Firstly a bit of theory
Ohms law: V = I x R (volts = current x resistance)
Joules law: P = I^2 x R (power = current squared x resistance)
In the link above Hastie had gone for 65 Watts, which seemed a little high to me, so I decided to aim for 55W which seems be what the Heat5Jackets kit puts out.
A roll of 30 AWG wire from RS came to about £17 delivered. This stuff should be about 0.3 to 0.4 ohms per meter. I could have got cheaper wire but I used the teflon coated stuff which is good to 200 degrees C and should be a lot tougher than a normal plastic coated wire.
A bit of maths gets us R=V^2/P, and my 900 puts out about 14.6V, so I want about 3.8 ohms to get my 55 watt target. I cut a 12m length, measured the resistance, then worked out that 10.5 meters should give me about the right resistance.
A lot of the people I'd found online had built the element in to thier jacket liner, but not wanting to knacker my jacket liner I found a cheap sleeveless fleece in M&S for £15. I laid out the wire using tape to hold in position to start with, then when I'd worked out where it all needed to go marked up with a felt tip and then spent a couple of evenings practising my needlework (us 900 riders have a reputation to maintain you know). I went for a similar layout to Hastie, a vertical pattern with an extra horizontal loop at the base to allow me to line it all up correctly at the end.
The wires end in the right hand pocket, where they're soldered to a thicker cable which pokes through the fabric, I've then used a standard DC power connector to join it to a longer cable with a DIN socket on the other end to plug in to the bike. The idea being if I forget to unplug it the inline connector should come apart without damaging anything, plus when I get off the bike and wander off I don't have several feet of cable trailing on the ground.
Once finished I've ended up with 3.7 Ohms, which works out at about 57 Watts, and 3.9A, so similar to an extra headlight bulb. Increased resistance with temperature means at 40 degrees this drops to about 52W, though I've not measured the temperature of it yet.
I needed to fit a power socket to the bike anyway, so I've fitted one in the right hand side panel for charging the phone etc, and one in the left hand side panel for the jacket, which goes through a switch on the dash so I can turn it off. My auxiliary electrics are all on a switched live with a 15A fuse, but I've also put a 7.5A fuse on the line to the left hand socket.
Took it for a test ride yesterday for the first time, and at a balmy 9 degrees C I had to turn it off at town speeds as I was getting too hot, fantastic at higher speeds though.
I'm now temped to add in the extra 1.5m of wire I've already got cut, and have a way of switching the power between the full 12m (which should give about 50W) and the current 10.5m (57W) to give me a high and low power setting. To do this I'd need a either a 3 way connector (both inline and the the connection to the bike, if anyone has any ideas let me know) that can cope with 4 amps, or the easier method might be to switch it in the pocket so I'd have decide if I want high or low before I set off, but can still turn it on and off when required. Alternatively I'd need to fit a pulse width modulator (as used by some heated grip controllers) but they're expensive
Total cost about £35 plus a couple of evenings of my time, but I've got enough wire to make a second one for the girlfiend so it's only really cost me about £27.
Edited by ChrisG, 14 February 2011 - 08:01 pm.
