Extra Mass on Bike Wheels Is Worse than on Frames
wired.com
wired.com
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TL;DR Considering reasonable wheels: Wheel stiffness is more important than weight. A too light wheel, is not stable.
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Wired's physics are correct. However, neglect mechanical dynamics & handling.
Probably the biggest downside of light (& aero even more) wheels is: When your wheels are very light, they are prone to cross winds and "jump around" a lot due to their low inertia. And, well, you always have cross winds and bumpy roads, because only very few of us are track riders.
Considering the acceleration, you lose a lot of energy by "twisting" the wheel. That is: the hub starts to turn because the chain you pull with the crank arm in front is forcing it to do so. Then, all this tension is transferred to the spokes, rim and finally the tire. In that process most wheels lose a significant amount of energy by simply storing it via elastic torsion and releasing the energy without contributing to movement at all. As already said in another comment, we "pulse" the energy to the back wheel, as the maximum pulse comes due to our own biomechanics. And we have two pulses per revolution with a zero at almost vertical crank arm alignment!
Therefore you want to have stiff wheels! Which stands in stark contrast to light wheels.
The company Lightweight basically made its money by creating aerodynamic and very very stiff wheels for the weight they had. These wheels, however, are not maintainable any more & very expensive [2].
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Wired's physics are correct. However, neglect mechanical dynamics & handling.
vs other comments denying that like
This is one of the most persistent and silly myths in cycling.
Which shall it be? A combination of the two?
Yes, when accelerating, wheel mass is worse
than frame weight, but only a tiny amount.
Edit:
To expand on this a bit: This "tiny amount" is a lot when you are among people close to your performance. And I'm talking about stiffness vs. weight. Stiffness is just so much more important, because the sum of these infinitesimal losses is just huge:
Modern riders rpm: 100 which equals to 200 energy pulses per minute, because each leg has a rest phase and a full force phase per revolution. In a 4h race: 240 * 200 which are 48000 pulses. Now imagine you have a super light but not very stiff wheel (because it's light, you know)... that's a lot of lost energy.
> What's the estimated lost energy?
From my point of view, that's almost impossible to calculate. But one can measure it. See:
Stiffness and Damping in Mechanical Design
by Eugene Rivin
Or Gavin's paper I cited in the other comment.
> Where does it end up as the wheel flexes?
Stiffness & Damping are closely related, as the book title above already implies. A part of the energy is lost in heat and structural changes. And another part is basically unused as it's unused by design:
Imagine the following experiment. You need to push a box using many impulses over a specific length. At the side of the box is a plate, which you are punching, which is attached to a spring which itself is attached to the box. Now, let's go the extreme. Your impulses are so short and weak, that the spring absorbs most of it and the box hardly moves. The energy quantum you transferred via impulse, was used up by the spring and the spring released it by its own = Lost by design. Now, let's get your impulses bigger and longer. You'll move the box better. And now let's gradually fixate or, say, stiffen the spring. Each hit moves the box directly.
That's stiffness.
You hit the pedal and you experience how the bike just instantly starts to accelerate your body forward. Which a very stiff wheelset and bike would do. Now get on a fully. Thick tires, springs everywhere. Hit the pedal. You'll feel like cycling under water.
PS: I'm not Eugene Rivin.
e.g.: For now, let's assume that the wheel is a torsional spring, we're putting a force of 1000N (~100kg*g) at the rim, (roughly 300NM torque) and it deflects 1 degree. The energy that goes into the elastic deformation of a torsional spring is k(theta)^/2, t=k(theta), so energy = t(theta)/2. Plugging the numbers, you get 2.5 joules.
Now, 1000N is a lot of force, and you're only likely to have that level of force with a heavy rider in a very low gear. (i.e., your tires have a coefficient of friction around 1, so that's implying 100kg on the back tire just to prevent wheelspin) But 2.5 joules starts to sound like it _might_ be measurable, and maybe look at the numbers a little more closely to see if they're reasonable.
I'm only a fairly decent rider, but even then that's only about 1% of my 5 minute output, (if you assume 1 power stroke per second).
5s: 23
1min: 11
5min: 7.5
20min: 6.5
5s: 20.28 (that's touching UCI Div 1/2 - sprinter type)
1min: 10.14
5min: 6.43
20min: 5.14
Side note: Rumors were that the big LA had 20min+ values above 7.5, which is pure alien. The 5min test is hell, because one stresses _all_ energy systems in the human body to their maximum effort. So you ride in pain after something like 30 secs till the finish, which feels eternal. The 20min test is more easy compared to that, as it is very close to your general endurance. Thus, a top rider these days would rip his legs & mind out, when climbing besides the former LA.
