Power meter output readings

The foundations for successful riding

Power meter output readings

Postby Arlberg » Fri Sep 06, 2013 9:14 pm

Is it possible to ride up the same hill twice, ride it slower the second time around than the first time around, yet have a higher power output on the slower ride than on the faster ride? (Using an actual powermeter to measure the power outpuy, not a Strava guestimation).

If it is possible, how is it?
Arlberg
 
Posts: 266
Joined: Mon Oct 03, 2011 5:43 pm

by BNA » Fri Sep 06, 2013 9:30 pm

BNA
 

Re: Power meter output readings

Postby Strawburger » Fri Sep 06, 2013 9:30 pm

Head/tailwind
Zero offset difference
Temperature
n=10 (2013 & 2004 roads,2010 track,2x 2009 foldups,1990 hybrid,1992 trainer,2007 rental,1970's step through,1980's zeus)
User avatar
Strawburger
 
Posts: 1195
Joined: Tue Aug 26, 2008 12:19 pm
Location: Dulwich Hill, Sydney

Re: Power meter output readings

Postby sim-o » Fri Sep 06, 2013 9:39 pm

...
Tyre pressure
Passing traffic
Mechanical/equipment/gear
Body weight
Water bottle levels
Power curve (i.e. sustain same power output whole time, or higher at start, or higher at end)
Riding line (i.e. straight whole time or a bit wobbly)
2011 Merida Ride 93
2012 Apollo MTB
Image
User avatar
sim-o
 
Posts: 122
Joined: Fri May 07, 2010 1:19 pm
Location: Wanneroo, Perth

Re: Power meter output readings

Postby DoogleDave » Fri Sep 06, 2013 9:55 pm

Different gear selection and cadence between the two rides?

Wouldn't using a higher gear with lower cadence require more power than a lower gear with faster cadence, yet potentially achieving the same (or slower) speed?

Dave
2012 Felt F75 | 105 | ProLite Braccianos | GP4000S
Image
User avatar
DoogleDave
 
Posts: 519
Joined: Mon Mar 12, 2012 6:56 pm
Location: Taylors Hill, Victoria

Re: Power meter output readings

Postby Arlberg » Fri Sep 06, 2013 10:17 pm

I should clarify, I rode the same hill twice in succession, so no issues with tire pressure, bodyweight, temperature, wind, water bottle weight etc.

Doogle Dave I have often wondered about that too. Climbing in a bigger gear means pushing harder on cranks, which means more flex in the chainring or crankarm to be measured by the strain guages, which translates to a higher power reading on the head unit and more than likely a slower speed. Perhaps that is the answer to my question right there.
Arlberg
 
Posts: 266
Joined: Mon Oct 03, 2011 5:43 pm

Re: Power meter output readings

Postby vander » Fri Sep 06, 2013 10:36 pm

DoogleDave wrote:Wouldn't using a higher gear with lower cadence require more power than a lower gear with faster cadence, yet potentially achieving the same (or slower) speed?

Dave


In a word no. A watt is a watt. Higher gear lower cadence requires more force, but same power.
Image
vander
 
Posts: 1211
Joined: Thu Jul 28, 2011 12:35 am
Location: North Strathfield, NSW

Re: Power meter output readings

Postby sim-o » Fri Sep 06, 2013 11:00 pm

Arlberg wrote: I rode the same hill twice in succession, so no issues with tire pressure, bodyweight, temperature, wind, water bottle weight etc..


I wouldn't be so sure. Unless you happened to be measuring all of the above at the time, then it would be difficult to say whether any of the above were affecting your speed or not (apart from water bottle obviously, but even then a minute amount may have evaporated 8) ). It may have seemed like the same conditions but any minor change in any of the above may have affected the resulting speed.

In addition to this, is your power curve exactly the same? Did you apply slightly higher/lesser power output on the steeper/flatter parts of the climb?

Seriously, there are so many factors that effect speed. Assuming your sensor is correctly calibrated etc, then power will be the best indication of your performance, and not "your performance+the environment."
2011 Merida Ride 93
2012 Apollo MTB
Image
User avatar
sim-o
 
Posts: 122
Joined: Fri May 07, 2010 1:19 pm
Location: Wanneroo, Perth

Re: Power meter output readings

Postby Arlberg » Sat Sep 07, 2013 3:44 pm

vander wrote:
DoogleDave wrote:Wouldn't using a higher gear with lower cadence require more power than a lower gear with faster cadence, yet potentially achieving the same (or slower) speed?

