It’s been a few days since I’ve written anything. It’s been a very busy few weeks. Nothing overwhelming stands out but the enclosure design in progressing. I’ve also been prepping the Quarq + Accuity comparison unit.
Above is the first design attempt. A small amount of plastic wraps around the outside of the arm and protects the wires leading to the axle.
Generally it’s hollow inside and the enclosure supports an off the shelf battery holder. This contributes to the larger lower enclosure size. I’m concerned about clearances all around. I generally don’t like this design, however the board was specifically designed to go on the backside of the drive arm between the chainrings and the crank arm.
I attempted to diverge a little from the original design and try a setup with the board and battery above the crank arm. This is not idea because all the fragile wires from the strain gauges have a greater distance to travel. This leads to all sorts of issues. The most ideal is to keep things short. This comes back to trying to place the board inside the crank arm.
Having given up on the second design I’m revisiting how to make the first more effective. I’m integrating the battery holder into the design to minimize size and weight. There isn’t much room around the crank arm. Lots more work ahead.
On to the Quarq + Accuity unit.
First the S950 (S900 compact version) crank has it’s spider removed. Just 3 simple Torx bolts. I wish everyone would use this design. SRAM (well Quarq really) should push this as the standard for everyone who wants to make a crank with removable spider. Right now the Cannondale, FSA, and SRAM units are not interchangeable. We can use ANT+ but can’t have a physical standard?
Then I removed the Quarq unit from the S975 Cinqo. The Cinqo holds much much tighter tolerances and was on a bit more snuggly. I’ll chalk this up to China / Taiwan manufacturing versus American. While swapping the chain rings requires recalibration, removal of the whole setup does not. There has been some talk about the removal of the chain rings and torqueing the bolts over at the Slowtwitch forums. I haven’t done this myself and don’t plan too. In the event I do then I’d like to check the calibration with the iPhone app – which as soon as I get a Wahoo dongle I can as I was given a “broken” iPhone 3GS. The screen is crack but still functions fine. Amazing. Maybe I’ll just send it in to Quarq with the Accuity prototype in tow and get them to calibrate it then?
I prepared the crank arms carbon fibre just like Vishay’s tech notes say to and applied the strain gauges.
Another picture with a better view.
I also installed the inside torque sensing strain gauge. All gauges were installed very quickly but I haven’t tested them yet except for resistance. Carbon prep time is much faster as abrasives can eat into the epoxy very quickly. DO NOT USE CONDITIONER A. Conditioner A is a mild acid, and a mild acid eats the plastic. The neutralizer (base) is fine to use. When sanding using isopropyl alcohol only.
Recently I’ve been testing the boards and their connection and function with the Texas Instruments ADS1248. Remember the four boards I made up and transmitted fine? I didn’t test the ADC interface. There were some bad connections on this board with the ADS1248 which meant it was acting very stupid. Finally got it working just before bed time last night.
Here is a picture of testing the board with my four sensor setup Rival arm used for thermal and force sensitivity testing.
So that’s everything. I’m hoping in 1 – 2 weeks to have a Quarq to Accuity comparison but don’t hold your breath. I’ll get the bike on the trainer inside to do this and if I can find another Garmin Edge 500 or similar for a decent price I’ll setup them both up outside for a ride.