Calibrating Slope on SRM

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Geoff
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by Geoff

fankozola101 wrote:Geoff, are you saying...

If: You manually enter a new slope constant in the Power Control (head unit)

Then: the Power Control (head unit) sends a signal to the Power Meter, which changes the slope constant stored on the Power Meter.

And: This new slope constant stored on the power meter is what a Power Control (head unit) will detect (in future) when in "auto detection" mode.

Sorry for missing this (Canada Day, etc.) I also think that this answer may cover-off some of the other stuff you have asked. The answer is a qualified 'no'.

The 'auto-detect' feature is unique to the wireless SRM PowerMeter (i.e., not found on wired PowerMeters), so by default, only the PC6 and PC7 PowerControllers can 'benefit' from it. When you change the slope in manual mode using the SRM software, the slope stored in the PowerController is not then changed so that the 'auto-detect' feature will, thereafter, apply the new slope. Rather, the PowerMeter will default to the original, factory slope setting.

I know. It's lame. I imagine that this will lead to another round of comments about SRM seeking to further secure 'repeat' business. There is a way to do it, though...

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Vallinotti
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by Vallinotti

Tapeworm wrote:I pinched this from a forum somewhere, been sitting in my docs files for ages. Easy as it gets.

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.


Is it possible to make these steps with a Garmin 810 unit?

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devinci
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by devinci

I am pretty sure it is NOT possible without a SRM head unit

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Giant DK
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by Giant DK

It is posible and even easier with a Garmin Edge unit as it doesn't leave the zero offset screen in the middle of the meassurements.
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devinci
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by devinci

but then can you adjust the slope afterwards?

wetpaint
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by wetpaint

Yes, you can manually enter the slope on a Garmin

coldmirow
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by coldmirow

Hi, I know - it‘s an old post.
But is it possible to enter a different sensitivity value using the current pm9 version?


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Giant DK
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by Giant DK

coldmirow wrote:
Tue Nov 14, 2023 12:52 pm
Hi, I know - it‘s an old post.
But is it possible to enter a different sensitivity value using the current pm9 version?


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Not what I know of. Unfortunately. And you can't read the slope value anymore and therefor not check it you self. A down grade in my eyes.

And no auto zero offset - from what I know. Maybe they believe this is not necessary with their "Auto temperature compensation". But I have been strugling with this not working properly and am experiencing some drift - can be easily fixed wih a manual zero offset. This is easy to spot riding on Zwift when I have dual record together with my Kickr. A down grade too.

If you find solutions please let me know.
Last edited by Giant DK on Thu Nov 16, 2023 10:19 am, edited 1 time in total.
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coldmirow
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by coldmirow

Giant DK wrote:
coldmirow wrote:
Tue Nov 14, 2023 12:52 pm
Hi, I know - it‘s an old post.
But is it possible to enter a different sensitivity value using the current pm9 version?


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Not what I know of. Unfortunately. And you can't read the slope value anymore and therefor not check it you self. A down grade in my eyes.

And no auto zero offset - from what I know. Maybe they believe this is not necessary with their "Auto temperature compensation". But I have been strugling with this not working properly and am experiencing some drift - can be easily fixed wih a manual zero offset. But this is easier to spot riding on Zwift when I have dual record together with me Kickr. A down grade too.

I you find solutions please let me know.
Thanks for your reply.
Did you try using their pm9 app?


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RyanH
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by RyanH

Ugh... seriously? My PM9 is almost definitely reading high...

I calibrate all of my PM7s myself and have been for the past few years.

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Giant DK
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by Giant DK

Yes, no option in app to turn on some kind of auto zero offset. Don't know if a PC headunit can do that though. My Garmin can't. Had no issues with this with my older PM7. Auto calibration seems to be working. And I could test/calculate slope myself and update it through my Garmin headunit. This does not seems to be an option anymore.
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Guibarhu
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by Guibarhu

Hi everyone.
I’m looking for an excel spreadsheet, website or app to help me with the calibration process of my SRM.
All the links to the previous sites are not working. I need to check the slope of my powermeter after a battery change. Many thanks


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Giant DK
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by Giant DK

Guibarhu wrote:
Tue Jan 09, 2024 4:25 pm
Hi everyone.
I’m looking for an excel spreadsheet, website or app to help me with the calibration process of my SRM.
All the links to the previous sites are not working. I need to check the slope of my powermeter after a battery change. Many thanks


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Send me your email in PM and I send you a spreadsheet I have created to calculate the slope.
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Guibarhu
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by Guibarhu

Many thanks for your mail and help.
What’s the minimal weight to do the calibration?
20kgs? Thanks


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Giant DK
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by Giant DK

Guibarhu wrote:
Thu Jan 11, 2024 7:16 pm
Many thanks for your mail and help.
What’s the minimal weight to do the calibration?
20kgs? Thanks


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You are welcome.

As for the weight, if you google "SRM calibration weight" you get a lot of hits and most suggest weight at 15-20kg. As you might have noticed I used a weight at around 15kg. Theoretical any weight should do. But the common understanding is that if should be "big enough". Since the slope, that you want to calculate the the slope of the graph where the frequenzy you read are directly proportional with the torque (torque = load*crank lenght). So the frenquenzy is directly proportional with the applied load. Maybe the precision of the readings is better with heigher loads. If you wanted to do in perfectly you would make several readings with a broad range of loads within the within the spectrum that you can deliver on the bike. Then you'll get several data point along the graph to make a more exact graph. But remember, the way you powermeter calculate the power it assumes the power are 100% directly proportional with the load.
The most important thing in my mind is you have the exact weight of you weight.

This example with a 15,77kg weight that reads 989:
Torque=15,77kg*9,81m/s^2*0,1725m=26,71Nm
This gives a slope of (989Hz-444Hz)/26,7=20,4Hz/Nm

So a 20kg weight would theoretical have given a reading of 20*9,81*0,1725*20,4+444=1135Hz.

This is also a way to check your SRM. Check different loads and see if the SRM reads a predicted - directly proportional with the applied load.

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