Almost all upright handlebars have a diameter of 22.2 mm (7/8") and the shifters below are designed to fit that size only.
Thumb ("Top Mount" Shifters
From the earliest days of mountain bikes through the late 1980s, top-mount "thumb" shifters were the norm. These are basically similar to down tube shifters, except that they mount on the handlebars.
Below-Bar (Trigger) Shifters
In the late 1980s, Shimano introduced "RapidFire" below the bar shifters. The first version had two buttons below the bar, intended to be operated by the rider's thumb. (This caused confusion, because the older top mount units were known at "thumb shifters" but that name actually would have been more appropriate for the RapidFires, since the thumb was used for both upshifts and downshifts.
The first RapidFires were not well received, in part because the upshift and downshift actions were done with the same digit, going the same direction. Riders found this confusing.
Shimano soon introduced "RapidFire Plus" shifters that used a similar thumb button to shift to larger sprockets, but a small "trigger" operated by the rider's index finger to select smaller sprockets. This version was much more successful commercially, and remains the norm for most Shimano upright bar stystems.
Twist Grip Shifters
Twist grip shifters have been around for many years, but the first to be really successful were the GripShift units from SRAM corporation. ****** ESP
Older front derailer cables used a short length of housing running from a stop near the bottom of the down tube, looping down just above the left side of the bottom bracket shell, then up the back of the seat tube to the derailer. The down-tube stop was sometimes a braze-on, sometimes a clamped-on accessory.
Older front derailers incorporated a downward-facing housing stop to terminate this loop of housing.
Newer front derailers don't use housing in this area. Instead, they run bare cable under the bottom bracket shell, then up behind the seat tube to the derailer. There's usually a plastic guide of some sort underneath the shell, which guides both the front and rear derailer cables below the bottom bracket.
Newer front derailers work a lot better than older designs, but they lack the housing stop to make them work with the old-style cable routing. This is not a difficult problem, however. All you need to do is ignore the down-tube housing stop and run the bare cable under the bottom bracket shell.
If you want to protect the paint of the bottom bracket shell, you can cut a short length, maybe 2 inches (50 mm) or so of housing to slide over the cable where it runs below the bottom bracket. This length of housing won't be working in compression as with most housing installations, since neither end will be up against a housing stop, but it will protect the paint and reduce cable friction.
You might expect the "floating" bit of housing to shift out of position over time, but this tends not to happen in practice.
Front Derailer Indexing
All modern rear shifting systems are indexed
, but this is not the case with fronts.
Shimano STI brifters and Below-the-bar MTB shifters are indexed for the front as well as the rear. So are many of the twist grips supplied as original equipment on new bicycles.
Bar-end, down-tube and Campagnolo Ergo brifters are indexed for the rear, but use friction for the front. This is also true of most aftermarket twist-grip shifters. Older front derailers will generally be compatible with any friction shift system, but you do need to consider the chainrings to be used.
If your bike uses a double chainring, front indexing is generally not an issue, since there are only two basic positions to be used.
If your bike has a triple chainring, things get interesting! Although rear indexing is consisten with a brand, Shimano's front indexing is different between their so-called "road" and "mountain" lines. Here's what that means in practice:
- Shifters for drop ("road") handlebars only index properly with "road" front derailers.
- Shifters for upright ("mountain") handlebars only index properly with "mountain" front derailers.
This can lead to problems if you want to use different chainring sizes
than the particular shifters/front derailer are commonly used with.
Shimano does make one front derailer, the R440 model, that is designed to work with upright bar shifters and a crankset with a 52-53 tooth big ring.
The no longer make any front derailer that is compatible between drop-bar shifters and smaller chainrings.
Front Derailers And Chainring Size/Number
Front derailers are designed to work with a particular-size of big ring, and the bottom edge of the outer cage plate should follow the curvature of the big chainring.
Good front shifting depends on getting the cage low and close to the chainrings, but a mismatch in curvature will make this impossible.
- If your derailer is designed for a smaller large chainring, the back end of the cage will be closer to the chainring than the front end. This will make for slow, difficult shifting.
If your derailer is designed for a larger large chainring, the front end of the cage will be closer to the chainring than the back end. Having the back end of the cage too far from the chainring will require trimming the front derailer excessively as you shift the rear back and forth.