open topic, for anything cycling related.
Very slightly (almost negligible) since the assumption of constant crank velocity during a pedal stroke isn't really ever the case, however it's not nearly as much of an error as what non-circular rings introduce. Certainly not enough to explain the sort of power differential many suggest they experience.
Some years back Eric Lin did some high frequency measurements of velocity during some simulated flat and climbing. I tagged the pics for reference.
Here we can see some charts plotting the bike velocity (m/s) pseudo-sinusoidal variation while riding on the flat and on a climb (simulated). The horizontal axis is time at high sample rate - each peak and trough represents one half of a pedal stroke.
Keep in mind the shortened vertical axis is designed to highlight this variance. Plotted with axis starting at zero would see a very slightly wavy line.
We can see a maximum variance in velocity of ~0.5% on the flat:
and ~ 2% on a simulated +10% gradient:
Now that 2% variation in velocity <> 2% error in power, it will be less than that, but I'm not doing the maths right now.
Elliptical rings exacerbate this issue somewhat.
So I just had a FSA meter fitted and the guys at the LBS told me they had to take the Garmin Speed/Cadence sensor off so that it wouldn't cause any interference. I figured it wouldn't be a problem as the Stages pulls the cadence anyway, but what about speed?
I know the wheel magnet was used instead of the GPS on the Garmin as it provides a more accurate figure (especially if you're riding in tree coverage). So was the figure my Garmin was getting from the GPS or does Stages somehow calculate the speed you're going as well?
For crits I use Instant power, 30s power, cad, hr, time, and lastly speed solely because everyone else uses it and it is useful to know how slow you are riding if anyone is playing doggo or you are worried that the break is too slow. Sometimes the bunch breaks down so badly that you are 10kmh below average, and it is worthwhile knowing exactly what is happening.
I have two Stages on my two Giants which both have the inbuilt speed/cadence sensors in the rear stay. Given the stages measures Cadence I haven't worried about putting the magnets on the cranks arms, however still use the sensor for the speed with no issues what so ever.
Can imagine that a magnet attached to the crank arm could cause an issue given close proximity (butted against it).
I am about to build up a new, more or less 'dream bike' build... I am pretty lost as to what I'm going to do RE: a power meter. Dura Ace 9000 groupset with a Stages PM was pretty much the go... dunno now.
Depends how well the shop and distro deal with this issue now. I am half expecting to get a lecture about riding in the rain...
Might get some vector pedals but that will mean new pedals for all my other road bikes (switch to keo cleat).
Had mine 3-4 mths, it goes on all my bikes (commute/race/track) rain or shine, no water or cover issues so far.
Does chew through batteries, the last one lasted a month but that was a big month, I ordered a large dealextreme CR2032 batch for chump change so not bothered.
Nobody has said that, in fact the Stages website says otherwise (see below)... but the fact of the matter is that when you ride in the rain (or on a wet road), the battery goes dead within a few days.
I'm not one to stop riding when it rains, and I want power data for all my rides.
The battery is supposed to last 200 hours according to Stages. I haven't even got anywhere near 100 hours in 6 months of ownership. Nowhere near that.
Random dead batteries for a piece of hardware your paying ~$1000 on isn't right.
The fact that they stick a bunch of Vaseline in the battery compartment would indicate they knew this was a design flaw from the beginning.
As much as I hate to alter the silhouette of my bikes, and as much as I don't want to switch pedals on my two other bikes, I am becoming more at peace with switching to Garmin Vector pedals.
Last edited by boss on Thu Apr 10, 2014 3:00 pm, edited 1 time in total.
I've had occasional dead batteries as well, nothing to do with water though. I ride a crapload less then you, so its not hours. I suspect the unit is at times not going to sleep , maybe its picking up my phone BT or something, or if the bike is leant against a wall on the crank arm and it senses a load.
Last flat battery was found on my warm up for Sundays CS out at Woodside, great fun, lucky I carry a battery at all times.
Glad to see the battery cover was fully intact when I took it off though.
I'm a service tech for work and I come across loads of dead batteries or batteries with short life spans, poor stock rotation and cheap OS batteries would be my guess.
My stages has had a piece of electrical insulation tape over the battery cover from day one just in case of water ingress problems. I'm on my second battery in 5 months and changed the first one before it died because I was doing a long ride and didn't want it to die mid ride.
Don't buy one then, it's pretty simple, buy garmin pedals and when you knock the pods off complain in another thread
You buy a cheap meter you will have to deal with some form of shortfall. You want perfect buy a SRM..
All well and good to say that, and I do side with your opinion (i.e. don't buy it if you don't wanna)...
But when I'm sitting here, looking at the Stages website claiming both that the unit is 'waterproof' and has a 200 hour run time, and all the while my unit is chewing batteries like it's going out of business...
I mean we're past the point of saying "Well, you should have spent more and bought a SRM" and we're into the territory of false advertising, deception, etc.
Or to be blunt - I'm not complaining about the accuracy of the thing. I bought a Stages and accept that there are limitations in it's design with regard to accuracy. I'm (and others are, too) complaining about something much more simple than that - the expectation that I can take it for a ride rain/hail/shine and put it aside and then take it for a ride the following day. The Stages unit that I currently own does not fulfill that basic expectation.
I'd rather control the variable (knocking off a pod) than have something that is completely out of my control (either design flaw or quality control issue - take your pick).
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