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Thread: Pros & SRM

  1. #1
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    Pros & SRM

    One thing I've always wondered is why so many pros use an SRM, even if they are not sponsored by SRM. They may not use it on their race bike, but when out training I almost always see them with a SRM rather than powertap or quarq.


    Pros don't make that much money unless they are at the top and that is why I find it odd. I would expect most of them to instead use a powertap or Quarq simply due to cost (most I see use Dura ace or FSA SRM which runs $2800-3700 depending on whether they use the head unit compared to 1000-2000 for powertap or Quarq). Anyone have an idea why SRM seems to be the choice of powermeter for a large portion of pro cyclists?

  2. #2
    'brifter' is a lame word.
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    it's the best.
    you can use it on the race bike and swap wheels at will.
    a LOT of the pro racers i know and/or work with will do whatever it takes to get their hands on an SRM. they'll switch it back and forth between training/race/tt bikes (or they make me do it )
    i work for some bike racers...
    2013 Trek Madone 5.9 w/ '12 SRAM Red
    2010 Cervelo T1 sprint bike
    Ruger 10-22TD
    Smith&Wesson M&P 15-22
    Smith&Wesson M&P 9
    oh, those belong in another forum

  3. #3
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    Quote Originally Posted by cxwrench View Post
    it's the best...
    That's good to know. Can you elaborate on what characteristics make it functionally better than others?
    ... 'cuz that's how I roll.

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    Quote Originally Posted by looigi View Post
    That's good to know. Can you elaborate on what characteristics make it functionally better than others?
    SRM was the first reliable Powermeter and they have been around for years before anyone every thought about make a quarq. I have one that is getting checked out at SRM for any maintenance needed that I will be selling when I get it back. You send them in for a battery and they give them a good look over and they just keep running and running and running.

  5. #5
    'brifter' is a lame word.
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    Quote Originally Posted by looigi View Post
    That's good to know. Can you elaborate on what characteristics make it functionally better than others?
    more strain gauges=better accuracy, better consistency. Quarqs seem to lose their calibration, sometimes in a major way. i think they have a handle on that now, but some of the first ones would go out by so much it was crazy. you also have to use a Garmin as the head unit. SRM makes their own which is part of the reason they're more expensive, but they provide more information than you can get from a Garmin.
    i work for some bike racers...
    2013 Trek Madone 5.9 w/ '12 SRAM Red
    2010 Cervelo T1 sprint bike
    Ruger 10-22TD
    Smith&Wesson M&P 15-22
    Smith&Wesson M&P 9
    oh, those belong in another forum

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    So, more reliable, more accurate, and more information. Sounds better to me. Thanks.
    ... 'cuz that's how I roll.

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    Quote Originally Posted by looigi View Post
    So, more reliable, more accurate, and more information. Sounds better to me. Thanks.
    How much more accurate? I haven't had problems with reliability or accuracy with my Quarqs. When I compared the price of the Campy SRM, it was more than twice what a Quarq normally runs.

  8. #8
    'brifter' is a lame word.
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    Quote Originally Posted by spade2you View Post
    How much more accurate? I haven't had problems with reliability or accuracy with my Quarqs. When I compared the price of the Campy SRM, it was more than twice what a Quarq normally runs.
    i definitely wouldn't say it's twice as reliable or consistent, but SRM claim +/- 2% these days. another reason that most pros use them is that they were the first. and only, for years. Quarq is very new compared to SRM, and we all know how pro racers can be about equipment.
    Quarq is owned my SRAM, so to be sponsored you'd need to be sponsored by SRAM. SRM makes units compatible w/ pretty much every major crank brand, so more racers can use them and not have sponsorship issues.
    i work for some bike racers...
    2013 Trek Madone 5.9 w/ '12 SRAM Red
    2010 Cervelo T1 sprint bike
    Ruger 10-22TD
    Smith&Wesson M&P 15-22
    Smith&Wesson M&P 9
    oh, those belong in another forum

  9. #9
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    Quote Originally Posted by cxwrench View Post
    i definitely wouldn't say it's twice as reliable or consistent, but SRM claim +/- 2% these days.
    And Saris claims +/- 1.5%. So in what sense is the SRM more accurate. (especially in light of https://www.facebook.com/photo.php?f...type=3&theater )

  10. #10
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    I sell both SRM & Quarq (and am quite happy to recommend Powertap as well, although I don't sell those).

