PT and SRM question.

[ergott]

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

[flydhest]

ergott,
The PT wouldn't read anything. It measures torque applied at the hub and translates that to power through the speed of the wheel (at least that's my understanding). With the rear wheel not moving, the PT won't show anything (this last assertion, at least, I know to be true).

You are describing a fanciful test I proposed some time ago for looking at how much power is lost through bike design. People talk about such and such a bike being so stiff that all your power goes straight to the rear wheel . . . I don't buy it, so I would love to measure it. Alas, there would be a whole host of measurement issues to get through before you could be definitive, but it would be fun.

Hey, TooTall has an SRM and I have a PT. TooTall, whatcha doin' later???

[BdaGhisallo]

The SRM would also read zero power too if the crank does not move. It too needs movement of the cranks to use as the speed component in the power equation. The SRM powercontrol unit could tell you the torque in this experiment.

[Spicoli]

If you want to measure deflection of something, how about trying this one to measure how much your frame will deflect (or twist) for that matter. Its kind of a caveman idea but very easy to do, I think? Take a string and tie it to your cable guides on your headtube or downtube, whichever. Then tie the other end fairly taught around your bottom bracket. Now get on it in a safe place and watch the string slacken as your frame twists from one side to another as you pedal. It should work and I am sure with some tweaking someone smarter than I could figure out a way of measuring the deflection just like a big spoke tensiometer? Stupid thing to do but is a way of visually getting an idea of how stiff one bikes BB area is versus another? More slack in string = more twist in frame, say if you used it at a certain wattage or something by watching a p-tap?

Just a stupid idea I suppose but same concept as a few other measuring tools, I think? I never full on tried it but if someone actually does, let us know if you had seen a difference from one bike to another.

Cheers, "Not So Scientific Spicoli" :confused: