I bought my son a cheap mountain bike last week, it did 2 miles before the chain mashed up the drailleur. is it worth replacing with a better quality component. im a novice , but handy enough to upgrade. he loves the look of the bike but my question is , is it worth the effort..

  • A replacement derailleur is actually non-trivial to install. First you need to make sure the hanger is not bent and is properly aligned. You have to make sure your replacement derailleur is the proper type. Then installing it can be tricky if you've never done it. You'll need a chain tool to install the new chain. Setting the proper shift cable tension and upper and lower limits on the derailleur can be tough if you've never done it. etc. It's not an easy job.
    – RoboKaren
    Feb 8, 2015 at 4:19
  • Do not underestimate the learning benefit of doing it. Once you have installed a dérailleur once, you will be able to tune the gears up and do other basic maintenance yourself, without spending big dollars at a shop for what should be a 5 minute job. You may need to buy some tools, but a basic bike maintenance tool box is quite cheap. Workshop quality is best - but for home handyman working on low to mid range bikes cheap tools get the job done provide care is taken.
    – mattnz
    Feb 9, 2015 at 0:16
  • @RoboKaren: you don't need to break the chain and re-connect it. You can instead remove the bolts through the jockey wheels to disassemble the derailleur cage enough to get it around the chain and then reassemble by putting the bolts back in. I replaced a derailleur this way as a child with little difficulty. Feb 9, 2015 at 2:47
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    The old chain was eaten up when the derailler was mangled. You'll need a new chain. If you've figured out how to put a new chain on a standard bike frame without breaking it, I'll give you a medal.
    – RoboKaren
    Feb 9, 2015 at 3:04
  • @RoboKaren Question does not state the chain was eaten up.
    – paparazzo
    Feb 9, 2015 at 16:44

2 Answers 2


I basically agree with @RoboKaren's answer, but wanted to add a couple of things that would take up too much space for a comment.

  • For starters, you say that the chain mashed up the derailleur. So the derailleur was actually the "victim" of the problem, not necessarily the cause. So while you might need to replace the derailleur, that is likely not the only thing that needs attention

  • To put things into perspective, you could (arguably) split a bike into four parts - frame, brakes, gears and wheels. You're talking about upgrading a single part (your derailleur) of a single part (your gearing). You're simply not going to turn a bad bike into a good bike with that degree of change.

  • you don't say how much you spent, but in general, for a limited budget, it makes more sense to get hold of a decent secondhand bike, than to buy something new. Of course, the trick here is being able to spot something "decent", although there are plenty of past questions on this site which may help, and will tell you what the red flags are. I don't know where you'd stand as regards a refund - 2 miles is a pretty strong argument that the bike was badly put together, at least - but if that opportunity arises.....it is worth looking into. You can always ask for advice here if you don't see your query covered already

  • Cheers for that advice. If the hanger is bent I would be wasting my time. Is there any way of knowing by eye. The range of derailleurs on the market is massive, the bog standard tourney on it retails at a tenner. Your advice about symptoms rather that the course has halted my keenness to run out spending more. I've already spent more than if it would have been delivered in a box...
    – george
    Feb 9, 2015 at 10:02
  • @george no, unfortunately you might be stuck if money is the issue. You can get a gauge to measure whether the hanger is bent (and to fix it), but that tool is a few tens of £/$ iirc. Your best bet might be to go into a shop and ask nicely if they could check it.
    – PeteH
    Feb 10, 2015 at 12:00

If your bicycle is a BSO (bicycle shaped object sold at discount mass retailers), then likely no. You'd spend $100+ labor on the derailleur and a new chain, but then the next week the brakes would fail, the handlebar would come off, or the frame would crack. BSOs are money pits. Furthermore, it's unlikely you could just replace the derailleur, you'd have to also buy new shift levers or brifters, depending on what you're replacing it with. If the derailleur ate the chain, then it's likely that the derailleur hanger will also be bent or misaligned. I'd use the return policy of the store to return the bike. It was clearly misassembled.

If your son's bike was a quality used bike (like a late 1980s hardtail Specialized Rockhoppper) then it's likely that the "bones" are good and that upgrading some components would be worth your while. A better derailleur would lead to faster and smoother shifts. But you'd still run into the problem that by the time you get new derailleurs and shifters, you might have spent enough to buy a used 2000s Trek mountainbike. There's plenty of life in the older hardtails and many people prefer them to the newfangled things.

Update: A store return isn't possible as it was bought off of the street and that the owner has £85 in sunk costs for a bike that would be about £130 new -- i.e., a little bit above a £50 BSO but not yet into the middle end. With £85 in sunk costs, I think you want to keep your repairs to under £25 given that you already spent a bit on a new tire and tube.

The likely thing that happened is that the old derailleur's high or low-limits were not set correctly. Thus when your son unknowingly shifted up or down (likely up), the chain skipped merrily off the rear sprockets into either axle (from the highest -- i.e., smallest-- gear) or the spokes (from the lowest). This then jammed the rear wheel, yanked the derailleur into bits, and it's lucky your son didn't have an accident.

First, you need to assess the damage. The rear derailleur is mostly probably bent/broken beyond repair (while tempting you cannot unbend it). Check the chain carefully, it is likely to have some bent or damaged links. Check the rear sprocket, are any of the cogs damaged? Check the spokes if the chain damaged them. Check the derailleur hanger, is it still straight? Check the front chain ring and derailleur, do they look good?

If you're lucky, the damage is just limited to the rear derailleur and to the chain. If the chain appears to have any damage at all, it's safer to get a new one (a bend in the chain or kink will cause chain suckage again).

The problem is finding a new derailleur. You want to keep the old shifters if possible to save money. There are two factors: one is the number of gears (7/8/9) on the rear and the second is the shifting mechanism. If you want to DIY, then you could purchase a new rear derailleur (assuming you have a 7-8 cassette on the rear, they go for around £10-20 for the cheapest models, but again make sure it's compatible with your shifters). Then look online for detailed instructions and be sure to set the hi/lo limits on your derailleur correctly -- and make sure your derailleur hanger is straight.

If you need a new chain, then you'll need to get a chain breaker tool as well, so budget that in. Other tools you may need will include a set of hex keys (allen wrenches), screwdrivers, adjustable wrench, bandaids (for skinned knuckles), pliers (to crimp the ferrule of the derailleur cable, don't forget to get some ferrule caps), and a BIG adjustable wrench if your derailleur hanger is bent.

Again, it's not trivial to install and adjust a rear derailleur correctly and not really a project for a brand new bike mechanic, especially when the test subject is your son.


Find a local bike store that appears to have been around since the time of the Wright Brothers. The greasier the better. Go to it with your tail between your legs and ask if they might have an old, used derailleur that will fit your bike. Do not wear nice clothes when you go. Bring your son and have him make puppy dog eyes.

If your store is anything like the place that I have downtown, then they can dig out an old, compatible derailleur from their parts bin for £3 (US$5) and might even put it on for you for a few quid more. Be sure to patronize them in the future.

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    I made schoolboy errors in buying this. I got it from eBay for 85 quid . The fella delivered it and in hindsight it was a bit shifty. I tried it around the block and it seemed good. I paid 85 and it turned out to retail at 130. ( another oversight) all the comment have been great. The chain didn't snap but I'm not sure about the hanger. I've already replaced the back tyre and both inner tubes as he dragged it 2 miles backwoth a jammed back wheel.
    – george
    Feb 9, 2015 at 9:51

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