I got a used wired SRM off craiglist for $800. Got a few questions…
My knowledge with bike stuff is a lil limited. I don’t mind trying to install it myself, but would it just save me trouble to bring it to a shop and have them do it? Saw some vids on replacing cranks and a lot suggest torch wrenches, which I don’t have.
I have a 2010 Cervelo P2, do I need a new bottom bracket?
How hard is it to calibrate one of these guys?
What type of SRM is this? Would like to find the right manual online to get good instructions.
I got that book “training with a power meter”, but never saw the need to read it but this deal kinda came out of nowhere. If theres some issue I forgot to address please feel free to let me know.
Before you do anything else, test the crankset off the bike. The SRM manual describes this and is available as a free download from the SRM website. Basically connect the PCV to the cable, hold the crank with the white plastic plate facing up. Locate the dark black spot on the plastic disc which is the cadence reed switch glued to the inside of the SRM housing. Wave the power pickup sensor (the one marked POW in your photo) over that dark spot. The magnet in the power pickup sensor (which is in the far end away from where the cable sprouts out of the sensor) will trip the reed switch which wakes up the crankset. Momentarily press both the Mode and Set buttons to get into the torque zero (also used during slope calibration, hit them simultaneously if you hit Set first then you start and interval and will see an interval number pop up and you can’t enter torque check mode while an interval is running so if that happens hit Set again to finish the interval and then hit both buttons at the same time) mode and you’ll see two numbers one above the other which are the crank torque setpoint (bottom) and current reading in units of Hertz (Hz or frequency of the coded signal coming from the crank, they code the torque into the pulsing frequency of a Pulse Coded Modulation, PCM, signal). The top line will read zero until the crank is awake and you hold the one of the long sides of the power pickup near the outer rim of the white plastic SRM housing disc where you should be able to see a faint copperish colored band which is the power signal sending coil also glued to the underside of the plastic enclosure. Move the sensor around and you should see a value in the range of several hundred Hz or more as you move to different positions around the rim of the disc. If all of that happens then you know that the crank wakes up, the PCV can read torque, the cable can pickup those torque transmissions and send it to the PCV. IOW, you basically know that most of the functions work properly which helps a ton in terms of troubleshooting installation problems. No sense chasing cabling problems if it turns out your crank battery is dead or you didn’t position the power pickup sensor in a place where its magnet would trip the reed switch to wake up the cranks.
Installing a bottom bracket and crank isn’t too difficult but does require some special tools. Check out the ParkTools site for good information on basic bike mechanical issues. Your SRM crankset appers to be an Octalink design which should help you find the correct tutorials, tools and of course the bottom bracket itself.
The SRM cranks appear to be an older SRM Amateur model. The Pro models have a hexagonal metal plate, not the circular plate at the spider as shown in your photo. The SRM Track and Science models also use a circular spider plate design but you’d likely know if you’d purchased one of those (and the track wouldn’t have double chainrings) as they’re quite pricey with their extra electronics. It is likely an older 9 speed crankset but that doesn’t really matter as long as you use chainrings appropriate to your drive train but it can influence how you place the power pickup sensor as things changed a tiny bit between the 9 and 10 speed models in terms of pickup sensor placement. Bottom line, use the wiring harness shown or replace it with the same style if you buy another one unless you’re certain you have a newer 10 speed SRM because it can be tricky to get the newer BB mounted pickup placed correctly for the older 9 speed cranks, it can certainly be done but it’s a bit finicky.
Once the cranks are installed on the bottom bracket you’ll flip the bike upside down and figure out where you want to mount the power and wheel speed pickup sensors. The wheel speed is easy and just like any cycling computer that has to be near enough to the wheel spokes that a magnet mounted on the spokes will reliably trip the reed switch to report wheel revolutions. The power sensor is a bit trickier as it has two jobs, one is to make sure the reed switch (dark dot in the plastic cover) passes near enough the far (non cable) end of the pickup to reliably trip once per crank revolution. The other is to make sure one of the long sides of the power pickup sensor pass close enough to the power sending coil (outer rim of the white plastic disc) to reliably pickup the PCM signal. A little trial and error is usually all it takes but it depends on the specifics of your bike but in many cases the sensor has to be installed so the magnet (non cable end) is closer to the BB center so if your position is behind the cranks (like a chainstay) then the sensor looks funky with the cable going out towards the rear of the bike and doubling back so the sensor can place the magnet where it needs to be.
