Trek/Bontrager Duotrap and SRM (PC7)

Discuss light weight issues concerning road bikes & parts.
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by baldkingpin

Team Trek bike due to arrive shortly. Never had a Trek. Bike is a 6.9 SSL with the Duotrap in the chainstay. I've been told the Duotrap will work as a speed sensor for my SRM system (7900 wireless, PC7) so I don't need the bulky zip-tied pod. Is this true? Do I just pair like normal via the PC7? Oddly, I couldn't find an answer to this question easily online. Thanks, all!

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by Geoff

Yup. I have done that for a friend with one.

by Weenie

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by Wester-Ross

If i understand correctly, the BB mounted 'sensor' supplied with SRM is in just a magnet that lets the SRM unit transmit cadence.
Since the duotrap also transmits cadence does mean one no longer needs to install the SRM BB pickup ? (With internal cable routing, Di2 battery mounting, etc all going on under the BB shell , installing these SRM BB pickups has me resorting to hot glue and other hacks)

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by KWalker

You still need the SRM magnet since it trips the reed switch, however, you don't need it for cadence necessarily. The way I understand is that the magnet ALWAYS needs to pass through the sensor pickup on the back of the spider, but you can pair other sensors as secondary cadence/speed sensors. For instance, to use a Garmin speed sensor you have to pair it as a Duotrap, etc.
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by Geoff

The SRM system has three different 'sensors' on the bike (i.e., exclusive of HR):

1. the speed sensor;
2. the cadence sensor; and,
3. the power sensor.

The PowerMeter includes the cadence sensor and the power sensor in its housing, but speed requires a separate wireless transmitter, typically attached to the chainstay with zip-ties. The Duotap negates the requirement for the speed sensor, as it broadcasts speed data in ANT+.

In order to 'activate' the PowerMeter cadence sensor and, as Karsten mentioned, the reed switch for power, the PowerMeter requires a separate magnet positioned within the arc of the reed switch. Personally, I do not recommend using the stock SRM magnet for this purpose, as it is more intrusive than required. Instead, just glue a 1/2"x1/8" rare earth magnet to the chainstay/bottom bracket shell (depending where the reed switch is in relation to the frameset. If I recall, on the Madone, we put it high on the chainstay, as it was flatter there).

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