“Can anyone tell me what was wrong with the pre long-arm designs?”
I posit: nothing, provided the user has the wit to spin the cranks lightly for the duration of a front shift. This, though, is beyond the coordination of many riders as I see often on the road.
Therefore, in recent years Shimano has expended great effort to make front shifting work better under load.
A couple of observations:
Not only is the arm long but the cable clamp, pivot, and cable exit point at the bottom bracket are almost aligned. That’s why these derailleurs have the two clamp positions and need the Shimano TL-FD68 tool to determine which position works at all. All of that could be avoided by having the arm cant off to the side, so clearly it was Shimano’s precise intention to sail close to the wind here. So why? I can think of these reasons:
- To make the mechanical advantage rise sharply as an up-shift proceeds. This takes up the initial cage-chain clearance rapidly and then increases the leverage as the cage hits the chain, the chain (under pedalling tension) offers increasing lateral resistance as it bends, and finally the chain hits the large chainring for the shift proper.
- To encourage the rider to start the shift decisively. The initial mechanical advantage is poor, forcing the rider to push the lever firmly. Having started with some effort, the rest of the shift proceeds more rapidly. It’s basically a trick to get the rider to exert more effort from the start instead of gradually meting out more force in response to a slow, grinding shift.
- By varying the mechanical advantage in this way, the shift proper can take place with less lever throw, possibly aiding riders with small or weak hands.
And why did Shimano dump this design in favour of the toggle-link type, ironically marketed as “[matching] the force curve of the hand more naturally”?
I can think of the need for better tyre tyre clearance in combination with the longer cable pull (to reduce wasted effort on cable friction), the trend to wider tyres, and short chain stays. Anything else? I haven’t figured out how the plethora of levers affects the mechanical advantage throughout the shift. Measurement of the cage movement versus hand-lever movement might be the easiest way to discover that, but I don’t have a suitable derailleur to hand.