I have personally lost track of the number of pants that bicycle cranks have shredded, or at least branded irreparably with that dreaded chainring tattoo. Instead of securing your pant leg with a goofy elastic or wearing skinny jeans, however, you can simplify your bike's drivetrain and fashion a custom chain-guard by de-toothing one of the front 'rings.
First I'll show you how I did it using some cheap steel cranks where the chainrings are riveted together. Then, I'll explain how you would apply this to make a nice-looking chain-guard for your fixed-gear or single-speed bike, starting from a pair of regular road bike cranks.
Simplifying a multi-speed bike
Why would you want to have fewer gears on a bike, rather than more? Well, the number of speeds a bike has is largely a marketing gimmick, because as you increase the number of chainrings in the front and sprockets in the back there is more and more overlap between the gears. Combine that with the fact that most casual city riders are only dimly aware of the existence of a front derailleur and only ever ride in one of a handful of gears, and the case can be made for ditching one of the shifters and its associated cables and derailleur.
Here's how I did it on a department-store mountain bike from the 80s, the kind college students love to ride into the ground. I started by removing the crank. Then, I ground down every tooth of the larger chainring using a bench grinder. It made terrible noise and unleashed a shower sparks. The crank arm got really hot.
Here's the process part-way through. The freshly ground edges are still sharp and jagged. (This is about the point where this activity stops being fun and you start questioning everything.)
And this is the finished product. This Raleigh's transformation from a twelve-speed bike with front and rear derailleurs to a five-speed that uses only a rear is now complete. (I also removed the freewheel, replacing it with one with fewer sprockets, and added that shiny, holey spoke-guard known affectionately among aficionados as a "pie plate," or "dork disk.")
Now you have a city bike with fewer cables and things to break, while ditching those useless high gears and keeping the useful low ones. Incidentally, this drive-train configuration is also the only time it is acceptable to ride your bike cross-chain.
Making a fancy chain-guard for your single-speed
If you want to cheaply convert your road bike into a single-speed bike, consider re-using your existing crank, which generally comes with chainrings of 52 and 42 (or slightly fewer) teeth. The 52 ring we shall sacrifice. Undo the chainring bolts holding the two chainrings together, and start grinding off the teeth.
Actually, with aluminium chainrings you're better off clamping down the ring and using a hacksaw, as it cuts through the material like cheese. Afterwards, you can get fancy with a file to get a smooth and circular finish all the way around; bonus points for gently bevelling the sides so as to remove any sharp edges. When done, you can bolt the chain rings together again and comfortably roll down your right pant leg.
(The only other thing to be aware of is that you may need to use a slightly longer bottom bracket to obtain a straight single-speed chain line. For the bike in the top picture, for instance, I used a bottom bracket 122mm wide.)
Now you can maybe focus on learning how to sew back up all those shredded pairs of pants, sitting at the bottom of your hamper.
[Images by author]