We have been wanting to do some coast-down tests on the recumbent bike for a while in order to find out the drag coefficient, but have been unable to collect the relevant data. What you need is speed against time – on a graph then the gradient would be accelleration, and the area under would be distance.
We thought of a number of ingenious ways to do this, but none seemed very simple. Here is a quick summary of ideas we had.
- Using a cycle computer, shout out the speed at regular intevals to someone writing them down. We decided this was impractial.
- Put pressure sensors in the road at regular intevals connected to an old computer. The computers that we had data loggers for were old bbc’s – and they don’t transport on a bike very well unless we had 100m of power cable!
- Using a camcorder, record the speed on the cycle computer over the journey. The problem with this was finding a suitable mouting point on the bike for the camera (I spent ages hunting around at uni and shops for a 1/4 inch witworth thread, before I found one in my junk box). After thinking some more about this, we came to the conclusion that the cycle computer must average the speed over some time, and so the data is not as accurate as it could be.
- Mount some bright flags on some of the spokes, and then mount a camcorder on the bike. After filiming the motion of the wheel over the run, step through the frames and record the positon of the flags. Aside from the issue of mounting the camera over the wheel, I didn’t fancy stepping through (3 runs x 6 speeds x 30fps x 60seconds) 32400 frames and marking the position of flags. I am sure it would be possible to write a program to do this, but just don’t go there.
It seemed like what should be a simple task was getting needlesly complicated, until I had an idea.
At home I found an old wireless mouse, and with the middle finger scroll butted against the wheel and some gaffer tape around the bike, I span the wheel. A few lines of code later on my linux box, I had a lovely list of numbers in front of me!
I plotted the numbers, and found that they didn’t plot well. The mouse was trying to scream ‘I’ve just moved’ about 8000 times a second, and something didn’t like all that screaming.
My bike wheel is 700mm in diameter, and the mouse scroll wheel was about 20mm. At 30mph my bike wheel spins 6.1 times per second, and the poor mouse wheel would be spinning at 214 revs per second. That 12810 rpm, and it is no wonder something couldn’t cope!
Going back to the drawing board I decided that I must take a mouse apart. Not wanting to destroy my nice wireless mouse, I found an old ps2 mouse which I soon had in peices. I soldered a few more wires and a reed switch onto it, and did a bit of jigery pokery with magnets on my bike and hey presto, I had a data logger!
If only life was so simple. I turned my laptop on, transferred the few lines of code I had written to collect the data, and plugged my mouse in – or at least I tried to. Apparently ps2 was not on the menu when my laptop was ordered, and so that port that I had never used on the back of my lappy which I thought was ps2 on closer inspection turned out to be a s-video out.
Back to ebay, and a usb to ps2 converter solved the problem. It now all works well! Now to get this gear on the recumbent.
Now for the next issue – There is nowhere to put a bag on a recument, because your back is against a seat. It is possible to put a small bag on your front, but your knees bang it every cycle. Not really the treatment for a laptop, but hey, anything for science. Lets do it!
Not so fast! Overnight it had snowed. I couldn’t take the recumbent out in the snow – it is unstable enough in the dry, especially at the kinds of speeds I was hoping to reach.
Tomorrow, Tomorrow, Tomorrow. Maybe one day.