Hello,

Do any of you have an easy to understand set on instructions for how to check and recalibrate slope on a wired SRM unit? I have searched the wattage forum and could not find a good set of instructions.

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.

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Here's another set of directions with pictures too. There's a nice spreadsheet where all you have to do it put in the numbers and it will calculate the slope for you.
http://www.westwoodvelo.com/showthread.php?p=6447#post6447

Weights that are typically available can vary widely in their true weight versus what is marked on the side of it. I recommend using Ivanko Calibrated plates. You can use an old pedal spindle and hang the weight off that (don't forget to factor-in the weight of the spindle).

I have a question..

I have SRM DA7800 with PC7..

If I use the srm in two bikes(with the same type of bottom bracket) I must do the calibration every time I change(dissamble and mount) the crank from one to the other bike?

The operation is very fast form me, but only if every time i mustn't do the calibration..

thankyou for your reply

No, you do not need to re-calibrate the PowerMeter. The PowerMeter is sensitive to changes made to it directly (opening of the case, changing the crankarms, etc.), but is quite neutral to other changes, such as swapping the crank in its entirety from one bike to another.

I have found this site very easy to use to work out the slope. You just enter the numbers it does the rest

http://www.cyclingpowermodels.com/Power ... ation.aspx

How would one do this without a PC7...?

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There is also an Android App for calculating the SRM slope, although I use the Excel sheet.

I use a 20kg calibrated mass as it gives a torque value similar to what I see at my functional threshold power. I just hang the mass from a pedal using some safety wire and rotate the rear wheel until the crank is almost horizontal. By looking at the PC frequency value you can tell when the crank is dead on horizontal as the frequency value will hit a peak.

I do not (yet) own a SRM unit.

My apologies if these are silly questions.

I assume the above procedure is used to determine a (more) correct slope constant for a SRM power meter. This new slope constant is then manually set in the head unit, and no change is made to the power meter. Correct?

Can this procedure be used with PCV, PCVI, and PCVII head units?

With regards to "auto slope detection":
The PCVI and PCVII documentation says the head unit can detect a slope constant automatically from a power meter.
Is it only the PCVI and PCVII that does "auto slope detection"?
Is a slope constant stored only on wireless SRM power meters?

I have read some forum post suggesting that there are ways to change the value that is "stored" on the power meter. Has anyone done this?
Is a Garmin 500 the tool of choice?

Many Thanks

Frank

No, not silly at all.

The procedure referred to is the recalibration of the PowerMeter itself. Is need only be done periodically (like, annually, in e off-season, or whenever changes are made to the PowerMeter itself). The basic procedure is the same for any PowerController.

When that type of recalibration procedure is completed, the result is the calculation of a new Slope for the PowerMeter. The last step in the process is to record the new Slope in the PowerMeter by way of manually changing the existing Slope definition in the PowerController. This manual slope can be used with any PowerController.

The 'auto slope' feature is a wireless PowerMeter option. Personally, I do not use the 'auto slope' feature on the PC6 or PC7, electing instead to manually set the Slope.

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.

The slope is just a multiplier (stored in the head unit) used to calculate power. The power control (or garmin or whatever other head unit you use) just takes information (strain/stretch/flex measured by the strain gauges) from the power meter, and uses the slope to complete a calculation that results in power. Basically, a given weight (the calibrated weight you use for the procedure) produces a specific amount of flex, and the slope correlates that known value to all other possible values (smaller weights/forces = less flex, larger weights/forces = more flex).

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I have assumed that what is happening, at least whilst using this calibration procedure, is that the power meter (spider) transmits a frequency proportional to the toque at the power meter (spider).

Whilst you hang a weight, the power controller (head unit) is displaying the frequency of signal it is receiving from the power meter (spider) in real time.

The procedure enables you to determine a “more appropriate” slope constant [Hz/Nm] for a power meter (spider), and enter it manually into the Power Controller (head unit).

The Power Controller (head unit) needs to know this slope constant to display the power, which is calculated by methods not exactly known.

Question 1
Can all of the Power Controllers (head units) PCV, PCVI, and PCVII, displaying the frequency of signal it is receiving from the power meter (spider) in real time?



When I read in the documentation for PCVI and PCVII, it suggests to me that an “appropriate” slope constant is “stored” on the power meter (spider), and that this slope constant is transmitted by the power meter (spider) to the power controller (head unit). The PCVI and PCVII can then either use the transmitted slope constant whilst operating in "auto slope detection” mode, or they can use a manually entered slope constant.

Question 2
Do you concur, that it is only the PCVI and PCVII that has an "auto slope detection mode" option?

Question 3
Is a slope constant stored only on wireless SRM power meters?



I have assumed that the power meter (spider) is a transmit only RF device, and the Power Controllers (head units) are receive only RF devices. If this assumption is incorrect? Please correct me if this assumption is not correct.

Question 4
Can the OEM slope constant “stored” on the power meter (spider) be changed? How?

1: Yes, the number displayed when you do a zero offset is the signal being received from the power meter. All PCs show this (when you hit mode & set at the same time).

2: The PCV doesn't have the autoslope option. I believe VI and VII both do.

3&4: I don't know. My experience is with PCVs, so I'm not sure exactly how the autoslope works. Maybe I'm assuming incorrectly, but I thought the autoslope was still just some calculation that would be carried out in the PC based on information received from the power meter.

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