Enclosure and Carbon Fibre Cranks

The enclosure is getting closer to what I want. I’ve ditched the wrap-around plastic on the drive side arm and I’m contemplating using a wire from the BB Axle (plastic cap) to the electronics enclosure. This is similar to Pioneer’s design. It’s not great but it’s easier and may help with speeding up the process to beta and compatibility initially with other cranksets.

As for calibration of the carbon fibre crankset, it looks very linear at least initially. Way to go SRAM for QA. This’ll make the comparison of the Quarq unit to Accuity so much easier. It’s expected to be linear as a new crank but my background research shows that at higher usage factors it’ll go non-linear. I can’t get the specific answer out of any manufacturer I tired so far. If it goes non-linear it can be calibrated out, however a much more sophisticated calibration is needed – unlike the simple hanging weight test.

A load cell rated to 1000lbs is tested at multiple points including 1000lbs and quotes linearity. If anyone ever gets a power meter from me they will be getting documentation on the calibration values and it won’t just be a single weight test. I want it tested to at least about 170N-M of torque which correlates to a 100kg person’s entire weight.

The calibration values all come out to be nearly identical. The weights that were used were measured on a calibrated scale at work and confirmed (to the 0.1 gram) with my household scale to the gram. The linear regressions are almost perfect

I plotted the R^2 values. Looks nice and linear to me. Also, keep in mind that the number of counts is only on a 16 bit scale. I’m capturing at 24 bits, but  the last few are noisy. At 16 bits it’s noise free (it’s actually noise free to 18 bits, but it’s hard to do 18 bit math with a 32 bit microcontroller without pointless programming).

Back to the carbon crank, until there are some fatigue cycles on the crank, a new carbon fibre crankset is giving reliable and accurate readings. Time will tell once the carbon starts to fatigue.

Enclosures and More

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.

Can I instrument a Carbon Crank?

I noticed a lot of traffic from www.slowtwitch.com which I found out is a huge forum on cycling. One of the largest I’ve ever seen. Someone mentioned my summary post here. I figured I’d sign up and thank them for all the traffic. A question emerged and I’ve been working on it in the back of my mind. Can I instrument a carbon crank? I’ve learned that Stages have mentioned hysterisis which is that the deformation while loading is different than the deformation while unloading. I’ve done some searching and found out that part of the answer (well one here) to this question depends on essentially the loading cycles.

Lets assume a constant sinusoidal loading. Assuming that this is constant eventually the crank will fail at some point. This is referred to as fatigue life. Below is a curve for the fatigue life of aluminium. At a given stress (Left hand scale) it’ll only survive so many repeated loadings. E.G: 200Mpa Stress = 1000 cycles, or 150MPA = 10000 cycles. You’ll notice how a small loading change can make a significant impact in the life before fatigue.

More after the break.