Dave


In a word no. A watt is a watt. Higher gear lower cadence requires more force, but same power.


My powermeter (Stages) measures the flex in the crankarm using strain guages. If I am putting more strain on the crankarm because I am pushing harder in a bigger gear, then the crankarm must flex more than it would if I am just spinning in a small gear. Surely that would affect the power reading?
Arlberg
 
Posts: 266
Joined: Mon Oct 03, 2011 5:43 pm

Re: Power meter output readings

Postby toolonglegs » Sat Sep 07, 2013 3:58 pm

More strain on crank arm equals more torque, not necessarily more power... Peak power is usually reached at a highish cadence ... That's why it is genetic, it's not how hard you can push down on the pedals, it's how hard and fast.
Image
User avatar
toolonglegs
 
Posts: 14331
Joined: Fri Jun 29, 2007 7:49 pm
Location: Somewhere with padded walls and really big hills!

Re: Power meter output readings

Postby Xplora » Sat Sep 07, 2013 4:27 pm

I have a question for you - is this hill you have climbed a familiar one? Were you doing repeats in quick succession, or just go back to the spot twice during your ride? What were the conditions you were under? How far was this climb, how steep was it?

I will often punch out a much slower time on a climb I have never seen before, particularly big ones because you just don't know how you will feel later in the climb. You can read 10% on Strava but that's usually meaningless, and 10% feels different on different days. The other thing that might scare you is that you might be favouring a different leg on the first climb. This goes to the heart of the arguments about the Stages, unfortunately :( You might have pushed harder with your left leg, then pushed harder with your right on the second time up. It's normal to compensate as you recruit different muscles to do the work. You really can't say much based on what you've told us - but there are tons of reasons why you are getting this issue. Nothing to get stressed about though. Go and do some hill repeats again next week and find out if you were getting similar problems.
Xplora
 
Posts: 5650
Joined: Fri Dec 10, 2010 11:33 pm
Location: TL;DR

Re: Power meter output readings

Postby vander » Sat Sep 07, 2013 5:13 pm

Arlberg wrote:
My powermeter (Stages) measures the flex in the crankarm using strain guages. If I am putting more strain on the crankarm because I am pushing harder in a bigger gear, then the crankarm must flex more than it would if I am just spinning in a small gear. Surely that would affect the power reading?


There is your answer one time you were more dominant with the left leg one time you were not, often happens with fatigue.

No it wouldnt. Power = force/time although the force is increased so is the time. With the higher cadence the time is lower and thus the force is also.
Image
vander
 
Posts: 1211
Joined: Thu Jul 28, 2011 12:35 am
Location: North Strathfield, NSW

Re: Power meter output readings

Postby Strawburger » Sat Sep 07, 2013 6:46 pm

+1 here. I didn't realize you had a stages.

L/R balance is rarely 50% during a ride. You start to favour one leg when getting fatigued. With stages assuming Lx2 = power, readings will vary even when doing hill repeats within minutes of each one.
n=10 (2013 & 2004 roads,2010 track,2x 2009 foldups,1990 hybrid,1992 trainer,2007 rental,1970's step through,1980's zeus)
User avatar
Strawburger
 
Posts: 1195
Joined: Tue Aug 26, 2008 12:19 pm
Location: Dulwich Hill, Sydney

Re: Power meter output readings

Postby Crawf » Sat Sep 07, 2013 7:22 pm

Personally from some unscientific testing I have done with my PowerTap...
Faster cadence nets a higher wattage.
Compared to a grinding slow cadence up the same hill, this one hurts more though.
Crawf
 
Posts: 1647
Joined: Wed Jan 09, 2008 8:20 pm

Re: Power meter output readings

Postby Arlberg » Sat Sep 07, 2013 7:55 pm

Some very knowledgable people on here! Thanks!

While I have you knowledgable people here, with the powermeter settings, should I use:

1. Power - Non Zero Averaging or Zero Averaging?

It seems to make sense to have it set to Non Zero Averaging because then I can see my average power output based on the time when I was actually pedalling rather than having the power output reading diluted by times when I wasn't pedalling (while cruising downhill for example).

On the other hand I want to be comparing apples with apples when comparing my power output with other riders. If the 'norm' is to have it set to Zero Averaging it's not going to be an accurate reflection of how my power output compares with others if I am using Non Zero Averaging as it would artificially inflate my power figures.