My point still is, it's possible to calculate the upper limit on losses from wheel flex under power load, and I'd assert that they are small enough to not matter. And what's more, the effect of torsional spring type flex goes down (at a constant power output) as speed goes up.
Because my experience is that if someone is talking about bicycles and stiffness, most of the time they haven't done the math.
If stiffness were really the problem, no one would be using low spoke count wheels, or radial/2x rears, or any of the funky asymmetric ones.
Gavin, H. (1996). "Bicycle-Wheel Spoke Patterns and Spoke Fatigue.",
J. Eng. Mech., 10.1061/(ASCE)0733-9399(1996)122:8(736), 736-742.
There is little money for that kind of research. TU Delft is the only university I know investing here. I'd be happy about anybody pointing me towards groups doing that, as I'll finish my Dr.-Ing. (as an engineering PhD is called here) in Germany this year.
Furthermore, jacquesm's comment nailed it: It's an aero compromise, as many (jackmott, thedufer, slashdotdash, etc...) already more or less said: Multiobjective Optimization at its best.
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[1]: http://teamgiantalpecin.com/team-giant-alpecin-to-accelerate...
Looking at that paper, I've got a couple of comments:
1) I'm not clear where they're getting lateral forces when a bicycle is cornering, unless the rider is hanging off the side of the bike motorcycle style. (which is generally unnecessary as bikes have enough clearance to corner at ~45 degree lean angles).
2) It's unfortunate that they didn't instrument a trailing spoke as well, so that they could disentangle the torsional strains from the radial strains.
3) While the tangential/torsional strains are high, it's not clear what the loaded zone is there, nor how much energy is being stored in the wheel due to power pulses. I suppose I could pull out my old FEA model and port it to see.
Parts wise, I'd look at something like:
* 36 hole hubs, either deore or ultegra level. 32 if you can't find 36. Not less than that. Some tandems go higher. This is more important on the rear wheel, where half (drive side) of the spokes are doing most of the work, because the other side's tension is so low.
* 3 cross, double butted spokes. It looks like the Dt Competitions are 2.0/1.7, which is a decent balance between elasticity and windup. 2.0/1.8 used to be my go-to choice, but that's hard to find now.
* Deepish section double eyelet rims, such as the Mavic Open Pro series. They should have closed section, not just a U for torsional rigidity.
* Grease the spoke threads and the nipple bearing surface.
* Tensioned just to the point of elastic instability in the rim, then backed off.
Reasoning:
* The key here is that you _never_ want a spoke to go slack under load. Once that happens, bad stuff happens with fatigue.
* 36 spokes puts either more spokes in the loaded zone.
* 3 cross lacing makes the spokes come in tangential to the hub, which reduces stresses on the spoke holes and prevents tearing of the hub.
* Double butted spokes allow the spoke to stretch more in initial tensioning for a given force on the spoke. As the rim deforms under load, the spoke will remain under tension to larger deformations.
* Eyelets make the nipples easier to turn in the rim, greasing even more so. Double eyelets also get support from second wall of the rim. I've had single eyelet rims (e.g. old MA series) fail where the inner wall of the rim pulled away from the braking surfaces.
* Grease the spoke threads and nipples. You want the nipples to turn on the spoke threads, not seize up and torque up the spoke like a spring. If you first ride a new wheel and hear it pinging, that's the spokes unspringing and unscrewing. Likewise, a good builder will always back off the tension a bit till there's no torque in the spoke.
* All other things equal, a wheel with twice the tension on the spokes will hold twice the load before you unload the bottom spokes, and have a much better fatigue resistance. So, take the tension as high as possible.
Using 32 (front) and 36 (back) using your mentioned double eyelet rims (tubular) in Cyclo-Cross racing these days.
When biking at a steady speed, even when accounting for the fact that steady speed isn't entirely steady because humans pulse their power a bit, wheel weight is not worse than frame weight (the extra effort of each micro-acceleration is offset by the reduce slowdown between micro-accelerations and then some)
When climbing a mountain, wheel weight is not worse than frame weight
Yes, when accelerating, wheel mass is worse than frame weight, but only a tiny amount. source: physics: https://docs.google.com/spreadsheets/d/1N2aS3E0K4wrTa5jKqed7...
That is my spreadsheet, it is very very basic physics and it drives me crazy that people spew pseduo physics and comment on this "without taking the time to comapre the sizes of the effects"
take the time!
TLDNR; don't worry about wheel weight any more than frame weight. Addendum: don't worry about frame weight much either.
I wonder where this old wives tale about wheel weight comes from?
Fun wheel videos: https://www.youtube.com/watch?v=GeyDf4ooPdo https://www.youtube.com/watch?v=ty9QSiVC2g0
The suspension reason isn't relevant to lightweight bicycles but important to keep in mind for other bikes.
