# Rotating Weight - debunking the myth



## Jim Nazium (Feb 3, 2004)

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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## CurbDestroyer (Mar 6, 2008)

One thing is for sure Francesco Moser didn't care about it too much at one time


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## android (Nov 20, 2007)

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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## rruff (Feb 28, 2006)

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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## rogger (Aug 19, 2005)

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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## android (Nov 20, 2007)

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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## threesportsinone (Mar 27, 2007)

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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## B15serv (Apr 27, 2008)

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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## asgelle (Apr 21, 2003)

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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## rruff (Feb 28, 2006)

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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## Jim Nazium (Feb 3, 2004)

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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## PeanutButterBreath (Dec 4, 2005)

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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## Francis Cebedo (Aug 1, 2001)

This study seems to put some real numbers behind the math.

<img src="https://accel23.mettre-put-idata.over-blog.com/0/02/72/10/Tests-Acheteur/base-de-donnees/inertia_english_.jpg" width="800">

https://www.rouesartisanales.com/article-15988284.html

It seems to make a difference. Thoughts?

fc


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## Pieter (Oct 17, 2005)

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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## Pieter (Oct 17, 2005)

"
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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## Pieter (Oct 17, 2005)

Jim Nazium said:


> The importance of rotating weight (or, more accurately, polar moment of inertia) seems to be a persistent myth in cycling.
> 
> 
> 
> There is no such thing as rotating weight. You mean rotating mass.


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## boldwarrior (May 3, 2008)

the heavy or light rims makes sense 
anybody tried the Hammer throw in athletics? 
spin that light bit of metal as hard as you can and see if you can even hold onto it!! Heavy as....


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## Jim Nazium (Feb 3, 2004)

Pieter said:


> There is no such thing as rotating weight. You mean rotating mass.


Actually I mean polar moment of inertia, but "rotating weight" is the phrase most people seem to use.


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## Jim Nazium (Feb 3, 2004)

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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## stevesbike (Jun 3, 2002)

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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## den bakker (Nov 13, 2004)

francois said:


> Thoughts?
> 
> fc


1/2*m*v^2


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## KeeponTrekkin (Aug 29, 2002)

In a race where the winner is decided by a photo finish, every gram counts, including rotating mass. For us mortals, it's a horse to beat to death.

Jim got it right; for all but the elite, it's a myth. For the elite, it's still a pretty insignificant issue.


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## android (Nov 20, 2007)

francois said:


> This study seems to put some real numbers behind the math.
> 
> It seems to make a difference. Thoughts?
> 
> fc


The numbers are probably real enough, but the conclusions are not really well thought out.

Just because it can be measured, doesn't mean that it matters. 

A joule is tiny, tiny amount of energy. To put things in perspective, there are about 750,000 joules of energy in a 12 oz can of Coke. (not diet of course) So to spin up even those terrible 165 joule wheels to 30 kph takes you like 1 drop of cola compared to the best wheels that will only take 1/2 drop of cola.

So you could accelerate to 30 kph, stop the wheel, and do it again about 4500 time on a can of Coke.

If you have a power meter of any kind, you'll notice they report energy in kilo joules because a joule is so tiny. You'll also notice that kJ's click by pretty quickly if your pedaling with any force at all. A decent paced ride for most cycling enthusiasts is going to be 800 kilo joules an hour and up.

EDIT: I think some of you would be far more impressed if the graphs were in ergs. You could say things like, "It takes 5 million less ergs to climb 10 feet with my new carbon fiber climbing wheels" and your friends would be so envious....

I mean 5 million, that's a lot, right?


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## CurbDestroyer (Mar 6, 2008)

This all depends on what we are talking about.

If we are talking about accelerating. If you have two wheels, and one set has lighter rims & tires. The one with lighter rims and tires is easier to accelerate. Think criterium and the number of time you have to accelerate. 

