Originally Posted by
oldbobcat
Forget the spring or the normality of the derailleur. Whether the front shift is being actuated by your thumb, your index finger, or the spring, whether the spring is pushing to to big ring or the small ring, the job is the same--knock the chain off the current ring and drop it onto the one next to it. All derailleurs work by deflecting the chain away from the cog that it's about to engage toward an adjacent cog. Derailleur chains are flexible in order to facilitate deflection. The front derailleur works by deflecting the span of chain between the top of the rear cog and the top of the chain ring. This is the pulling span, the one that's doing the work.The tension on this span is a function of how much pressure you can apply to the cranks against the resistance of the rear wheel. For example, going uphill increases rear wheel resistance. Pushing harder on the cranks increases force from the other end. The bottom span is just along for the ride. This span is being pushed by the bottom of the chainring toward the tension pulley of the rear derailleur. That's negative tension. Pedaling harder increases negative tension. Did you ever try to push a rope? The only thing keeping that span from flopping uselessly is the tension spring on the arm.between the jockey and tension pulleys, unless you're coasting. The only thing that can increase tension on that span is pedaling slower or increasing spring tension by shifting to larger cogs. It doesn't matter whether it's front rear.because all you're doing is pulling on a spring.
First, there is no such thing as “negative tension”. Tension only works on way. People often confuse compression as the opposite of tension but the only “opposite” is the direction of the force vector. Something in tension has to go to zero before it can be compressed. The chain on the bottom side of the crank is still under a bit of tension because that’s what the arm on the rear derailer is supposed to do…keep it under tension so that it doesn’t flop around. Yes, it is just along for the ride but it is held in a little bit of tension so that it doesn’t hang slack.
Yes, I agree that the front derailer is meant to “derail” the chain from one ring to another. However there are lots of situations where there is too much tension on the system for the derailer to do that job especially when depending on a spring to do the job. When the spring eventually gets to the point where it can cause the chain to deflect off of the ring…either because of the teeth profile on modern chainrings or because the rider eases up enough on pedal pressure to allow it to drop…the chain can drop too rapidly causing that rear derailer arm to snap back resulting in the free chain coming in contact and entangling with the chain running through the jockey pulleys. That entanglement is what can result in the derailer snapping off.
The directionality of the derailer
does make a difference. When downshifting on the rear, the cable is forcing the derailer to move the chain, assuming, of course, a high normal derailer. Rear derailers don’t tend to lag like the front can during downshifts unless the rear derailer is a low normal (reversed) derailer. The failure of Rapid Rise was due to relying on a spring to derail the chain under tension…just like the front derailer.
More tension makes more friction which makes more resistance to making the good things happen and makes things wear out faster or bend and break more easily. Bicycle chains are strong when they're pulled in line. When you push against them sideways, they push back.
I have no problem with this explanation except that you are drawing the wrong conclusion. Yes, there is more friction and rather than depending on a cable to actively pull the chain off under high tension situations, we depend on a relatively weak spring to knock the chain off. Shimano front derailer springs are actually weaker than SRAM for example. SRAM front derailers tend to work better in my experience because of that strong spring. That said, a high normal front derailer actively pulls the chain off so there is no need for that silly easing of pedaling. Unfortunately high normal front derialers are exceedingly rare.
A bicycle chain is like a spoked wheel. The bike and rider hang from the spokes above the hub. The spokes below the hub just help keep the rim and hub aligned until they're on top again. You could probably hang a refrigerator from a single spoke, but if you apply negative tension (or push on it in line), it bends and collapses..
No argument from me on that other than to point out, again, that there is no such thing as negative tension. Tension can only be there or not. Compression isn’t the opposite of tension.