Lombard

And it's all your fault. Troublemaker!!

3alarmer 

...no offense intended, but once it went to wheels, it actually became an interesting discussion topic. Everyone, beginner or self styled expert, rides on wheels. And with the advent of newer materials tech and the R+D guys assigned to find a way to make an essentially un-aerodynamic package more aero. wheels have been all aero'd up for about the past 20 years, dating back to the exploding Synergy wheels that damaged a few people. All the aero wheel designs add some weight at the rim, so suddenly the discussion boards were full of this.



A lot of the more specious arguments put forth follow the model in this thread. Someone does a "calculation" of exactly how important weight on the wheel might be, in comparison to weight on the frame and rider. They use the commonly available equations for the acceleration force for spinning a wheel, versus the linear acceleration of the same weight on the bicycle frame. They get a relatively trivial answer. Not asking themselves whether this could be suspect as an answer, using their real world experience with bicycle wheels and tires, the question of where on the wheel this weight is added never enters the modeling. It turns out that a bike wheel, for some very practical reasons with regard to the conditions of the surfaces it travels upon, is kind of a poor design from the acceleration standpoint, with much of the weight out at the rim and tire, where it requires more force to spin. (This was the reasoning behind Moulton's designs, using small wheels.)

I forget now, but it's something like a factor of ten in difference, if you add the wheel weight at the rim on a 700c wheel, as opposed to adding it at the hub, at the center of rotation. Which is, I think, where that .3% number came from. Probably an engineer, or ex-engineer, who is used to letting the science guys do the modeling..."just give me the equations, damn it." IIRC, the actual number for requisite additional force is in the 2.5% area for something like a half pound at the rim. It's been a very long time since I bothered to calculate something like this, because aero wheels are all the rage, and they do seem to work, even if I don't use them. Offsetting advantages and disadvantages with them make them the obvious choice, when permitted in the rules for your event, and there is no drafting or quick maneuvering involved.

But all other things being equal (and they are not in this case), putting a pound of additional weight onto your bike by using tires that weigh that much more on your rims, will slow you down quite a bit more than sticking it in your pockets. Try it yourself, if you don't believe me.


How's that for mansplaining ? Good luck in your endeavors. I was a physical wreck from a lifetime of athletic abuse by the time I hit 50. Stay flexible, and remember your breathing.

tomato coupe

No, not close. Do the calculation.

Koyote

In an earlier post, I mentioned ordering some new cf-rimmed gravel wheels. This point was a key factor in my choice: the wheelset I ordered has the same rims as some much more expensive Specialized wheels, but cheaper and heavier (and likely more durable) hubs. As long as the hubs are reliable, I don't think their weight matters much. But lighter rims, especially when they're much wider than my current rims and hence will allow me to use a nominally narrower (and hence lighter) tire? Sure, I'll take that gain, even if it's not as high as 2.5%.

I think we all understand some of the epistemological problems with personal perception. I prefer science.

As a shameless wheelsucker, aero is less important to me. Just give me a large and strong friend to draft, and I'm good. That's yet another reason for wanting lighter wheels: sometimes the bastards try to accelerate away from me, and I have to catch them.

To be fair, you've been addressing plenty of other (presumably male) posters in the same manner in which you've addressed the OP, so I wouldn't accuse you of 'mansplaining.'

3alarmer 

...science is all about theory, verified by experimental results. The theoretical here is ( I think):

where in this case, not only the radius is increased, but all the weight is moved out to the circumference as well.

But the simplest experiment I can envision is to get some really heavy tires (not sure where you'd find tires that are heavier than your current ones by half a pound each). and mount them to your wheels. Then see how it rides. You could probably inflate the lighter tires to lower pressures to more or less the equivalent of the crummy heavyweight ones, but it will probably alter rolling resistance. So not an ideal experiment. Maybe just turn the rear wheel on a stand, using the different weight tires, and measure the required torque in each case. But then you lose sight of the real world aerodynamics. It's not a simple problem to isolate one variable like this in a complex system.

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Lombard

And once again, you would be wrong.

Carbonfiberboy 

Weight of neither the rim nor the tire will make a bicycle slower, other than the increased weight will slow it on climb, but no more than the weight anywhere else on the bike. This is really very simple, no need to do any calculations.

