I think everyone should read this first to put things in proper perspective: http://www.slowtwitch.com/...nd_Inertia_2106.html

The fact is, the rotational inertia of the rotating parts of a bike (including the wheels) account for a very small part of the TOTAL inertia of the bike+rider system...so much that the amounts of reasonable variation in the rotational inertia of the wheels don't actually matter.

Which all basically leads to the point that the demonstration shown in the video really doesn't apply to bicycle performance in any meaningful regard.

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http://bikeblather.blogspot.com/

well, that make for a short thread. you could have wait 3 or 4 pages before posting !!!!

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Jonathan Caron / Professional Coach / ironman champions / age group world champions
Jonnyo Coaching
Instargram

could you not, through a similar mathematical demonstration, prove that the entire weight of the bike - say, a 1 pound difference, or 2lbs, even 3lbs - is an inconsequential amount when considering performance?

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Dan Empfield
aka Slowman

Agree with Tom.
The physics is very clear.
This isn't bleeding edge quantum mechanics where there is any doubt. It is simple mechanics.

Saving a pound of wheel weight is certainly of some small substantive benefit in a crit, but it is not of significantly MORE benefit than saving a pound of ass weight, or frame weight.

I made a spreadsheet which all are welcome to copy and play with. You can input various wheel mass differences, and various acceleration scenarios, to see how much difference is made by the mass, and by the inertia of the rotation

https://docs.google.com/...;usp=drive_web#gid=0

As Josh from Zipp said - wheel mass isn't 5 times more important than frame mass.

It is about 1.1 times more important, and only when accelerating really hard.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

3lbs of frame weight can range from not consequential to extremely depending on the event. Ironman Florida - none, Savageman, pretty big deal

3lbs of wheel weight would be, to close approximation, exactly the same degree of consequential. No extra penalty.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

I ran through the numbers assuming a 400 gram wheel rim, a 75kg total mass (rider, bike, everything), and 15 m/s speed (about 54 km/h or 33 mph). At that speed, the kinetic energy due to translation (the linear motion over the ground) is about 8.4 kJ. The kinetic energy stored in the spinning wheel rim (at a rate required to go 15 m/s) is 45 J. So, the two spinning wheel rims together store about 1% of the kinetic energy. The tires would store another 1/2 to 3/4% and the spokes a little more, but still not totalling to more than about 2 to 3% of the translational kinetic energy. If your bike totalled 7.5kg, it then stores about 10% of the translational kinetic energy. Knocking a pound off the system, anywhere, would save you about 1/165 or 0.6% in stored kinetic energy, which would translate directly to requiring that much (or that little, depending on your viewpoint) less power to accelerate the whole thing.

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Less is more.

I'll cede any stupid comments to the mechanical engineers that know this shit.

Well...actually (as Jack noted above) yes. For the majority of purposes, with the exception of steep uphill-only events, those sorts of mass differences would only account for very small differences in performance (e.g. 1lb out of 185lbs total is just a 0.5% difference) overall.

The point is that changing the rotational inertia of the wheels without a mass change (as that video is intended to demonstrate), results in a whole order of magnitude lower effect. Even with a relatively large mass change (for example, in that article I used a 400g difference ALL at the rim), the differences in rotational inertia are so inconsequential that the only thing to actually consider is the mass difference.

The mass of bike components as an effect on bike performance (with that notable exception listed above) is a lower effect than either aerodynamics or rolling resistance (no matter what the weight weenies think). Rotational inertia is such a small effect even compared to mass that it's basically inconsequential (bike stand experiments be-damned ;-)

People tend to not take a "systems approach" when evaluating some of this stuff...

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http://bikeblather.blogspot.com/

As others have mentioned - the effect on acceleration/braking of a pair of 800g rims+tyres is pretty minimal compared to the effect of the 50-100kg sitting a bit further up.

A while back there was a discussion about cyclists vs runners for standing start sprints. For fun I did a 200m standing start (road bike, training wheels/tyres, bumpy road) to get the following data



It took me 19s to go from 0 to 54.1kph during which I averaged 1250w (after the 3s lag for powermeter to kick in)
Acceleration starts high but I'm at 35kph after 6s and from there acceleration is tiny



Unsurprisingly, I have a handy moment of inertia calculator for wheels and can look at the overall effect in this situation



Hed Jet featured twice to show the effect of heavier tyre/tube, of course - the heavier tyre and tube will have higher CRR which will far outweigh the MoI effect.
Even building a stupidly light Tubular Enve/Extralite wheel gains very little for this example. And has a marked aero penalty.


The full picture requires considering aero and CRR data to compare the options. Unfortunately I haven't finished that part of this particular model (and it's not a major priority). Obviously aero is a small impact in the first couple of seconds when going slowly but dominates the last 10s of 40+

I've also looked at the major accelerations I've had to do in races (like the final sprint) and the rates are pretty low - it's more about who can hold it the longest.

