| The job of the front derailleur is
to initiate shifts. The job of a chain guide is to ensure that
shifts are completed successfully, and also to eliminate spontaneous
derailments. This is my assessment of the
various guides made for preventing inward derailments. Ring Guides Theoretically, the best guide for preventing inward derailments would be a ring guide, larger than the smallest chainring, mounted directly to the crank. It would be unaffected by crank flex, it would prevent both forward and reverse derailments, it would require no adjustment, it would experience the least wear, and it would have the shortest distance between guide face and mount, as well as a tight profile, so there would be little chance of something knocking it out of position. However, for most cranks, there simply are no inner ring guides made because there are very few bikes which have enough room for such a guide between the crank and the chainstay. To get around the problem of the chainstay pinch-point, AJ's Frame Guard is a non-rotating ring guide with the rear portion of the ring removed. The AJ's guide is a well-made, reasonably light unit that can offer the same forward and reverse protection of a rotating ring guide, and I have recommended it* on many occasions as a quality alternative to the Jump Stop. The chief difficulty with the AJ's guide (aside from price) is that many bikes simply won't accept it. You need to have a BB with a fixed cup flange, you need to have BB locking ring that can sacrifice sufficient threads of engagement when the BB is moved over to make room for the guide, you similarly need enough clearance between the left crankarm and the left chainstay to move the BB over, and you need to have an inner chainring which has a compatible horizontal position. And it is an inherent property of BB mounts that flex and weight problems increase as the chainring size increases. To minimize this problem, AJ's guide specializes in covering only the smaller ring sizes. (I did not find a largest chainring size listing on their site, but from the photos, it looks to me like the max would be around 24 - 26T.) Metal Plate Guides The idea here is to cover the entire engagement zone with a long-wearing metal guide surface. There have been several attempts to integrate a plate guide into front derailleur design, but they all had design problems which, in my opinion, were best solved by making the guide unit separate and fully independent from the front derailleur. The Moots Chain Backstop was the first independent plate guide that I know of (introduced while I was still tooling up for the Jump Stop). It was expensive, heavy, and difficult to install and adjust, but it had Moots first-class handmade workmanship, and it was probably strong enough to survive a nuclear blast. So far as I know, this guide is no longer made. The only seat-tube mounted plate guide I know of being offered at this time is the Jump Stop. The main advantages of the Jump Stop are high effectiveness, long wear, low cost, reasonable weight, easy installation, and a wide range of lateral adjustment, backed by a no-fault warranty. It can't prevent reverse derailments like a ring guide can, but it is designed to catch them and make them easy to correct. The chief disadvantage of the Jump Stop design is that there are some frames it simply won't fit, such as bikes with no room between the front derailleur clamp and the downtube joint, or bikes with suspension elements in the way. Many carbon fiber frames will also not accept the standard Jump Stop clamp, but I hope to have an adapter kit for those soon. E-Type Plate Guide If a seat-tube mounted guide is out of the question, and the AJ's unit seems a bit expensive, there is a new entry from J-Tek called the DropStop, which mounts under the fixed cup flange. I have found no reports on the performance of this unit, but just looking at the the pictures, it appears to cover a taller range than the AJ's unit, though it eliminates reverse derailment prevention. I don't know how lateral adjustment is achieved, and I'm sure there are several of the same compatibility problems that the AJ's unit has, but this does provide another possible option, and I certainly have no argument with stainless steel as a guide material. Plastic Horn or Tooth Guides These guides have a forward-curving plastic structure which serves as the guide. These are generally the lightest guides, and often the cheapest. The first of these was the Chain Watcher, from Third Eye. Its worm drive clamp fits almost any size seat tube, and it has the largest guide face of any of the plastic guides, but its hollow horn is both flexible and short-wearing. The curvature of the horn provides a cam-type adjustment system, where lateral adjustment is accomplished by rotating the unit around the seat tube. Unfortunately, this sometimes places the horn far enough forward that the chain can hook an inside edge of a tooth on the small chainring. When that happens, this guide is notorious for flexing out of the way to let the chain pass through, and then popping back into place to hinder efforts to re-thread it. Even so, apparently some people have found this guide satisfactory. The solid plastic imitators of the Chain Watcher (Deda Dog Fang, Redline, and various nameless Chinese OEM units) are less flexy and don't wear out quite as quickly. Unlike the Chain Watcher, they have size-specific clamps, but they do have the same cam-type adjustment--with the same drawback. However, at least these units tend to rotate out of the way when the chain pushes through, so rethreading is easier. The solid plastic guides also have the smallest guide faces of all, so it is not difficult for the chain to hop right over the top of the tooth--at which point the tooth basically just serves as a chain hanger to keep the chain from falling all the way down and getting stuck between the crank and frame. None of the plastic guides cope with reverse derailments very well. Which guide is best? Depends. If your top priority is saving every gram, the plastic guides are definitely the lightest (roughly half an ounce). But if you want to completely eliminate derailments, permanently, your best options would be a crank-mounted ring guide (if you can find one) or the AJ's Frame Guard, or the Jump Stop--whichever works best with your bike. *I have no affiliation with AJ's. They are a competitor. But their reviews are solid and I can see the merits of their design for those who can use their guide, especially for bikes which will not accept the Jump Stop. |