Originally Posted by
cyccommute
The front is slow and clunky because it is operated wrong. The front derailer depends on a spring to knock the chain off to the lower gears under torque. Shimano’s Rapidrise suffered from the same problem. The spring just isn’t strong enough to knock the chain off under high torque situations and, thus, the derailer just clatters against that chain until torque decreases enough that the spring can knock off the chain. If front derailers worked the same way as rear deraiers…i.e. the cable drags the chain off the larger gears to the smaller ones…the shifts on the front would be as lightning fast and efficient as the rear. Suntour made a few of the high normal front derailers (as did Shimano) that worked really well back in the days before pins and ramps on the chainrings.
It doesn't quite work this way.
Going to the small ring, the derailleur move the chain over and lets it hang over the smaller ring (so the teeth on the larger ring can't catch it). The rotation of the ring causes the chain to drop on the smaller one,
Going to the larger ring, the derailleur move the chain over so it can catch on the pins and ramps, which lift the chain up so the teeth pick up the chain.
Because there's more friction, more force (and enough) is needed. It might be more reliable to use muscle power for that since the amount of force can be varied. A spring can't vary its force, which means a spring that happens to be or get weak might not work.
Originally Posted by
cyccommute
Originally Posted by
Velo Mule
It pretty much comes down to mechanics. It would be difficult to put a cluster of gears on the crank. The other issue is that when shifting the chain on the front derailleur, it is under tension whereas when shifting on the rear the chain is slack. The most we see on the front is three chainrings.
Nope. You can get
four.
I know about those. They are so rare that it's not exactly wrong that 3 is the limit (what the vast majority of people will ever see).
They aren't being made any more either. So, saying you can "get" them is misleading.