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I broke the rear derailleur on a borrowed bike. I'm trying to understand why.

Until two months ago I knew nothing about bicycles. I bought a worn down cheap second hand bike. It took me way more time and effort than I thought it would, but I finally managed to ride down park lanes and quiet streets, take wide turns, brake, shift gears, with no major incidents other than scraped knees.

This past weekend I borrowed another bike and rode almost entirely on road, just a little going off road and no off road slopes. Then tried a few slopes on the road. All was great. This bike is newer and feels much nicer than mine. It's similar to this but an older model:

https://www.decathlon.ro/p/bicicleta-mtb-st-530-27-5/_/R-p-311274?mc=8582845&c=NEGRU

I got the confidence to roll down a long rather steep slope, on road, with a few turns and hairpins. It seemed like I went really fast, felt like well over 50 kmh, though I'm not experienced enough to tell. I kept engaging the rear brake gently and managed to keep control all the way downhill, then stopped smoothly.

Then I started climbing back up. I changed gears, then only went a few steps from the base of the slope when bang! The rear derailleur (as I now found out it's called) snapped in two. It's not the hanger or the chain; these look fine to me. It's not bent. It's broken like a twig. As if it was made of plastic. It can't be plastic though, can it?

This is the type of derailleur that broke:

https://www.decathlon.ro/p/schimbator-spate-shimano-altus-9-viteze/_/R-p-199939

Some more details, as I remember them:

  • The bike was bought new, but the person I borrowed it from says he "swapped a few parts". Haven't asked what and I don't want to bring it up again.

  • For the past two years or maybe more the bike sat unused in a shed. To my eye it doesn't show any visible signs of wear or decay, other than the set of sprockets at the rear is partly rusty. They still turn fine I think.

  • I dusted it off, inflated the tyres and sprayed the chain and sprockets and the front and rear with a spray labeled "anti rust", in the naive belief that it would remove the existing rust somehow.

  • I read somewhere that going "big-big" is a no-no. I wasnt't. I was using the small chainring (of three) and the second largest sprocket at the rear. So I was going "small-second big", if you will.

  • I set the saddle a bit low for my taste, but as I'd been using it interchangeably with my kids, I thought I'd just leave it at a height somewhere between what they needed and what I needed. So a bit low for me. This made me tire more easily.

  • The bike has a thumb lever gear switch. The gears switched fairly smoothly one step at a time, but a few times pressing the lever had no effect, then pressing it a second time made the chain jump up or down two steps at a time.

  • Before I was confortable switching gears, I would get off the bike, press the lever, lift the rear wheel by the saddle with one hand and used the other to turn the pedals, thus switching gears. Then I got comfortable doing it while cycling.

  • The stretch of road where it happened is used by lots of cyclist, including kids and older men and women riding ancient rusty bikes, sometimes carrying bags on the handlebar. So it wasn't like some extreme piece of road.

  • The road had fresh tarmac, felt very smooth.

  • I weigh 92 kg, that's about 200 lbs.

Questions:

  • Was I in the wrong gear? Small front sprocket out of three, second largest rear sprocket out of nine. So second speed overall.

  • Did I overuse the rear brake going downhill? Could this have anything to do with it?

  • Am I too heavy for this bike (200 lbs)?

  • Did I ruin the derailleur by changing gears "manually" a few times? (see above)

  • Is it a low quality derailleur? I thought Shimano is a good brand, correct?

  • Was it the wrong (too low) saddle height?

  • Was is the rusty sprockets at the back?

  • Was it the "anti rust" spray?

I'm really trying to understand what I did wrong because I don't want this to happen again. My bike is a lot more flimsy and worn out than this one, and now I'm afraid to even get on it at all. This one looked good and sturdy and it turns out it's so fragile.

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  • 3
    "Is it a low quality deraiileur? I thought Shimano is a good brand, correct?" Shimano has the whole spectrum from very cheap to very expensive for professionals. Some of it is very old-fashioned technology (e.g. 6-speed freewheels and associated Tourney derailleurs). Altus is still among the relatively cheap ones although much more modern. Sep 11, 2023 at 11:02
  • 1
    Seeing the photo of the broken derailleur could help to guess what might have happened. It is hard to say just from the description. It should not happen, but bad things happen, for example, when the derailleur hits the wheel spokes. Sep 11, 2023 at 11:08
  • We need pictures of your actual broken derailleur to identify what went wrong, such as whether you just got really unlucky
    – Chris H
    Sep 11, 2023 at 13:31
  • There's a lot of questions here - might be better to join Bicycles Chat and work it through a bit at a time.
    – Criggie
    Sep 11, 2023 at 18:55
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    That's a really surprising break. That bit of the derailleur isn't particularly stressed by anything you can do while riding, and even catching something in the chain affects the cage rather than that part IME. I've got Altus parts with thousands of km on them, and have always assumed that bit to be aluminium. I'm sure it's not steel, but a tough plastic could actually work there quite well. To me that looks like impact damage, though a too-short chain could put it under stress
    – Chris H
    Sep 12, 2023 at 6:03

9 Answers 9

17

You didn't break the bike. The problem is poor configuration or maintenance.