    They are all good and have their pros/cons, but mostly pros. What's the best choice for each person will depend on a range of factors. A few things stand out for me with SRM:

    - ease with which one can self validate and amend slope calibration of the power meter. You can do it with a Quarq too but you need additional non-free hardware (iphone and a ANT+ dongle) and free software to do it. You can check a Powertap's calibration but not amend it if it is out.

    - longevity of the equipment. My track SRM is approaching 10 years old and works perfectly, all my other SRMs are 4-8 years old and also work perfectly - and SRM will continue to service them as required. Can't say same about Powertaps, which IME have a much shorter lifespan (I've owned about a dozen Powertaps) and they discontinue service for non-current models after a period. Quarq has not been in business long enough for us to know. There is nothing to suggest longevity will be a problem, but only time will tell.

    - SRM provide choice of using their own head unit, which just works and hence not subject to a third party changing things that impact power reading/accuracy/reliability. It doesn't have GPS though, if that is important to you (but of course it will also work with compatible ANT+ head units such as Garmin if you want that feature).

    - SRM have by far the greatest range of crankset options, and units specifically designed for non-road applications such as BMX, MTB, track and so on.

  11. #11
    rebounder
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    i had heard 1.5% for the garmin pedal-based unit, the vector (still to be released to the masses). if the pedal based systems can offer even a fraction of the features and accuracy of crank-based units, the ease of use alone will make them popular for many people. but, if they cant take 10k miles a year or more the pros wont ride them.
    on the other hand, you have different fingers

  12. #12
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    I was asked in a PM about how to calibrate an SRM.

    The following is how to check the slope calibration of an SRM. The same basic principle applies to all power meters, except the formula for each will be slightly different.

    The fundamental principle for checking calibration on all power meters is:
    1. Set the power meter's handlebar computer in the mode to display the raw torque values
    2. Apply a precisely known torque to a crank arm (sufficient to replicate reasonable power output)
    3. Record the raw torque reading for that precisely known torque value
    4. Record the raw torque reading for when there is no torque applied to the crank arm
    5. Check both cranks arms and both chainrings, several times.

    Calibration slope = (loaded torque reading - unloaded torque reading) / Actual torque applied.

    Compare that to the slope quoted, and if you have an SRM, update the slope in your Powercontrol, or use the Garmin to update the stored slope in the power meter.

    The formula needs to account for the units used. SRM uses SI units (i.e. metric), Quarq uses a mix of metric and imperial.

    Power2Max quotes their slope as the inverse of the above.

    SRM Slope Calibration Check

    These are instructions I've written that go with a spreadsheet I designed. I'll just give you the formula for SRM. Quarq, Power2Max and Powertap use different formula. Quarq have an instructional video on their website on how to use their Qalvin iPhone App.

    You will need to set up bike so you can hang an accurately known mass (at least to nearest 50g) of at least 20kg from the pedal spindle of a horizontal crank arm. Gym plate weights and a cord for hanging are good, all items weighed on calibrated postal scales.

    Placing the bike in a turbo trainer is good for performing calibration tests. It keeps the bike stable so you can manage the weights. You can raise the front wheel of the bike up onto a chair to give more clearance for hanging masses.

    You will need a means to prevent the rear wheel from turning. Either have someone hold the brake, or use something else to prevent the wheel from turning when needed.

    Record both the mass used and crank length in sheet provided. If known, note the previously known or factory slope calibration value (usually on a sticker on the inner plate of the power meter).
    *
    For SRM & Powercontrol (V, VI & 7):

    1. Select the gearing for the test (chainring and rear cog). For road power meters, recommend testing in both large and small chainrings.

    2. Wake up the SRM by turning cranks several times (forwards for wireless SRMs), and ensure Powercontrol is on and receiving a signal from the power meter.

    3. Press Mode & Set buttons together to display the Zero Offset value (Hz).
    Make sure the Powercontrol is not in interval mode as zero offset canít be displayed in interval mode. If using a different computer, then the "calibration" mode of a Garmin needs to be visible.

