Competitive Cyclist SRM Power Pickup Bracket
Even though the Competitive Cyclist SRM Power Pickup Bracket was specifically designed for carbon frames with massive bottom bracket shells, we were looking forward to testing it on an aluminum frame because we aren't crazy about the standard SRM mounting of the power sensor on the right chainstay. SRM includes two thick rubber bands with the wiring kit for their power meters. When you see them, it's hard not to feel they cheezed out on a mounting system for the power and speed sensors. The nice thing is they stretch around pretty much any frame tube or fork leg. But they're hard to stretch into the plastic holder that the sensors go into, they are a pain to work around beefy chainstays, they dry rot. And it's nearly impossible to replace the rubber band that holds the power sensor without removing the crank. The last reason is why we're using zip ties on the chainstay-mounted sensor.
Add to these woes the beefy bottom bracket juncture that is becoming increasingly standard on carbon fiber frames. Sometimes, the area around the bottom bracket seems like a giant carbon cylinder with extra-large carbon cylinders attached to it. There's simply no room to put this sensor in the way SRM had originally imagined. The SRM Service Center likes these brackets so much that they buy them from Competitive Cyclist and sell them to customers & supply them to their sponsored teams such as Discovery and CSC.
Latest generation aluminum framesets are not much easier for mounting than the new generation of carbon fiber framesets. Large-diameter chainstays are the norm, and they're often made so the crank spider barely clears the stays.
Into this world jumps the SRM Power Pickup Bracket. Since it's a Competitive Cyclist creation, and a response to a need experienced, we can't be hard on its origins or purpose. This was not a marketing-driven creation or modification. As the text on the product indicates, the widget started in metal and progressed to plastic. We imagined that the only "drawback" to using this system is that the bracket will have to be removed temporarily if we service our bike on a workstand that contacts the bike at the bottom backet; there are very few of these in use and we don't have one.
Since weight is a concern for most of those buying the frames onto which this bracket can be affixed, we did weigh both the SRM original part and our replacement. The rubber band and plastic bracket weigh in at 2g. The screw on plastic bracket weighs 8g.
Assembly is pretty easy. Start by turning the bike upside down and taking out the screw holding the under-bottom-bracket cable guide in. The bikes we checked had an extremely short screw holding the guide in place, so we had to substitute a longer screw. CC says you need an M5 bolt. For those who aren't familiar with the terminology, M5 bolts are usually used to hold water bottle cages to the "braze-ons" (or "mold-ins" or "press-ins" in the carbon fiber world). We always have some aluminum water bottle cage bolts handy, but only one is necessary. That one weighed in at 2g.
Our current ride is made of thinwall aluminum. And we wanted to run this bracket on our bike. We don't particularly like the zip-tie mounting we have to do because of the way the sensor cable loops behind the sensor and how the ties might dig into that cable. We also don't like how grit gets caught up in the ties and cable and how hard it is to give it a good, solid scrubbing. Turns out our frame has such a thin bottom bracket shell that it couldn't be threaded; the cable guide has a clip rather than a bolt holding it in place.
The bracket goes in so the slot is facing up when the bike's wheels are on the ground. While the clearance looks close, there seemed to be enough room for the speed sensor to slide in. The sensor itself is about 8mm tall and the cable guide seems to be about that height, but since the bottom bracket shell curves, the sensor won't have to fight the shell much, if any, and the angle adjustment screw in the back of this bracket is easy to set. We found the fit of the sensor pretty tight, and the length of the sensor channel in the bracket only microscopically longer than the sensor itself.
We're a bit paranoid about stuff rattling loose. Having the power sensor come loose is nothing we want, because every second of data capture counts. Worse is waiting a day or four for a replacement sensor cable, especially when we have VO2 max intervals or an important race on the calendar. So Loc-Tite on the bolt is essential for us. We can't give you a torque spec, but we found that "secure" seemed to be when we noticed the plastic bracket bowing ever so slightly under the pressure of the bolt head. We haven't had it in position long enough to see if this will eventually crack the bracket, but we think it won't. The bracket is machined from Delrin plastic stock and should take this kind of force no problem.
There's a bolt on the Delrin bracket to adjust the angle that the power sensor is held has a lock washer and is easy to screw down hard. Most people will run it so the dovetail joint for the sensor it facing up and the sensor is mounted above the plastic bracket. Since the piece holding the sensor is threaded all the way through, it can be reversed if the common mounting does not place the sensor close enough. And with the dovetailed Delrin threaded all the way through, the bracket itself can be reversed, turned upside, down, or adjusted any which way to array the sensor in such a position as it can pick up the signal from the crank.
We pushed in the power sensor to see how the two pieces fit together. It went in ok, but once all the way in, the fit was tight. So tight, we had a hard time getting it out. After struggling, we found that by bowing the Delrin mount ever so slightly with two thumbs, pressing against the mount towards the sensor, should loosen the lock on the sensor enough to slide it out. Still, next time, we're putting a little wax-based lube in the bracket to make sliding out easier.
This little bracket is pretty much a universal solution,and it's one that can be used by most anyone who wants to simplify the SRM mounting process -- the most potentially frustrating part of owning an otherwise delightful training tool.