I recently caught my rear derailleur on a log and snapped it in two. I have done some recent work on some department store bikes and noticed that some come equipped with an axle mounted guard around that surrounds the derailleur. Does anyone have any experience with these. Do they work or is it just something else to get snagged by brush and vines?
I had a mate fall off with one attached it got bend and jammed the derailleur so bad we had to walk home, as the tools we carried were not good to get it off. I have seen many prangs where the derailleur took bigger hits with no problems. Most that I have seen (Department store ones) are too light and cheaply build to be anything other than cosmetic.
It also depends on the bike - better bikes have a derailleur hanger that is sacrificial and bends, protecting the derailleur in many common prangs. Department store bikes often don't have a hanger, hence the derailleur takes the full impact.
I have though about it, but with years of Mountain biking, many falls (some big), only ever bend a hanger. They may work, but I asked - why do no mid/high end bikes them. Is it purely weight, do the want to sell more parts, or is it just not worth weight and money required to make a bike so strong it cannot be broken in a fall.
I decided (for me) they a not worth while.....
I had one on my bike in Japan. I never had problems with them, my derailleur stayed intact. I didn't take it through many bushes, but I did drop it a lot on the right side (naturally get off that side, and I never use a kick stand) and have bumped against posts and stuff. The riding I do now is in the mountains, lots of rocks but not much brush so I don't ever think about it anymore.
The one I had was bolted to the dropout, but covered the same area as the ones I have seen on the axle. If you are going through some pretty tough brush, it might not protect against everything, but for the protection it does give, it is definitely worth the effort.
I have seen some bikes with the derailleur being bent because the bike fell on the derailleur side. So I usually recommend to have one so that your derailleur is protected, and it proved useful for me a few times, but some guards are just too light and they bend and then bend the derailleur, so it must be heavy to really protect your derailleur.
Yes it makes your bike heavier, but what the heck, to gain weight on your bike just wear short pants instead of jeans, you'll gain more weight than using costly parts on your bike.