Technical FAQ: Rear derailleur explosion
Lennard Zinn addresses a question about how and why a Campy rear derailleur exploded during a bike ride.
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Editor’s note: This article has been updated since it was originally published.
I was out cycling last week with a friend doing our usual “coffee ride.” We had gone around 50km and stopped at a red traffic light. Started off and there was a loud bang. The rear derailleur had sheared off the frame and gone into the back wheel.
It was tipping down with rain (sod’s law). Peter, who I was out cycling with, offered to cycle home and get his car; it would take him around 20 minutes. I said no, as I could walk home in that time, which I did with the rear derailleur/chain getting dragged along the road while I did. Took the bike to my very friendly bike shop the next day where I had it built. [They said], “Never seen anything like this; we will send it back to the [Campagnolo] importer, as it’s still under warranty.”
So they did.
Can a Jockey Wheel fitted the wrong way can do this amount of damage? Or is it an escape route? Talked to them on the phone — they did not want to know. So much for Super Record EPS. I had the bike built 2.5 years ago, and checking my Strava rides it has not done more than 7,000km.
I never knew what Sod’s Law was before! Thanks for that!
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I changed my answer this week after looking more carefully at the photos and seeing that my assessment of incorrect derailleur cage assembly was wrong (a big thank-you to those of you who pointed it out). I had originally thought that the jockey wheel cage was upside down from your comment about a jockey wheel fitted the wrong way combined with low-resolution photos. (The resolution on the letter from Campagnolo was too low for me to read.) An upside-down jockey wheel cage can destroy a rear derailleur by hooking on a spoke. On most derailleurs, the upper and lower jockey wheel bolts are the same size, so it is easy enough to invert the inner cage plate when you overhaul or replace jockey wheels.
While the cage may have been on right, I do think that chain was improperly routed over it, however. The photos I received are low-res, not any higher than you’re getting on this web page, so blowing them up gets pretty grainy. But I think Mike Varley of Black Mountain Cycles in Point Reyes Station is correct when he pointed out that, “Regarding that exploded derailleur, what I’m seeing is a groove worn into the cage tab at the upper pulley that might indicate the chain was wrapped over the tab instead of under it — see photos E3 and E5. Not sure how that could have contributed to the derailleur going into the spokes, but under extreme tension, it might have twisted the cage just enough. The vast majority of exploded derailleurs I’ve seen have been due to incorrectly set limit screws — or perfect-storm situations that you explained based on failures of your derailleurs.”
I think that the chain was indeed wrapped over that tab on the outer cage plate, and the chain riding there may have over time bent the upper part of the cage outward (toward the spokes), which could explain how I had last week misinterpreted photo E3 to be showing that the cage was upside-down. It could also just be the angle from which the photo was taken … It does make sense, though, that under high pedaling load, that the chain riding there could pry the cage plate over toward the spokes.
The derailleur clearly went into the spokes. This was almost certainly due to pedaling hard from a dead stop in your lowest gear. This is an extreme high-torque situation on the rear wheel that simultaneously produces high lateral flex, creating the perfect storm to wreck a derailleur that would have been exacerbated by misrouting the chain over the tab on the jockey wheel cage.
Your high pedaling force in low gear against a stopped wheel winds up the rear hub in a forward direction (clockwise when seen from the drive side), causing half of the spokes to be de-tensioned and to bow outward. At the same time, standing with all of your weight on your right pedal while at a dead stop will flex the wheel so that bottom of the rim moves laterally outboard relative to the frame. This brought those loose spokes so far outboard that one of them snagged your derailleur. Then your continued pedaling with your derailleur tangled with the spokes sheared it into pieces. Something had to give, in this case the derailleur, and, perhaps, the derailleur hanger and a spoke or two as well.
While on the subject of the derailleur hanger, it is worth mentioning that it could have played a role. On most modern road bikes, the derailleur hanger is a sacrificial piece meant to bend or break off in the case of an impact to the derailleur. This is supposed to save the frame by protecting the dropout itself from bending or breakage. It also may save the derailleur. In the case of the derailleur going into the spokes, the rapid failure of the derailleur hanger can also reduce the damage to the derailleur, wheel, and frame, but it still probably cannot break quickly enough to avoid ruining the derailleur in that instance.
