Shimano long-arm front derailleurs: purpose of design?

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Samuel D
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Shimano long-arm front derailleurs: purpose of design?

Postby Samuel D » 26 Jun 2020, 3:46pm

In another thread, ElCani asked the question:

“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:

  1. 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.
  2. 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.
  3. 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.
Any other suggestions?

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.

Samuel D
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Re: Shimano long-arm front derailleurs: purpose of design?

Postby Samuel D » 26 Jun 2020, 3:52pm

Oh, and a few other things Shimano has done to improve front shifting under load:

  • greater cable pull of front shifters
  • extra front-derailleur bracing against the frame, with an aluminium plate glued to the seat tube to take that load
  • hollow chainrings to enable greater lateral stiffness at low weight, especially with the smaller BCD chainrings that have allowed smaller chainrings but at the expense of lateral stiffness with larger rings
  • shifting by electric servo motor with great force.
No doubt there are more.

Jamesh
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Re: Shimano long-arm front derailleurs: purpose of design?

Postby Jamesh » 26 Jun 2020, 6:29pm

Also a trim function which allows you to trim the FD according to rd position.

Cheers James

fredN4
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Re: Shimano long-arm front derailleurs: purpose of design?

Postby fredN4 » 26 Jun 2020, 6:59pm

well I am sure you are all right but I have never had a problem with Shimano shifters, although I much prefer Campag and lately, Sram. Having to swing the whole lever has always seemed crude to me.

peetee
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Re: Shimano long-arm front derailleurs: purpose of design?

Postby peetee » 26 Jun 2020, 9:00pm

In my experience nothing beats the shift you get from a 7/8 speed XT front mech, the FD-M735. The one on my mountain bike still changes faultlessly and silently after many thousands of hard miles.
In contrast, I have found the long arm 10 and 11 speed mechs finicky to set up and on some bikes, where the cable approach angle is less than idea, they can be heavy to operate even with the polymer cables -which incidentally are only low friction for as long as the coating is intact after which they are a right pain in the rear. So, looking at the big picture, 30 years of wasted technical ‘refinement’ in my opinion.
Current status report:
Back on two wheels in deepest Pastyland and loving every minute. Mission: to enjoy big, bad hills again.

Samuel D
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Re: Shimano long-arm front derailleurs: purpose of design?

Postby Samuel D » 26 Jun 2020, 9:57pm

peetee wrote:So, looking at the big picture, 30 years of wasted technical ‘refinement’ in my opinion.

It’s wasted on me too. I don’t have a problem to pedal gently while shifting (for one second?) and to shift down before my cadence drops to nothing. My interest in this is mechanical curiosity.

All the same, I often see other people having trouble with front shifting, and not just beginners. So I see why Shimano has been trying to do something about it.

Brucey
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Re: Shimano long-arm front derailleurs: purpose of design?

Postby Brucey » 26 Jun 2020, 10:12pm

I think that the argument that it provides a high speed initial mech movement may well be correct, or at least that may have been shimano's main intention; why else would you set yourself up for making the whole installation so sensitive to cable routing, chainline etc? Not to mention hard work to initiate the shift sometimes.

This strategy obviously doesn't fit with any type of triple chainset, but maybe that isn't important since they are not at the cutting edge right now (apparently)….

cheers
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