acceleration and deceleration climbing a hill would be much more pronounced with less (shorter) "maintainence" energy. For instance... and this is anything but scentific....

Let's say you are pedalling a slow (but mathmatically easy) cadence of 60 with each full revolution of your crank taking 1 full second.

Say the downstoke of your drive side crank arm takes 4/10 a second as does the downstroke of your non-drive side crank arm.

The "lull" or transitional period where the stroke is not as smooth (which I am sure would not be near as pronounced with a pro rider) is a tenth of a second on each side.

So here we are with 8/10 of a second spent on downstrokes of each piston and 2/10 of a second on the "transitional" time.

Assume the 3 of the 4 tenths on each downstroke is spent accelerating and 1 of the 4 tenths on the downstroke and the tenth of transitional time is spent decelerating. (there would be two much smaller fractions of a second where there would be neither acceleration or deceleration)

Wouldn't that mean we have spent 60% of our revolution as acceleration and 40% as deceleration? Surely at least on a micro level?

Does this then mean that rotational weight could be more important than formulas that use "constant" speed prove?

See the same post over in Wheels 'n' Stuff, in the Tubie vs. Clincher thread.

 

[dogmeat]

650c wheels don't rule the mountain stages

If rotational inertia was all that important, then racers would have discovered long ago that smaller diameter wheels climb significantly faster in a race. Guess what?

Take the rotational inertia arguement to the absurd- build a climbing bike with those little folding bike size wheels, and see if your climbing times go down!

Get two sets of wheels and a coin. Go to a big hill, flip the coin: heads=wheels set 1, tails-wheel set2. Climb and time, record. Repeat. Do like 100 climbs on each set... it is going to take a lot of time and a little statistical analysis. Know what you'll find?Your legs are getting stronger!

1 experiment is worth 100 calculations

'meat

 

[Reynolds531]

No, no you need a flywheel

A flywheel will even out those micro accelerations. Get some real heavy rims and tires to smooth out your power delivery.

 

[Lifelover]

What I find interesting..

..is that when the discussion of rotational weight comes up a ton of people claim that we don't pedal smooth and most of the force comes on the down stroke.

If the discussion turns to pedals than the same people claim that you will see a major increase in efficiency with clipless pedals over platforms because it allow you to use the up stroke.

So which is it?

The bottom line is that for 99.99999% of us all this talk of micro this and micro that don't me crapolla.

When all other thinks are considered wheel weight will not be the difference in making it up the hill or winning the race.

 

[asgelle]

After reading the very heated clinchers vs. tubulars debate with a sub debate on rotational weight perceptions and realities I came up with a couple of my own ideas. I am very good at math but I am not a physics major, so I would welcome any comments.

Put in some reasonable numbers; calculate total work, time, or some other metric; and show the significance of the effects you propose. Otherwise you're just waving your hands and asking us to believe you.

 

[danl1]

To answer in analogue, there's a reason that engines have flywheels. The analogy doesn't work perfectly unless you're riding a fixed gear, but the essence is still there.

Get a heavy wheel spinning, and that rotational weight will provide a measure of inertia to help keep it spinning at a higher speed during the less-powered portions. So a heavier wheel will 'pulse' less than a lighter one, and a lighter wheel will get up to speed somewhat faster than a heavier one, though the amounts are pretty small.

But for overall speed, it doesn't matter a whit. Building inertia is essentially lossless - conservation of momentum assures that those watts find their way to the road one way or another, the only variable is time - and here we mean portions of a second.

The meaningful difference in lighter wheels is that they are lighter. It's important not because of it's rotating nature, but because it's about the easiest place left to lose meaningful weight from a bicycle. Because a pound of rotational weight savings 'feels' lighter than a pound of frame savings, it was assumed to be so, and the math was used to produce that expected result, mostly by looking at discrete time slices rather than the whole ride.

