Power Meters and calculations

Bicycle related chatter & discussion
User avatar
mikesbytes
Posts: 6991
Joined: 13 Nov 2006, 13:48
Location: Tempe
Contact:

Postby mikesbytes » 21 Jan 2013, 07:59

Came upon an interesting article about power meters and non circular rings

http://bikeblather.blogspot.com.au/2013 ... rings.html

While its a little hard to understand, it seems to say that a spot measurement is made and then a calculation is made over the rest of the pedal stroke, which means that if you run non circular rings that the power is inaccurately represented.

I'm thinking that if this is the case, then there will be a variation in the figures given between a rider who pushes the down stroke with a return up stoke vs a rider who pulls on the up stroke.

It also make me wonder how Velotrome (spelling?) do the power stroke analysis, it provides a graph of each leg thru the entire stroke. Perhaps that power meter takes multiple power samples?

Bottom line is that I wouldn't buy a power meter without doing a whole bunch on analysis on how they work and and what information they provide

User avatar
weiyun
Posts: 4173
Joined: 17 Nov 2006, 22:32
Location: Birchgrove
Contact:

Postby weiyun » 21 Jan 2013, 13:42

Given that you are not competing on your PM read out in races, it's irrelevant. PM numbers are used for personal training and you are your own reference. You just need consistent and reproducible numbers. That's what counts.

wallman
Posts: 298
Joined: 17 Nov 2007, 15:10
Location: Marrickville

Postby wallman » 21 Jan 2013, 14:54

I'm thinking that if this is the case, then there will be a variation in the figures given between a rider who pushes the down stroke with a return up stoke vs a rider who pulls on the up stroke.
I don't think so. If you sample once per revolution based on, say, a LHS crank magnet then you are basing your calculation on the power output of the left leg as it pushes down, plus any (debatable) contribution from the right leg as it pulls up. The cranks are connected through the bottom bracket, thus the contributions of both are captured by the measurement irrespective of pedaling style.
It also make me wonder how Velotrome (spelling?) do the power stroke analysis, it provides a graph of each leg thru the entire stroke. Perhaps that power meter takes multiple power samples?
I have no idea what a Velotrome is, but it would have to take many samples per second to produce a meaningful result across the pedal stroke at 100+ rpm, there's no perhaps. For example the Powertap samples something like every 0.127s from memory.

shrubb face
Posts: 1010
Joined: 09 Sep 2008, 01:43
Location: Marrickville

Postby shrubb face » 21 Jan 2013, 15:01

I’m fairly certain that the majority of power meters have a far higher sampling frequency than once a second. From a quick check an SRM seems to have a frequency of 1ms. This number is likely then averaged out over a period of say 1-2 seconds, which is then logged and reported by the head unit. Given that you don’t really care that much about how your power changes over a period shorter than that – except for check your pedal stroke smoothness, it is pointless for the head unit to log more frequenly than this as it would simply chew up precious space.

User avatar
mikesbytes
Posts: 6991
Joined: 13 Nov 2006, 13:48
Location: Tempe
Contact:

Postby mikesbytes » 22 Jan 2013, 07:25

So that article may be misleading as to the sampling

User avatar
weiyun
Posts: 4173
Joined: 17 Nov 2006, 22:32
Location: Birchgrove
Contact:

Postby weiyun » 22 Jan 2013, 07:43

No, the article exposed the myths surrounding the instant power gains of oval chainrings. Put simply, it's artificial and there's no free lunch.

User avatar
mikesbytes
Posts: 6991
Joined: 13 Nov 2006, 13:48
Location: Tempe
Contact:

Postby mikesbytes » 22 Jan 2013, 15:17

That is because, it says that there are calculations based on sampling. If the actual power was measured thruout the complete revolution, then the oval rings would not cause misleading figures

shrubb face
Posts: 1010
Joined: 09 Sep 2008, 01:43
Location: Marrickville

Postby shrubb face » 22 Jan 2013, 15:43

You cant measure continuously, there is always going to be a sampling rate. Thats the fundimentals of digitalising a signal, in this situation the rate its more than adequate. The calculations are done by the power meter itself and transmitted to the headunit as a 'power' value at a much lower frequency. The article is fundementally wrong in its assumptions. Whether or not Oval rings allow you to produce more power overal is a different arguement.

User avatar
Toff
Posts: 1215
Joined: 20 Sep 2007, 14:34
Location: Stanmore

Postby Toff » 22 Jan 2013, 22:36

Came upon an interesting article about power meters and non circular rings

I'm thinking that if this is the case, then there will be a variation in the figures given between a rider who pushes the down stroke with a return up stoke vs a rider who pulls on the up stroke.
No-one pulls on the up stroke, except very briefly on steep climbs or at the start of a standing sprint. Analysis of the pedal stroke of professional cyclists shows that there is a reduction in downforce on the pedal when going through the upstroke, but the net force on the pedal is still downward. This also makes sense from a muscle efficiency point of view, given that it takes more energy to pull up a pedal than to rest the foot on the upstroke, and save that effort until the more efficient downstroke.

