Elsewhere, someone claimed that down-shifting the front derailleur under load, somehow catches the chain, which then can yank the rear derailleur and snap the hanger.

I can't picture how that's possible. Thoughts ?

i think i'll have a bowl of ice cream for dessert.
why is that dude on the 250 ninja riding by 5 times in a row?
i think my G. shep Mona may have farted... i'm heading outside.

The initial description ("down-shifting the front derailleur under load, somehow catches the chain") sounds like a description of a typical chain suck. Yes, it happens when downshifting in the front. Most of the time the stuck chain goes a full circle, severely scrapes the chainstay and then slams into the FD cage, deforming it significantly. I.e. it's the FD that gets damaged, not the RD. But, I guess, under some chain length and RD cage length combinations damage to the RD is also theoretically possible. Including the hanger.

This is what chain suck looked on my bike back in the day



And in my case the FD cage took the damage. It used to happen from time to time with this FSA crankset, no matter how I polished the chainrings or cleaned the chain. Once I switched to SRAM cranksets it just disappeared for good.

yet another thing bad about FSA cranksets... i thought the too large screw holes in the chainrings, teamed with the too large BCD cut, causing repeated creaking/loose/worn screws was bad enough.

the wiped out rear ders. i've seen happened at CX races, in the heavy mud/grass, usually after it quits raining.
the mud/grass builds up, a clump breaks free, and the chain wraps up on the cassette... SNAP! instant DNF.

Is this Gabe? New version? (That "01" addition?)

I've seen more rear ders damaged from chain suck than fronts.

This chain suck problem is one big reason to learn how to shift better. Andy

Howdy 😀 Andy

I have never had chain suck on a road bike, and I used to ride them pretty hard racing & training. And only once or twice on my ancient mtn bike.

I have. It involved lack of maintenance / chainring clag, as well as shifting under too much load.

Chain suck isn’t a shifting problem. It’s an equipment problem. The chain hangs up on burrs on the teeth of the chainrings and is pulled upwards. Often it will release with tension but not always. It’s also worse with smaller aluminum rings.

Yet adjusting one's shifting technique can/usually does "fix" the problem... I agree that worn ring teeth (including when they become burred) worsen the chance for chain suck. And I have replaced many rings to fix the problem for my customers. And I have explained how to better avoid this in the future.

Gabe makes a good point, even if he didn't focus on it. Chain suck really became a "common place" problem as mountain bikes became more popular. My speculation that a few factors were at play, combining for a "perfect suck", so to speak. First is the smaller rings that MtBs typically have (compared to road bikes, which were the "standard bike" prior to the mid 1980s), smaller rings wear faster and make the chain links pivot through more rotational range. Second is that the riding conditions tended to be far more abrasive, mud and sand. Third is the trail "flow" tends to be far more lumpy/hilly and the rider has to place more extreme efforts on shorter notice often. Which leads to the fourth factor, which is there often being less time, less crank revolutions to accomplish a shift when off road. Thus shifts are often done in a far harsher manor as the rider struggles to maintain momentum. And the last injury the bike sees is being "put away wet" after the ride, and when ridden next often not being cleaned or lubed before. Like so many devices, past performance works well, till it doesn't any longer.

Equipment will address some of the above. Good maintenance will help too. But rider technique (be that by purpose and good heads up riding or by reactive needs) stands above all IMO. Andy

You shouldn't shift the front derailleur in either direction under load.

Well no but sometimes you get caught out

Back in the days of friction shifting, perhaps. Even then, however, the problem was more due to the configuration of the chain ring than to anything a rider could actively do.

Although it can still happen today, the original flat rings of cranks used in mountain bikes up until the late 90s was part of the problem. The metal was soft and the flat teeth could be easily damaged on up or down shifts as the chain scraped along the inner part of the ring. Chain suck on the large ring was largely a nonissue but aluminum 22 tooth inner rings were especially prone to damage because of the high torque nature of using them. It’s easy to twist a tooth ever so slightly or damage it so that the chain catches on the tooth and is sucked up into the frame. Ramps and pins as well as shaped teeth did a lot to alleviate those issues as did better chainring metal. Stronger springs on front derailers also help since they would knock the chain off easier rather than let the chain just sit in the middle ring and grind away (mostly a mountain bike problem). Clutched derailers have also helped since their springs are stronger too.

Under load, the span of chain that runs from the rear cluster to the chainring is tight. Shifting in either direction, the front derailleur knocks the chain off of one ring and drops it onto the other. So when you shift under load, chain tension is suddenly momentarily released, possibly snapping back toward the rear cluster, and then suddenly re-engaged. Possibly, this snapping back could cause a wave in the chain that could conceivably, jump over the cluster.and put some weird tension on the jockey pulley, especially in a small-small combination. But I've never seen it happen because all of my observed shifts happen on the workstand and my left arm can't apply the same kind of force as a 160-lb rider standing on the cranks. Regardless, shifting the front under load can cause damage to chains, chain rings, and front derailleurs, and booggered shifts.

It takes a lot of force to derail and re-engage a tight chain, because it's less flexible than a loose one. It's asking the front derailleur to do a lot work.The rear derailleur has an easier job. The only tightness it's working against is the spring pressure from its tension arm. No matter how hard you're riding, the tension on the lower span of chain is no greater than what that spring can provide. That's pretty light compared to a 160-lb rider standing on the cranks.. I know you don't want to hear this, but your equipment will last longer if you momentarily back off the pedal pressure through your front shifts.
And for chain suck, check the wear on your inside chain ring.

I don't think anyone suggested shifting the front under load as normal practice.

