Dev,

So let me get this straight, you have been riding your SRM for for a good bit of time and have never
validated the slope? They are pretty notorious for being off right from the factory and I would certainly
do a recalibration after any change in cranks or chain rings. Just changing the torque on the chain ring bolts
has been know to affect the slope.

Learn to calibrate the unit! It is very quick and easy. Someday you will want to compare your now current data
to the then current data. Most folks eventually run through several different units over the years and its very nice
to have them be accurate as well as precise in their measurements.

Hugh

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Genetics load the gun, lifestyle pulls the trigger.

...and with any other PM you may happen to use in the future.

Sigh. This thought process never ceases to amaze me...especially when referring to a PM where it's actually possible to make sure it's reading correctly. Why wouldn't you do so? Not checking the calibration is just being lazy.

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http://bikeblather.blogspot.com/

If the slope is correct, then it matters not what the crank length is.

But if you change cranks (whether or not the change is same length or not), you may affect the power meter's slope calibration.

When doing a slope calibration check, then of course you need to know crank length and mass being applied to check a known offset value versus the zero torque offset value.

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Training Plans -- Power Meter Hire -- SRM Sales Australia -- cyclecoach.com -- My Blog -- Sydney Turbo Studio

Actually you are correct. If I look at the math, the crank length cancels itself out. To answer the other questions, no I have never recalibrated the slope because I just assumed the factory got it right, and I've never changed the chainrings and did not realize that normal usage would change the slope on an ongoing basis. Looking at the process, while it is not difficult, it entails having a known weight that is reasonably large measured to the "gram". Where do I get one?

Anyway, I can't say that my performances are any different having a powermeter or not. I think if you've been racing triathlon enough years, you get to know your body reasonably well. In long course triathlon (half IM and above), there are so many other things that impact final race results, that having or not having a powermeter is something that is "in the noise" if you already have a good appreciation for your levels of exertion.

I realize that this is a big if, but it actually is possible to be reasonably well dialed in.

At Ironman France it just confirmed to me that the "perceived pace" for the long climbs was in line with the required pace to succeed on the run.

Swim fitness ,swim pacing, run fitness, run pacing, nutrition, transitions and a host of other execution items put the powermeter "somewhat in the noise". It is nice to have, and perhaps for someone new to racing it will have a huge impact. For the eVent that I really train for (half Ironman), I can't say that having power data in real time has changed my final results in any quantifiable way. I've done 5 half IM's since getting the SRM and my results really are not that different than when I did not have one. It just confirmed for me, what I already new....at times when I start to go a bit too hard I better dial back and sometimes when it starts feeling easy with a tailwind, I need to hurt as much as when there is headwind....there is no diff in effort either way....just don't look at your speed....feel your effort.

As such I am not too worried about perfect calibration as long as my readings are consistent ride to ride. Then it can be used effectively as a stupidmeter and perhaps save the run split from going into the tank.....and even then the scare tactics from the PM centric community about "do a few too many surges on the bike and your bike split is toast" is somewhat overrated.

I will readily admit that after getting a PM in pure bike TT's I have had PB's across the board. It probably is much more critical in a bike TT (just like good pacing early in a 10k), when you are measuring performances in seconds or 10's of seconds...but that's the not the sport I compete in.

Anyway, I will try to get the correct weight and do a proper calibration and see if after swapping my cranks arms out if my PM converges back to a slope of 19.2 Hz/N-m as they are now with 165's on there (calibrated at factory).

Not certain about Canada but in the States we can take something into the postal service and have it massed to the gram.
I picked up a cheap 20 kg dumb bell plate from Craig's List. You could make a mass with a bucket and concrete mix as well.
Once you've done the calibration process once you'll find it so easy you'll wonder why you waited. If at any time in the future
you doubt the accuracy of your unit you can quickly put those fears to rest. I can't imagine spending the $$$$ on one and not
bothering to verify it periodically.

YMMV

Hugh

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Genetics load the gun, lifestyle pulls the trigger.

Thanks, that is a great idea. I suppose I can take three 10 lb plates, put them in a cloth bag and have the entire thing weighed.

OK, next task is to go and get a weight of 15-20 kg measured to the gram and actually go through the recalibration. I swapped out the 165s for 170s and zero'd it out and rode with the old slope of 19.2Hz/Nm. Numbers are off for sure, and tended to read lower across the board for a given level of perceived exertion.

Since my computainer and my previous SRM calibration were within a watt or two of each other, before I actually recalibrate, the SRM, I'll ride it on my computrainer to get an appreciation for how far things are off. It 'seemed' to me that around 200W I was reading around 15W low based on perceived exertion....but who knows, which is why I want to ride on the CT before I go through the recalibration.

