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Long trousers are one of the reasons why they are a good idea, the reason why they are not optional is that the legal standard for "City and trekking" bikes is different from the one for racing bikes. The racing bike standard says that chain guards are not normally fitted to racing bikes, but then defines the parameters if they are. The city and trekking bike standard says a chain guard must be fitted and then defines the same parameters. Spotter's home build doesn't have to conform to the standard but does, PX's OTP bike should conform to the standard but doesn't.
What exactly do you mean? Because people ride them in long trousers a lot?