    4. Rotate the cranks until one crank arm is horizontal and towards front of bike . Record the unloaded (zero) offset Hz value. It will typically be between 400-800Hz

    5. Brace the rear wheel so that it canít rotate.

    6. Hang the known mass from the pedal spindle of the forward facing crank.
    Record the loaded offset Hz value.

    Optionally, you can very slowly rotate rear wheel until you attain the maximum offset value reading while the wheel is not moving.

    7. Remove the mass from the pedal spindle.

    8. Repeat steps 4 through 7 using the opposite crank arm.

    9. Repeat steps 4 through 8 one or two more times to validate readings.

    10. The sheet will calculate the slope of the power meter (Hz/Nm)

    I'm not including the sheet but here is the simple formula:

    Slope is simply:

    (Loaded offset Hz reading - Unloaded offset Hz reading) / (Actual torque applied)

    where:
    Actual torque applied (Nm) = mass (kg) * acceleration due to gravity 9.81 (m/s^2) * crank length (m)

    e.g. let's say this is what you have/get from checking one crank:

    loaded offset value: 1320Hz
    unloaded offset value: 565Hz
    mass used: 22.50kg
    crank length: 0.170m
    gravity: 9.81m/s^2

    Slope (Hz/Nm) = (1320 - 565) / (22.50 * 9.81 * 0.170)
    = 20.12 Hz/Nm


    you get that value several times for each crank arm and then average the values.
    Ideally you don't want much variance in the slope measure for each crank arm, nor when checking slope with chain on different chainrings.

    You can also validate using different masses to assess the linearity of the slope calibration, but I rarely bother as IME the slope on SRMs is almost always very linear.

  13. #13
    The Slow One.
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    I own a Quarq, and one of the selling points for me was the ability to change batteries instead of sending it back to manufacturer. Having never owned a SRM, what's the turnaround time for one, and how often do you send it in? I've heard nothing but good things about them, but the price put me off a bit.

    So far my Quarq has been flawless, and I'm contemplating buying another one to be shared between the commuter (same as my race bike) and my TT bike. Swapping cranks during stage races can be a pain, especially on days with 2 stages, and it would be nice to get power data for my commutes.

  14. #14
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    Quote Originally Posted by Alaska Mike View Post
    I own a Quarq, and one of the selling points for me was the ability to change batteries instead of sending it back to manufacturer. Having never owned a SRM, what's the turnaround time for one, and how often do you send it in? I've heard nothing but good things about them, but the price put me off a bit.

    So far my Quarq has been flawless, and I'm contemplating buying another one to be shared between the commuter (same as my race bike) and my TT bike. Swapping cranks during stage races can be a pain, especially on days with 2 stages, and it would be nice to get power data for my commutes.
    Price and being able to do my own battery changes made sense. I ultimately picked up a 2nd Quarq because I run different gear ratios on my race only road bike and my TT bike. I also don't like wrenching between races. These 2 Quarqs were less expensive than the Campagnolo version.

  15. #15
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    Quote Originally Posted by Alex_Simmons/RST View Post
    I was asked in a PM about how to calibrate an SRM.

    The following is how to check the slope calibration of an SRM. The same basic principle applies to all power meters, except the formula for each will be slightly different.

    The fundamental principle for checking calibration on all power meters is:
    1. Set the power meter's handlebar computer in the mode to display the raw torque values
    2. Apply a precisely known torque to a crank arm (sufficient to replicate reasonable power output)
    3. Record the raw torque reading for that precisely known torque value
    4. Record the raw torque reading for when there is no torque applied to the crank arm
    5. Check both cranks arms and both chainrings, several times.

    Calibration slope = (loaded torque reading - unloaded torque reading) / Actual torque applied.

    Compare that to the slope quoted, and if you have an SRM, update the slope in your Powercontrol, or use the Garmin to update the stored slope in the power meter.

    The formula needs to account for the units used. SRM uses SI units (i.e. metric), Quarq uses a mix of metric and imperial.

    Power2Max quotes their slope as the inverse of the above.

    SRM Slope Calibration Check

    These are instructions I've written that go with a spreadsheet I designed. I'll just give you the formula for SRM. Quarq, Power2Max and Powertap use different formula. Quarq have an instructional video on their website on how to use their Qalvin iPhone App.

    You will need to set up bike so you can hang an accurately known mass (at least to nearest 50g) of at least 20kg from the pedal spindle of a horizontal crank arm. Gym plate weights and a cord for hanging are good, all items weighed on calibrated postal scales.

