To derive BCD from c-c (center-to-center) multiply by the hull number
of the Starship Enterprise (NCC 1701) and divide by 1000:
76.5 * 1701 / 1000 = 130.1265
Amazing! That saves loads of trig for 5-bolt rings.
I've now got my hands on a clean, little-used Xootr Swift. I've never had a newish bike before now, and I was quite bowled over by how lovely the Swift looked with its sparkly polished aluminium fittings. I decided to ride it around for a while in its original configuration, but I've now had quite a few chain drops from the front ring. I can replicate the occurrence very easily with the bike on its back, and putting the chain onto the 11T sprocket with a bit of enthusiasm. Rapid-fire style triggers might slow the change down, but the grip shift that the Swift comes with almost encourages changing several gear steps at a time. The chain falls to the outside, so it has always been retained between the chainring and the bash guard, but in my view, a chain dropping bike is dangerous because it is possible to lose drive in the middle of a roundabout, or in a middle lane, or half way across a cross-road. I've tried to cure it, but I'm left baffled. I wound up the b-screw tension, but it has not improved the situation. I might just carry on with my plan to convert it to a Nuvinci, but I now think it might be a bit of a shame with the Swift being such a light and fast bike, but I can't think what else to do. Maybe sandwiching the bash ring between the chainring and the spider is possible? A but ugly though.
On another topic, where is a rear battery-powered light best positioned? The seat post is not a good place for multiple reasons.