SRM Calibration - questions

bcarroll said:

I recently purchased an SRM Pro after riding with a PT for the last year and a half. I did several rides with both powermeters together and the SRM is reading about 20% lower than the PT. I have decided to perform a static calibration of the SRM but have several questions.

1. I obtained a known weight of 20kg. Is this a sufficient amount of weight? I have read different descriptions of how much weight to use and am confused. Generally, I read more is better so just wondering if I should get another 20kg or if what I have is sufficient?

2. In the calculation formula obtained from SRM's website, it says to "take the average of all 6 zero-torque offset readings, and use in the calculations as the zero torque offset for both sides." Is this the average of the readings obtained with weight or am I to record the zero offset between each reading and average those (i.e. average the zero offset without weight)?

3. What is the proper gearing to use during the calibration? (i.e. 53 x 11 or 39 x 21, etc...) Is it necessary or beneficial to record the load test using different gearing? If so, do you take the average (assuming different readings)?

Any help you can provide would be appreciated. Thanks.

How do you know it's 20 kg? I would use the heaviest weight I can accurately measure, and handle during the calibration. For me this is about 52 pounds within something like 0.1% with this:
http://www.cyclingnews.com/tech.php?id=tech/2005/reviews/ultimate_scale

You take the offset measurement without the weight and with the weight on each side three times. If you read it carefully it says the zero offset without weight on each side should not change more than a very small amount - if it does one has to suspect something is wrong with the install or something further up the chain which should be remedied before the calibration is completed.

This is why you need to pick a weight you can handle while the weight is applying pressure on the crankarm, you may have to play quite a bit with rotating the arms to get the max reading six times.

Why do you think the gearing matters if they don't even mention it and it doesn't appear in the formula? FWIW in the picture it's in the 53x12.

Woofer said:

How do you know it's 20 kg? I would use the heaviest weight I can accurately measure, and handle during the calibration. For me this is about 52 pounds within something like 0.1% with this:
cyclingnews.comtech.phpOpen ↗

You take the offset measurement without the weight and with the weight on each side three times. If you read it carefully it says the zero offset without weight on each side should not change more than a very small amount - if it does one has to suspect something is wrong with the install or something further up the chain which should be remedied before the calibration is completed.

This is why you need to pick a weight you can handle while the weight is applying pressure on the crankarm, you may have to play quite a bit with rotating the arms to get the max reading six times.

Why do you think the gearing matters if they don't even mention it and it doesn't appear in the formula? FWIW in the picture it's in the 53x12.

I know the weight is 20kg because I purchased an Ivanko plate which are Olympic competition calibrated weights. Their maximum tolerance on a 20kg plate is 10grams (0.0005% accurate). Additionally, I have weighed it on a digital scale as you mentioned. I am quite confident that I have a known weight.

I did read the instructions very carefully but did not think that point was clear. Thanks for explaining. It makes sense.

I know they don't mention the gearing in the instructions, however, I have read different posts where people mention taking the reading in different gearings. I was hoping to gather some more information regarding this point from others who have experience.

gearing does not matter

bcarroll said:

I know the weight is 20kg because I purchased an Ivanko plate which are Olympic competition calibrated weights. Their maximum tolerance on a 20kg plate is 10grams (0.0005% accurate). Additionally, I have weighed it on a digital scale as you mentioned. I am quite confident that I have a known weight.

I did read the instructions very carefully but did not think that point was clear. Thanks for explaining. It makes sense.

I know they don't mention the gearing in the instructions, however, I have read different posts where people mention taking the reading in different gearings. I was hoping to gather some more information regarding this point from others who have experience.

yzfrr11 said:

gearing does not matter

According to a test performed by Andy Coggan, the SRM is sensitive to alterations in the chainline (lists.topica.commessage.htmlOpen ↗). This is why I had ask the question and was wondering how most people factor this in their calibration, if at all.

bcarroll said:

According to a test performed by Andy Coggan, the SRM is sensitive to alterations in the chainline (lists.topica.commessage.htmlOpen ↗).

Geez, what a nerd!

Anyway, in case it isn't obvious, there's a mistake in that description...what I do is repeatedly remove and remount the right crank while keeping the left pointing straight ahead. As stated, the slope of my road crank varies by 3.4% around the circle when assessed this way. My track crank (assessed in only four positions, since it is square taper) shows a bit less positional sensitivity, however, as it varies by only 1.8%.

Note that when you're pedaling, the torque applied to the crank is highest when the cranks are horizontal, or nearly so...an argument could therefore be made that you should use the slope derived by testing the crank in only these two positions, such that the procedure described in the above post is a waste of time.

acoggan said:

Geez, what a nerd!

