Stem and downtube shifters are both friction shifters and were both in use at the same time in the 70s. Not sure if one came first, but I'd guess downtubes. Stem shifters came on low-end bikes for casual riders who wanted to sit upright. They came on bikes that also came with "safety levers" on the brakes (not always, but the same type of riders who wanted stem shifters often wanted safety levers). Luckily, stem shifters went out of fashion, as being on a moving stem made them awkward to use. Downtube shifters remained the norm for decades. Many stem shifters took cables with the now almost obsolete lug ends that you'll only see on double-ended gear cables.
The D-ring on the side of the lever is just tension adjustment for the lever -- too loose and the derailleur won't stay in gear, too tight and you can't hardly shift. Do make sure there is no significant cable slack when the levers are all the way forward.
Stem shifters can be replaced with downtube shifters if you can find the clamp-on variety (and be sure to clamp them good and tight or they'll slide down the frame from cable tension and scrape your paint). I find downtube shifters much easier to control in terms of making fine derailleur adjustments, and they put your hand in a more natural position. You can also use bar-end shifters if you can find a clamp-on cable stop for the downtube.
Last edited by DebW; 04-07-2010 at 05:48 AM.
Oil is good, grease is better.
2007 Peter Mooney w/S&S couplers/Terry Butterfly
1993 Bridgestone MB-3/Avocet O2 Air 40W
1980 Columbus Frame with 1970 Campy parts
1954 Raleigh 3-speed/Brooks B72