If you are a member of the Google Wattage group, check the files section as several people have uploaded Excel spreadsheets that perform all the math for you.

Oh man, that was too easy (and I'm an idiot for not looking there)...thanks mate.

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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

Why didn't I think of that....

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My insomnia destroying blog can be found here http://tfalin.blogspot.com/

I have the wireless SRM and the Garmin 705. I searched SRM's site and Google Wattage but can't find out if it is possible to check the slope without a different head unit. I saw some info on how to enter the slope, but not how to test it.

I guess this is not possible with the Garmin as the head unit?

After reading the posts about calibrating the slope of the SRM (as well as Tom A's comment that not calibrating the slope yourself is lazy), I decided I needed to do this, especially as my readings have been a bit hard to believe. For background:

Sent my SRM unit back to SRM last spring. I have a wired unit, and the sensor broke in July, so I got a new sensor cable and installed that. No issues (at least that I was aware of) for the rest of the season. Put my bike on the Computrainer and started to ride with both the CT and the SRM units on 2 months ago. There was a big discrepancy in Wattage, with the CT reading about 20 Watts higher. This didn't seem right, as I had read that the CT "should" read a little bit lower, due to the measurement at the rear wheel instead of the crank, and, previously, they had read much closer to each other.

I used a 40 pound dumbbell (4 x 10 lb plates + bar) and went to the local UPS store to have it weighed, figuring they weigh heavy objects fairly accurately. I got a weight of 44.25 lbs. Put my bike on a trainer on top of a table, and went through the whole calibration drill with the help of my 9 year-old son. He thought it was fun to grab onto the brake as hard as he could.

First thing I found out: my sensor was in the wrong spot. The two different horizontal readings (9 o'clock and 3 o'clock) were much different. When I changed the sensor location, that solved the first problem.

Second thing I found out: my slope was definitely different than the slope that SRM had, going from 29.9 --> 29.2. When I changed the slope on the SRM and then tested it against the CT, the readings came out nearly identical.

I'm much happier now, especially since it appears that my power readings have gone up.

I wanted to say thanks to everyone, because I trust my power readings a lot more now.

Richard

Congrats...always good to have accurate power readings.

I too recently re-calibrated my SRM (I used 4 x 25 lb weights, increasing and decreasing in 25 lb increments, repeated 3 times for each crank), and this enabled me to check linearity and repeatability, and with a linear regression analysis achieved a coefficient of determination, R^2 of 1.00.

But, if you remove your chainrings (I did so to do an annual clean of the SRM etc) then it is important to torque each bolt to similar settings and then RE-calibrate the SRM. The zero offset will change too after messing with the chain rings. I found it best to ride 10+ miles ride (with a few power efforts) after re-installing chainrings, and before re-calibrating, as the zero offset was more consistent after "settling in".

I've got a new (used) SRM Dura Ace 7900 meter and a Power Control 7. I want to swap the crank arms to a longer size, and have the 7800 wireless unit available to do the swap. I read in the manual that the wireless systems automatically re-calculate / calibrate the slope. Can I rely on this automation to true everything back up? Thanks
John

Hmm, having had six SRMs over the last six years and having checked the slope somewhat religiously whenever I changed chainrings or swapped the cranks out to another bike I have determined that the factory values are more than adequate for my needs. I came to this conclusion after doing a bit of math comparing very similar slope numbers and their affect on the recorded power values. Since I can never be sure my 'known' weight is actually the weight I think it is (very difficult to gain access to an 'accurate' scale in these parts) I can never be sure the values that come out the other end of the calculation are 'better' than the factory calibration since they are typically within a tenth or two and I don't believe I am somehow better than the factory at determining what the slope actually is so I just spend that time happily riding my bike rather than fretting about a couple of nanowatts here and there.

.

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I know enough to know I don't know enough...

At least you checked it ;-) As others have noted above, your experience is apparently not exactly "universal".

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http://bikeblather.blogspot.com/

I never claimed my experience was universal. I merely stated my experience is that not ALL SRMs are inherently suspect when it comes to the slope setting from the factory. I also found that changing chainrings and crank arms didn't affect the slope to any great extent even when taking the entire process to extremes (torquing all bolts to the same value, riding prior to calibrating, etc., etc., etc.). The values I get are typically close enough that I don't have the time or patience to determine who is wrong (me or SRM) so I just accept the values and ride my bikes. I have put my road SRM on my time trial bike and my time trial SRM on my road bike and done efforts on the LeMond Revolution just for yucks and I found the power values to be pretty much identical (+/- 5 watts) for a given speed so I consider the accuracy of both to be acceptable (it was raining out and I needed to find a way to motivate myself to get on the trainer otherwise I wouldn't have bothered even going this far).

