How do I correct the cadence?

I fitted some PowerTap pedals to my old lifecycle and it works beautifully- no more sore bum!
BUT I don’t know if it matters but Zwift is showing 76 cadence when I am really doing 70.
PowerTap let’s me set a crank length but I can’t notice much difference. Presumably cadence has nothing to do with crank length as I guess the PowerTap pedals feel the rotations regardless.
I will try the PowerTap forums as well but they are a bit clueless. If I get an answer ai will post here.

So have to set the correct crank length?

How do you know what cadence you are doing?

Unless you have a cadence sensor paired, Zwift will estimate your cadence. The Zwift estimate could be spot on, way off, or somewhere in between.

I believe all PowerTap pedals support cadence. Do you have your PowerTap paired in Zwift for Cadence as well as Power? I seem to remember some weirdness where you might need to pair the PowerTaps for Power before they will appear as an option for Cadence. Then after pairing the PowerTap for cadence, you can change your Power to something else if you want (e.g., Kickr, Neo, etc…). I believe this was/is the result of a lack of support for both Cadence and Power on one Bluetooth channel (i think i said that right) :man_shrugging:

Crank length doesn’t make a noticeable difference
The LifeCycle produces a cadence figure
Yes the PowerTap does transmit cadence so I have power, cadence and not controllable

It’s your pedals that estimate the cadence then not zwift. Check for a firmware update.

How do you know the cadence is wrong?

The LifeCycle reports the correct cadence based on the rotating cranks.
Zwift is getting cadence from the PowerTap pedals. So it’s not a Zwift issue but a PowerTap issue (and I fully realise I should be in a PowerTap forum but the Zwift users are good at this stuff).
I don’t really know if it matters that PowerTap has the cadence a bit high. I guess it means I am riding more kilometers than reality but I can’t get my head round it.

Ok gotcha. Sorry I didn’t notice you had already said that.

It’s pretty hard for the cadence sensor in the lifecycle to be wrong but there is always that chance (you could count revolutions for a minute to check, use a clock that isn’t on your lifecycle as that may be wrong too!) Probably more likely the estimation used in the pedal is just a bit out. Nothing you can really do there.

I wouldn’t assume that the Lifecycle has accurate cadence.

Oh gosh I hadn’t considered that, I just assumed it would be a magnetic pick up near the crank and therefore 100% accurate.

Does anyone know what difference - if any - the crank length setting makes on the PowerTaps as far as Zwift is concerned? I guess it needs the crank length to get my speed right?

Check out the review of the pedals here https://www.dcrainmaker.com/2019/03/powertap-pedals-review.html#:~:text=The%20company%20opted%20for%20some,power%20meters%20is%20often%20missing.

That write up may lead you to the information you are searching for. There will always a margin of error when it comes to data collection so if one device is known to operate within a certain threshold and the other is relatively close to it and behaves similarly, then you could be looking at both of them being “good enough.” The difference between 70 and 76 could just be within the margin of error with one reading a little low and the other a little high or a combination of various levels.

Crank length won’t matter to the pedals though.

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I’m sure I’ve read that these pedals need to know the crank length to be calibrated.

From memory they measure torque and so need the distance to determine the power.

You speed in Zwift is calculated from your power. Your PowerTap pedals require the correct length to calculate the correct power value. Thus if your crank length is incorrect, your power values will be incorrect which will lead to incorrect speed values.