If we are talking about a time trial, then the weight doesn't make that much difference.


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## Francis Cebedo (Aug 1, 2001)

KeeponTrekkin said:


> In a race where the winner is decided by a photo finish, every gram counts, including rotating mass. For us mortals, it's a horse to beat to death.
> 
> Jim got it right; for all but the elite, it's a myth. For the elite, it's still a pretty insignificant issue.


I can live with that :thumbsup:.

I abide by the 95% rule. 95% of cycling is training and fitness. 5% is the bike. So the bike does make a difference but mostly if the other part is taken care of.

fc


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## stevesbike (Jun 3, 2002)

after 82 hours of racing, Contador retains the pink jersey by 4 seconds, Di Luca in third at 21 seconds back. While there's a lot of tactical decisions etc. involved in these gaps, that's a gap that can be accounted for many times over -- just on the final climb -- in terms of savings due to rotational inertia, efficiency (ceramic bearings etc). The same principle would apply at any level of racing where the difference between winning, placing, or not is small.


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## asgelle (Apr 21, 2003)

CurbDestroyer said:


> If we are talking about accelerating. If you have two wheels, and one set has lighter rims & tires. The one with lighter rims and tires is easier to accelerate. Think criterium and the number of time you have to accelerate.
> 
> If we are talking about a time trial, then the weight doesn't make that much difference.


It doesn't matter in a Crit either 10^6 times 10^-12 is still 10^-6.
The data are here. http://www.biketechreview.com/archive/images/appd.pdf


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## Kerry Irons (Feb 25, 2002)

*Wt. vs aerodynamics*



CurbDestroyer said:


> If we are talking about accelerating. If you have two wheels, and one set has lighter rims & tires. The one with lighter rims and tires is easier to accelerate. Think criterium and the number of time you have to accelerate.


Unless of course the heavier wheel is more aero, then it swamps the lighter wheel. If you did a 40 km crit on a 1 km circuit with 4 turns per lap, and had to brake 10 km/hr off your speed for each corner, then wheels that saved 800 gm would reduce your total effort by 0.5% 7 calories or 2 watts. That's for braking 160 times. If you only had to brake 16 times, your energy savings would be 0.05% which is about 1 calorie/0.2 watts. Hard to measure numbers this small


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## LosAngelino (May 30, 2008)

For a brilliant article comparing common wheels for aerodynamics and inertia, check out this site: http://www.rouesartisanales.com/article-15441821.html

Their conclusion: the most expensive wheels save you 7 watts of effort per 10 seconds to achieve the same speed compared to the low-end racing wheels.

Here's an anecdotal experiment you can try for yourself: Go to your local tennis shop and get 300 grams of lead tape. Place the tape in your saddle bag and go out on ride including some sprints, tempo, climbing, and descending. 
Now take the lead tape and wrap it around your back wheel between the rim and the rim tape. Replace the tube/tire and go out to do the same ride. Next, take the lead tape and weave it around your spokes as close to the hub as you can. Again, do the same ride. Then, just for kicks and grins, take the tape and try to concentrate as much as you can on one spoke on one side of the wheel as close to the rim as possible. Do your ride.

At the end of the tests you will probably reach these conclusions:
1. Rotational weight matters more than dead weight.
2. Rotational weight matters most during periods of inertial displacement (acceleration).
3. Disbursement of weight relative to rotational axis affects inertial displacement (the further away, the bigger the displacement force required)
4. Balance of rotational weight is an underrated factor in quality wheel building.

Of course, none of this counters the claim that inertia(rotational weight) is less important than aerodynamics. For the average club rider, rotational weight may indeed be a negligible factor, but in the end, every rider has to make their own cost/benefit analysis of the pricier whirlies, taking into account other aspects, such as stiffness and rolling resistance, and of course, disposable income. But then, that's half the fun of riding, right?


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## Jim Nazium (Feb 3, 2004)

LosAngelino said:


> Here's an anecdotal experiment you can try for yourself: Go to your local tennis shop and get 300 grams of lead tape...


... but you've already decided what you're going to conclude. The experiment I'd really like to see is similar, but involves a 'blind test':

Take two identical wheelsets, and on one of them, put 150 grams of lead tape inside each rim. Have someone else (your assistant) mount identical tires and mark one pair 'x' and the other pair 'y'. Have your assistant mount one pair on your bike, then you go for a ride. Switch pairs as many times as you want and do as many rides as you want, just always have your assistant switch the wheels so you can't directly compare the weight. Whenever you're ready, decide which wheelset you think had the extra weight, then take the tires off and see if you were right.

Of course we'd need to repeat this many time to see what percent of the time the tester was correct, but my opinion is that it would be close to 50%, i.e., the same as random guessing.

300 grams is not quite the weight of five Powerbars, and I just don't believe that you really feel slower with five Powerbars in your pocket. Nor do I believe that wheel weight is more important beacuse it's rotating, which was the point of the OP and which I think was proven by the studies cited in earlier posts.

I'd like to see a similar blind test conducted on frame materials, but there are so many other variables it isn't really practical. That's a rant for another thread anyway.


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## Jim Nazium (Feb 3, 2004)

LosAngelino said:


> ... in the end, every rider has to make their own cost/benefit analysis of the pricier whirlies, taking into account other aspects, such as stiffness and rolling resistance, and of course, disposable income. But then, that's half the fun of riding, right?


I agree with all of the above, and I don't want to spoil anyone's fun if they want to put the lightest equipment on their bike. Nor do I want to discourage manufacturers from engineering better products. I just want to encourage people to separate wishful thinking from objective facts.


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## Francis Cebedo (Aug 1, 2001)

Jim Nazium said:


> ... but you've already decided what you're going to conclude. The experiment I'd really like to see is similar, but involves a 'blind test':
> 
> Take two identical wheelsets, and on one of them, put 150 grams of lead tape inside each rim. Have someone else (your assistant) mount identical tires and mark one pair 'x' and the other pair 'y'. Have your assistant mount one pair on your bike, then you go for a ride. Switch pairs as many times as you want and do as many rides as you want, just always have your assistant switch the wheels so you can't directly compare the weight. Whenever you're ready, decide which wheelset you think had the extra weight, then take the tires off and see if you were right.



Why 150 grams??? 

I would say 500 grams!

What are we afraid off? The rider can tell the difference? The problem with small deltas is the rider has to be very experienced and very fit to notice them. Top athletes will be able to detect 50 grams. But if we really want a delta, put 500 grams in one wheel and do a blind test. It either makes a difference or it doesn't.

I say it does.

fc


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## rruff (Feb 28, 2006)

francois said:


> What are we afraid off? The rider can tell the difference? The problem with small deltas is the rider has to be very experienced and very fit to notice them.


You can *feel* the difference with heavy rims/tires because the centrifigal force makes the wheels more steady and turn less quickly. This is a completely separate issue from how it effects your speed, though.


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## djh2 (Mar 5, 2008)

Just think of it in terms of energy. Energy in = Energy out, conservation of energy. Also, Energy In = Energy out + Energy lost due to friction.

So, the energy you put into the system by your legs, m*g*h, the rotational energy by you spinning circles in light shoes vs heavy shoes etc etc etc. This Ei is "constant" for right now. 

If you look at the Energy out, you will see there is Kinetic energy in the wheels as well as the bike as a system moving forward. This energy will be modeled as (Rotational Kinetic Energy) + (Linear Kinetic Energy). We know that Rotational Kinetic energy = (1/2)(Moment of Inertia)(angular velocity)^2 and Linear Kinetic energy = (1/2)(Mass)(Velocity)^2. 

Without going into specific Moment of inertia/rotational inertia formulae for spoked wheels, disks etc and how that works, you can still see how the energy as a broad term works.

