Hi Agent B.,
I make the Jump Stop chain guide, and among my customers have been top pros riding the best equipment set up by the best mechanics. You may very well be correct that you cannot obtain a front derailleur adjustment which will eliminate your chain drop problems. The front derailleur inner cage plate shape and position is optimized for upshifts, not for preventing inward derailments. Sometimes the bike and component configuration permits a front derailleur adjustment that works acceptably well most of the time. Sometimes not.
The surest way to prevent inward derailments is to have an anti-derailment guide in place. Theoretically, the best guide would be a ring guide, attached directly to your crank, but on most bikes there simply isn't enough room for such a guide between the crank and the chainstay. The AJ's frame guard is a well-built frame-mounted version of a ring guide, and I have recommended it on several occasions as a good alternative to the Jump Stop. The chief difficulty with the AJ's guard (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 to make room for the guide when the BB is moved over, and you need to have an inner chainring which has a compatible horizontal position, and the inner chainring must also be small enough. (I did not find a max chainring size listing on their site, but from the photos, it looks to me like the max would be around 24 - 26T.)
Seat-tube mounted guides are lighter, cheaper, easier to install, have more lateral adjustability, and they work with any size inner ring, but being positioned at the top of the ring, they cannot prevent reverse derailments, like the AJ's can. There are principally three types of seat-tube mounted guides: the hollow plastic Chain Watcher, the solid plastic imitators of the Chain Watcher (Deda Dog Fang, Redline, and various nameless OEM guides) and my Jump Stop (
www.n-gear.com). The Chain watcher is one of the lightest, it has a fairly large guide region, and its worm drive clamp will accommodate any tube size or shape in its range, but the hollow plastic tooth does not wear well, and its cam positioning system will sometimes put the tooth far enough forward that the chain can catch a tooth before it hits the guide, and then the chain just pushes aside the flexible tooth--as happened here:
http://www.precisiontandems.com/tiptimingchain.htm Even so, judging from customer reviews of it at mtbr.com, apparently some people have found it quite satisfactory.
The solid plastic tooth guides are more rigid and wear a little better, but they have the same cam-type adjustment system with the same drawback. However, if positioned such that the chain can catch a tooth, these guides generally rotate on the seat tube. Although this can be hard on frame paint, it does at least move the guide out of the way so that you can get the chain back up. The solid plastic guides also have the smallest guide face, so sometimes the chain can hop right over the top of the tooth. When that happens, 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.
At 30 grams, the Jump Stop is the heaviest of these seat-tube mounted guides, it is mostly limited to standard tube sizes, and it also will not work with some rear suspension designs or bikes that have no space between the front derailleur and a frame weld. But for bikes that can accept the Jump Stop, it is very difficult for the chain to get past its large, rigid, stainless steel guide plate. (The Jump Stop, also, is reviewed at mtbr.com.)
If your top priority is saving every gram, your best bet is one of the plastic guides. If robustly eliminating derailments is your top priority, your best options would be a crank-mounted ring guide (if you can find one) or the AJ's guard, or the Jump Stop--whichever works best with your bike. (I don't know about the availability of the other products in South Africa, but direct mail overseas is no problem for me.)
Cheers,
Nick
N-Gear
ngear@gvtc.com