Anyone who picks wheels by weight is missing the point.

...or, by rotational inertia for that matter.

And yet, there are cycling publications who in a review of wheels use BOTH mass and rotational inertia as a very large portion of their rankings. For example:

http://velonews.competitor.com/...or-purchase_264284/1

They've got a moment of inertia tester, and they're damned well going to use it!

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http://bikeblather.blogspot.com/

So would I. Just because more data = better :-)
Of course, if I were building a test lab I'd focus on the important stuff first so the MoI tester probably wouldn't feature.

Why are steep uphill only events an exception? Why would there be a noticeable performance difference in that situation?

Those are the exceptions for mass.
Not inertia.

The only time inertia even begins to be substantive is massive accelerations. Like 1,500 field sprints that start from a slow speed (which is rare). In those cases the intertia differences of wheels can amount to a watt or two.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

Low speed = no material aerodynamic between different setups. So you might as well reduce the number of joules of potential energy at the end of the hillclimb. :)

And by low speed, we're talking ~10 mph or less.

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The question of who is right and who is wrong has seemed to me always too small to be worth a moment's thought, while the question of what is right and what is wrong has seemed all-important.

-Albert J. Nock

Because only then does the work required to lift the mass up the hill start to become a significant contributor to the overall power requirement alongside the aero drag and rolling resistance.

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http://bikeblather.blogspot.com/

It's kind of a basic physics demo. Nothing super exciting to folks that understand moment of inertia. However, it looks like huge masses were used for the demo, which of course would create a big moment of inertia difference. Which makes it highly unrepresentative of real riding on real bikes.

Far more compelling would be how much difference you would see if you had a zipp 808 wheel (so a wheel with a typical high mass rim) vs a good climbing wheel with a low mass rim but a climbing wheel with some tiny weights added to the hub so that both wheels' total masses were equal. But such a demo would be far less visually interesting, as the difference in acceleration of the two wheels would be tiny, even in such a lab type demo.

And nearly immeasurably tiny when one considers in the whole system of rider plus bike on a real course.

Of course, not so with aero drag ...

Greg @ dsw

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Advanced Aero TopTube Storage for Road, Gravel, & Tri...ZeroSlip & Direct-mount, made in the USA.
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I totally agree...and the problem with a "demo" such as that is that it unfortunately encourages mis-optimization based on an incomplete understanding of the system and orders of magnitude.

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http://bikeblather.blogspot.com/

Thank you very much! That graph is super interesting, and counter-intuitive to me (proving how little I know). I had thought that even at a 10% grade, the vertical vector component of the propulsive force would still be pretty small.

Oops I duplicated

I did something similar for our tri clubs discussion of the issue a few years back.

To make it easier for the average Joe to understand I modelled two known wheels (I think I used a lightweight sram and an s60) and a local course with my power output from actually riding it. I then converted the difference into time lost due to accelerations at the 180s over the course of a sprint distance race. I also included the expected aero advantage of the s60 over the same race to complete the comparison.

Using time and known courses helps people take in the concept. Similar to how the wind tunnel data is normalised to 40k tt time.

I will have to see if I still have that spreadsheet kicking around, but the results were surprising only in that I was surprised how much energy the whole rider system takes to accelerate.

I think this sort of analysis would better respond to Slowman's query
It will also be able to show why the smaller 90 deg corner accelerations don't add up to much, or the smaller accelerations over the minor elevation changes, passing etc.

Well more importantly is how the smaller 90 corner accelerations are overall *Easier* with a heavier aero wheel anyway.

About the only time this question even comes up is choosing tubies vs clinchers, (or maybe Flo vs ENVE) that is the only time intertia would be very different at the same degree of aeroness. But even then the weight hurts you 10 times worse than the inertia.

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Kat Hunter reports on the San Dimas Stage Race from inside the GC winning team
Aeroweenie.com -Compendium of Aero Data and Knowledge
Freelance sports & outdoors writer Kathryn Hunter

This is why I love Tom A. As always, good stuff.

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Chris Thornham
Co-Founder And Previous Owner Of FLO Cycling

Non-marginal gains.

There is a very small oscillation in speed (since pedaling is a pulsed power input). Ironically, a bike with higher inertia, will minimize the amplitude of these oscillations, which would result in a lower aerodynamic drag. This was Moser's justification for using a massive rear disc when setting the hour record. Of course this effect is so minor, I doubt it's even worth 1sec./40K.




That 5sec. video should be 10sec, showing how the wheel with the higher inertial moment decelerates less on an upward slope, and catches the first wheel.

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ECMGN Therapy Silicon Valley:
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