srm or p2max for general training usage

A light bike doesn't replace good fitness.

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mr_tim
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Joined: Sun May 31, 2009 8:47 pm
Location: durp durp durp

by mr_tim

I am trying to decide between a SRM based meter on Rotor 3d / 3d+ or a P2Max meter on Rotor 3d / 3d+

The weight difference seems relatively minor to me, perhaps 100gr which I won't miss if racing (tub wheels will get me below 6.8kg anyway) ... so is there any difference in what the two meters can display / record / push out in terms of data? SRM would be with PC7 whereas the P2max would be likely to be on a garmin500.

Will be using principally for training & using my limited time as well as possible.

I don't really want to spend on the srm unless its going to have definite benefits so wanted to ask the question of ppl that have experience or used both.

Lastly - I can get either Rotor 3D or 3D+ .. for a 75kg rider will you notice the difference?

Chiva
Posts: 341
Joined: Sat Apr 03, 2010 11:05 pm

by Chiva

I have a PT, but on the google wattage forum they discuss this a lot. With the SRM you can recalibrate it (reset the slope) you can't do that with the P2max or w/ my PT either. It would have to be sent back to Germany for any re-calibration/maintinence(sp?) issues. I was going to buy a P2max but already have an old wired PT so I am waiting for the Metrigear/Garmin pedal power meter or the Brim brothers cleat based power meter. Everyone says the P2max is accurate, etc, just can't change the slope which for many people that are hard-core is a deal breaker. Hope this helps.

by Weenie


rchung
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Joined: Sat Jul 09, 2011 5:01 am

by rchung

Chiva wrote: Everyone says the P2max is accurate, etc, just can't change the slope which for many people that are hard-core is a deal breaker. Hope this helps.

I don't think everyone says the P2M is accurate.

Here is an on-the-road comparison between a PT and the P2M. The PT is in black, the P2M in red.
Image

Here is an on-the-rollers comparison between the same PT (black) and P2M (red):
Image

And a zoomed-in view on the difference from the roller test:
Image

At my suggestion, the rider swapped out and carefully reinstalled with new chainrings--however, for this test he'd loaned out his PT so this plot only shows the P2M. (I should have shown the plot in red rather than black, sorry).
Image
What you'll see with the last plot is that the "drift" of the P2M power readings appears smaller than before but still present. I concluded that the P2M can be sensitive to proper installation but that, even so, some drift still occurs.

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CoachFergie
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Joined: Sat May 30, 2009 12:54 am

by CoachFergie

Robert analysed the data on the set of P2M cranks I have been loaned by the Oceania distributor. They come without rings so I initially put the FSA rings from my current cranks on and the inner ring has a cut out specific to the FSA cranks. I has a 36t older truvative ring so used that. I was noticeable that the drift was greater for the inner ring than the outer and as Robert has shown even when matching the SRAM P2M cranks with SRAM rings there is still some drift.

My flatmate has a Dura Ace 7800 Wired SRM so I might do a roller test using the Powertap on his bike to see what the differences are.
Hamish Ferguson
Cycling Coach

Geoff
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Joined: Sat Dec 13, 2003 2:25 am
Location: Canada

by Geoff

I haven't tried the Power 2 Max PowerMeter yet, but have over 10 years on SRM. I believe that they are on the right track with a crank-based system. The reason that I am such a staunch supporter of SRM is that I can vouch for the quality of the build and the robustness of the design over the long term. In another 10 years, your SRM will still be going strong and the Corporation will still be there, too.

Chiva
Posts: 341
Joined: Sat Apr 03, 2010 11:05 pm

by Chiva

SRM is the "gold" standard.

mrfish
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Joined: Wed Mar 28, 2007 12:49 pm
Location: Near Horgen, Switzerland

by mrfish

Hi Mr T,

I think the posts above are interesting. But no power meter is perfect even the SRM. The real issue is whether the P2Max readings are stable over time, and that's not clear, and I'm not sure it ever will be.

Personally I'm holding off for Metrigear, but if you're desperate for Power I would personally get an old SRM or a Powertap with one of the later properly sealed hubs and sell it later at a £50 loss or something. Go for the SRM as it's much more pro.

Geoff
Posts: 5093
Joined: Sat Dec 13, 2003 2:25 am
Location: Canada

by Geoff

The PowerMeter is just a tool. Like any other tool, it is only as useful as the person who wields it. I like the SRM because it has served me so well for over 10 years. Others have their favorites, too. The point is that you can't get the use of the tool if you don't have it.

mrfish
Posts: 1617
Joined: Wed Mar 28, 2007 12:49 pm
Location: Near Horgen, Switzerland

by mrfish

Point taken. But for me it's more inability to train while sitting in an office or hotel. I just need to do more than 2 sessions per week on the bike rather than spend more money.

by Weenie


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