Not sure about it on bikes? I think it's mostly a styling exercise - seems to have become very popular with the "Steampunk" fraternity.

Makes sense on a car as it theoretically keeps engine bay temperatures down - not really an issue on a bike. Also the wrap stays dry on a car. It quickly turns to a cr@ppy looking mess when exposed to rain, mud etc. It's also an absolute nightmare to remove if you change your mind.

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