Input energy = rotational kinetic energy + linear kinetic energy. If it takes more energy to spin the wheels, then you're taking energy that could go linear into kinetic energy. This translates into less velocity. So in theory, a light wheelset with a lower rotational inertia will accelerate faster because less energy is going into spinning the wheels, as is propelling you forward. The converse goes as more energy goes into spinning heavy wheels, less energy propels you forward.


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## LosAngelino (May 30, 2008)

Jim Nazium said:


> Of course we'd need to repeat this many time to see what percent of the time the tester was correct, but my opinion is that it would be close to 50%, i.e., the same as random guessing.



All this would prove is that some cyclists aren't as sensitive as others when it comes to their ride. It would hardly 'debunk the myth'. Unless your version of the myth is, "the only thing between a club rider and a pro is a set of Lews".

One of the best riders I know runs a set of Shimano R-550's. There is no combination of components that will allow most riders to keep with him. But if you offered him a pricey pair of aero wheels, he wouldn't turn them down.


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## LosAngelino (May 30, 2008)

Jim Nazium said:


> ... I'd like to see a similar blind test conducted on frame materials, but there are so many other variables it isn't really practical. That's a rant for another thread anyway.


This probably is for another thread. But I agree, it would be interesting. In general, it's good for your pocketbook to be skeptical about the claims of manufacturers in the cycling industry, and this is especially true of wheels.


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## Jim Nazium (Feb 3, 2004)

LosAngelino said:


> All this would prove is that some cyclists aren't as sensitive as others when it comes to their ride. It would hardly 'debunk the myth'. Unless your version of the myth is, "the only thing between a club rider and a pro is a set of Lews".


If the result is very near 50%, as I've conjectured, it would prove that the riders in the study could not reliably distinguish between a 1400 gm wheelset and a 1700 gm set. This would debunk the myth, perpetuated in numerous cycling magazine articles, that X wheelset is 'noticably faster' or 'spins up appreciably faster' or 'feels livelier'. That's the myth I'm referring to. Lots of people claim to be able to feel 100 or even 50 grams, and I don't buy it. For a recreational cyclist, if you can't feel the difference, what's the point?

If the result is much higher than 50%, then I'm wrong.

If you're racing, and you're coming within a tiny fraction of winning, then sure, you should look at all your equipment for that extra edge. I just think people would be wise to be realistic about the performance gains they can expect from equipment. If I have to spend 3000 US dollars on a wheel upgrade to save no more than 7 watts, I'd rather spend the money on a coach or even a sports psychologist instead.


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## LosAngelino (May 30, 2008)

Jim Nazium said:


> If the result is very near 50%, as I've conjectured, it would prove that the riders in the study could not reliably distinguish between a 1500 gm wheelset and an 1800 gm set. If I have to spend 3000 US dollars on a wheel upgrade to save no more than 7 watts, I'd rather spend the money on a coach or even a sports psychologist instead.


Given the OP tabling the myth of rotational weight, the study would first have to determine the threshold weight that the riders could reliably distinguish. Then that same amount would then have to be compared as dead weight and as rotational weight to determine if there is a difference. In this test, any difference at all, no matter how slight, would demonstrate that the conventional wisdom about rotational weight is not a myth. On the other hand, if the contention is that rotational weight is oversold as a criterion for better cycling performance, that's a spoke of another color...


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## asgelle (Apr 21, 2003)

djh2 said:


> the Energy out, you will see there is Kinetic energy in the wheels as well as the bike as a system moving forward.


And heat? Please review first law of thermodynamics.


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## rruff (Feb 28, 2006)

djh2 said:


> So in theory, a light wheelset with a lower rotational inertia will accelerate faster because less energy is going into spinning the wheels, as is propelling you forward. The converse goes as more energy goes into spinning heavy wheels, less energy propels you forward.