I think your spreadsheet makes several assumptions that are only true in theory.
As other have said, acceleration costs more energy. You don't get that back when braking.
More importantly, in practice, the road is not flat at all. There are small movements at every brick / stone that you cross, slightly rotating the bike forward and back. The momentum (MOI) multiplied with the weight of the bike for those those small movements is lost in energy. Weight at the ends of the bike (mostly the wheels) increases the momentum of the bike far more than weight in the center of the bike. I don't remember the exact energy formula, but think there is a power of 4 with the distance from the center in it...
I'm no expert but I would say that wheels are closer to a ring. You're underestimating the moment of inertia of the wheels by a factor of about 2 by doing this. Might be well enough to throw off your conclusions. In fact if I read your spreadsheet correctly this would bring the rotational energy to about the same as the translation kinetic energy.
First three pedals are the only ones that count. All we ever, ever do is accelerate from a dead stop and cut people off at the 30ft line. I'm running front-specific rims front and rear to cut weight, and it's working.
This may not be applicable to cycling at large, but it's extremely important for BMX racing.
As a sidenote, the picture looks to be a late-1880s Columbia of some sort. Yup, same Columbia from whom you can buy a bicycle today (well, at least something bicycle-shaped; they haven't made good bicycles in over 100 years).
At my bicycle co-op, we call these BSOs, bicycle shaped objects, to describe Huffys and other Walmart department store bikes :-)
Completely wrong. Biggest bang for the buck is low CRR tires, then skinsuits, then, I don't know, maybe helmets. Then 100 other things, then light wheels.
Skin suits? Who let the triathletes in? :-) Horses for courses, I guess. For a crit, road race, and generally just riding around, I'll take a nice set of wheels before squeezing myself into a sausage casing.
Air Resistance:
> However, this force will produce a greater change in speed on the bike with less mass.
But the wheels act as (unsurprisingly) flywheels. Extra weight in the wheels adds both momentum and angular momentum, while frame mass adds only the former. This is exactly symmetric to their acceleration argument, so it's kind of shocking they skipped it.
Internal Friction
> More mass on a bike can increase the friction in the bearings—but again, this won’t matter where the mass is located.
As an extreme example, a massful wheel with a massless frame clearly has no bearing friction. I'm pretty sure wheel weight does not contribute to bearing friction.
Does anyone have any insight into why these matter less than the acceleration bit?
But of course most people riding a bike are not racing. Even still acceleration is quite important. When you are on a relatively uniform gradient you can judge your power easily and make sure you don't go anaerobic. It's when you have to push hard to get over a small bump, or sprint ahead to get out of traffic, etc that you get into the red zone. Making those situations easier will make your whole ride easier.
But it is fair to point out that "good" bikes are really only "better" for certain situations. For example, my wife almost never travels faster than 20 km/h. Many of the things that are absolutely necessary on a "good" bike that is travelling at 40+ km/h make absolutely no difference for her. Lot's of people tootle around at 10-15 km/h. Any well maintained bicycle will be completely fine. In fact a "good" bicycle will be unsatisfactory because it will be hard to handle.
It's all about context. And in the context of talking about which bicycle is "better", the convention is to assume you are talking about road racing unless otherwise stated.
There's only so much that toying around with slightly better uprights can do.
Also: modern bikes have clipless pedals, not toe clips.
So yes, I had problems on hills, because there was not enough weight on the driven wheel.
But mine was a very special kind of recumbent bikes, without a classic handlebar and instead steered with the feet. Most recumbents are driven on the rearwheel.
For the curious: I used to ride a flevo racer with a very unique steering mechanism. (See eg https://www.youtube.com/watch?v=eyTP5I8hv5Q)
It's a very curious bike to ride, with a long learning curve.
Accelerating, as the article mentions, is the one place that their theory holds. My question was why that's considered more important than holding speed (which heavier wheels makes easier) or the extra bearing friction.
Here's the thing with "I'm a print subscriber but hate having to log in on every device I might read your site on".
No longer. Screw minimizing wheel weight. I want my bike to be an extension of myself. I want to be able to push it and for it to deliver. If I need to spend some extra joules on angular momentum, fine. My thighs need the workout anyway.
Downhill bikes weight 40 pounds, BMX's have 36 spoke wheels for 20" rims, etc.. for good reason: they are built to be good at what they do. You want to go race faster, you get light wheels, with medium to deep section rims.
That is all before you get into the category of exotic and expensive deep profile CF rim wheelsets that cost $1500+/pair...
The science of bikes is pretty great.
That said, got mine used for $1800. Sold them 2 years later for.... $1850.