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I know enough to know I don't know enough...

A handy link for doing the recal math:

http://www.cyclingpowermodels.com/...eterCalibration.aspx

Right...but you wouldn't have known that if you hadn't checked it. So, like I said, it's helpful to be able to, and to actually, check it :-)

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http://bikeblather.blogspot.com/

resurrecting a zombie thread, but looking for a related answer. in doing the calibration to adjust for new chainrings, does one do the unloaded zero offset with the old rings or the new? big ring, small ring, or both? i am changing my big ring (normal round road) to a solid tt style q ring. thanks.

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________________________________________________________________________
"that which does not destroy me will only make me stronger" Frederick Nietzsche
andrew peabody
http://BREAKAWAYMULTISPORT.COM

Here is my idea for checking and calibrating an SRM. I have a digital readout torque wrench. I'll tape the head unit right above the torque wrench readout so I can see both readouts at the same time. Set the wrench to read N/M. Lock the brake/ immobilize the cranks. Put the torque wrench on the center crank nut and apply torque. Pull the wrench to some higher value while recording the readouts with my phone/tablet. Watching the playback, I can reference any point of torque/hz and figure the slope. Advantages I see are: crank length is irrelevant, as is crank position. No fumbling hanging weights. Easy to do one handed. A recording of the torque sweep, easy to catch and replay. Next I'll have to actually try it and see if it works. I can't think of why it wouldn't.

Few things come to mind

What center crank nut? On a few units those are to be just barely tight. This would work for Sram but not for Duracci and Rotor
What's the accuracy on your torque read out?
How can you be sure you are applying consistent torque during the time in which the offset updates?

Crank position is a trivial issue. Human eye can easily tell if something is 5 deg off, and that represent an error of 0.4% max. Both the mass and crank length are also static and don't change during measurement, unlike the torque you apply.

Seems nobody bothered to answer you. You do the whole calibration process (unloaded and loaded) with the new rings. Do the calibration on both the big ring and small ring to see how their slopes compare. They should be close. To account for any difference you have a choice of either inputting the average slope for the two rings or, if most of your riding will be in one of the rings, using just that ring's value. This at least applies to round rings. For Q rings AFAIK SRM have suggested you should calibrate with round rings and then put on the oval rings. However others disagree (check Wattage forum archives). If you did go this way I'd think would be better to choose a round ring with about the same stiffness, IOW solid in your case.

thanks. that's what i thought.

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________________________________________________________________________
"that which does not destroy me will only make me stronger" Frederick Nietzsche
andrew peabody
http://BREAKAWAYMULTISPORT.COM

I can't think why you'd need to do this. When doing static torque checks, the SRM spider doesn't care/know about the chainring shape. All that matters is measurement of the torque applied to the spider, which is independent of the size and shape of the ring. All the chain and chainring do is prevent the spider from turning when you hang a weight from the crank arm.

Of course some rings might flex a little more/less and this causes a slight variance in measured torque but you'd be better off doing that check with the rings you actually intend to use.

SRM calibrates by hanging mass off of a chain, thus actually mattering. they probably forgot that point when instructing people who calibrate by hanging mass off of a pedal

Here is a photo of the SRM test rig (Colorado Springs facility) from CyclingNews. Has anyone rigged up something similar?

Interesting...I guess that picture means that they don't bother to see if there's a difference in calibration between force applied to the DS and NDS of the crankset in the factory cal?

Oh, and yeah...calibrating by hanging the weight on the chainring instead of the crankarm isn't going to work too well with non-round rings.

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http://bikeblather.blogspot.com/

This reminds me of the picture from a few months ago of the pc7 assembly in the "clean room" with no electrostatic discharge protection. Not exactly lab quality........

i'm no expert in this area, but SRM specifically says to calculate slope for those systems with round rings installed, then switch to non-round chainrings.

YMMV, but in my experience the slope of SRM tends to change very little based on ring choice (brand and size). e.g., when i switch between 53/39 and 56/44 rings, the slope difference is <=0.4%. the difference is even less when ring size is the same.

you raise an interesting point about factory calibration perhaps not considering DS v NDS differences. i always test the units that pass through my hands straight from the factory (DS, NDS, both rings, weighted and unweighted) and often notice a small difference.

It should make no difference for calibration, round rings or not, with the weight applied to the crankarm since the chainring size doesn't even come into the calculation, just the crankarm length.

Doing it the way shown in the pic above, that's not the case...and is probably why they recommend what they do...even though it's just wasted effort.

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http://bikeblather.blogspot.com/