2. Cadence - Non Zero Averaging or Zero Averaging? If Im riding with a cadence of say, 80rpm it seeems silly to dilute that reading by including in the average all the times when Im not pedalling at all.

3. Generally- Smart recording or Every Second recording?
Arlberg
 
Posts: 266
Joined: Mon Oct 03, 2011 5:43 pm

Re: Power meter output readings

Postby vander » Sat Sep 07, 2013 8:09 pm

Always zero average! Non-zero average tells you absolutely nothing and is pointless they need to remove it as an option.
Image
vander
 
Posts: 1211
Joined: Thu Jul 28, 2011 12:35 am
Location: North Strathfield, NSW

Re: Power meter output readings

Postby Arlberg » Sat Sep 07, 2013 8:13 pm

Xplora, I know the hill very well, it is the Zoo Hill in Mosman. It is not long or steep but it is convenient to where I live.

By the way, DCRainmaker tested the Stages very thoroughly and was unable to influence the power output readings when actively trying to put more and less power through the LH crankarm compared with the RH crankarm. He also found the Stages to be within 2% accuracy of the SRM powermeter.
Arlberg
 
Posts: 266
Joined: Mon Oct 03, 2011 5:43 pm

Re: Power meter output readings

Postby Arlberg » Sat Sep 07, 2013 8:19 pm

vander wrote:Always zero average! Non-zero average tells you absolutely nothing and is pointless they need to remove it as an option.


But if I want to calculate my power output on a climb and ride hard up a hill for 20 minutes at say 350 watts, then coast back down without pedalling for the next 20 minutes, my power output for the hill climb was not 175 watts, it was 350 watts.
Arlberg
 
Posts: 266
Joined: Mon Oct 03, 2011 5:43 pm

Re: Power meter output readings

Postby nickobec » Sat Sep 07, 2013 9:01 pm

Yes, I did it three times in today's race.

1st time chasing back on after a mech, hit the bottom hard and powered over the top, 2nd highest wattage, 2nd highest speed
2nd time to show that the chase back did not take it out of me, came off another wheel at the bottom and eased back hitting the top, lowest wattage, fastest time.
3rd time a bit later in the race, rode alongside another rider at steady pace, halfway up attacked, highest average wattage, lowest speed.
User avatar
nickobec
 
Posts: 1435
Joined: Thu Mar 26, 2009 11:51 pm
Location: Perth or 42km south as the singlespeed flies

Re: Power meter output readings

Postby vander » Sat Sep 07, 2013 9:45 pm

Arlberg wrote:
vander wrote:Always zero average! Non-zero average tells you absolutely nothing and is pointless they need to remove it as an option.


But if I want to calculate my power output on a climb and ride hard up a hill for 20 minutes at say 350 watts, then coast back down without pedalling for the next 20 minutes, my power output for the hill climb was not 175 watts, it was 350 watts.


So lap for the climb then, your power for the ride is 175W but your power for the climb was 350. To show your average for the ride to be 350 would artificially increasing your power a lot!

As for DCrainmakes N=1.
Image
vander
 
Posts: 1211
Joined: Thu Jul 28, 2011 12:35 am
Location: North Strathfield, NSW

Re: Power meter output readings

Postby toolonglegs » Sat Sep 07, 2013 10:21 pm

Isn't that what the lap feature is for?... hit lap at the bottom of the climb and watch your readings on the way up.
Don't get rid of the zeros... that's what NP is for.
Image
User avatar
toolonglegs
 
Posts: 14331
Joined: Fri Jun 29, 2007 7:49 pm
Location: Somewhere with padded walls and really big hills!

Re: Power meter output readings

Postby Xplora » Sat Sep 07, 2013 10:53 pm

OP, I had a quick sticky on Strava at the segments near the Zoo and it's a reasonable distance. I'm guessing you were doing repeats because the area is not conducive to centuries without covering the same ground :lol:

I'm going with compensation as the explanation because it's a long hard climb if you are pushing fast; yes, I 100% agree that the Stages is accurate within 2% but the leaderboard shows people putting out 400+W average with powermeters. That comes to 8W variance of actual power, and it's hard to say if the reference is out as well... calibration is important. Anyways, the point is if you're concerned about your readings, then you're unlikely to be satisfied if you are trying to compare same day efforts and you aren't willing to accept internal variations (you pushing the pedals differently).

