Why elusive, you ask?
Just walk into your local bike shop and count the number of bikes with chain guards. Yes, plenty of kids bikes have them, but they seem to vanish when you get to wheel sizes over 12 inches. And if you do happen to have a chain guard sighting it will probably be a partial coverage one that just covers the top half of the chain.
Now there’s no question in my mind that chain guards are essential. They keep your pants clean and remove another barrier from just hopping on your bike and riding. OK, so there are other solutions that people have suggested like cuff rolling, pants strapping and knicker wearing. But a BFROU is about using your bike for transportation. You wouldn’t think about special clothing modifications for driving your car, so why should your bike be any different?
Chain guards have other functions like keeping the lube on your chain and the dirt off of it. If you’re really lucky you’ll find a bike with a chain case. Chain cases enclose the chain on both sides and keep the weather out, extending the life of the drivetrain. Dutch bikes are commonly equipped with chain cases because, like our beloved cars in the US, are made to sit out in the weather for many years without frequent maintenance.
Chain guards and chain cases are not without their drawbacks. They add an extra step to removing your rear wheel. Access to your chain for inspection, cleaning and lubrication will be hampered as well. At one time chain guards were fashionable and made to enhance the appearance of the bike; however, now the bare lines of the chain are the desirable visual cue thanks to the dominance of fixie and track bikes.
I’ll mention that if you want to add a chain guard to your existing bike they’re hard to find and can be a challenge to retrofit. They range from the very functional and plastic (SKS) to the handcrafted and unique (Velo-Orange). Soma even has a modular one that they say works with front derailleurs. With the resurgence of internally-geared transportation bikes we hope to see more chain guards and more BFROU along with them.