Anyway, in case it isn't obvious, there's a mistake in that description...what I do is repeatedly remove and remount the right crank while keeping the left pointing straight ahead. As stated, the slope of my road crank varies by 3.4% around the circle when assessed this way. My track crank (assessed in only four positions, since it is square taper) shows a bit less positional sensitivity, however, as it varies by only 1.8%.

Note that when you're pedaling, the torque applied to the crank is highest when the cranks are horizontal, or nearly so...an argument could therefore be made that you should use the slope derived by testing the crank in only these two positions, such that the procedure described in the above post is a waste of time.

Makes sense. Thank you for the information.

bcarroll said:

Makes sense. Thank you for the information.

BTW, in case it wasn't clear the "geez, what a nerd!" comment was directed at myself. (I thought I'd tossed in a smilie to help make that clear, but apparently not.)

acoggan said:

BTW, in case it wasn't clear the "geez, what a nerd!" comment was directed at myself. (I thought I'd tossed in a smilie to help make that clear, but apparently not.)

I thought you may have meant that, however, it wouldn't be the first time I was called a nerd so I wasn't really offended.🙂 Thanks for making that clear. BTW, I did my first calibration last night and it was very easy once I got everything set-up.

bcarroll said:

I thought you may have meant that, however, it wouldn't be the first time I was called a nerd so I wasn't really offended.🙂

I'm relieved to hear that!

bcarroll said:

BTW, I did my first calibration last night and it was very easy once I got everything set-up.

So, how did it turn out? Was the factory slope close to correct, or did you come up with something significantly different?

acoggan said:

So, how did it turn out? Was the factory slope close to correct, or did you come up with something significantly different?

The factory calibrated slope was 18.2 and I came up with 17.9. I did it twice and got the same results both times so I feel good about the process. It was closer than I originally thought (from riding with both my PT and SRM for several rides and SRM being lower) but I am glad to have the peace of mind that I calibrated it myself and also to learn the process.

I also did the "stomp" test on the PT with my known weights and came up with a 5% error (torque was higher than calculated) so I think this might be part of the difference I was seeing between the two units (assuming I did my math right).

bcarroll said:

The factory calibrated slope was 18.2 and I came up with 17.9. I did it twice and got the same results both times so I feel good about the process. It was closer than I originally thought (from riding with both my PT and SRM for several rides and SRM being lower) but I am glad to have the peace of mind that I calibrated it myself and also to learn the process.

FWIW, I think the best way to compare two powermeters is to use them both during an incremental exercise test on a trainer, with each stage being long enough to allow you and/or the equipment to "settle in" and to get a good average (I usually allow 3 min, using only the last 2.5 min). In my experience, this allows you to pick up on smaller differences than riding outside and just looking at the numbers on the display periodically.

bcarroll said:

I also did the "stomp" test on the PT with my known weights and came up with a 5% error (torque was higher than calculated) so I think this might be part of the difference I was seeing between the two units (assuming I did my math right).

The reason that I first proposed the "stomp" test is because your body mass is likely the largest mass you have available to apply to the pedal/crank. The 20 kg mass you used is certified to be very precise, but it is too small to fully overcome the fact that the PowerTap computer only displays the torque in in-lbs to the nearest whole number. Or, to put it another way: it may be that your PowerTap is really only high (at that torque) by 2.51%, but that torque gets rounded up such that it appears to be off by 5%. Using the lowest gear (e.g., 39x25) possible helps some (since that magnifies the torque transferred to the rear hub), but to get really accurate results you need to apply more than 20 kg of force.

Anyway, since you now have your SRM calibrated you could repeat the comparison between the two, in which case they should agree to w/in ~5-10 W of each other, depending on what gear you're in and the condition of your drivetrain. If the PowerTap really is reading high by 5%, you'll see much less difference than this, or you might even find that it is reading higher (which should never be the case).

bcarroll said:

The factory calibrated slope was 18.2 and I came up with 17.9. I did it twice and got the same results both times so I feel good about the process. It was closer than I originally thought (from riding with both my PT and SRM for several rides and SRM being lower) but I am glad to have the peace of mind that I calibrated it myself and also to learn the process.

I also did the "stomp" test on the PT with my known weights and came up with a 5% error (torque was higher than calculated) so I think this might be part of the difference I was seeing between the two units (assuming I did my math right).

Aw, but the important question is - what gearing did you use? 🙂
Just kidding. I only calibrate on track bikes and I only race on one or two gears but good to hear all the hypothesis nonetheless.