Wednesday, May 15, 2013

PCB or Enclosure–Chicken and the Egg

After spending 3 hours literally looking at bike cranks, holding them in very different ways, in different light, cutting out pieces of cardboard and tucking them in different places I can honestly say I have no idea which to work on first – the PCB or the enclosure. I think that the design of both items has to happen concurrently, each iteration causing the other to update until they converge. So I’m moving forward concurrently as best as I can.

The sad thing is that where I want to instrument, and where I can place electronics / battery / antenna are two very different things. Luckily I’m down to about a 5 component setup (battery, nRF51422, Balun, hall sensor/gyro, ADS1248). The bulk of these things are small and I can fit all of them except the battery into a space about 2 – 3 square centimetres. The battery, that’s still another story as it’s not fully decided. It needs more power measurement work.

While I want to keep both the pcb and enclosure as universal as possible, so that I can instrument various different cranks without too much work, I’m going to have to choose one to start with. SRAM Rival. Why? I already have 4 unused ones that have been purchased. BSA / 68mm threaded, are the most common BB’s and compatible with things like BB90 and BB30 via adapters (though I’ve heard mixed feelings on these, some good, some horrendous). I’ve also noticed a lot of BB30 cranks are three piece cranks now, or that the shaft is now on the non-drive side arm. This complicates things a bit for me. It’s not impossible, it’s just annoying. Learning from my Trek Madone about BB90, I have to say that I’m warming up to BBright and BB386. 30mm spindle is great, but wider BB is better in my mind for stiffness – and more importantly for me installation strain gauges inside

Where the problem is in designing something that doesn’t interfere with the legs motion. In picking up a used Quarq unit I noticed shoe scuff marks in what I thought was the most impossible place. My shoe has never rubbed my crank while cycling, but I’ve seen the result and it certainly happens. I don’t know why, if it’s acceptable or if it’s something that should be corrected in their cycling gait, but they are there. If my electronics package sticks out anywhere it could get caught. It would be pretty lame business practice to not thing about this before building.

Other issues such as the chain. If it comes off the big ring due to an over shift then I need to keep the electronics out of the way, or baring that, make them small and durable enough that it isn’t an issue. The first is preferred.

I’m back from a ride and looking at a SRAM Rival, S950, and S975 Quarq Saturn Cinqo and still lacking ideas.

As for my Trek Madone, it is now together (though with a temporary rear derailleur cable stop) and feels so different from my aluminum frame. The wheelbase is shorter so its more responsive, but honestly the big change is in stiffness and damping. It feels smoother with the crank, and I feel less vibrations over rough roads. Being a heavier guy, I have longingly wanted a stiffer bike, tapered headset, and wider BB. It feels so much nicer but I keep wondering what bikes like that are like in the hands of people who are much more capable riders than myself. I need to adjust the setup and I only have a temporary magnet setup for the Quarq with sticky tape, so I’ll have to fix that if I want to keep gathering background data to compare in the future.

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