The logic that allows lower rotational weight to be faster than the same static weight creates a logical problem with average speeds on hills. If an uphill slows you the same amount as the downhill speeds you, your average should be the same as over flat ground, right? Obviously, it doesn't work out that way, because you end up spending less time on the downhill than you do on the uphill. The rotational weight argument suffers the same fallacy, flipped around a bit.

All that said, there is a tiny advantage to losing rotational weight, based on the additional amount of kinetic energy lost in braking operations. But that's related to energy efficiency rather than racing speeds, so not meaningful to the normal course of this conversation, and it's really tiny anyway.

 

[estone2]

Know what you'll find?Your legs are getting stronger!

HAHAHAHA

 

[KeeponTrekkin]

danl1 beat me by 5 minutes

I have a degree in Physics but do not currently work in the discipline. However, I have given considerable thought to the matter, as has Leonard Zinn, Technical Writer for Velonews, see: http://www.velonews.com/tech/report/articles/9727.0.html.
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In short, I agree with the second writer, Kerry.
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You have to make a few simplifying assumptions, like perfect wheel bearings, but that’s not a big stretch for me as high quality wheel bearings are darn good.
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What Kerry doesn’t say is that any rotating mass functions as a flywheel. A perfect flywheel does not consume energy, it receives, stores and returns energy. Thus, neither cadence nor wheel weight (assuming identical bike weight) effect average top speed in the flats or while climbing. The discussion about micro accelerations, etc., is basically correct and irrelevant. At the extreme, the rider with the lighter wheels might have a momentarily higher road speed but would also decelerate more to a slower slow speed because during the “lull periods” as his light wheels have less inertia and store less energy and thus propel his bike less than the rider on heavy wheels. Ultimately, average speed depends solely on the energy being delivered to the system by the engine or, more specifically, the rider.
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Kerry’s point about the Crits is a good one. Basically, if a race is won on the final acceleration, it is possible that the lower rotational moment of inertia of the light wheel will make a difference. However, subjective analysis and skepticism tell me that aerodynamic wind resistance and the overall power to weight ratios of the competitors will be much more significant factors.
<o> </o>
Too many brain cells are toasted on the debate of the merits of super light wheels vs light wheels. Get sensibly light wheels that won’t fail at the wrong time and give good road feel; then, go work on what matters.

 

[danl1]

Another thought

That I don't have the energy to math out, but is worth noting:

Assume a 'lumpy' pedal stroke compared to a smooth one generating equal power. Look at the effects in a 'light' wheel vs a 'heavy' one, and to keep things constant we'll handicap the light bike like they do racehorses.

Average speed is assured to be the same, in the absence of other factors. But an 'other' factor that is overlooked is air drag. That is, the light wheels would 'pulse' between say 19 and 21 mph in getting an average of 20mph, while the heavy wheels would pulse less - say between 19.5 and 20.5 to the same average.

But since aerodynamic drag moves as a function of the cube (more or less) of speed, the higher variance in speed will net a slightly higher amount of drag, requiring slightly more power. Same sort of thing as the uphill-downhill argument - you don't save as much at the lower low as you spend at the higher high.

This example overemphasizes the differences in speed fluxuation, and the effect would be small, but it does exist.

 

[handsomerob]

First, let me thank everyone that contributed to my understanding of this concept. I very much appreciate the time and your knoweledge.

Second, I just won a pretty light wheelset on eBay for very cheap (<$200 shipped). Basically, I will be shedding about 650g from my current wheelset. I know that the engine is exponentially more important than the equipment, but I like to know as much as I can about the effects of different changes.

Lastly, as a cyclist and novice "wrench" I feel much better understanding the mechanics of things on a bike.

Thanks again.

 

[KeeponTrekkin]

Effects of weight shaving...

are proportional to the weight change of the rider & bike package. A 1 pound saving for a 200 pound rider & bike is worth a 1/2% improvement. A 650 gm shaving for the bike and especially on the wheels will make the bike feel much more responsive. Congrats on your purchase. Post a report on them after you've ridden a few hundred miles!