The tech eventually filters down and I can now buy wheelchair wheels like http://www.spinergy.com/content/wheelchair/products
Is this always true regardless of wheel radius? At first I was thinking it has to depend on radius, but then thought about how the ground contact point has a zero velocity, and the exact other side of the wheel is always double the velocity of the bike frame relative to the ground, regardless of wheel radius. I'm not sure if this is a reasonable intuition. Maybe it makes sense that a gram added to the rim is always double the difference in effort that a gram added to the axle or frame or rider is.
For racing/road bikes, maybe. You're leaving out the 26" wheel adult bikes (which over time has encompassed 5 different rim diameters[1]) and 24" wheel bikes that are targeted at youth, but smaller adults sometimes prefer (well, those that have been give the option).
My bike pedaled contraption is a rear suspension compact long wheelbase recumbent with two real 20" (406mm rim) tires and they're spectacular.
Yes, the kinetic energy of a uniform ring on the outside of the wheel is exactly double of the kinetic energy of an equal mass elsewhere on the vehicle. You can prove this by integrating around the circle; the key is that the ratio of the velocity of a point on the ring to the velocity of the vehicle depends solely on the angle. (The point in contact with the ground isn't moving, while the point at the top of the tire is moving at 2v.)
Note that this applies only to the outside of the wheel; there is a cost to weight closer to the centre of the wheel, but not as much. (Also, things get weird with non-rotationally-uniform wheels, but people tend to avoid those for other reasons.)
https://en.m.wikipedia.org/wiki/Moment_of_inertia
It's discussed in terms of inertia and momentum, but these quantities are the useful quantities when discussing rotation about an axis.
Speaking loosly, applying a force perpendicular to the radius of a rotating body changes its angular velocity, which is a factor of momentum. The moment of inertia determines in part how large the torque on the mass is for a given force about the center.
"In summary, wheels account for almost 10% of the total power required to race your bike and the dominant factor in wheel performance is aerodynamics. Wheel mass is a second order effect (nearly 10 times less significant) and wheel inertia is a third order effect (nearly 100 times less significant)."
http://www.biketechreview.com/reviews/wheels/63-wheel-perfor... http://www.slowtwitch.com/Tech/Why_Wheel_Aerodynamics_Can_Ou...
Riding in a bunch, race or the city. Not so much. In Crits (80 laps times 4 corners at least) or a city ride you don't move fast most of the time, but bike handle, accelerate & brake most of the time.
"... the point of all this was to demonstrate the relative unimportance of wheel rim mass and why aerodynamics matters even when accelerating"
[1] http://alex-cycle.blogspot.co.uk/2014/12/the-sum-of-parts-ii...
[2] http://alex-cycle.blogspot.co.uk/2014/12/the-sum-of-parts-ii...
[3] http://bicycles.stackexchange.com/questions/15047/are-aerody...
Today I learned that a VPN counts as an ad blocker. Hooray.
On the flip side, this is also exactly the same reason why flywheels outside of automobile land, when used as energy storage devices, are hugely massive.
Better yet, get a high resistance tire. Wide knobbies will do it.
Naturally this doesn't, of course, diminish the fact that lighter wheels give you more bang-for-the-buck than lighter anything-else, due to the physics described in the article. But if the difference between heavy, cheap wheels and light, expensive ones is a few hundred grams and a few thousand dollars, losing a pound of body fat is likely a better bet.
Bicycle weight only helps if you don't push yourself already. Going fast is fun, so why not go light?
Here's something they don't mention in the article: the location of the mass in the wheel has a direct relationship to torque. The same mass located in the hub will take less torque to move than if the mass were located in the rim. So acceleration could in part be improved merely by relocating mass in the wheel. (But that's just wheel acceleration torque; you'll still need to do the same amount of work to move the same total mass of the bike at the same speed)
Also, for the same weight tire rubber, wider tires would create increased friction and slow the tire down more.
[1] https://en.wikipedia.org/wiki/Unsprung_mass#Effects_of_unspr... [2] http://www.formula1-dictionary.net/unsprung_weight.html
The same conclusions about mass hurting acceleration also occur with the car, but the relative masses are so different, it's mostly irrelevant (cars are a lot heavier than bike frames).
The main difference comes with the ability of a lighter wheel to be able to fully track the road, and not leave it on a bounce. This gives better traction. (But somewhat greater vibration in the cabin.)
Unsprung mass is relevant to vehicles with suspensions (including mountain bikes). The inertia of the vehicle is set against the inertia of the wheel when it's being pressed back down on the backside of a bump.
Humblebrag: I could bunny hop my 28 pound trial bike 2 ft, and sidehop 30 inches too.