1> SRM Professional PCV
2> Typical range is between 1010-1035 depending on tempeture, median offset is 1020 (although I uninstalled from the bike, removed the change rings and cleaned everything, and after remounting seems to be around 870)
3> Easy to check offset.
4> Not much - hardly enough to notice other than intellectually
5> Yes, I have done it about every six months since I got it. Factory calibration is 21.8. I measured it at 21.7 (rounded down from 21.74…)
Basically you hang a known weight from the pedals, and record the offset number. Based on the 0 offset and the known weight you can calculate the slope.
PM with your email and I can send you the spreadsheet I used to do mine.
How to recalibrate SRM?
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.
Calculate the weight in Newton. E.g. 30 kg = 30 * 9.81 = 294.3 N 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).
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.
Notice zero of Powermeter (MODE + SET, right number)
e.g. F0=500 Hz.
Bring crank in horizontal position and hang the weight on the left pedal.
Notice frequency output-left of Powermeter (MODE + SET, right number)
e.g. Fleft=1450 Hz.
Bring crank in horizontal position and hang the weight on the right pedal.
Notice frequency output-right of Powermeter (MODE + SET, right number)
e.g. Fright=1550 Hz.
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.
Calculate average slope of Powermeter.
Slope = 1000 Hz/50.77 Nm = 19.90 H z/Nm.
This slope is the calibration of Powermeter and must be set in Powercontrol.
only changes 20 points over the year, temps I ride in 35F to 95F.
Super easy, unclip (while riding), spin the cranks backward few times (just lightly kick the pedal), let the number settle on the display, then Set the new number. Takes 30 sec or less.
have not done this.
Yes, changed the factory setting by .2 I took a 25lb plate to the post office and got a weight to the nearest .01
-mb
I would think that you would weigh a series of weights - like 5 kg, 15 kg, 25 kg, 35 kg (up to about 120% of FTP at a cadence of 60-100) and then see if the slope matches. I was only able to do it with light weights - 7 1/2 pounds and 15 pounds as my offset on the 705 maxes out at 1000. I didn’t check L vs. R so will look at that, too.
For all those with SRM’s - can you please answer/post any/all of the below:
Post type of SRM and computer.
How much does your offset change and under what conditions? (For example - drops 20 from 545 to 525 if I go from 40 F to 90 F or estimates of T:).
How easy is it to recalibrate while riding?
If you have done the calcs - how much does the above offset change your watts if you do not recalibrate?
Have you ever tested the accuracy of your SRM by hanging weights? Results?
Thanks,
David Bassler
4 x SRMs:
1 x SRM Amateur with variable length cranks with PCV on my indoor ergobike
1 x SRM Pro wired, to be reinstalled on training bike
1 x SRM Track with PCV
1 x SRM Wireless with PCVI
Not a lot, although the wireless is in bed in stage and I had a fair drift happen last weekend of around 15Hz as things warmed up. Currently riding along with Powertap. Track is pretty stable, maybe drifts 4-5 Hz. Indoor amateur is very stable. The pro was same when it was on ergobike before I swapped them over.
RTM. You can’t calibrate while riding. You can only check / re-set zero offset, which is not hard to do. The new model wireless/PCVI has an auto zero feature.
RTM. If you mean ZO, then that depends on ZO, slope and power you are riding at.
Power watts = ((Freq_loaded Hz) - (Freq_zero_offset Hz)) * Cadence rpm * 2 * PI / (Slope Hz/Nm * 60)
All of them. SRM’s supplied calibration numbers are consistently wrong. Always check your SRM’s calibration.
Supplied calibration numbers = slope? They’re often wrong??? That seems whacked. You check it with weights on the pedals as mentioned above? How do you change it (with SRM computer vs. Garmin?). I don’t think I can change it with Garmin - actually I don’t think I can even check it with a garmin (705) as I can only use weights up to about 17 pounds (Garmin max offset is 1000 and zero is about 630).
How does the auto zero feature with the new SRM computers work?
Supplied calibration numbers = slope? They’re often wrong??? That seems whacked. You check it with weights on the pedals as mentioned above? How do you change it (with SRM computer vs. Garmin?). I don’t think I can change it with Garmin - actually I don’t think I can even check it with a garmin (705) as I can only use weights up to about 17 pounds (Garmin max offset is 1000 and zero is about 630).
How does the auto zero feature with the new SRM computers work?
Dave
SRM’s own slope calibration might be right when it leaves the factory on whatever rig they test it on but once installed on your bike, then many many users who have checked their SRM calibration over the years have consistently found it to be different. I have a 20kg mass (19.525kg actually) and a 11.680kg mass to use for this.
I did a ride on Wednesday (I still have my PT on there as well). My SRM gave my 2.5% higher power than my PT (which as far as I can tell from cal checks is within spec). That’s about right for the difference in drivetrain losses between crank and hub.
I can’t comment on the Garmin, I use a PCVI with the wireless SRM.
The auto zero offset (according to the manual if you’d like to read it, it’s on the SRM website) looks for periods of coasting and adjusts if it’s not right out of whack. I’m a bit dubious about that given you are still clicked into pedals.
I’ve asked Garmin to add “Torque” to their values that you can read on the computer (there are 8 values so you could easily look at torque on 1 of the 2 main screens) - so if you are coasting - you can see if there is any torque on the pedals (or negative torque) - then you’d know that the offset had drifted. I can’t use watts as if its <0 I think the 705 says 0 and if cadence = 0, then watts = 0…so you have to spin lightly and see if its says 1-2 watts but I’m not sure how good that is…a good autozero feature would be nice. The Quarq resets offset with 4 backpedals.
Its all somewhat frustrating as you want to know that your power numbers are good…
I did just put my old PT on my SRM bike so am comparing numbers again…should drivetrain losses be a %, a set amount of watts (I always hear “10 watts”), or a combination?
I did not get the SRM computer as I heard a new version was coming…
Playing with the offset and pressure on the pedals has shown me that a little pressure will throw it way off, but thats pressure on 1 pedal with a locked wheel…seems like if you have balanced pressure on 2 pedals then the offset would be reasonably accurate…but I’m still playing around with looking at that.
Playing with the offset and pressure on the pedals has shown me that a little pressure will throw it way off, but thats pressure on 1 pedal with a locked wheel…seems like if you have balanced pressure on 2 pedals then the offset would be reasonably accurate…but I’m still playing around with looking at that.
Thanks for your input.
Dave
I still have my doubts. But I have it now (bit of an unscheduled purchase) so will see how it goes. Turns out to be a blessing in disguise as my pedalling form improved when I put on the 170s from my normal 175s (riding with a prosthetic no doubt has changed things for me). I’m gunna do same to my other road bike - I have an SRM Pro sitting here with 170mm cranks that can go on it.
I think drivetrain losses should be a % but it’s probably a bit of both: fixed + %.
A few factors come into play - what gear combos are being used etc.