I use hot glue to mount SRM sensors as it’s easy to use, does not damage the frame’s finish and is easily removed for repositioning purposes if you don’t get the placement just where you want it.
Once it’s mounted test things out by first seeing if cadence is reported correctly, you may have to wake up the PCV by hitting the Mode button if you don’t have the wheel speed sensor and spoke magnet installed yet or haven’t spun the wheel but see that cadence seems reasonable as you spin the cranks. Spin the cranks at 30 rpm or higher as the SRM doesn’t report very low cadences.
Then go back into the torque zero screen (simultaneously hit Mode and Set) and check a number of different positions of the cranks around the pedal stroke but at least check the four major directions (0, 90, 270, 360 degrees). Let each position settle for a few seconds but you should see the same value in Hz reported within a range of say +/- 10 or +/- 15 Hz all the way around the pedal stroke. If not you’ll have to reposition your sensor to get a more stable set of torque zero readings around the crank cycle.
That’s the big stuff for checking out and installing a wired SRM, read the manual for information on how to use it and ALWAYS zero the torque before rides as there is no auto torque zero function on the wired SRM and it does change.
Thanks a lot Dave. You seem to know a lot about it. After my hard ride on Sat i’ll try to get it going. Although I don’t have a bottom bracket thatll work for it so I may need just go bring it to a shop to everything. Doubt theyd setup the whole power meter, for cheap.
You don’t, if the head unit goes blank it means the battery has drained - get the PCV on the charger before you get to that point or you’ll have to reset the date and time.
Most shops will install the BB and crank for a nominal charge but unless they’ve installed wired SRM sensor wiring before you’re better off doing that part yourself to make sure it works correctly. I’ve had to reposition the sensor on a handful of systems that were shop installed where the shop guys figured it was just another speedometer sensor down there under the BB shell and made some real basic mistakes like positioning the sensor so there was no way the magnet would trigger the cadence reed switch and wake the unit up.
Yeah, but realistically the PCV isn’t ‘on’ all the time. Yes, time and date are displayed so it’s ‘on’ the way a digital watch is on all the time. The actual PCV wakes up when you spin the wheel and it detects speed but otherwise is in standby mode. In fact if your wheel magnet slips and you don’t get speed readings the PCV won’t automatically wake up and you’ll need to hit the Mode button to see power, and if it happens mid ride and you coast for a while the PCV will go back to sleep without speed data unless you do special things in the setup menus which isn’t a great idea as it’s good that the PCV goes into standby when you’re not actually riding.
The PCV gives you an estimate on remaining battery life if you hit the Mode and Pro buttons simultaneously. A fully charged PCV shows 29 hours of battery life (30 when it’s fully charged and still connected to the charger) but take that with a grain of salt and I’ll try to get mine back on a charger ASAP if the battery life estimate gets below 10 hours or so. The SRM manual tells you to leave the PCV on the charger all the time when it’s not in use but that’s generally not a great idea with rechargeable batteries so I tend to charge every few days but not constantly.
If the PCV does totally discharge you’ve got to reset time and date but most settings including the slope setting (which is how you calibrate your PCV to match your crankset in addition to zeroing the torque at the start of every ride) is saved as are things like wheel circumference, storage interval and such. Get a free download of SRMWin from the SRM site and you can connect to your PCV for things like setup and setting the clock quickly to match your computer clock but also to see and or change the custom settings like what shows in interval or regular display modes and how often it toggles back and forth between things like time and distance ridden. And definitely download the manual and learn how to use the thing.
BTW, in terms of the shop question. Since you don’t seem to own any bottom bracket tools and haven’t done that kind of work before you should definitely have a shop do at least the basic crankset install as the tools alone will likely cost you more than their shop fee. If they’ve done wired SRM installs then have them do the whole thing and have them show you the basics of how it works, if not find someone else that knows how or do it yourself.
Ok well I got the BB installed and got the speed and cadence getting picked up, but power is a little issue. If it picks up the cadence should I be close to where it’s supposed to be? I’ve only tried to get the power reading when I have had the bike upside down so I dont know if I need to put more power into it or something to make it read. Any tips are welcome.
I am running 2 wired SRM’s one on my road bike and one on my TT rig.
In theory if it is picking up the Cadence it should also be picking up the Power.