Big-big can break the derailleur if the chain is too short. That's not your fault. You said you didn't do that so we'll move on.

A loose or bent hanger can allow the derailleur to shift into the spokes if you don't have a spoke protector. If this happens you will have broken or damaged spokes.

A poorly maintained derailleur can fail in multiple ways.

None of these issues are your fault, but if I were in your shoes I would pay for a replacement derailleur. If you think it's expensive, look at the cost of a Dura-ace and consider yourself lucky you're only replacing Altus :-) I ride Campagnolo and just spent over $200 for a new rear derailleur.

BTW - congratulations on getting out there and riding. It's a wonderful hobby.

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The rear derailleur generally doesn't break there - often it is strong enough to act as a lever to bend the bike's frame itself.

I suspect that the root cause of your problem is:

For the past two years or maybe more the bike sat unused in a shed....

And "something" happened to the bike to damage it. Perhaps a gentle shove from a car, or something else hit it and damaged the derailleur such that it was already cracked and weak, and that your riding in the normal manner was enough to finish it off.

Good news is that this part isn't super-expensive, and refitting it might only need some metric allen keys/hex wrenches. It may even be possible to refit a replacement without splitting the chain, just by dropping out the lower jockey wheel temporarily, and sliding it in.


I also notice the COIL of spare inner cable - no bike shop would do that, which leads me to believe someone has already worked on this area of the bike. You're not the root cause of this broken part.

The jockey wheels are suspiciously clean as are all the other nooks and crannies. I'd bet that the rear mech was already replaced by a handy but poorly-equipped person.

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  • The rolled/uncut derailleur cable is another indication that someone has already done some work on this part of the bike.
    – Rеnаud
    Sep 12, 2023 at 8:16
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    I have actually replaced a very similar RD without splitting the chain, by dismantling the cage. It was a new chain, of the type held together with a special rivet, and I had no spares or suitable quicklinks. But the chain length should be checked to ensure that in the big-big combination the derailleur isn't fully stretched out. Incautious amateur repairs (which seem likely) could have left it with a too-short chain
    – Chris H
    Sep 12, 2023 at 8:33
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Hi and welcome to the wonderful world of cycling where things break and you often down know why and in this very moment. You will receive more extensive answers but my gut feeling says, no, by your honest description, I don't think you have mis-used a bike in a way that the rear derailleur should disintegrate like that.

Was I in the wrong gear? Small front sprocket out of three, second largest rear speocket out of nine. So second speed overall.

No - that's exactly the gear us mortal cyclists use on ascents. In theory, you should be able to use any gear combination without short-term damage, there is the concept of cross-chaining ("weird" gear combinations as large chainring at the front, largest sprocket at the back - or the other way around), it just increasingly wears the chain because it is running at quite an angle on the teeth. Gear ratios usually overlap quite a bit, so you can just shift down at the front and up a few sprockets at the back to have the same ratio in a more drivetrain-friendly way.

Only if the chain slips off and gets caught, this "jamming" might damage or even rip off the the rear derailleur but it sounds as if you already made the shift and it just collapsed while pedaling. Did it hit the spokes, were there any marks/damage on the wheel after the incident?

Did I overuse the rear brake going downbhill? Could this have anything to do with it?

No, I can't see a reason how this is related.

Am I too heavy for this bike? 200 lbs

I don't think so, most road bikes I've seen limit rider weight at 100+ kgs, I don't think this is any different on your brother-in-laws bike.

Did I ruin the deraiileur by changing gears "manually" a few times? (see above)

No.

Is it a low quality deraiileur? I thought Shimano is a good brand, correct?

No - even the cheapest Shimano derailleurs should be able to take a beating before falling apart.

Was it the wrong (too low) saddle height?

No. I don't think so, the only thing you could ruin with that is your knees.

Was is the rusty sprockets at the back?

It's not ideal but this would just create some unwanted noise, slow you down a bit (drivetrain efficiency) and wear out components that are not yet rusty and ready for the bin because of increased friction.

Was it the "anti rust" spray?