We knew that already... the point is that the difference is trivial compared to other factors.


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## rruff (Feb 28, 2006)

LosAngelino said:


> Given the OP tabling the myth of rotational weight, the study would first have to determine the threshold weight that the riders could reliably distinguish.


Why? Riders might not "distinguish" by feel an aero benefit that makes them 5% faster, yet they can feel a difference in wheel MOI that doesn't effect their speed at all.


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## Noupy (Aug 25, 2003)

*It matters when accelerating.*

Take two rear wheels lets say 1600 gm each , same spokes ,design,
one has a heavy gauge steel hub and a light titanium rim 
the other a light titanium hub and a heavy gauge steel rim

which one would you want on your bike when riding with a couple of friends and one decides to sprint ?

Put a bike on a fluid trainer and try to turn the cranks with someone putting pressure
with his pinky on the tire
then ask him to to use his thumb and index to try and stop the chain from moving.
its all in the rim and tire 
I like cars and at the local speed shop you see young drivers bring in their Civics
and swap out the steel rims and tires 14 inch for 16 inch mags and low profile tires ( to have the same diameter) and then come back and cry about their car being so slow to accelerate

Why ? because the mass of the wheel is 2 inches further away from the hub .


When i ride with my friends there is always an attack brewing and I am very happy
to have my shamals on my bike


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## asgelle (Apr 21, 2003)

Noupy said:


> I like cars ...


Bicycles are not cars.


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## LosAngelino (May 30, 2008)

rruff said:


> Why? Riders might not "distinguish" by feel an aero benefit that makes them 5% faster, yet they can feel a difference in wheel MOI that doesn't effect their speed at all.


Your comment about the subjectivity of the rider is a different subject altogether and would make an interesting thread. Mathematically, it seems an established fact that rotational weight makes a greater difference than dead weight if we can trust the calculations of the tests at the website I posted earlier.

The only question remaining then is, what is the threshold at which this rotational weight can reliably be perceived by the rider in the form of improved acceleration, and is this greater acceleration worth the additional cost and likely compromise in stiffness/strength?


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## asgelle (Apr 21, 2003)

LosAngelino said:


> The only question remaining then is, what is the threshold at which this rotational weight can reliably be perceived by the rider in the form of improved acceleration ...


Why do you make perception by the rider the sole metric for improved performance?


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## rruff (Feb 28, 2006)

LosAngelino said:


> The only question remaining then is, what is the threshold at which this rotational weight can reliably be perceived by the rider in the form of improved acceleration, and is this greater acceleration worth the additional cost and likely compromise in stiffness/strength?


There is no way that a rider will be able to tell if the bike's acceleration *rate* is 1% faster because his rims are 2 lbs lighter... but he will be able to feel the difference in MOI.


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## LosAngelino (May 30, 2008)

asgelle said:


> Why do you make perception by the rider the sole metric for improved performance?


As the testing suggests, we already know that acceleration is increased by the additional 7 watts per 10 seconds of effort in the best wheels in comparison to the low priced wheels. What we don't know, is at what point any given rider, or an average rider, etc, will actually perceive the bonus acceleration.

Seven watts, when you're pumping out 400, is not much. The placebo effect of thinking your wheels give you the edge, might amount to a greater jump in performance than that from the wheels themselves.

So in effect, a blind study would be the most effective way to determine the weight difference at which the lighter wheels are perceived by all the testers. This is a different test however, from the test to determine the threshold at which most riders can tell the difference between dead weight and rotational weight in general.

Screw it. Let's just ride.


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## CurbDestroyer (Mar 6, 2008)

If you are accelerating two objects, and one is lighter than the other. You can accelerate the lighter one faster. It's that simple. . . . If that's the argument.

Now maintaining the speed is a different story.


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## CactusJackSlade (Jul 3, 2004)

Talk and calculated to death.... ugh...:idea: 

I say go try a set of light wheels and then post up. 