However, the very weakness of the derailleur hanger can increase the likelihood of the derailleur going into the spokes. All it takes is a relatively mild impact to the rear derailleur that the owner might not even consider could have damaged anything, like the bike falling over in the garage, to bend the derailleur hanger a bit toward the spokes. It doesn’t take much of a bend for it to move the derailleur dangerously close to going into the wheel.
Once it went into the spokes, it would not have mattered what brand or model of derailleur it was; it was going to break. I know of no derailleur warranty from any manufacturer that will cover that. The fact that the derailleur that went into your spokes is made entirely out of carbon fiber meant that it got torn apart more dramatically than one made out of aluminum, but this process would still have rendered an aluminum derailleur unusable. And, because the carbon-fiber one shattered so rapidly, the damage to your wheel is likely to have been less than with an aluminum derailleur. It is a shame to lose one of the world’s most expensive derailleurs this way, of course …
I’m not sure riders are generally aware of how close the rear derailleur is to the spokes in low gear. Furthermore, the smaller the largest cog is, the closer the derailleur is to the spokes. The clearance has become tighter yet with some 11-speed freehubs; there are a lot of gears squeezed into a narrow space, which competes with the desire of a wheel manufacturer to maintain as much hub-flange width as possible to keep the stance angle of the drive-side spokes as wide as possible.
Due to this tight clearance, there are a lot of ways to put a rear derailleur into the spokes and thereby destroy it. All it takes is shifting to the largest cog when the rear derailleur’s inner limit screw is not tightened in enough to prevent it from going into the spokes. Another way is to flex the rear wheel hard by starting up out of the saddle in low gear on a steep hill, or shifting into low gear in that situation.
The wheel also plays a role. The more that some spokes bow out when the wheel is flexed increases the likelihood of the occurrence. Most rear wheels have crossing spokes on the drive side, and when the hub is twisted forward by chain force, the “pulling spokes” become tighter, while the “static spokes” become looser. These static spokes will tend to bow outward more the more they are de-tensioned, and the greater the driving torque on the hub, the greater this tension change. This is one argument (in addition to more even stance angle left and right) for radially lacing drive-side spokes and putting the crossed spokes on the non-drive side. It’s another reason for lacing the drive-side pulling spokes out of the inner side of the hub flange, so that when they pull, they pull the static spokes inward (because they cross over them).
I have exploded a rear derailleur by putting it into my spokes four times that I can remember.
One time, 36 years ago, I jumped a train track on my bike at maybe 20mph. At that time, I had not trained myself to jump my bike with my right foot forward. The heel of my (big) right foot kicked my rear derailleur into the spokes and stopped the wheel in the air. When I came down, I heard an explosion probably louder than the sound I imagine you heard. Pieces of derailleur shot all over the road, and my rear wheel was now D-shaped, as the derailleur being jammed in there when it landed with all of my momentum driving it forward drastically bent those spokes, which pulled one side of the rim inward.
Another time, I was riding on a cold, snowy day, and my cassette had become packed with ice so I could only use the one cog the chain was on. I came around a sharp, blind turn into a steep uphill. Forgetting that my cassette was iced up, I tried to shift into my largest cog. The chain just skittered across the cogs, looped into the spokes, and pulled my rear derailleur in with it. It was a carbon derailleur, so it snapped in two without damaging my wheel or derailleur hanger.
The other two times, I started from low speed on a steep hill in low gear. One was in cyclocross with a Mad Fiber tubular rear wheel on the bike and one was on the road with a Mad Fiber clincher rear wheel. The big, broad, flat, carbon Mad Fiber spokes would bow way outward when the wheel was flexed laterally. Accelerating from low speed by standing on the pedals and yanking on the bars flexed the rim far to the side and bowed those flexy spokes way outward. Both times, one of those broad, sharp-edged carbon spokes (that are super strong in tension but weak in compression) bowed out, hooked onto the rear derailleur, and tore it apart. One of those times was with a carbon derailleur, snapping it in two without damaging my wheel or derailleur hanger.
Your bike shop should have said that a derailleur going into the spokes is not really a derailleur-warranty issue. They shouldn’t have left it up to Campagnolo to give you the bad news.