You seem to be misunderstanding how a front derailer works. For downshifts, the spring on the derailer is doing all the work. That spring is too weak to actually knock the chain off under tension. The derailer will usually clatter on the chainwheel until such time as the rider eases up enough to allow the spring to knock the chain off. Downshifting from the middle ring to an inner ring is usually the hardest shift to make because of the weak spring on the derailer. A lot of the problem with chain suck (and with rear derailers snapping off) is due to the poor design of the front derailers. Rear derailers are almost always high normal and the cable pulls the chain from low torque gears to high torque gears as it moves across the cassette. Rapid Rise was mostly a failure because they were depending on the spring to move to high torque gears which is the same problem as the front derailer.

If the front derailer were a high normal, there would be fewer issues with shifts because the cable pulls the chain from the high gear/low torque gear to the low gear/high torque gear. Shifts would be cleaner and crisper. It would also alleviate the “wave” issue that can cause the rear derailers to snap back and perhaps catch the chain in a loop that results in ripping off the derailer.

Chain suck, by the way, almost never happens on an upshift because the chain is being actively dragged from one ring to another. Not that chain suck is the cause of a derailer getting ripped off. A sucked chain is going to be under loads of tension which doesn’t allow the rear derailers to snap backwards and get entangled with the chain.

Honestly, it doesn’t take a lot of force to to derail and reengage a chain. Your rear derailer does it all the time. Modern indexed derailers can be shifted under load without that momentary pause that front derailer require. If front derailers were designed for high normal, they wouldn’t require that momentary pause either.

The basic chain suck problem is that the "pitch" between the teeth on the large chainring and the small ring is random, and is almost never exactly an integer of link length. So the best you can hope for is that the chain drops to a tooth that is less than X number of links away. But if it just over the length of the links and you are standing on it, that's when you can wedge the chain between the chainrings.

At low torque this doesn't happen because the pedaling force isn't enough to force the chain down onto that too-far tooth and it just slips into the next one.

For the sake of a constructive conversation I'm going to ignore your first sentence. You can thank me later.

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.

Now here's the experiment to show how the front derailleur feels about this. A workstand helps.Have a friend hold back the rear wheel, or lock the rear brake. Now push on a crank arm with all your force, and deflect the chain behind the front derailleur with your other hand to simulate shifting the front derailleur under load. Try to deflect it enough that the chain lines up with the cog next to it, just like your derailleur. Don't hurt your fingers. Now try it with minimal pressure on the crank. Easier, n'est pas? This is what happens every time you shift your rings. With a tight chain you've got more tension on the cable (or return spring), more tension between the inside of the derailleur plates and the outside of the chain plates, and more tension between the chainring teeth and the inside of the chain plates. 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.

Now while the bike is still on the stand, apply minimum force on the crank with one hand and deflect the lower span with your other hand. Pretty easy, huh? Now do it again with maximum force on the crank. Notice any difference? Of course not. That span is just along for the ride. The rear derailleur deflects the chain to the new cog before the crank can apply tension to it. I stand by my last post.

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..

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.

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.



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.

You're just baiting me with semantics about negative tension. The chainring pushes the chain toward the tension pulley on the rear derailleur. The spring on the tension arm takes it up, hopefully with enough force that the chain won't go slack. I have never seen the chain go slack from "shifting too hard," just like I've never seen it happen from the other end as I described, but I won't say this would never happen. But the only way to apply more tension on the bottom span of the chain is to shorten the chain, get a stiffer spring, i.e., a clutch, or pedal backwards. And the relationship between the chain and the cogs doesn't care whether the shift is initiated by cable tension or a spring. The shift will occur when there's sufficient deflection in the chain to move it from one cog to the other, and that would be facilitated by applying more force from the derailleur or slackening the chain.

Pedaling in small to small, with an excessively long chain, the chain sags toward the ground. Not much pushing going on.

Remove a connector link from a chain and turn the cranks (preferably by hand, with the bike on a repair stand), and the chain drops straight down to the floor off the front of the chainring.

No, that's just gravity saying, "I'm the boss of this."

My point is, front shifts go smoother and makes the equipment last longer when you momentarily soft pedal while the chain is switching over. Period.

It's like the low-mileage Ford Focus ST that I bough eight or nine years ago. Little did I know that the previous owner (who had installed a very nice cold induction system, thank you) was power-shifting it, so the clutch blew up after 30k miles.

Or to put it another way:

Yes, that's gravity saying, "I'm the boss of this."

My point was that, even if your claim that the chainring pushes the chain toward the rear derailleur were accurate, which my examples showed it isn't, whatever happens to the lower span of the chain has no effect on front shifts.

That said, your final statement ("front shifts go smoother and make the equipment last longer when you momentarily soft pedal while the chain is switching over") is indisputably accurate.

[QUOTE=Trakhak;23590491

My point was that, even if your claim that the chainring pushes the chain toward the rear derailleur were accurate, which my examples showed it isn't, whatever happens to the lower span of the chain has no effect on front shifts..[/QUOTE]
Well, yeah, it's also getting pulled through the rear derailleur cage by the rear cog, which is being driven by the top span of the chain being driven by the crank. OK, I see what you mean.

What you say is true. However the reason that we have to momentarily soft pedal during a shift…and only really during a downshift…is because the spring doesn’t have enough strength to over come the friction of the system under load. Ramps, pins, and shaped teeth have decreased that problem to a certain extent but it still exists.

On the chainrings, the ramps and pins are only on the big one to help the upshifts. Or the middle if you have a triple. On downshifts, the chain floats free to the smaller ring.