My sub 1200 gram American Classic Mag 300's with light tubes/tires (160 gram tires w/ 50 gram tubes) "respond" MUCH better than my training wheels. By respond I mean they are so much lighter in comparison the lessened gyro effect made them feel "twitchy" until I got used to them. Although I know I can't measure it objectively I feel they accelerat a LOT faster. I dropped over 1 pound of rotating weight... and most of that was rim, tire and tube (where it counts).

I ride the same ride every Saturday and my timed hill climbs seem to confirm this.

As far as being less of a factor for us "mortals" I disagree... I'll take all the help I can get!!!  They will make a difference over the long day's ride... at least I have "felt" so  

Scientific: not! but my bike feels GREAT!


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## DrSmile (Jul 22, 2006)

Light wheels are all nice and good until a crosswind blows you off the road... How much energy is lost to friction counter steering against the wind?

At best 7 watts while you accelerate, then no difference except for aerodynamics... yeah I think that's well within the margin of imperceptibility. Training one week probably gains you more watts.


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## gatorling (Jun 25, 2008)

I don't think 500 grams will make the difference between a person getting blown off the road or staying on the road.
Frictional resistance is directly proportional to the total weight of the system multiplied by a static coefficient of friction (can't be bothered to look up the relatively simple formula).
Those 500grams are pretty moot compared to the 70kg rider on top of the bike.


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## DrSmile (Jul 22, 2006)

gatorling said:


> I don't think 500 grams will make the difference between a person getting blown off the road or staying on the road.
> Frictional resistance is directly proportional to the total weight of the system multiplied by a static coefficient of friction (can't be bothered to look up the relatively simple formula).
> Those 500grams are pretty moot compared to the 70kg rider on top of the bike.


The weight of the rider is fairly immaterial. The wheel staying straight depends on the rotating mass and the speed that it's spinning at. That is the basic concept of a gyroscope.


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## gatorling (Jun 25, 2008)

DrSmile said:


> The weight of the rider is fairly immaterial. The wheel staying straight depends on the rotating mass and the speed that it's spinning at. That is the basic concept of a gyroscope.


Although a rotating mass does help you stay upright it's fairly immaterial compared to the effects of having more trail on your fork.
[Physics Today, April 1970]

A bike with a second wheel mounted parallel to the front wheel and spun in the opposite direction was found ridable. A bike however, with a negative trail was very difficult to ride.
Increasing the trail on the fork.

Also a few grams on your rim isn't going to produce enough of a gyroscopic effect to determine whether or not you're going to blow over. Cross sectional area would be a much bigger factor.


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## DrSmile (Jul 22, 2006)

You're changing the premise. Trail matters BECAUSE of the gyroscopic effect of the wheel. If the wheel isn't spinning, you can have any trail you want and the handlebar will turn just as easily. And yes I agree that cross sectional area matters a lot. But it's the ratio of the area to the weight of the wheel, and again here a lighter wheel loses, and to reiterate that this lighter wheel will have less gyroscopic effect to counter the crosswind.

The test you're referring to (by David Jones I believe) doesn't address the tracking, just the stability. The bike WILL be less stable without the gyroscopic effect.You have to read the wording very carefully. I would also add that David Jones SPECIFICALLY indicates that gyroscopic action is minimal at LOW speeds, which is hardly applicable to road bicycles. In fact his entire article is misleading to use on a modern road bicycle as it does not address handling at all, but rather stability at low speed. When you think about it it makes perfect sense that castor (trail) would be very important at low speeds, as this keeps the bars from turning. At speed, the gyroscopic effect becomes more and more important as this effect increases with the speed of rotation (and the wheel's mass!).


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## gatorling (Jun 25, 2008)

I disagree, the gyroscopic effect might be a component but the extra 100-200g per wheel is insignificant compared to the normal force exerted by the rider and the bike. Resistance due to friction wins in this case.


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