I got into another discussion and have a question for those in the know.

Let's do an experiment.

1) Take a bike that has a Powertap, and a SRM and place it in a trainer (someone here must have both).

2) place the gears in the 39/25 combo (or whatever the low you have is).

3) fashion a way of preventing the rear wheel from turning by holding onto the rim. Without a tire, a hook in the valve hole could do this.

4) have rider mash as hard as possible on the pedals. It might be a good idea to have the rider pull on something else besides the handlebars to prevent them from breaking.

5) repeat 2-4 with the gears in the 53/19

What would break first??!! Let's now assume nothing is destroyed just yet.

Would the two meters read the same? The SRM reads power right from the legs. What about the PT? Does it need speed and the strain gauges to eliminate the influence of different gears?

I'm sure some of you might be able to answer the above without actually doing the experiment. If strain gauges were incorporated into the spokes somehow, I'm sure some great data could be gathered on the effect of spoke count, hub construction, spoke gauge, spoke count, spoke pattern, and rim construction on the wheels ability to transfer torque. The obvious answers can be verified by actual data and the extent of which each variable effects torque transfer could be known.

Perhaps the bike and rider would have to be on the ground so the wheel is properly weighted. If the rider could be assisted in staying on the bike and the bike was prevented from moving forward you could get more real world data.

Thoughts?

-Eric

I know this. They would read the same for power - 0. No motion, no power.

If you were looking at torque, they would not read the same. The SRM would read the torque applied to the cranks. The Power-Tap would read the SRM torque adjusted for the chain ring/rear cog ratio. In fact, hanging a weight from the pedal with the wheel fixed and checking torque is how you check the calibration of the Power Tap or SRM.

I think the fixed weight on the crank is the way to go to do this sort of experimenting. I also was a little unsure if you would get any torque readings. I was also unsure how each meter converted torque to power. Does the PT use your speed reading to get to power?

-Eric

I think the fixed weight on the crank is the way to go to do this sort of experimenting. I also was a little unsure if you would get any torque readings. I was also unsure how each meter converted torque to power. Does the PT use your speed reading to get to power?

-Eric
"Does the PT use your speed reading to get to power?" Yes. - TF

I think the fixed weight on the crank is the way to go to do this sort of experimenting. I also was a little unsure if you would get any torque readings. I was also unsure how each meter converted torque to power. Does the PT use your speed reading to get to power?

-Eric
Kind of. It uses the rotation rate of the rear wheel to calculate power. If the circumference of the wheel is entered incorrectly, the speed will be wrong but the power will still be correct.

"Does the PT use your speed reading to get to power?" Yes. - TF

And similarly, the SRM uses the crank rotation rate (cadence) combined with crank torque to calculate power.

The whole power thing is a little new to me. Thanks guys,
Eric