If it wasn't a bike-specific product, it might have been a bit something more aggressive but I doubt it would be as aggressive that it makes a sturdy derailleur snap.


While you (as a "beginner") may have still a learning curve in terms of cycling, maintenance and riding technique (use both brakes!), I can't seen anything that has definitely cause the RD to snap.

We don't know the previous state of the bike, it sitting in storage for some time in less-than-perfect conditions means it could already have some damage/knock or mis-adjustment that existed before you even got on the bike and just pushed it over the cliff...

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  • 1
    Do you mean hardest in the front and easiest in the back for your cross-chaining example? (Big in front and small in back is not cross-chaining)
    – RLH
    Sep 11, 2023 at 14:03
  • @RLH Yeah, definitely. The terminology mixup of small/large cogs/chainrings vs. small and big gears throws you off after so many years. Thanks for spotting!
    – DoNuT
    Sep 11, 2023 at 14:07
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I can't answer every single question, but here are some critical things.

  • As Vladimir said in comments, Shimano makes a full range of cheap to top tier components. Altus is on the cheap side. I'm not familiar with Shimano's lower tier components, but I believe Altus is better than what you'd find on a Walmart bike or similar.
  • Altus won't regularly break for no reason under everyday riding. You may have got a stick sucked into the rear derailleur. Or the derailleur may have been defective - manufacturing defects are rare but they can affect components at any point in the range. Anyway, derailleurs are made of aluminum. We weren't there, so it's impossible to diagnose why the derailleur failed.
  • My read of the situation is that this was unfortunate, but you can't assign fault.
  • None of the following things could have snapped the derailleur by themselves or in combination with anything else: anti-rust spray, rusty cogs, saddle height, braking, being in the wrong gear (including big-big), being on the heavier side, nor simply sitting in storage.
  • Riding in big-big is not ideal. The bike will almost certainly not explode because you're in big-big1. You will get a lot of drivetrain noise, especially with a triple front ring. So, when you hear that noise, you'd shift to a lower gear in front.

I think that this would be less likely to happen on a better bike, but this can still happen on occasion. Generally, if something failed, the hanger would snap, and you'd call a ride home, and you buy a new hanger. They're not expensive. Many people could install a hanger themselves.

We could make cheaper bikes and components that are much more resistant to failures. However, there are always tradeoffs. One of the main ones is that while we can exercise for very long periods, which is apparently how we hunted back in the day (persistence hunting), we don't have a high power to weight ratio. Many trained humans can probably produce a sustainable power of about 1/3 horsepower (about 250W). More durable components would be heavier, and riding would be less enjoyable. Many internal geared hub bikes could be more suitable for the sort of riding you're doing (beginner, bike is in storage most of the time, you don't want the drivetrain getting damaged), but they're less efficient to pedal - and a trained cyclist has more reserve power than you do.

Footnote 1: To be fair, last year I shifted to big-big and my rear derailleur got sucked into my wheel. This locked it up, bent the hanger (mine is steel), and destroyed the RD. Post-incident, the RD was loosely threaded onto the hanger. It must have somehow got sucked into the rear wheel, perhaps with the loose mounting contributing. Again, this is a rare occurrence. I do typically shift out of big-big, but sometimes you're pedaling up a hill and you don't think to shift in the moment.

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  • 1
    I assume (@Weiwen) that your chain is long enough to run big-big?
    – Chris H
    Sep 11, 2023 at 13:32
  • 3
    Hint: the part where the derailleur is mounted is different from the building where aircraft are stored.
    – mkrieger1
    Sep 11, 2023 at 20:43
  • 1
    @mkrieger1 what, I store all of my aircraft in a hanging position‽ Sep 11, 2023 at 21:22
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This image has quite a few indicators of a fatigue failure where a crack slowly propagated then failed suddenly:

enter image description here

The bottom area where the derailleur broke off looks different from the narrow top edge of the break - the top edge area appears much rougher and even has a very distinct demarcation where the texture changes.

That's likely the area of the derailleur that failed last.

Also note the lighter-colored area on at the bottom, farthest away from the axle. That could very well be where the crack started and the lighter color is from longer exposure of that region of the crack to outside elements. There also appear to be striations typical of fatigue failure near the border where that lighter-color area merges into the larger darker failure area.

It's also possible the darkest area nearest the axle is where the crack originated, with the darkest colors indicating the longest exposure to outside elements. It's impossible to be sure without knowing the exact composition of the metal and perhaps even the length of time the crack existed and what it was actually exposed to.

The differences in color and texture, and the presence of parallel striations are all classic symptoms of the slow propagation of an existing crack that failed suddenly.