User avatar
mikesbytes
Posts: 6991
Joined: 13 Nov 2006, 13:48
Location: Tempe
Contact:

Postby mikesbytes » 23 Jan 2013, 08:09

No-one pulls on the up stroke, except very briefly on steep climbs or at the start of a standing sprint. Analysis of the pedal stroke of professional cyclists shows that there is a reduction in downforce on the pedal when going through the upstroke, but the net force on the pedal is still downward. This also makes sense from a muscle efficiency point of view, given that it takes more energy to pull up a pedal than to rest the foot on the upstroke, and save that effort until the more efficient downstroke.
I pull on the up stroke and I also pull the pedals backwards, so I disagree that no-one pulls the pedals. It also depends on where the strength in your leg muscles is distributed

wallman
Posts: 298
Joined: 17 Nov 2007, 15:10
Location: Marrickville

Postby wallman » 23 Jan 2013, 13:56

I pull on the up stroke and I also pull the pedals backwards, so I disagree that no-one pulls the pedals. It also depends on where the strength in your leg muscles is distributed
Indeed. Perception and reality can be two different things. Strength and technique certainly are.

Chris96
Posts: 62
Joined: 10 Dec 2010, 19:51
Location: Ashfield

Postby Chris96 » 23 Jan 2013, 14:27

An interesting article on pedaling: http://www.perfectcondition.ltd.uk/Arti ... 0Notes.htm

patn
Posts: 247
Joined: 07 Jun 2010, 15:47

Postby patn » 23 Jan 2013, 16:42

As far as human motor skills go, i would have thought pedalling is pretty simple.your foot is attached to the pedal, and the pedals can only go round in circles. like in that article i'm trying seriously hard to find any difference between figures 1,2 and 3!? or am i missing something?

Chris96
Posts: 62
Joined: 10 Dec 2010, 19:51
Location: Ashfield

Postby Chris96 » 23 Jan 2013, 18:06

As diagram below attempts to show, a small change in foot position can result in a big change in where the force is applied. The spindle is indeed under the ball of the foot in fig 1 but as soon as the heel drops (fig 2) the ball moves behind the spindle and any further pressure will cause the heel to drop further. Clutch pedal, not Claw. The answer is to position the cleat further back, as in fig 3. The problem is that some cycling shoes do not permit this. (I spent a lot of money finding this out and have modified the soles of most of my cycling shoes to get my cleats in the right place)
This is the text that goes along with the 3 figures which look quite similar.

Chris96
Posts: 62
Joined: 10 Dec 2010, 19:51
Location: Ashfield

Postby Chris96 » 23 Jan 2013, 18:25

Although pedaling does seem like a simple motor skill it there are significant gains in efficiency to be made through changes in technique. Look at the way that a guy like Lance pedals. You will see LFC pedaling. The way this differs to normal ankling or horizontal foot pedaling is that through every part of the pedal stroke muscles are engaging. While ankling the riders is asking the body to put a lot of force through a small muscle group (calves) when the quadriceps and glutes are much larger. This also tires out the hip flexors. The traditional pedaling style leaves two dead-spots in the stroke and relies on one leg to push the other through these. Rotor and Osymetric rings were developed to try and combat this. The Lift, Flick, Claw method eliminates flat spots with the calves while still utilising the major muscle groups in the leg (quadriceps, glutes). The down stroke remains similar but from 6-12 'O clock the calf engages to lift then flick the leg. The best way to think of this is attempting to throw your knees over the bars. Watch Gilbert, Armstrong or Indurain. It is very pronounced. It was documented that Lance increased his pedaling efficiency by 8% from 1993-1998 by adopting this method. It is important to note though that this style of pedaling is most efficient at higher cadences (95-105) so it won't be as useful to you as a sprinter. However for roadie this sort of pedaling style is quite beneficial.

Chris96
Posts: 62
Joined: 10 Dec 2010, 19:51
Location: Ashfield

Postby Chris96 » 23 Jan 2013, 18:28

http://www.perfectcondition.ltd.uk/Arti ... /index.htm - Visual comparison of the techniques.

User avatar
mr mojo
Posts: 124
Joined: 18 Aug 2010, 17:24

Postby mr mojo » 23 Jan 2013, 19:57

Don't want to pull the thread off topic but the Lance 8% efficiency gains had nothing to do with pedalling efficiency. The author Coyle attempted to explain what he had measured over a seven year period (using different instruments). The paper and conclusions are flawed.

Lances efficiency gains came from his EPO assisted aerobic capacity gains allowing him to spin at a higher cadence taxing his aerobic engine harder. Ashenden explains below.

http://nyvelocity.com/content/interview ... l-ashenden

From what I have read very little power if any is generated in the upstroke, but unweighting the foot does allow for an increase in efficiency of the downstroke.

patn
Posts: 247
Joined: 07 Jun 2010, 15:47

Postby patn » 23 Jan 2013, 20:34

Chris you make some good points, but what the lance years showed is that it all comes down to two things: body weight, and the capacity of your heat + lungs. pedalling is a red herring. get fitter, and lighter. full stop.