    Placing the bike in a turbo trainer is good for performing calibration tests. It keeps the bike stable so you can manage the weights. You can raise the front wheel of the bike up onto a chair to give more clearance for hanging masses.

    You will need a means to prevent the rear wheel from turning. Either have someone hold the brake, or use something else to prevent the wheel from turning when needed.

    Record both the mass used and crank length in sheet provided. If known, note the previously known or factory slope calibration value (usually on a sticker on the inner plate of the power meter).
    *
    For SRM & Powercontrol (V, VI & 7):

    1. Select the gearing for the test (chainring and rear cog). For road power meters, recommend testing in both large and small chainrings.

    2. Wake up the SRM by turning cranks several times (forwards for wireless SRMs), and ensure Powercontrol is on and receiving a signal from the power meter.

    3. Press Mode & Set buttons together to display the Zero Offset value (Hz).
    Make sure the Powercontrol is not in interval mode as zero offset canít be displayed in interval mode. If using a different computer, then the "calibration" mode of a Garmin needs to be visible.

    4. Rotate the cranks until one crank arm is horizontal and towards front of bike. Record the unloaded (zero) offset Hz value. It will typically be between 400-800Hz

    5. Brace the rear wheel so that it canít rotate.

    6. Hang the known mass from the pedal spindle of the forward facing crank.
    Record the loaded offset Hz value.

    Optionally, you can very slowly rotate rear wheel until you attain the maximum offset value reading while the wheel is not moving.

    7. Remove the mass from the pedal spindle.

    8. Repeat steps 4 through 7 using the opposite crank arm.

    9. Repeat steps 4 through 8 one or two more times to validate readings.

    10. The sheet will calculate the slope of the power meter (Hz/Nm)

    I'm not including the sheet but here is the simple formula:

    Slope is simply:

    (Loaded offset Hz reading - Unloaded offset Hz reading) / (Actual torque applied)

    where:
    Actual torque applied (Nm) = mass (kg) * acceleration due to gravity 9.81 (m/s^2) * crank length (m)

    e.g. let's say this is what you have/get from checking one crank:

    loaded offset value: 1320Hz
    unloaded offset value: 565Hz
    mass used: 22.50kg
    crank length: 0.170m
    gravity: 9.81m/s^2

    Slope (Hz/Nm) = (1320 - 565) / (22.50 * 9.81 * 0.170)
    = 20.12 Hz/Nm


    you get that value several times for each crank arm and then average the values.
    Ideally you don't want much variance in the slope measure for each crank arm, nor when checking slope with chain on different chainrings.

    You can also validate using different masses to assess the linearity of the slope calibration, but I rarely bother as IME the slope on SRMs is almost always very linear.
    Alex,

    I have a Quarq S975 (been using it for 3 months) and besides calibrating on my Garmin 500, I haven't done any real calibration. To my knowledge the calibrate feature on the Garmin accounts for temp/pressure. I have no idea if a reading of 300 watts is really 300 watts, but I'm assuming that it's close. The slope measure that you discussed, is it usually a small difference or could it be significant? In other words, would a 300 watt reading on the Garmin change to 250 or would it most likely equate to under 10 watts? I'm just seeing if it's worth the effort given that I'm not a serious racer.

  16. #16
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    Quote Originally Posted by Alaska Mike View Post
    I own a Quarq, and one of the selling points for me was the ability to change batteries instead of sending it back to manufacturer. Having never owned a SRM, what's the turnaround time for one, and how often do you send it in? I've heard nothing but good things about them, but the price put me off a bit.
    I've been using SRMs since 1998 and have never sent one in.

  17. #17
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    Quote Originally Posted by new2rd View Post
    Alex,

    I have a Quarq S975 (been using it for 3 months) and besides calibrating on my Garmin 500, I haven't done any real calibration. To my knowledge the calibrate feature on the Garmin accounts for temp/pressure. I have no idea if a reading of 300 watts is really 300 watts, but I'm assuming that it's close. The slope measure that you discussed, is it usually a small difference or could it be significant? In other words, would a 300 watt reading on the Garmin change to 250 or would it most likely equate to under 10 watts? I'm just seeing if it's worth the effort given that I'm not a serious racer.
    Remember that the "Calibrate" feature in the Garmin (when using a Quarq) is not really calibration but rather , the setting of the zero offset.

