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Old 09-26-08, 08:56 AM   #1
KendallF
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Powertap "stomp test"? Static torque test?

All of these threads about powermeter accuracy have gotten me curious. I zero my Powertap regularly but haven't compared it against anything else, and I sold my second PT hub a while back so I don't have anything to compare.

I've seen mention of a "stomp test", but don't really know what that means.

My thought is just to level the crank arms, hang some weights from the cranks, set the PT on torque mode and calculate the torque I should be seeing. Do this with a few different weights, plot the line.

Any better methods?
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Old 09-26-08, 09:06 AM   #2
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That's really the best method.
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Old 09-26-08, 09:07 AM   #3
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Oh, Stomp test means *you* are hanging from the pedal, rather than weights.

If you want to reduce inaccuracies, make sure you know how heavy your weights really are, by the way, and measure your real crank arm length.
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Old 09-26-08, 09:08 AM   #4
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From midweek FAQ

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First, check that the transmission icon is on, and if not, give the rear wheel a spin. Then, enter the torque mode by holding the “Select” button down for 2 seconds or longer (the “WATTS” designation will disappear from the top line.) Apply the rear brake sufficiently to lock up the rear wheel. Now, measure torque as follows: with the cranks exactly horizontal (right crank at 3 o’clock), hang a known weight of at least 50 lbs from the right crank, or simply stand on it – hence the name ‘stomp test’! Measured torque = (weight in lbs) (crank length in mm) (1 in/25.4 mm) (cog teeth/chainring teeth). For a 159 lb rider standing on a 175 mm crank, with the chain on the 39 tooth ring and the 23 tooth cog, 159 lbs 175 mm 1 in/25.4 mm 23/39 = 646 in-lbs. Compare this to the displayed value by calculating % error as (measured torque - displayed torque)/measured torque.
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Old 09-26-08, 01:57 PM   #5
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Quote:
Originally Posted by KendallF View Post
My thought is just to level the crank arms, hang some weights from the cranks, set the PT on torque mode and calculate the torque I should be seeing. Do this with a few different weights, plot the line.
Better than trying to level the cranks, start with them slighty below horizontal then rotate the wheel backwards while watching the torque until they're a little above level. The maximum value will be when the arms are horizontal.
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Old 09-26-08, 02:34 PM   #6
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Quote:
Originally Posted by KendallF View Post
All of these threads about powermeter accuracy have gotten me curious. I zero my Powertap regularly but haven't compared it against anything else, and I sold my second PT hub a while back so I don't have anything to compare.

I've seen mention of a "stomp test", but don't really know what that means.

My thought is just to level the crank arms, hang some weights from the cranks, set the PT on torque mode and calculate the torque I should be seeing. Do this with a few different weights, plot the line.

Any better methods?
That's what I did with the Quarq...
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Old 09-26-08, 03:47 PM   #7
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Quote:
Originally Posted by asgelle View Post
Better than trying to level the cranks, start with them slighty below horizontal then rotate the wheel backwards while watching the torque until they're a little above level. The maximum value will be when the arms are horizontal.
That's a good idea...thanks!

I was going to try and do this before leaving for Six Gap, but I don't have time to go chase down a scale tonight. Guess I'll try it some time next week.
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Old 09-26-08, 03:53 PM   #8
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Note that when rolling the wheel back, you are accelerating the weights, so torque will read higher. Go slow or maybe even just go a cm (at the tire) at a time and stop.
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Old 09-26-08, 07:03 PM   #9
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Or just have two of them. You'll know pretty well whether or not one is off from the other.
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Old 09-26-08, 07:48 PM   #10
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Quote:
Originally Posted by asgelle View Post
Better than trying to level the cranks, start with them slighty below horizontal then rotate the wheel backwards while watching the torque until they're a little above level. The maximum value will be when the arms are horizontal.
Correct although is the powertap that sensitive to get readings of just the torque created by the just the crank and pedals?
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Old 09-26-08, 09:30 PM   #11
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How accurate is Powertap when it comes out of the factory? So far I've only done the torque zeroing thing.
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Old 09-26-08, 09:33 PM   #12
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Quote:
Originally Posted by waterrockets View Post
Note that when rolling the wheel back, you are accelerating the weights, so torque will read higher. Go slow or maybe even just go a cm (at the tire) at a time and stop.
I'd have to be slinging the wheel around like roulette to get significant centripetal force.

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Correct although is the powertap that sensitive to get readings of just the torque created by the just the crank and pedals?
I believe Asgelle's suggesting doing this with the weight attached. Move the wheel slowly and watch the torque readings; when you see a max, that's when the crank's level.
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