PeteHski

Do you brake very often when climbing 7% gradients? I know I don’t.

The fact is that wheel inertia has little or no additional effect over and above the static mass of the wheels when climbing. You can totally ignore inertia when calculating how fast you would climb Alpe d’Huez. All that matters when climbing is total static mass, rolling resistance and aero to a lesser degree as it gets steeper. The models predict the outcome very well.

Wheel inertia only becomes a factor when there is a significant amount of braking and acceleration involved.

terrymorse 

I've given this a fair amount of thought, and I suspect there may be a feature of climbing where wheel inertia matters.

Let's call it "micro decelerations-accelerations". Whenever the grade goes up even slightly, the speed drops quickly, and the rider must put extra force on the pedals to get back to speed. These very short bursts of acceleration add up, and if the wheels have more inertia, that effort requires more peak pedal force and total power.

One might reply that the heavier wheels also decelerate less quickly. Sure, but they require more power (and force) to bring back up to speed, and that's the part that causes leg fatigue.

I've climbed with light wheels and heavy wheels, and the heavy wheels definitely were more fatiguing.

Just a thought.

PeteHski

Those micro-accelerations and decelerations simply cancel out unless you actually use your brakes. So they are irrelevant.
The only thing that does matter is the actual static mass of the wheels as part of the total mass. This is what you are really feeling with heavier wheels when climbing.

You've actually said it yourself above. The heavier wheels decelerate less quickly and accelerate less quickly. So they actually damp out any micro-speed changes. In reality you are climbing at a steady speed with minimal acceleration or deceleration anyway. You get to a steeper section and you lose a few kph. The change in inertia of the wheels is trivial. It's not like say accelerating from 10 kph to 50 kph, where you might actually measure a real difference with wheel inertia.

venturi95

I disagree. You are assuming the rider's output is a mathematically perfect model. In reality there is a pulsating nature of power output, twice each crank revolution, from beginning to end of the climb. With a cadence of 80 rpm, 160 times a minute the rider is trying to accelerate upwards. "Micro accelerations" do not simply cancel each other out.
Take two identical riders and bikes. Have the first climb a steep 3 mile hill at 6 mph (30 minutes). Have the second climb the same hill in 30 minutes, but have him coast (no brakes) to a stop every 100 feet. Your argument says they do the same energy expenditure, perhaps mathematically they do, but we all know that's just not how cycling uphill works.

PeteHski

Your speed variation with this "pulsating" power output is so minuscule when climbing that it really doesn't matter and it actually does cancel out anyway. Only on BF do people regularly dispute basic physics. I knew this would happen, it always does. But if you want to disagree and pretend something else is happening then fine.

Coasting to a standstill every 100 feet and then having to accelerate to a higher speed to maintain a 6 mph average is not really proving your point. That's really more of a comparison of high power intervals vs steady state power application i.e. testing the rider's physiology rather than the physics of mass and inertia.

I just had a look at my speed variation on a steep 0.5 km section of a fairly steep climb and it's less than 1 kph across the entire segment with no measurable speed variation across individual 1 sec intervals. So changes in wheel inertia are totally insignificant. You might see more effect from being hit by flies.

You are not trying to "accelerate" upwards 160 times per second, you are in fact maintaining a fairly steady speed (a very close approximation to zero acceleration)

venturi95

So if I put 2,000 grams of water in my waterbottles before a climb it is no different than adding 1,000 grams of mass per rim?
I am not trying to over rule basic laws of physics. You are ignoring laws of inertia. Only on BF would someone try to remove human physiology in a discussion about riding.

asgelle

Absolutely not. Kraig Willett actually did the experiment and his measurements show adding 1000 gm to each rim is equivalent to adding anywhere from 2000.2 and 2002 gms of static weight. Huge.

Trakhak

That asgelle's post was meant ironically will likely be lost on some reading this thread. Here's the article where Kraig Willet explains that wheel weight is of comparatively low significance and that wheel inertia is 10 times less significant than wheel weight. From that article:

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).

PeteHski

I'm not ignoring the laws of inertia at all. I'm just saying that you can ignore wheel inertia when there is little or no acceleration involved e.g. steady state climb. For changes in inertia to matter, you need to be measuring significant accelerations. Any "micro-accelerations" during your pedal stroke are by definition "micro" i.e. approx. zero. And for every one of these micro-accelerations, there will be a micro-deceleration where the energy is recovered. It's one of the fallacies that persists in this type of discussion. I've heard it before and no doubt someone will mention it again next time. But the amount of power you need to climb a hill at an average speed can be predicted accurately from total mass, rolling resistance and aerodynamic drag. You don't need to know wheel inertia and it will make zero difference to the prediction, except maybe a tiny fraction of time while you initially accelerate from 0-15 kph or whatever on the climb.

So yeah, I am saying that 2 kg of water will have the same effect on your time as 2 kg of wheel mass on a climb. Actually the water will be more useful if it's a hot day.

PeteHski

Good article thanks, which puts it all into perspective. Bicycles don't accelerate very much and even less so when climbing. Therefore changes in wheel inertia are negligible. But will some people accept this simple truth? No, of course not!

The key points in this article relating to wheel inertia are:-

"In bike racing this peak acceleration is about .1 to .2 g’s and is generally only seen when beginning from an initial velocity of 0 (see criterium race data in Appendix D )"

"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).

That was a summary for all types of terrain. For the climbing part, 50% reduction in wheel inertia was worth 0.01W over 237W. So we can just simply ignore it in a climbing model.

venturi95

My whole argument was about climbing on a 7% grade being faster on aero wheels. The work in the link is referring to a climb with a 16.8 mph average speed, and 1/100th of a watt? Probably beyond the resolution of the instrumentation.
Almost everybody knows this. Merckx was furious, he had never lost a time trial to Moser:
The lenticular carbon-fibre disc wheels weighed 4.6kg between them and were intended to work like flywheels, allowing Moser to maintain a constant pace once up to speed. The total weight of Moser’s bike was 9.6kg — nearly twice the 5.75kg of Merckx’s machine.
Good luck on a real climb with these wheels, you would be hating life.

PeteHski

You were actually arguing about the effect of rotational inertia above and beyond the effect of simple static mass. Hence your heavy water bottle vs heavy wheel climbing challenge.

Why don't you just admit that you were wrong, learn a little and move on? I'm a professional mechanical engineer, so I get paid to know this kind of stuff and I see a lot of people get confused about the importance of all these various parameters. I probably would if I did something else for a living. There is only shame in denial.

A mathematical model will predict the effect of a difference in total mass on a 7% climb, regardless of whether or not it includes wheel inertia - which again I will repeat is insignificant. It will also equally well predict the effect of a reduction in aero drag, so the net result will be valid. If your climbing wheels did weigh 4.6 kg of course they would be slower, but only as slow as carrying 4.6 kg of water bottles or whatever else. Back in reality, modern aero wheels have a weight penalty of only a few hundred grams, so aero could well overcome that if the rider is capable of riding fast enough.

Trakhak

Yes, and Ondrej Sosenka beat Chris Boardman's Best Human Effort hour record (i.e., the record requiring the use of a Merckx-style pre-aero-era bike) using an even heavier bike. From this page:

"In his attempt, Sosenka was using a 54 x 13 gearing, a 3.2 kg wheel and 190 mm cranks, with his bike weighing a total of 9.8 kg. The reason for the heavy wheel was that although it was harder to get up to speed, it was easy to maintain it. (Remark: This paragraph was taken from an article in cyclingnews.com. Hmmm, the good old flywheel effect)... "

But, as demonstrated in the article I linked to earlier, you'd be hating life climbing on a bike with higher overall weight regardless of whether most of the weight was in the wheels or in the frame. Or in the water bottles.

venturi95

Oh yes you are, and not just accelerate upwards, but forward, and trying to overcome the inertia of the wheel. And by wheel I think we all know I mean the outermost components, the rim and the tire.
Go to about 6:10 for a over an over-dramatazation of the phenomena known as a human pedaling a bike uphill. Obviously you are %100 correct in your assertations of a steady state model, but no one makes power and propels a cycle uphill perfectly smooth.

venturi95

AND FURTHERMORE!!! (jk) I think the Kraig Willet numbers, all of them, look too low. I have never used a power meter but have had a passing interest in power output, numbers and instrumentation.

Trakhak

If those numbers "look too low," you're welcome to devise your own experiments and provide your own measurements. Or you can fall back on your earlier assertion that "everyone knows" that wheel weight is more significant than weight elsewhere on a bike.

PeteHski

I can see you are stubbornly determined not to understand this. He isn't accelerating at all. He's merely maintaining a very low constant speed against the force of gravity. If you want to invent some alternative reality, then you are going to have to come up with a far more convincing argument. How about you model the wheel inertia (or find someone who has) and show your calculations of how much it affects a steep climb. You couldn't have picked a better example of where rotational inertia doesn't matter i.e. very low rotational speed combined with almost zero acceleration. Mass is all that matters here, whether it's on the wheels, frame, water bottles or the rider.

venturi95

Thank you for getting snarky and muddying up the waters, RE: everyone knows - I was referencing the flywheel effect of Moser's TT bike. I maintain my assertation that tire and rim weight are more significant than bits that don't spin when climbing a %7 grade. Got anything to say about the climbing vid?

PeteHski

Too low based on what exactly?

venturi95

Oh, FFS, do you not see the guy slowing at the bottom of his pedal stroke? You are the one inventing here, HUMANS DO NOT PEDAL PERFECTLY 360 degrees, more so when stressed a bit climbing.

venturi95

Sorry I mentioned it now, the only reference I have is elite level riders. I meant all of the numbers, aero numbers as well, I apoligize if I wasn't clear.

PeteHski

FFS do you not understand that the micro deceleration is irrelevant? All other things being equal he would actually have a smoother pedal stroke with a higher rotational inertia (although that would also imply higher mass too which is obviously bad for steep climbing)

You are just making a fool of yourself here, but keep going if you wish.

PeteHski

I haven't looked at all his numbers in details, but they seemed pretty reasonable to me. His explanation also fits in with actual Newtonian physics. Yours doesn't.

venturi95

If I am reading it correctly, Willits is getting a savings of about 2.4% of 200 watts with both aero wheels? I thought the aero wheels would be dramatically better than 32 hole wheels. You guys must know right off the top of your head the difference.

PeteHski

So about 5W saving on an average speed of 19 mph. What did you expect?