This is a thread that could keep linguists going for months. As a cyclist, I have always been impressed by the number of variations which English speakers manage to find to pronounce dérailleur. This is understandable as the standard French pronunciation ([derɑjœ:r] for those familiar with IPA) actually contains four phonemes (sounds) which do not occur in English. If one anglicizes the French, one gets 'derailer' which I find retains a good linguistic resemblance to the original and gives a meaningful description of what the device actually does. Doubtless railway enthusiasts will think it is a device for causing train wrecks but, for me, a French dérailleur can find a perfectly good home in English as a derailer.