I've been slowly going through the process of converting my old hardtail mountain bike into a commuting bike. One of the things that I'd like to do is to fit a chain guard so that I can ride the bike while wearing long trousers and not have to worry about chewing up the trouser cuffs.
A complication is that the bike (like most mountain bikes) is fitted with both front and rear derailleur gears. I'm not keen to replace these with an internally geared hub system as I have been trying to keep the changes relatively modest in their cost.
Are there chain guards that you can add to bikes with derailleur gears?
SKS Germany - a company that make great bike products, including the fenders that I put on two of my bikes, make a product called the Chainboard - a chain guard designed to accommodate front derailleurs.
I don't have one, but you can google SKS Chainboard and find several reviews.
In general it can be done. My Unibike Voyager has both a chain guard and a front derailleur.
I see two possible difficulties: You might have trouble finding a chain guard that fits your particular geometry (size of sprockets, model of the derailleur etc.) and you might have even more trouble attaching the chain guard to your frame, as it might need custom attachment points (for example my chain guard is screwed to the bottom of the bottom bracket).
You can DIY a solution if you need to. The combination of a "chainring protector" and a chain tube will do 80% of what you need at relatively low cost, or you can fold one up out of metal or plastic.
A friend of mine obsessed about this for a while and went through several iterators of folded plastic guards, but none really worked for long. Eventually she bought one of the plastic chainring protectors you see on city bikes, just a ring of plastic that sits outboard of the chainring to keep your clothing off it, and a length of polypipe over the top section of chain, held in place by a clamp on the pipe and a bit of string tied to the bike frame. It looked very DIY but had been on her bike and working for about 6 months of touring by the time I saw it. You could add another length of polypipe on the bottom to help keep muck off the chain if you wanted to.
Great idea to add chain guard to 21 speed bike. Easy, if you are handy with cutting and grinding. Just remove the 'back side' of the chain guard that is closest to the frame. Your pants will thank you, and your friends will say, OH, no more funny looking pant leg clamps!