Wednesday, November 18, 2009

Power Tap Static Torque Test

I performed a static torque test on my Power Tap at lunch today.

I took three weights:

1 - a 45 lb olympic plate (which actually weighed in at 45.5 lbs)
2 - two 45 lb olympic plates (90.5 lbs)
3 - my body weight

I ran three tests. The instructions say to use 50-lbs or more, which might explain why the first test at 45.5 lbs was off a little. In the first two tests, I hung the weights from the right pedal when it was parallel with the floor and the rear brake was locked. In the third test, I balanced myself in a door jamb such that all my weight was on the right pedal and the front brake was locked. (hanging 90 lbs from the pedal of your bike is more difficult than you might imagine).

It turns out that my PT is DOBA (dead on balls accurate) at the two higher weights. That makes me feel better about my recent testing. Now I need to do the same thing with my teammate's PT to see if I can eliminate a source of potential error with his results.

My results:

Test 1: expected 69.96 in-lbs, measured 63.0 in-lbs (thought maybe I was in trouble here)
Test 2: expected 138.39 in-lbs, measured 139 in-lbs (DOBA)
Test 3: expected 239.86 in-lbs, measured 240 in-lbs (double-DOBA)

Here are instructions for performing the Power Tap static torque test (I swiped them from a wattage Q&A posted by Dr. Coggan):

Technically, the PowerTap cannot be user-calibrated, but its accuracy can be checked using a simple test that is similar to the SRM
calibration check. 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.

So if you find your strain gagues are not calibrated properly, there's nothing you can do but send it to Saris for recalibration, I guess. You can rezero the thing yourself (actually most PTs are set up to re-zero themselves while you are coasting), but you can't modify the strain gauge settings yourself.

Ok, Grasshoppa - time to rush home and hang weights on your cranks to see what's up.

1 Comment:

Robert said...

If you have the bike clamped into a stationary trainer it's often easier (for a klutz like me) to hang the weight from one of the pedals then back the rear wheel s l o w l y until the torque reading stabilizes at a maximum value, which occurs when the cranks are horizontal.

Then check a couple of different cogs to make sure the readings are linear across the cassette.

One 40-lb plate should be enough.