Why? Consider the pendulum. What determines the period of a pendulum? Only the length of the string or arm, not the weight of the object at the end of the string or arm. That makes no difference at all:
https://www.acs.psu.edu/drussell/Dem.../Pendulum.html

It is possible to prove this through calculation, but it requires calculus. Perhaps some more recent graduate than I can provide the equations.

But why is that true, anyway? It's simple. The acceleration of gravity accelerates the weight from the end of its swing. That increases its kinetic energy. It then uses that kinetic energy to propel itself up the rest of the swing, against the acceleration of gravity. At the end of the swing, it then has a certain amount of potential energy, which it then coverts to kinetic energy as it swings down to the center point. The amount of weight involved makes no difference, there's only the conversion of one type of energy into another.

It's the same thing with a bicycle rim. It's true that it's harder to accelerate a heavier rim, but then it gains kinetic energy which makes it want to continue to spin. So a heavier rim/tire combo will be harder to accelerate at the start of the sprint, but will make no difference once top speed is attained, and thus also makes no difference when holding a steady pace on the flat or on a climb. Eric F, in post 64 noticed this effect and described it very well. When one pedals OOS, especially pedaling hard, one only hammers the downstroke. Maximum acceleration happens very briefly, when a pedal is at 90° on the downstroke. The bike then rather coasts a bit until the other pedal hits that point. A bike with heavier rims will coast better and seem to pulse less, both if speed over the ground and in perceived pedal speed. Eric said that "it felt like they were pulling me along." Exactly.

Of course a lighter bike is faster on climbs, and accelerates faster, but weight is weight and it's distribution does not matter except when accelerating the bike's average speed, when we have to add a little (very little) additional energy to spin up the rims and tires. Holding a steady pace, rim and tire weight do not matter - though is the extra weight happens to improve the aero quality of that rim/tire combo, then the bike will be faster.

Those super light Helium rims came and went very quickly. Riders quickly discovered that those rims weren't faster, in fact box section rims are slower. All the pros use aero rims on climbs now. The little bit of extra weight is more than offset by the aero advantage, even at climbing speeds. Look at 2:08 for example:

As an aside: note how still the top riders' upper bodies are now. Nothing like appropriate gearing and being smooth.

3alarmer 

.
...wow.

Nice talk. I stand corrected.

son_of_clyde

Looks pretty negligible. This isn't to 30 kph, but rather a 90kg bike/rider modeled with a constant 50N propulsive force (~828W at ~37mph) with CdA = 0.3. Rolling resistance not accounted for. Two wheel weights were considered: light set at 1500g, heavy set at 2000g. All weight concentrated at rim. EOMs used : dxdt(1) = x(2), dxdt(2) = (1/(m + 2*I/r^2))*(F - 0.5*rho*CdA*x(2)^2) with rho (air density) = 1.23 kg/m^2, r (wheel radius) = 0.343m, I (wheel moment of inertia) = mass_wheel*r^2, m (mass bike+rider) = 90kg, and F (propulsive force) = 50N. Many details were not included in this model, so it's not a highly accurate representation of reality.

vespasianus

But what if the wheels were 1200g instead of 2000g?

And yes, I am being sarcastic. When the MTB world moved from 26" wheels to 29" wheels, people said the extra weight of the bigger wheels, tires and frame would make them really slow to accelerate and such, and as we know, that is not the case. Bike weight is sooo over-rated for anything on the bike. In fact, for off-road riding, a heavier bike (and heavier wheels) actually feels much better - again something the MTB world learned years ago.

Lombard

You're getting there.

Lombard

IMO, I prefer a 26" mountain bike, but it has nothing to do with weight. A 26er is able to turn much tighter than a 29er which feels ungainly to me. I believe one of the reasons the MTB world embraced 29ers is because they can roll over objects easier than 26ers.

vespasianus

Now, a 26er versus 29er discussion will reduce the inflammatory nature of this thread! (:

I also still have a 26er that I ride often but also a modern 29er (Ripley). Both are fun. The wheelbase on the Ripley does impact the ability of tight trails and you definitely feel the gyro-effect of 2.6 inch tires on 29ers. I can feel it on my gravel bike as well (700x45 tires), which is the only reason to have lighter tires (maybe?).