See this Wikipedia image:

enter image description here

Note here the aluminum crank arm is darkest where it was exposed to outside elements the longest.

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My derailleur also once cracked over the massive metal section as if made from glass. It did this when riding slightly down, low load, on a small sprocket, not even shifting gear at that time.

As the cracked surface was black and not shiny, I suspect there are sometimes just defects in the metal.

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If you change gears under load it can put a lot of stress on parts. Especially if you change several gears at once (either because the first lever press had no effect (because of bad adjustment or maintenance) or if you have levers where you can press hard to switch 2 or even 3 gears at a time).

The rusty cassette and the fact that whoever took care of the bike didn’t cut the shifter cable (instead just coiled it up) kind of makes me suspect sub-par maintenance and adjustment.

All of that shouldn’t be enough to break a rear derailleur, but combined with a 92kg beginner putting full power into the pedals it might just do it. Experienced riders instinctively ease up on the pedals for a tiny moment when they shift. When you shift to a bigger sprocket and it fails to engage you can press the lever a tiny bit further (not far for a second *click* and gear step, just a tiny bit to move the derailleur).

In any case, the good news is that compatible rear derailleurs (any Shimano 9 speed, long cage derailleur should do) are cheap (~20€), readily available and easy to install. You don’t even have to open the chain to thread it through, instead you can take out the two small sprockets (pulley wheels) of the derailleur. The hardest part will be adjusting the limits and cable tension but there is a nice Park Tool video tutorial for that. This assumes that the derailleur hanger, cassette and chain are not bent or otherwise damaged.

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  • 3
    While it's true that shifting under load is bad for parts (in particular, it wears out the cassette and risks snapping the chain), it's not really putting more stress on the derailleur. The derailleur, sitting under the cassette, doesn't actually experience the high chain tension at all. The only thing that stresses the derailleur is shifting too many gears before the chain has time to move to the corresponding part of the derailleur. Sep 12, 2023 at 7:21
  • 3
    @leftaroundabout and "too many" is a rather large number. After all, Altus shifters are designed to shift down multiple sprockets by pushing the lever further - up to 3 in one go in the model I have here
    – Chris H
    Sep 12, 2023 at 8:37
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This is a bit speculative, but it's the best answer I can give from the pictures.

The fracture looks like that part of the derailleur is cast iron, a metal which is kind of infamous for its tendency to crack. A bit surprising; that material is not normally found on bikes. But it could be that Shimano chose it in this model because cast iron is cheap and stiff, and therefore a reasonable option to keep a budget derailleur stable in alignment. The brittleness may have been tolerated because a) you wouldn't expect a bike with such a derailleur to be ridden hard offroad, b) the derailleur hanger should relieve stresses in a crash and c) the high mass isn't critical in this price segment.
Again, this is speculative – if anybody has reliable information on the material, by all means comment.

If it is indeed cast iron, then it could easily be that it got damaged earlier by something like a rock whirled up by the wheels, which would have caused a tiny, at first unnoticed crack. That crack would then steadily grow, and you seem to have been unlucky to ride the bike at the very moment when it finally failed entirely.

Whether that's really what happened or not: as the other answers said, you didn't do anything wrong. Just forget about it, it will almost certainly not happen again.

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  • 2
    It's easy to test whether it's cast iron, with a magnet. But it probably isn't. It's probably a cheap but fairly light alloy that just looks dark for some reason, or a plastic light polyacetal. The outer surface looks too smooth for cast iron - it's not an impossible finish but hard enough to achieve that the cost benefit goes away. Also there's no sign of rust.
    – Chris H
    Sep 12, 2023 at 8:35
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    There's aluminium alloys that break in a similar way as cast iron. For example, aluminium parts of office furniture.
    – Opifex
    Sep 12, 2023 at 12:06
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    @Opifex like many Shimano parts with complex shapes, those chair parts are cast aluminium alloy. The colour at the break is probably something to do with the grain size as the casting cools, as well as the composition of the alloy. Casting aluminium alloys have much more silicon in them than typical machining alloys. I wouldn't be surprised if that made them look darker at a fracture
    – Chris H
    Sep 12, 2023 at 14:52
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It is more likely your chain ring was worn and it grabbed the chain up from the bottom, hence it pulled the bottom loop of the chain back into the from chain ring, then the chain will yank the derailleur forward and bang something will brake and it was the derailleur. Change the rear derailleur and check whether the chain rings have a shark fin like shape.

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  • The chainring would have to be a lot more worn than the sprockets we can see. As it's been well-used and had things done to it, that's not impossible, but it doesn't seem all that likely
    – Chris H
    Sep 14, 2023 at 12:55

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