I have just noticed something, and feel stupid for only just noticing, given I am using my current power meter for 2 years now.
I have a Tacx Neo where I train indoors and have my road bike attached. On my TT I have a Stages Power Meter (left crank).
I screwed up badly 2 IM these year: IM Austria and IM Switzerland. Fared well on the swim and bike and messed up the run. After much introspection and thinking I decided to try to setup a 10min workout with powers ranging from 100w to 350w and use both power meters Tacx Neo and Stages. I am shocked. Stages reported an avg 18% lower than the Neo. I then went online and noticed I need to do a Zero reset (oops). I did that and repeated the procedure. Stages is now reporting an avg that’s 8% lower than the Neo. I understand they won’t be exactly the same but 8% sounds way too much. We are talking about the Neo reporting 145w avg and Stages reporting 133w.
This might explain why I destroyed myself on ride in both IMs, since I was trying to achieve 75% of my FTP on the bike, which is around 175w (as measured on the Neo).
Anyone had problems like this? What do you think of the power difference? Any suggestions for the future?
Any help is welcome. Doing IM70.3 Zell-am-See and it’s my last chance this year to do well.
I have just noticed something, and feel stupid for only just noticing, given I am using my current power meter for 2 years now.
I have a Tacx Neo where I train indoors and have my road bike attached. On my TT I have a Stages Power Meter (left crank).
I screwed up badly 2 IM these year: IM Austria and IM Switzerland. Fared well on the swim and bike and messed up the run. After much introspection and thinking I decided to try to setup a 10min workout with powers ranging from 100w to 350w and use both power meters Tacx Neo and Stages. I am shocked. Stages reported an avg 18% lower than the Neo. I then went online and noticed I need to do a Zero reset (oops). I did that and repeated the procedure. Stages is now reporting an avg that’s 8% lower than the Neo. I understand they won’t be exactly the same but 8% sounds way too much. We are talking about the Neo reporting 145w avg and Stages reporting 133w.
This might explain why I destroyed myself on ride in both IMs, since I was trying to achieve 75% of my FTP on the bike, which is around 175w (as measured on the Neo).
Anyone had problems like this? What do you think of the power difference? Any suggestions for the future?
Any help is welcome. Doing IM70.3 Zell-am-See and it’s my last chance this year to do well.
I have stages as well. I have a tacx vortex smart trainer at home. And I go to a computrainer cycling class once a week. stages is always lower than vortex and computrainer. since I race based on my stages power meter my power tests go off that reading.
You are measuring power at different locations. You would expect the highest power at the crank or pedal (Stages) and a lower power at the rear cassette because you have the frictional power losses. Chain friction from a freshly cleaned and waxed chain is about 5 watts. Dirt, wear and poor lube can put you at a 8-10 watt loss.
I have always thought the fixed trainers seem to measure a bit high. I would suggest seeing if you can borrow a powertap and looking for a consistent difference. That will tell you which PM is most likely correct.
The Stages power meter just measures the left leg’s wattage and doubles it to come up with a total. If your two legs don’t produce exactly the same wattage the Stages will read either higher or lower than your actual output. It’s not uncommon at all to be say 55/45 or 45/55 in regards to balance between the legs and have the amount vary with intensity as well as fatigue. So if one has a 55/45 left right balance and you’re actually outputting 80 watts with the left leg and 65.5 with the right, your Stages power meter with read 160 watts while you’re actually outputting 145.5. Conversely if the balance is 45/55 your Stages will show 131 watts while you’re still outputting 145.5.
On top of that, 75% of FTP may well be too aggressive a target wattage. What sort of bike time did you do?
This might explain why I destroyed myself on ride in both IMs, since I was trying to achieve 75% of my FTP on the bike, which is around 175w (as measured on the Neo).
I’m not a huge fan of one sided power meters, but that aside, what made you think you could hold 175 watts during your IM bike? Did you not do some outdoor long rides using your stages PM where you were targeting 175 watts for 5+ hours and feeling like you could run a marathon after that? Or did you just do a FTP test, and figure you would hold 75% of that number regardless of what your outdoor long ride power was?
You should have done indoor and outdoor power FTP tests. I use the stages indoor and outdoors and your indoor power will almost always be lower than your outdoor power with the same power meter.
Go outside and do an FTP test. Use indoor numbers for indoors and outdoors numbers for outdoors. It’s not a matter of your equipment, it’s how you’re using it.
To reiterate, even if your two power meters had 0% error, you’re still going to run into a problem when your interchange indoor and outdoor power.
I would recommend looking at DC Rainmakers site. He goes into great detail and rides multiple PM’s at once. I pretty sure he posts periodic reviews of his findings comparing all these different PM’s.
The other area to look at is the Wattage board. (google). 8% may sound like a lot, but I’m not sure what the delta between crank based and rear wheel PM’s. Depending, on your indoor training platform, you should be following the wattage on your Stages, especially data from your FTP tests.
I’ve been using a power2max for a couple years and just recently bought an Elite Drivo. I’ve noticed while doing ERG workouts with Elite’s app and monitoring my P2M on my Garmin that there is a noticeable variance. After my workout when I upload the files for each the Drivo shows the exact watts the segment was suppose to be (20min at 240w shows exactly 240w) but the same segment from my P2M shows typically 7-10w, but at times as much as 15w less.