DCRainmaker is just one example, and he has an enormous amount more work to do with publishing his data on L/R splits to make meaningful comment. Bear in mind, that we just haven't had L/R power until recently; his data wouldn't focus on that. As a triathlete, he's concerned about pacing more than all out efforts.
Xplora
 
Posts: 5650
Joined: Fri Dec 10, 2010 11:33 pm
Location: TL;DR

Re: Power meter output readings

Postby thearthurdog » Sun Sep 08, 2013 5:53 am

Arlberg wrote:Some very knowledgable people on here! Thanks!

While I have you knowledgable people here, with the powermeter settings, should I use:

1. Power - Non Zero Averaging or Zero Averaging?

It seems to make sense to have it set to Non Zero Averaging because then I can see my average power output based on the time when I was actually pedalling rather than having the power output reading diluted by times when I wasn't pedalling (while cruising downhill for example).

On the other hand I want to be comparing apples with apples when comparing my power output with other riders. If the 'norm' is to have it set to Zero Averaging it's not going to be an accurate reflection of how my power output compares with others if I am using Non Zero Averaging as it would artificially inflate my power figures.


2. Cadence - Non Zero Averaging or Zero Averaging? If Im riding with a cadence of say, 80rpm it seeems silly to dilute that reading by including in the average all the times when Im not pedalling at all.

3. Generally- Smart recording or Every Second recording?


Zero

Zero

Every second
Cervelo s5
Giant Trinity Composite 1
http://www.crosswindmissile.blogspot.com.au
thearthurdog
 
Posts: 208
Joined: Wed Jan 25, 2012 7:51 pm

Re: Power meter output readings

Postby mick243 » Sun Sep 08, 2013 11:45 am

toolonglegs wrote:More strain on crank arm equals more torque, not necessarily more power... Peak power is usually reached at a highish cadence ... That's why it is genetic, it's not how hard you can push down on the pedals, it's how hard and fast.


Power = torque x rpm x conversion factor
mick243
 
Posts: 210
Joined: Wed Mar 20, 2013 10:44 am

Re: Power meter output readings

Postby toolonglegs » Sun Sep 08, 2013 2:16 pm

mick243 wrote:
toolonglegs wrote:More strain on crank arm equals more torque, not necessarily more power... Peak power is usually reached at a highish cadence ... That's why it is genetic, it's not how hard you can push down on the pedals, it's how hard and fast.


Power = torque x rpm x conversion factor


Isn't that what I said ... with out the crank length bit. Not much science in my head :mrgreen: .
Image
User avatar
toolonglegs
 
Posts: 14331
Joined: Fri Jun 29, 2007 7:49 pm
Location: Somewhere with padded walls and really big hills!

Re: Power meter output readings

Postby Alex Simmons/RST » Sun Sep 08, 2013 7:34 pm

vander wrote:
Arlberg wrote:
My powermeter (Stages) measures the flex in the crankarm using strain guages. If I am putting more strain on the crankarm because I am pushing harder in a bigger gear, then the crankarm must flex more than it would if I am just spinning in a small gear. Surely that would affect the power reading?


There is your answer one time you were more dominant with the left leg one time you were not, often happens with fatigue.

No it wouldnt. Power = force/time although the force is increased so is the time. With the higher cadence the time is lower and thus the force is also.


Just to clarify:

Power is not force/time.

Power = energy / time
Power = force x distance / time = force x velocity
Power = torque x rotational velocity

To answer Arlberg:
If you are riding at same speed up a climb (ceteris paribus) and the only change is gear, then power will be the same. You might be applying more torque to the cranks if you choose a larger gear but at the same bike speed you are doing so at a lower crank rotational velocity (reduced cadence). The increase in crank torque will be inversely proportional to the reduction in crank rotational velocity.
User avatar
Alex Simmons/RST
Expert
 
Posts: 3376
Joined: Fri May 02, 2008 3:51 pm

Next

Return to Training

Who is online

Users browsing this forum: No registered users



Popular Bike Shops
Torpedo 7 Torpedo7 AU
Ground Effect Ground Effect NZ
Chain Reaction Cycles CRC UK
Wiggle Wiggle UK
Ebay Ebay AU



InTouch with BNA
“Bicycles BNA Twitter
“Bicycles BNA Facebook
“Google+ BNA Google+
“Bicycles BNA Newsletter