Lombard

Whoa! A new argument - 26ers rule, 29ers drool!!!

Eric F 

As climbs get steeper, and speeds continue to reduce, wouldn't gravity reduce the contribution of the flywheel/pendulum effect, so you're effectively having to accelerate the wheel again on every pedal stroke in order to keep moving? It seems to me that a lighter rotating mass to accelerate becomes more of a factor as grades increase. I'm not very smart on the theory stuff, so I'm probably wrong. When I was racing, there was a choice of light or aero. These days, there are wheels that are lighter than the old "light" wheels, and more areo than the "aero" wheels. It's like having both cake and pie!

Eric F 

I ride both a 26er and a 29er. Both are hardtails that weigh fairly close to the same. To me, the difference in handling is not significant. The rollover capabilities of the 29er are definitely appreciated, however.

3alarmer 

...that's very interesting, and much more detailed a calculation than any I've seen put forth elsewhere.


One question, if you will permit me. Your first graph (above), there seems to be no difference at all in the two wheelsets, even accelerating from low speed.

I'm not questioning either your expertise (I don't know you), or your mathematics. But this seems to fly in the face real world experience, even of the staunchest "wheel weight doesn't matter" folks here. All of them have argued that it does, at least, matter in accelerating from a standing start. In fact, a couple of them took great offense at my suggestion we go back to steel as a wheel rim material. Am I reading your graph wrong, or is there some other explanation ? If I am going to be wrong about something, I'd at least like to learn from it. And I freely admit, I learned little about pendulums here.

Is 6 or 7 MPH the magic number where this weight difference no longer matters ? There seem to be no blue line values below that.

Carbonfiberboy 

No. You still get that energy back. The only way to destroy energy is to turn it into heat (brakes). The heat death of the universe . . . is something we'll never see and good on us. That pendulum without a periodic injection of extra energy from the clock works would slow and eventually stop. Why? Because of friction with the air, which generates heat.

But of course one will feel the deceleration much more on steep grades. Which is a good reason to use low gears, pedal fast, and keep the bike moving. On really steep stuff, when I've run out of gears, I prefer to sit and pedal circles. I'm not good at standing for long periods or I'd probably like that better.

3alarmer 

.
...two more discussions on this same topic. One for bicycles, and one for cars (which is pretty much where I originally learned this stuff.)

3alarmer 

.
...and here, for all the world to see, is the sort of writing that set me on the wrong path in this world of wheel weights.

We Can Prove Why Extra Mass on Bike Wheels Is Your Worst Enemy

Eric F 

Hmmm...I'll take your word for it. My dumb brain hasn't wrapped itself around it yet, though.

3alarmer 

...as a physical model, there are many places where energy is lost on a bicycle. Tires, rolling resistance, aero resistance because we don't ride in a vacuum, general friction losses all come to mind. I'm not arguing your general point that in a perfected theoretical model, mass continues to go in the same direction, and at the same speed, once you get it moving. I'm just saying that in this particular case that we started out discussing, (a 112 mile tri race over rolling countryside, with 20-30 mph gusting winds), it's a much more complicated picture of how often you need to introduce new accelerative force into the system.

tomato coupe

Why in the world would you post a link to an article that you think is misleading, and put it in a gigantic font to draw more attention to it?

Carbonfiberboy 

Well no, energy is not lost, it's just turned into heat. But we were talking about rims (I thought). There's a little bit of energy lost due to atmospheric resistance on the surface of a rotating rim, and the deeper the rim, the more is lost, but that's a very small amount of loss that's only due to rotation. Vastly more is lost to the spokes just from rotation. What you're saying is true, but it has nothing to do with rim weight. In all the cases you point out, less energy is lost to an aero rim, no matter its weight. Though one must also point out that next to the losses due to the bike and its rider, the contribution of the rims is quite small.

tomato coupe

Let's not rewrite history. The rolling hills with 20-30 mph winds scenario is just some spaghetti you threw at the wall on page 5 of this thread. This whole journey down the rabbit hole started when several people pointed out that your (much) earlier statement was incorrect: