conducting a study on how crank stiffness effects speed

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djconnel
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by djconnel

First off, EXCELLENT! I really want to see results of this sort of test.

First, people advocate the strain energy approach. This isn't good enough: energy going into strain can be released in the drivetrain or into other reservoirs, including heat. Juanmoretime asserts, and I agree, that most will be released into the drivetrain. This will result in a delay in application of a few watts, but the watts aren't lost. The whole point of the experiment is to see if this is true.

For power, you won't get dissipation upstream of the measurement. So a powertap would be useless. By the time power gets to the wheel, it's going to move the bike forward. Maybe powertap for testing spoke flex, not crank arm flex. Same deal with the spider: if you lose energy in the crank arms, the spider won't even pick that up. So you want to use a pedal-based power meter. Even then, if there's a difference in internal body dissipation, you won't pick that up. You'd need to measure metabolic indicators like CO2 exhaled to detect that. But at least the pedals will tell you something about the effect of crank flex.

By the time you're ready to go, you'll be able to use a Garmin Vector, hopefully, so that's what I'd use.

You can use the powertap to compare with the pedal-based system. They should differ more if there's more transmission loss through the cranks.

I'd do the test on a trainer with a huge flywheel or even better, a treadmill. Most power meters work better when cadence doesn't vary much through the pedal stroke. That implies a high-inertia state, with round chainrings.

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kbbpll
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by kbbpll

I'm not an engineer and failed as a physics major, so feel free to disregard. As purely a "thought experiment", it seems to me that the spring effect would result in a loss of energy going into forward motion. Energy would go into the spring during the power part of the stroke, and then spring back when power from the foot eases. Since one end of the spring is bone and muscle, I would think your foot/ankle/leg would absorb the energy rather than it going into crank rotation. If, however, you did the experiment with a pedaling machine with a high degree of stiffness, I think the spring energy would in fact go into crank rotation, since stiffness at the pedal would remain more constant throughout the stroke. Well, it's fun to think about.

istigatrice
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by istigatrice

The Garmin vector sounds like an interesting product, hopefully cost won't be prohibitive on that.

With someone's suggestion earlier on of not using custom cranks and standard market cranks, what cranks are generally known to be stiff and which ones not so? I've read the fairwheelbikes crank shootout and I was hoping there would a crank that flexes much more than the cranks on test... does such a crank exist?
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basilic
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by basilic

to follow up on djconnel, why not measure power both at the pedal and at the hub? the difference is what you wasted in the crank, chain, sprocket.

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djconnel
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by djconnel

Right -- that would be a good approach if you trust both power meters to be accurate enough.

On dissipation in the leg: the leg follows a certain trajectory with a stiff crank and a slightly different trajectory with a flexible one. I see no reason to believe one versus the other would result in more or less internal dissipation. For perfect stiffness it's a perfect circle, for flexy it would be slightly eccentric. But it's not as if the foot is stationary in one case (stiff) and moving in the other (flexy).

eric
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by eric

verycreativeusername wrote:I was hoping there would a crank that flexes much more than the cranks on test... does such a crank exist?


I used to use FSA carbon ISIS cranks which flexed WAY more than anything else I have used.

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djconnel
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by djconnel

I'll bet Rene Herse flex a bit:

Image

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djconnel
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by djconnel

Here's the loser in the Fairwheel test:

Image
Image

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Juanmoretime
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by Juanmoretime

I find myself coming back to this thread every time a post occurs on it. I do use a powermeter on my tt bike but really wanted to breakdown the force that a human body really creates. So I Google searched for a watts to horsepower calculator.

http://www.rapidtables.com/convert/power/watt-to-hp.htm" onclick="window.open(this.href);return false;

If my goal is 300 watts for a time trial I'm putting out about .402 of a horsepower. A 1600 watt sprint equals a little over 2 horsepower but this type of wattage cannot be maintained very long.

I then wanted to put this into perspective by looking at other things I have some knowledge of that create power. My motorcycle creates 102 hp and 110 fp of torque. My car is rated at 260 hp. I then look at the number of mounts that hold my motorcycle engine in place verses the power and it makes a bicycle crank look pretty beefy. Really the same goes for my car.

I still feel the stiffness has really no effect on speed and have not felt flex in crank since my early days of multisport where I could get my chainrings to rub on the derailleur cage when really pushing big gears. That could have been a combination of bottom bracket, crank arm, chainring and frame flex. Considering I was winning races overall I don't think it slowed me down.

I'm really just thinking aloud here but do feel free to throw rocks.

PS It does seem logical that the weakest link would flex first such as a tire since its a soft structure and visibly flexes just pushing hard on the bike.
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istigatrice
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by istigatrice

@Juanmoretime, interesting observations there, with your days in multisport, where your competitors on similar equipment to you? It could just be that everyone was experiencing the same "loss" as you.

Also, what are people's opinions on the Look Keo Power pedal? Since I have a Polar computer this would probably be easiest, but I'm concerned by the accuracy of it
I write the weightweenies blog, hope you like it :)

Disclosure: I'm sponsored by Velocite, but I do give my honest opinion about them (I'm endorsed to race their bikes, not say nice things about them)

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