You guys are right! You want your unit reading reasonably accurately, so you can compare current rides with past and future rides. One ride with suspect data was enough to convince me!!! Now the quest to go measure a weight! Here are shipping at work, the scale only goes down to half pound increments (which frankly I am surprised by).

One doesn't need to measure to the gram. 25g in 20kg is 0.125% error.

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Training Plans -- Power Meter Hire -- SRM Sales Australia -- cyclecoach.com -- My Blog -- Sydney Turbo Studio

Alex, why is 15-20 kg a magic number? What happens if the weight is only 5 kg (for example). Is the strain gauge not sensitive enough to pick up 5kg*0.17mx9.8m/s**2 = 8.33 N-m (which I'd be surprised by) , or is the issue that when you divide by this lower Torque number you get a measurement error that is ~ 5x the measurement error on a 25 kg weight.

I'm just trying to figure out if I can take a 10 lb weight, put it in a small bag and be done with it (after measuring the 10 lbs weight).

Dev

1. the greater the difference between the loaded and unloaded offset values, the less error in the calculated slope value
2. the higher mass more closely replicates the typical forces exerted at around threshold power levels*

of course you can use different masses and validate linearity of slope.

* e.g. the average effective pedal force for both legs on 170mm cranks at 300W / 90rpm is 19.1kg. The peak force per leg is ~ double that.

I use ~ 35kg when checking power meter calibration for myself and/or clients and have some special "pedals" that enable me to easily place weight plates on them.

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Training Plans -- Power Meter Hire -- SRM Sales Australia -- cyclecoach.com -- My Blog -- Sydney Turbo Studio

I'm tearing my hair out here...where can I find the SRM calibration instructions?

The westwoodvelo link in the OP won't open for me (office, home, wherever), and google just points me to chatter about calibration, not the instructions themselves.

Anyone? TIA...

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blood, sweat...and big gears

I hated every minute of training, but I said, ''Don't quit. Suffer now and live the rest of your life as a champion.''
- Muhammad Ali

Show poney, cut and pasted from the westwood velo link:

How to recalibrate SRM?

1. Look for a weight of about 30 to 40 kg. Hang it on a cable of about 25 cm, so that the weight does not touch the floor when it is hanging on the horizo ntal crank pedal, or take a longer cable and put the bike on a table.

2. Calculate the weight in Newton. E.g. 30 kg = 30 * 9.81 = 294.3 N 3.

3. Calculate the torque you get when the weight hangs on the horizontal pedal crank.
E.g. at 172.5 mm cranks : 0.1725m * 294.3 N = 50.77 Nm (Newton meter).

4. Switch the Powermeter on by pedalling backwards. Do this with a middle gea r so that the chain is in a line, e.g. 53/15.

5. Notice zero of Powermeter (MODE + SET, right number)
e.g. F0=500 Hz.

6. Bring crank in horizontal position and hang the weight on the left pedal.

7. Notice frequency output-left of Powermeter (MODE + SET, right number)
e.g. Fleft=1450 Hz.

8. Bring crank in horizontal position and hang the weight on the right pedal.

9. Notice frequency output-right of Powermeter (MODE + SET, right number)
e.g. Fright=1550 Hz.

10. Calculate frequency change of Powermeter with this weight as (Fleft+Fright)/2 -Fo
e.g. (1450+1550)/2 Hz - 500 Hz = 1000 Hz at a torque of 50.77 Nm.

11. Calculate average slope of Powermeter.
Slope = 1000 Hz/50.77 Nm = 19.90 H z/Nm.

12. This slope is the calibration of Powermeter and must be set in Powercontrol.
We recommend to check the slope monthly.

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Alex, as an experiment on my ride home from work, I simply scaled my original slope number (19.2Hz/Nm) by 165/170 and the perceived exertion seemed to be in line with the wattage readings. Looking at the math again, the crank length should make a diff in the slope number, because we are using

{(Fleft+Fright)/2 - Fo}/Torque where Torque is Mass*9.8*(crank length in meters)

So the crank length in meters is in the denominator, which means that the slope in inversly proportional to the percentage by which your crank length grows or shrinks during a crank arm swap out.....or are Fleft and Fright also effected by the crank length itself (in which case crank length effect cancels itself out.

Alex,

I just went through the process as an experiment. Used two 10 lbs plates with a velco strap running through the holes in such a way that I could hang the entire weight contraption off my pedals with crank arm at 90 degree

F0 = 440 Hz
Fleft = 730 Hz
Fright = 734 Hz
crank length = 0.17m
gravity = 9.8 m/s**2
mass = 20 lbs (mass) = 9.091 kg

Using these values, I got a slope of 19.27 Hz/Nm

My original factory calibration was 19.2 Hz/Nm when they returned the unit to me with 165 mm cranks.

