El Texican

How does the PowerTap measure the cadence when in "hub measurement" mode? How accurate is it?

R900

In the ballpark. When looking at data after the ride, PT cadence is close to actual measured, usually a couple rpm low.

ElJamoquio

You put out two torque spikes per revolution. I'm sure it doesn't measure perfectly, but I've never noticed a difference between hub mode and sensor mode. I'd bet if I looked at the data the sensor info would be smoother, but on average over longer periods (one minute and up), I bet sensor = hub.

patentcad

Seemed far less accurate to me than the regular crankarm sensor I just installed. Big difference.

merlinextraligh

I have 3 bikes set up to use the PT. On one of them the cadence pickup isn't working, so the cadence is hub only on that bike.

In my experience, the cadence isn't very accurate at low power outputs, from just the hub. But when you're working, (above about 200 watts) it settles downand is pretty consistent with what I get on the other bikes.

My theory is that at low power output the two torque spikes aren't as pronounced, leading it to jump around.

ericm979

For me the hub cadence is totally useless. I can sometimes find isolated sections of seated climbing where it's close, but it is often at 200 or above. I spin low gears but not that low!

I'd like to think that it's my incredibly smooth pedal action that is throwing off the hub but it's more likely lack of power.

cslone

Mine is within a couple rpm's most of the time. When I do spin ups though it is way off. I always have a max reading of 244 rpm.

waterrockets 

Of note, hub cadence will read a trackstand as pedaling. One time I had the thing in cadence mode while I was trackstanding at a light and it had me pedaling at 17 rpm.

The torque pulses were there, just from me rolling back and forth to keep my balance.

cslone

While the PT geeks are here, does anyone else's PT turn the cranks when they spin the wheel backwards? Seems odd to me.

waterrockets 

Yeah, the freehub isn't the smoothest at low speeds. You must not have an outboard BB

My Ultegra Octalink will spin forever, but my wife's FSA outboard has always been tight.

cslone

SRAM outboard, just installed last night.

And I was just making sure that it wasn't too out of the ordinary. I was having some problems today and while testing it out, I noticed it. I was on the phone with Cycleops, but forgot to get around to asking them.

waterrockets 

Never mind...

Anyway, I've handled only two PowerTaps, and both had the same feel to the freehub body. Must be the seals in there to keep water off the power components. Eat another Twinkie and you'll still outcoast your teammates

cslone

Sounds good enough for me.

And no twinkies for me. I'm down 10 lbs on last season, with another 10 to go.

asgelle

Don't all wheels do that? Isn't that how they do roll outs?

waterrockets 

Ah yes, I was thinking of the wheel rolling forwards and spinning the cranks.

cslone

Hmmmmmm........never really thought of it. Man, I need to get off the trainer.