In other words, if it is positioned where the sensor is reading the unit it should read both, have you tried riding it? even just around the block just to see if it’s reading, sometimes it might not pick up really low power readings, so if you are spinning it upside down with your hand you might not get power numbers showing.
what issues are you having with the power readings?.
+1, the magnet trips the cadence switch to wake up the SRM crank but you’d never know that if the power pickup sensor wasn’t close enough to the power sending coil in the crank to receive the signal. So you’re definitely receiving the transmission at least some of the time.
The next important test is to go into torque display mode (press both the lower buttons on the PCV simultaneously, if you accidentally hit the middle button first you’ll trigger an interval, a small interval number will show up on the left side of the display and the unit will not go into torque display mode till you hit the middle button again to stop the interval and then hit both simultaneously). In torque display mode you should see the reported torque, with no force applied to either pedal note the torque, then rotate the pedals 90 degrees, let the unit settle for a second or two and then note the torque again. Go all the way around the pedal cycle, going back into torque mode with the double button press if the torque display mode times out. The unloaded torque should be within +/- 10 or +/- 15 units (Hz) all the way around the pedal cycle, if not then the sensor should be repositioned.
If all of that is good, then do a test ride on the trainer or outside to verify power reporting and get into the habit of zeroing the unloaded torque every time you ride as the SRM PCV does not have a torque auto-zero mechanism like the PT hubs and it does change with temperature.
I just spin it upside down. Got to pick up zip ties tomorrow since i dont have any with me. Can get cadence but no power with spinning it. Can try it tomorrow once i got it tied together.
Put it on the trainer. Did some adjusting with the pow sensor and some tape and got some cadence readings. So i did a spin and got some power readings, but I dont think its right. It showed at most 75 but really fluctuated a lot. Do I still need to adjust it? If so how? I read in the manual that spinning backwards resets the torque? Do I do that?
You apparently read the wrong manual, in fact that sounds like the Quarq manual not the SRM manual.
The wired SRM units require you to manually zero the torque.
Spin the cranks to wake the unit up
Simultaneously press the lower two buttons to display torque (and if you accidentally hit the ‘set’ button first you’ll trigger an interval, hit it again to end the interval before hitting them both at the same time)
Make sure there’s no force on either pedal, let the display settle for a moment or two and press the center button to accept the torque value displayed in the upper line, the lower line should now read the same value
Hit the lower (mode) button to return to normal mode
That’s how you torque zero a wired SRM.
But also check the zero torque all the way around the pedal stroke as described in my previous post to make sure your sensor is properly placed or all bets are off.
Also make sure the correct slope is programmed into the CPU, you can do that via button presses or use SRMwin to set it from your computer via the data cable.
Well got it going. A lot of the stuff you guys have said makes more sense now. Thanks for that.
So why does the torque always change day to day? Will it be ok if I set the torque in transition before a triathlon swim then be on the bike ok?
It seems that whenever I hook up the powercontrol v unit it starts to do some constant rapid flashing with the digits on the screen. Stops when its not mounted and hooked up. This normal? The battery was replaced and such when I sent it in for tests to SRM.
Anyone got any good mounting setups on aero bars? Mine looks pretty ghetto right now. Thinking about maybe using double sided velcro to stick it to both sides of the computer and bars…
The torque zero point mostly changes with temperature because the crank spider expands or contracts with temperature changes and the crank spider is the actual strain element that is being measured. If the temperature is stable, the torque zero will be stable so if the temps aren’t changing much during the day then yes you can zero the torque and the value should stay good for a long time. But it only takes a moment to zero the torque and the bike can be rolling as long as there’s no pressure on either pedal. So you could zero the torque as you walk/run the bike out of the transition if you want to really be certain.
I don’t know what you mean about random digits flashing.
Yes you can choose metric or imperial units. The easiest way is via your computer and the download cable from SRMWin under the setup menus.
You really should read the manual, all of this info and a lot more is in there given what these things cost you might as well figure out what it can do and how to do it.
I use training peaks to upload my SRM as their software is rather hard to read. Sometimes I can’t get NP or the VI. It reads pretty much everything else but around 25% of the time that doesn’t read. I also do a lot of 5x bike/run bricks and the non showing up the NP/VI happens more often during these for some reason? It’s usually bike, put bike in car, run for 15ish minutes, then get it out and bike again. Anything I can do to prevent that from happening?