I've given this a passing thought too. I'm not an electrical engineer (different flavour) but I think this would

1. require some pretty special skills as you'd have to solve the very non-trivial problems of reverse engineering SRM's transmission protocol and power calculations
2. be very time-consuming to construct by hand (i.e. as expensive as a wireless SRM if you pay for labour)
3. not be mass producible as the market of SRM V users who would (1) buy this gadget + an ANT head unit and (2) can't afford a wireless SRM VI is too small

Sorry to rain on your parade, maybe someone who knows more about the subject can correct me?

Booo... Parade rainer!

Actually what got me thinking about this was a previous discussion of Di-2 that briefly veered towards the SRM. I think Rappstar had an EE friend and they created some sensors and wiring when he was using SRM's that were better than stock and cheaper too. I think the computer is what translates the signal so the calculations would be done there. I'm thinking reed sensor connected to Ant tranceiver sends signal. With Ant computers coming into the 200 dollar range and soon less I'm sure. I think it would be cheaper than a 3500 wireless SRM. There were a lot of those wired SRMs sold between 99-2008. There might be a market?

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Heath Dotson
HD Coaching:Website |Twitter: 140 Characters or Less|Facebook:Follow us on Facebook

To convert raw strain gauge readings into power numbers requires SRM's special computer. The strain gauge numbers are pretty much meaningless unless you know SRM's secret formula. For the SRM V this computer may be in the powercontrol V or the crank. I suspect that some processing is done in the crank and some in the head unit to calculate the power number that is displayed. So, you can't just send the signal from the wired SRM crank to an ANT+ head unit - the head unit wouldn't know what to do with the signal or how to interpret it.

(The face that no part of the wired SRM is compatible with the wireless SRM, and that you can't use a powercontrol VI with a wired crank tends to support this theory). For the wireless SRM's this computer resides entirely in the crank, which does all the processing and then sends a simple and clear power number via the ANT+ signal to any head unit.

Even if all the processing is done in the wired SRM crank the cable transmission doesn't use the ANT+ protocol - so you have to build a translator between wired SRM-speak (which could be really complicated) and ANT+ speak (which is probably fairly simple). This is the really difficult step.


What Rappstar's EE friend did was make a better connecting cable between a wired SRM and the Powercontrol V. This cable however does not do any processing of the signal it's carrying.

Pretty much the only way to do this would be to crack open a powercontrol V, take the input to the display and send this to an ANT+ transmitter. So, Reed sensor -> Powercontrol V -> ANT+ transmitter sends signal.

...and it would be hard making this small enough, and the PCV alone is around $800. Ouch!

BTW I'm making educated guesses in regard to how the wireless SRM and ANT+ works, so feel free to put me right or confirm :)

Not that hard at all. In theory SRM could do it- but to do so they figure they would have to hire a bunch of techs and they would do mostly changeovers instead of new sales.

maybe the EE's have a clever tool that can read the powercontrol V's programming directly off it's chips and then replicate this programming on a generic microcontroller? Which would solve all the problems?

I think the other thing that got me thinking about it was that SRM had a wireless crankset before the the PCVI was ready so they shipped it with the wired PCV. Then I also read on SRM's site you could use the PCVI with the old wired units by plugging them in. The PCVII has a usb and no five pin port so it is wireless only. So I'm guessing not much happens in the way of calculations in the crank itself. I'm not sure though since the cranks are cross compatible with any ANT+ computer. Maybe there is a specific setup you have to do with the ANT+ computers.. Someone with a Garmin might be able to tell us if you have to specifically program it to accept the SRM signals.

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Heath Dotson
HD Coaching:Website |Twitter: 140 Characters or Less|Facebook:Follow us on Facebook

Hm, that I didn't know! I was under the impression that the powercontrol VI couldn't be used with the wired crank. That you can plug in the wired crank is very interesting.

The powercontrol V has only 6 hrs of memory @ 1s recording which is REALLY annoying doing a stage race. The VI has much more memory, is cooler.

I thought I read on SRM's site though that the VI was not backwards compatible? will check

Nope - PCVI is not compatible with older wired SRM cranksets due to the fact that the older SRM's wiring harness and PCV have a 5-pin connection whereas the PCVI has only 4 pins.

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Yes, it involves eBay and working a little overtime.

When I talked to SRM they said it would? Maybe you just need a different wiring harness. I think initially they were not going to make the unit backwards compatible then they changed their mind.

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Heath Dotson
HD Coaching:Website |Twitter: 140 Characters or Less|Facebook:Follow us on Facebook

So you are saying it is possible and you'd be able to do it? Or that it is just possible?

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Heath Dotson
HD Coaching:Website |Twitter: 140 Characters or Less|Facebook:Follow us on Facebook

That's a good point - anyone try to read a wired power meter via harness with a PCVI?

I still do not think it is possible - SRM only markets one wired harness although there are options for front wheel or rear wheel speed sensors this harness is for use with the the PCV. As an owner of two wireless SRM systems, I am pretty certain that the 4-pin connector on the underside of the PCVI is for downloading data and recharging the battery - at least until someone can demonstrate otherwise.


While I agree that given the costs for an SRM, such a retrofit would be nice for owners of an older wired SRM, there is an inhibitor to this plan. As bike/frame manufacturing progresses and the prevalence of the various bottom bracket standards such as BB30, BB86, and BB90 becomes more prevalent, owners of older wired SRM's will find they can no longer use their SRM with new frames/bikes. They will find as I did with our 09' Giant TCR Advanced SL framesets, that unless they have one of the last batch of 08' model wired system, the opening in the torque ring for older SRMs is not large enough to accomodate the wider bottom bracket shells for frames using anything but ISIS, Octalink, or External bearing/Mega Exo style bottom brackets.

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Pretty old thread so I'm hoping some advances have been made regarding the ability to convert a wired SRM to ANT+. Any news out there? I sure would like to turn my wired SRM PCV into a 310xt or 500 compatible system.

Nope.. The only advances that have been made are less expensive crank based power meters like Quarq.

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Heath Dotson
HD Coaching:Website |Twitter: 140 Characters or Less|Facebook:Follow us on Facebook

This is what I was thinking this evening....did a google search and this ST thread came up. Too bad that SRM is unwilling to make a unit that simply converts the output from the crank to something that ANT can interpret and then transmits that to any ANT compliant device. I'd pay a few hundred dollars for that. At least they would keep me on SRM and get some incremental revenue from me, rather than have me bolt over to another supplier/system.

I agree with you, as I have a great PCV version that I want to keep using, but pretty much only use my garmin units (trying to just use the 800 for training and the 500 for racing). But, now that I am upgrading my P3 to a new bike, I will be forced to buy a new PM or put that cable version on my super bike....

Cheers,

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I would love to have the PCV into ANT+.
Perhaps some real good engineer can figure it out if SRM will not give it to us...

It really shouldn't be that hard to do this but I imagine that the return on investment would so small that it isn't worth the time for SRM. All you would have to do is put something in the bottom bracket area that picks up the signal and converts the data to something that an ANT+device can read.