Given the accuracy that the Drivo claims and the fact that it’s measuring from the wheel I would have figured the crank based P2M would have read higher, not lower.
The graphs do look very similar as far as spikes and valleys, it’s just the amount of watts that each is showing is not in line with what I was expecting (figured P2M would read higher than Drivo).
What sciguy says is right. But there’s a little more to it. Stages always reports double the left leg power, as do all of the one-sided power meters. And you likely have a dominate leg. But the ratio between your legs may not always be constant and further, the same leg may not always be dominant. In testing, I found that might right leg is dominate in low cadence, high power efforts, while my left leg is dominate for high cadence, lower power efforts.
All of this makes it very difficult to accurately translate a controlled effort on a smart trainer to the proper effort on the road with a one sided power meter.
The Stages power meter just measures the left leg’s wattage and doubles it to come up with a total. If your two legs don’t produce exactly the same wattage the Stages will read either higher or lower than your actual output. It’s not uncommon at all to be say 55/45 or 45/55 in regards to balance between the legs and have the amount vary with intensity as well as fatigue. So if one has a 55/45 left right balance and you’re actually outputting 80 watts with the left leg and 65.5 with the right, your Stages power meter with read 160 watts while you’re actually outputting 145.5. Conversely if the balance is 45/55 your Stages will show 131 watts while you’re still outputting 145.5.
On top of that, 75% of FTP may well be too aggressive a target wattage. What sort of bike time did you do?
Hugh
Thanks, that analysis makes a lot of sense.
I am doing 236w FTP (measured on the turbo) and aimed at 170w IM. Never managed to output those values for the whole ride. End up doing avg 155w NP, 146 avg. That’s for a 6h05m in Austria. Aiming for the same in Switzerland but destroyed myself on first lap and ended up doing 7h in Switzerland.
This might explain why I destroyed myself on ride in both IMs, since I was trying to achieve 75% of my FTP on the bike, which is around 175w (as measured on the Neo).
I’m not a huge fan of one sided power meters, but that aside, what made you think you could hold 175 watts during your IM bike? Did you not do some outdoor long rides using your stages PM where you were targeting 175 watts for 5+ hours and feeling like you could run a marathon after that? Or did you just do a FTP test, and figure you would hold 75% of that number regardless of what your outdoor long ride power was?
My outdoor long rides 5+ hours were always easy. Never tried to do them at race power. Never even looked at power meter during them. Mistake in hindsight but… hindsight is always 20/20.
You should have done indoor and outdoor power FTP tests. I use the stages indoor and outdoors and your indoor power will almost always be lower than your outdoor power with the same power meter.
Go outside and do an FTP test. Use indoor numbers for indoors and outdoors numbers for outdoors. It’s not a matter of your equipment, it’s how you’re using it.
To reiterate, even if your two power meters had 0% error, you’re still going to run into a problem when your interchange indoor and outdoor power.
Not many good (read safe) places to ride 20mins outdoors while killing yourself. Maybe in the US with those long stretches of road but I live in the mountains in south Germany. Very hilly and bendy around here.
If you live hilly then there will we a quiet nice steep where you go up for 20 mins to get an estimate.
Ain’t optimal since you want to test in the aerobars for these races but better than 2 different systems to guess your numbers.
This may be contributed to by measurement margin of error but is almost certainly affected more significantly by improved cooling outdoors and having many multiples of inertia outdoors. Particularly if using a basic trainer or erg mode on a sophisticated one.
Tacx neo on slope mode and two big ass fans will close the gap significantly
I have a tacx neo and tried 2 different stages PMs on my trek SC this year. Even had them checked with stages and they said it worked fine! But, they were at least 10% lower and erratic at higher efforts versus the neo. Using power match in trainer road was a complete mess and if I was doing 220w efforts using stages, it would be like 240-250 on neo.
I switched everything to quarq and couldn’t be happier. It’s not as smooth as just using the neo but it’s almost exactly the same Power numbers. Soooooooo much better and I know I don’t have much L/R imbalance. Even when I ride the quarq outdoors, it feels almost exactly like my indoor efforts with the neo. There isn’t much difference in price, if at all, just don’t use stages with a neo.
Last 20 Min test I took using the Neo (about 6 weeks ago), I was 237 too! I took the test in a fatugued state because I don’t know how to rest enough during leak training for a test (offseason should be possible). I tested my quarq outdoors and rode 85 and 90 mile rides (mostly flat) at 175 and 190w NP. I’ll shoot for 175-180 during my IM in a few weeks.
I think your “long rides” need to include race pace and/or some harder sets. Unless you have the time to do a ton of easy hours and recover well.
I have two computrainers, against my PowerTap wheel one is consistently 5-6% low and one is consistently 2-3% high after a warm up and roll down calibration. I use the same one all the time now so it’s not a big deal but I use outside performance to set power goals.
Even without all the issues with single side power and discrepancies between power meters 70% of ftp is a bit on the high side.