The problem with using different power meters

Apologies for not posting, etc!  The season fast approaches, so if you follow this blog you’ll be getting spammed soon enough, and longing for the fallow winter months 🙂

Virtually my entire season is planned out.  I’m hopefully beginning with the King’s Cliffe Flyers Winter Warmer 12, which is next Sunday 22nd February.  I’ll mainly be riding in the N&DCA (Northampton & District) points series, mostly because they are local events and it’s good to measure yourself against the same riders I reckon.  Oddly I haven’t planned to ride many fast 10 courses this year, indeed I might not go to Hull at all.  I spent a lot of last year chasing the sub-20 minute 10, I suppose I decided it will come if and when it comes.  The main reason is that there’s 10 N&DCA events and I want to ride as many of them as possible.  There’s typically two per month, so if you want to commit to that, it limits what else you can do.  I might change my mind later of course, but at the moment I want to focus on the local races.

A quick update on my winter training.  I have been beavering away doing the sessions set for me by my coach Matt Bottrill.  I will write a longer post on my experiences with Matt soon, but suffice to say working with him has been a revelation.  Of course so far I have achieved nought, and results will be the barometer, but he has planned training for me that has been interesting, eye opening, and has pushed me to power numbers I haven’t seen before.  Not night and day differences, but I am certainly producing more power than I was 12 months ago.

Now to the actual intended subject of this post.  I own a Power2Max power meter, mounted on a Rotor 3d+ crank.  I love the thing.  If I had to keep only one component from all the cycle-related things I own (which believe me, is quite a lot) it would be that.  It’s been attached to my feet through most of my training and all my races so far, and has been one of the key tools I use to measure performance.

During the week I use one of 3 WattBikes that we have in the gym at work.  All last year I found that I could not make the same power on the WattBike as I could on the road through my Power2Max, either on a road bike or my TT machine.  I put this down to overheating.  The gym at work is air conditioned, but there’s no direct fan, so I get very hot and sweat buckets during the 60 minute sessions I tend to do.  Towards the end of 2014 that started to change – I found that I could at least equal the Power2Max power on the WattBikes.  This was a good thing, and I congratulated myself on adapting to the WattBike.

Into the new year, and I started to see a further swing in favour of the WattBike.  To the point where you couldn’t mistake it – I was making more power in the gym at work than I was able to produce in any configuration through the Power2Max.  Road bike, TT bike, on the road or on the turbo.  It seemed unmistakeable.  My very good friend Mark “Eddie” Amos has a PowerTap wheel that was recently serviced and calibrated by Paligap, and he kindly offered to lend it to me to conduct some tests.  The good thing about the PowerTap is that you can measure power both through the Power2Max at the cranks, and the PowerTap at the hub simultaneously.  I have two Garmin Edge head units (A 500 and an 810) and I thought it would be useful to record a couple of rides simultaneously to see if I could discern any measurable difference.

First up was a 20 minute FTP test.  I loathe these with all my heart.  To the point where I am ready to refuse to do them.  I set my Dolan TT training bike up in the kitchen, made sure both meters were being recorded and set off.  The highest power I have recorded on the turbo before for 20 minutes was 302w in January 2014.  During this test I monitored the PowerTap reading and did not look at the Power2Max.  By coincidence, the resultant 20 minute reading on the Power2Max crank was 302w.  But the PowerTap read some 14w higher at 316w.  I think if you have two power meters, one measuring at the crank and one at the rear hub, if both are accurate, you should get a higher reading from the crank, because it is measuring power before any losses in the drivetrain (chain, gears, cassette), whereas the hub meter will “deduct” those losses.  Indeed Matt told me his PowerTap reads typically 5-10w lower than his Quarq crank.  14w is nearly 5%, and if the PowerTap is accurate, the true difference might be as much as 20-25w.

The next day I took the bike on the road for a sweet spot session (around 90% of FTP).  Again the PowerTap read higher, 12w this time over the course of 2 hours.  It seems likely to me that, given my subjective experience with the WattBikes at work, and the direct comparison I did with the PowerTap, that my Power2Max is reading low.  I am going to send it back to Germany to have it serviced.  I have discussed it with Power2Max, and they aren’t too impressed with my comparisons, but hopefully they will check the calibration and change it if necessary.  Unlike some other power meters, it’s not possible to calibrate the Power2Max by yourself.

The question that begs is, if the Power2Max is indeed reading low, when did it start?  Did it affect any of the readings from my races last year?  Am I making more power now or not?  It’s impossible to tell.  I am going to rent a PowerTap for a few months from so I can regularly compare, and at least have some idea of consistency.

Thanks for reading, I’ll be back after the race next weekend with a report.  I’m hoping to get round this time and not suffer another puncture.  Fingers crossed…


Posted on February 13, 2015, in Uncategorized. Bookmark the permalink. Leave a comment.

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