Stomp Test is off -- what now? - Page 2

Hmmm...if that's the case, how do you know your body weight to any accuracy?

The plates are just a suggestion. Another variable weight source you could use is a bucket filled with a measured amount of water. Just need to figure out how to elevate the bike so you can hang the handle from the pedal spindle.

Here's a link to a BTR thread on this subject that includes some pics from a torque test.

http://biketechreview.com/phpBB2/viewtopic.php?t=1951

You can rock back and forth for maximum torque readings, but then it's unlikely a dowel will be at just the right place to lock off the rear wheel. The length of the straps, chains, whatever you use to connect the scale to the cranks determines how the spokes line up relative to horizontal cranks. You could just hold it steady and take the readings, but you'd either have to crane your neck to read both instruments from back there or get a helper.

Horizontal is a good starting point and I considered a bubble level on the cranks as others have suggested but 90 degrees between the straps to the scale and the crank is really what you're after with this method. A peak torque reading is still the goal and you'll get that when the force(in this case the tension through the scale) is at a right angle to the crank.

I also scratched a bit of trig to see how big an effect an angle error makes on either method to justify my eyeballing approach. It takes +/- 8 degree error in angle relative to horizontal cranks or perpendicular force application to create a 1% error in effective crank length. And the created torque will always be lower than desired regardless of which direction the angle is in error. IOW, either method is fairly tolerant to small angle errors and cranks will effectively shorten with angle error not lengthen. A +/- 5 degree error in force application results in an error of ~ 0.004, well below the accuracy of power meters.

I'll try to post some pics.
-Dave

That's a good question, and the answer is that I don't. My thinking is that I can estimate my body weight within a couple pounds, which represents a pretty small relative error because of the large weight involved. I could see that inaccurate weight/force would be a potentially big source of error, but I'm not really seeing that crank angle or the inability to rock over the peak is going to introduce a noticeable effect (beyond the 1-3% accuracy that I would expect from a PT). I wonder if the post office would let me climb on their scale?

My questioning of weightlifting plates was just based on comments I've seen that they can even be off by a couple pounds, which would be a much larger error from a 25lb plate. Calibrated plates like the BTR pics would be ideal if I could find a set.

Interesting discussion, thanks. Looks like common sense prevailed in the end. I could see a huge source of error from someone standing over their bike on one foot, with the opposite hand holding the brake steady. That's why I clamped the brake and stepped off a chair to place right foot on left pedal to be sure that I was able to stay upright and centered over the pedal. The displayed torque was definitely stable when I was balanced steadily with no bouncing.

What about the comments from the one crazy guy about the torque setting (offset, right?) in the PT computer itself? It looks like that just adjusts the zero value, and as long as the unloaded torque shows zero there shouldn't be anything to change there, right?

Thanks everyone for the comments and help.

that 'raw' torque setting should be around 512 ???? or so - definitely within a certain range or it indicates there's definitely trouble.

I had an m/b hub in which that 'raw' torque value decreased to 300 or so. Saris told me to send in the hub. Symptoms first were that the power was reading WAY WAY low.

Just a total tosser of a thought but are you ensuring you zero your PT before doing any of those stomp/weight tests. err .. stranger things have happened . . . ... ... . .

__________________
rmur

They're readily available but can be so expensive (I've seen some that are $600) that it would be far cheaper to buy a digital scale and accurately weigh weightlifting plates.

Or take some cheap plates to the post office and weigh them there

__________________
Custom Training Plans -- cyclecoach.com -- My Blog -- Power Meter Hire in Australia

The last time I was at a post office, their scale didn't read in grams.