User avatar
mikesbytes
Posts: 6991
Joined: 13 Nov 2006, 13:48
Location: Tempe
Contact:

Postby mikesbytes » 23 Jan 2013, 20:40

Don't want to pull the thread off topic
Nah, go ahead, the threads topic has been discussed

There's a thread that discussed ankling here

patn
Posts: 247
Joined: 07 Jun 2010, 15:47

Postby patn » 24 Jan 2013, 10:36

actually, i do have a question that IS back on topic (well on the topic of powermeters and calculations). I'm told SRMs and presumably other powermeters give torque as an output you can download - my question is how you calculate the force using it? what is the length thats being used to calculate the torque? i.e. what do you divide the torque by to get measures of force. do powermeter programs like golden cheetah or whatever do that?

shrubb face
Posts: 1010
Joined: 09 Sep 2008, 01:43
Location: Marrickville

Postby shrubb face » 24 Jan 2013, 11:38

Here is a good quick overview of how they work Pat. http://www.srm.de/index.php/us/technology/function

The length of the crank is irrelevant because the strain is calculated by the deflection of the spider. The distance that the strain guages are located from the centre point of rotation is known and fixed. The strain value to torque relationship is set by factory, and using that, the guage distance and the angle displacement (determined by velocity over time) give you the power.

patn
Posts: 247
Joined: 07 Jun 2010, 15:47

Postby patn » 24 Jan 2013, 13:05

thanks for that!

so does that mean the relevant 'distance' is the distance between the spider and the strain gauge? so lets say that distance is 5mm (so 0.005 metres). if for example the srm output says the torque is10Nm, then the force is 10/0.005=2000 newtons?

thanks

patn
Posts: 247
Joined: 07 Jun 2010, 15:47

Postby patn » 24 Jan 2013, 13:14

sorry,i mean the distance from the strain gauge to the bottom bracket axle...

shrubb face
Posts: 1010
Joined: 09 Sep 2008, 01:43
Location: Marrickville

Postby shrubb face » 24 Jan 2013, 13:24

Yea that is right, it becomes a little bit confusing because that formula for torque is assuming a point load, obviously the theoretical force is spread over the entire cross section of the spider. But we can work backwards from that, assuming no deflection of the cranks, to say that if the torque is 10Nm the force at the pedal is 10/0.175 =~58N or about 5.8kgs. The length of your cranks only real matters if you wanted to know your force applied at the pedal, it doesnt influence the strain calculation.

The strain guage works by measuring the strain in the spider cross section, strain is generated a variation in 'length' of the material caused by a force which induces a stress, either positive or negative in the material. So as the force of your pedal stroke acts on the spider material, it induces a stress which is a function of the material area and the force. This stress distorts the material, and the strain guage which is attached to it.

Lizanne
Posts: 1178
Joined: 15 Sep 2010, 13:58
Location: Wolli Creek

Postby Lizanne » 24 Jan 2013, 13:52

could you guys go away, have a few beers, then talk about this using real words?
i've been lost for a while

shrubb face
Posts: 1010
Joined: 09 Sep 2008, 01:43
Location: Marrickville

Postby shrubb face » 24 Jan 2013, 14:08

Lets imagine it in a real world lizanne situation.

Force: How hard you poke Ben when your trying to annoy him
Stress: How angry and upset he gets
Strain: How much it forces your relationship apart compared to your normal everyday life.

Better?

Lizanne
Posts: 1178
Joined: 15 Sep 2010, 13:58
Location: Wolli Creek

Postby Lizanne » 24 Jan 2013, 14:23

how many beers did you have to come up with that?
and yes heaps better.

but how does that make me ride my bike faster?

User avatar
weiyun
Posts: 4173
Joined: 17 Nov 2006, 22:32
Location: Birchgrove
Contact:

Postby weiyun » 24 Jan 2013, 14:34

...but how does that make me ride my bike faster?
It doesn't. It's no EPO. :mrgreen:

PM is like an accurate beer counter with full alcohol and caloric content analyser for an intoxicated drinker. By knowing the precise count and therefore total intake, the drinker will be able to accurately progress to a bigger or smaller beer belly. So back to cycling, PM accurately records the volume and intensity of training thereby provides guidance to an optimised training plan. It's benefits aren't limited to just this though but is good enough for a starter.

User avatar
paul
Posts: 247
Joined: 03 Feb 2008, 21:43
Location: Leichhardt

Postby paul » 25 Jan 2013, 11:59

My high school physics teacher taught us that power is something you lot (ie my class) never do: work over time. P=w/t. A crap joke that was in his lesson notes, and used every year.


Return to “Conversation”

Who is online

Users browsing this forum: No registered users and 5 guests