I have no clue how close to 10 lbs those actual plates are, but I was surprised that my slope number came so close to the original. I am guessing that they are a bit lower or my perceived exertion is way out of whack after having done an Ironman 2.5 weeks ago and not quite feeling recovered (today was my first ride with a powermeter).

Now I have to find a place to get those weights weighed. I can add a third plate for the actual measurement. You guys are going to turn me into a calibration Nazi, and now I'm going to be questioning the validity of everyone's data :-) :-) :-)

Dev

Well to give you some idea of how bad they can sometimes be (depends on quality of weights bought), a 20kg weight plate I have actually weighs 19.525kg or 2.4% less than stamped.

lol

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Training Plans -- Power Meter Hire -- SRM Sales Australia -- cyclecoach.com -- My Blog -- Sydney Turbo Studio

thanks dude

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blood, sweat...and big gears

I hated every minute of training, but I said, ''Don't quit. Suffer now and live the rest of your life as a champion.''
- Muhammad Ali

What scale that can handle weights that heavy but still be relatively accurate did you use?

Also, forgive me if I'm being an idiot, but how do you hang a weight from a horizontal crank arm and keep it horizontal? hold the rear brake the whole time? Put a stick between the spokes and the seat stays? Otherwise it seems as if the crank arm & pedal would just rotate down to the 6 o'clock position, no?

The crank is in the 3 O'Clock position. I used a velcro strap through the hole of the weights and wrapped it around my pedal. You just hold the seat of the bike to keep it from moving forward. The crank stays at 90 degrees and basically there is a static load on the crank (mass of plate*9.8m/s**2)*crank length in meters.

Now that Alex has turned me into a calibration Nazi, I actually have to go find a proper scale. The one here at work only displays in half kilo increments, which is totally useless for the task at hand. It seems like the harder part in this entire task is not recalibrating the SRM, but actually finding a means to calibrate the weight that is being used as a basis for coming up with accurate power numbers!

Gotcha, that makes a bit more sense. In my mind, I was thinking of the bike being on a trainer as the original post stated, and was thinking the crank would just continue to rotate through to 6 o'clock.

I hang my bike from a stand and lock the rear brake by backing the brake barrel adjuster way out. If your barrel adjuster is gummed or corroded up then you should deal with that first. That seems to be one of the little maintenance issues that many of us let slide too long anyway. My bike can't sit on the floor because the 20kg plate I use is too large in diameter to clear the floor.

YMMV

Hugh

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Genetics load the gun, lifestyle pulls the trigger.

Hey there....the math is the fun part of the entire process.

I still want to know how the Fright, Fleft and Fzero are generated by the strain gauge. I take it that the F "number" is a conversion from the instantaneous torque being measured by the strain gauge into a unitless real number that represents that torque, assuming a known "slope". Why would the SRM not just transmit the actual instantaneous torque number to the powercontrol rather than the "F" number and then take the first derivative of this number to get the instantaneous power = dW/dt = (Nm/s)....or am I missing something glaringly obvious?

Is the F number there to hide SRM's strain gauge measuring algorithm so that its not easy to reverse engineer their design? Just seems counter intuitive to be pumping out meaningless "F" numbers, when there are perfectly good and understandable units of measure in Newton Physics!!!!

Dev

It's to the nearest 25g. I used two different postal scales, which agreed within 25g of each other.

Bike in trainer and rear wheel locked by brake (having a mate there to help is good) or something in the wheel. For wireless SRM it helps to be able to spin cranks forward every so often as that wakes up the power meter if it's gone to sleep.

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Training Plans -- Power Meter Hire -- SRM Sales Australia -- cyclecoach.com -- My Blog -- Sydney Turbo Studio

"F" is the output frequency of the strain gage circuit, which is proportional to the torque applied. To convert that to torque, the head unit uses the frequency slope value. So, the "F" value IS the output of the measurement device in it's "rawest" form.

Also, the first derivative of this frequency will NOT give you the power...that would just tell you the rate of change of the frequency (or torque value). To calculate the power it needs to multiply the torque (as derived from the circuit frequency measurement multiplied by entered torque slope) by the rotational speed of the crank (i.e. the cadence). THAT gives you the power. Torque x rotational speed = Nm x radians/s = Nm/s = W.

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http://bikeblather.blogspot.com/

I just use one of these across the brake blocks of the rear caliper to squeeze the caliper closed...(the clamp, NOT the model truck ;-)

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http://bikeblather.blogspot.com/