  18. #18
    chica cyclista
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    impressions from someone who has used various powertaps over the past decade, and has had various friends/coaches on SRM units:

    The SRM is bombproof, has a much, MUCH more reliable head unit, is easy to maintain (like, zero maintenance) and allows use of whatever wheelset the rider prefers. It's not AS easy to swap bike to bike (assuming wheel compatibility), but it's definitely faster to swap out a crank than to rebuild a rear wheel, and I'd argue it's as easy as, say, converting a freehub from SRAM/Shimano to Campy for use on different bikes (something I've had to do frequently). This is crucially important to most elite riders as they swap wheels frequently. They are typically pretty far out of the price/value set of your average amateur racer, tho, so without a major bit of luck or a pro hookup, you're generally not going to see one on Joe Cat 3's bike.

    The PowerTap setup is much more affordable and like SRM has a long well established track record. Most (not all, but most) amateur racers don't have quite the sponsor hookups of the pros, don't care about pulling down data during key events and prefer to shed the excess weight and cost instead. Major con to a PowerTap is their stock head is ghastly, horrifically bad and the included PowerAgent software is, well... like, don't even bother with it bad. I've personally never had one that worked correctly, the wireless ones constantly "lose" pairing, they eat batteries, on and on. It's way more worth it to get a Garmin head (hub + 500 is only slightly more expensive than buying the entire PT setup cold) and you'll likely never again ride home swearing cos you lost half the data off of a crucial coaching session (this has happened to me more times than I care to admit, with both wired and wireless PT heads). Since pairing my PT hub with my Garmin head, I've had exactly zero problems.

    I have nothing to report on Quarq units; they're too new for me to have had any experience with; my husband and I both got wireless PowerTaps in 2010 and I don't know anyone offhand using one. If they can bring a similar accuracy to the table with the same Ant+ versatility/reliability as the PT, they'll likely become the stock standard for everyone but the pros / brodeal dudes and rich old Masters' guys.
    Grandpa LFR: "Kid, don't wrestle with pigs; you'll just get covered in crap, and the pig enjoys it."

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  19. #19
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    Quote Originally Posted by Alaska Mike View Post
    I own a Quarq, and one of the selling points for me was the ability to change batteries instead of sending it back to manufacturer. Having never owned a SRM, what's the turnaround time for one, and how often do you send it in? I've heard nothing but good things about them, but the price put me off a bit.
    Turnaround time will depend on where you are located and which SRM service centre you choose to use (they have dedicated service centres in many countries). Usually every 2-3 years is typical, although some will go for longer.

    The very latest SRMs have new batteries which are rated to 1900 hours of use, which would be several seasons for many people.

    It is possible to do your own battery change if you have reasonable skill with basic electronics/soldering and have the right tools for the job, but I usually recommend people send them in - SRM will change the battery and usually check anything else/service as needed and recheck the calibration.

    It's normally a job you schedule for a part of the season where you won't miss having a crank for a week.

  20. #20
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    Quote Originally Posted by new2rd View Post
    Alex,

    I have a Quarq S975 (been using it for 3 months) and besides calibrating on my Garmin 500, I haven't done any real calibration. To my knowledge the calibrate feature on the Garmin accounts for temp/pressure. I have no idea if a reading of 300 watts is really 300 watts, but I'm assuming that it's close. The slope measure that you discussed, is it usually a small difference or could it be significant? In other words, would a 300 watt reading on the Garmin change to 250 or would it most likely equate to under 10 watts? I'm just seeing if it's worth the effort given that I'm not a serious racer.
    As RC28 says, the unfortunately named "calibration" function on a Garmin is just the means by which the torque zero (or zero offset) or the unloaded torque value is checked/set.

    It is not a slope calibration check.

    Analogy: Bathroom scales.

    Checking the torque zero of a power meter is akin to making sure your bathroom scales read zero when you are not standing on them. If they don't read zero, then one can expect that the reading will be wrong by that same amount when you stand on them.

    Checking the slope calibration is akin to not only knowing the scales read zero when you are not standing on them, but also that when you stand on them, the scales weigh you accurately. Only way to check that is to place an accurately know mass on the scales and see what the scales report.

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