alo

I have a 26 inch fat bike which I consider to be decent quality, but not excessively expensive. It had a Shimano rear derailleur. I have been riding the bike for over two years. I was told the bike was made in Taiwan, so I expect the derailleur was made in Taiwan.

Recently I was out riding, and my pedals locked up. I got off to have a look. The derailleur was in a strange position. I rotated the chain with the pedals, and it went back to normal, so I continued riding.

A little while later, the rear wheel locked up, and the pedals locked up. I got off to have a look. The derailleur had bent, going between the spokes, been taken around with the wheel, and the lower half was ripped off. When it got to where the chain goes away from the wheel, everything just locked up.

I started to analyze what had caused this. Here is what I believe.

The derailleur has two plastic wheels which the chain goes around. In time the center of these wear. They can then wobble. You finish up with the chain dragging against the inside of the derailleur.

I believe the chain caught on the inside of the derailleur. This caused the derailleur to bend, which resulted in it being caught more solidly. It kept bending, getting caught between the spokes, which ripped it off as the wheel went around.

I consider this to be a design flaw. I believe, where the chain enters the derailleur should have a curved surface, so a chain can never catch on it.

Old derailleurs were often cast, and had rounded edges, so a chain could never catch on them. This is a modern one, made out of pressed sheet steel.

I finished up going to a mechanic to have the chain shortened, so I could ride the bike home as a single speed.

Sometime soon, I plan to fit a new chain and derailleur.

I haven't had a thorough inspection, but I don't think any spokes are broken.

shelbyfv

I'd be surprised if your analysis is correct. Your mileage over two years is unlikely to have worn out any "decent quality" component. However, if I'm incorrect, replace your derailleur with Deore or above. It seems that is the level where long term durability over significant mileage and truly decent quality begin.

dedhed

The upper pulley on all Shimano indexed derailleurs is designed to move side to side within the cage to assist in shifting. Most Shimano derailleurs are manufactured in China, Malaysia, or Singapore.

If your RD is going into the spokes you either bent your hanger and/or high limit stop was incorrectly set. When you replace that make sure the hanger gets aligned.

CliffordK

And, that may well be a big part of the problem.

The low end derailleurs aren't "decent quality".

All the better quality derailleurs have either steel or ceramic bushings, or sometimes roller bearings on the jockey wheels.

The low quality derailleurs inevitably simply have plastic jockey wheels over a steel pin.

The low quality derailleurs are both flexible making them difficult to properly adjust, as well as made with inferior parts.

Plus, the obvious upgrade path for a 9-speed or less derailleur would be a Deore RD-M772, but in Shimano's marketing wisdom, they have dropped that upgrade path. So, one is stuck with low quality parts to support the 9-speed or less drive trains.

Most low quality bikes will have some kind of spoke protector:



However, those are designed to protect against chain over-shifts, not derailleurs in the spokes.

Crankycrank

I agree with the other posters that this is a classic sign of either the derailleur hanger got bent at some point, could be from leaning it against a wall, a tipover, a heavy handed wheel removal/replacement, etc. or the derailleur went out of adjustment causing it to come in contact with your spokes. Could be either or both of these problems. Low end derailleurs usually have some extra slop but this can be compensated for somewhat with proper adjustment. Also price for sure can determine quality but being made in Taiwan doesn't mean poor quality. Some of the best bike products come from there and Shimano has been farming out production for some of their high end pieces to other Asian countries for a few years now.

HillRider

I agree that a bent hanger could be the problem or that the low limit stop could be incorrectly set.

cxwrench

No, 99% it was a bent derailleur hanger. It's pretty rare for Shimano to experience a 'design flaw' of that nature.

3alarmer 

.
...an "inexpensive fatbike" is not exactly the best example of bicycle production to use as your data set for analysis. Most of the componentry will be marginal, but still serviceable for everyday use if you are careful with it. The "pressed steel" derailleur plates are another example of production at a certain bargain price point, and the suggestion to shop higher on the food chain for your replacement is a good one.

But I agree that you've probably bent the derailleur hanger somewhere along the way, which is not hard to do with the long cage models used on most fatbikes, expensive or cheap.

If you can make it work as a single speed for your riding, you'll be happier with it. Less to worry about with moving parts that are exposed and subject to misadjustment. But if not, you need someone knowledgeable to advise you on what's involved in your repair, how bent the hanger is now, whether it's replaceable, and really, whether the repair is worth doing iff you need to pay a mechanic to do it and buy the parts at retail prices. Everyone here mostly does his own work, with used parts, so we tend to fix stuff that might not make sense to fix in a retail environment.

dedhed

I think I was high when I wrote that LOL

70sSanO

I doubt it was the limit screws. With an index system that shifts fine for 2 years, the limit screws don’t mysteriously move, especially to the extend need to drive a derailleur, against spring tension, with the shift cable into a wheel. It just can’t get there.

My guess... really crappy shifting under load over time caused the chain side plate(s), one or more, to flair out and that is what caught the derailleur and dragged it into the spokes.

A good chain can still work in the worst conditions, a flared chain will destroy the best drivetrain money can buy.

John

alo

I am 100% certain this was caused by the derailleur catching on the chain. The chain tried to take it up around the back or the wheel. But the chain is not long enough, so the derailleur bent, causing it to go into the wheel.

Before it happened, the derailleur worked fine.

Before it happened, the derailleur was nowhere near the spokes. It was definitely not caused by being out of adjustment.

It happened twice. The first time the pedals stopped when the derailleur went back, and nothing was broken. The second time it bent, going between the spokes.

Other things are debatable.

Before it happened, it did not look bent, but I can't say for sure it was not bent a little. It was definitely nowhere near the spokes.

I am not familiar with all of the Shimano derailleurs, but it may be the least expensive type.

alo

I had the chain on the large front sprocket, and the middle freewheel sprocket. The chain was a bit too loose, and would jump teeth on the freewheel when going up steep hills, or trying to accelerate quickly.

However, after a few hours of riding, it changed by itself down a gear, to the next largest sprocket. The chain was too tight, and there was too much resistance while peddling. I was concerned about bending the axle, but it didn't seem to. If I continued riding this, it would wear quite rapidly, and loosen up. But the gear is too low, and I finish up riding too slow.

I have been losing weight. The gear I am in now, would have been good last year, before I lost the weight. But my bike goes faster now.

So I plan to fit another derailleur and chain.

CliffordK

If you weren't riding in the lowest gear (largest one or two rear sprockets), and weren't shifting at the time, then it is unlikely the derailleur simply jumped into the spokes.

You are the only one that can judge how much slop there is with the Jockey wheels and their ultimate condition. I'd also expect to see some wear on the derailleur cage side plates.

A couple of photos might help.

I like to check my chain for stretch every 500 to 1000 miles. And, whenever I replace the chain, I also clean, inspect, and lube the Jockey wheels.

Trakhak

Obviously no design flaw is involved, since literally millions of bikes with the same derailleur have operated and continue to operate flawlessly around the world, but you know best.

Before you install a new derailleur and chain, have a shop mechanic correct the bent dropout/derailleur mounting tab or else the problem will recur.

Edit: when you buy your new derailleur, check the "slop," so-called, in the upper (jockey) wheel. You'll find that the movement of the jockey wheel of the brand-new derailleur, regardless of where it falls in Shimano's parts hierarchy, is identical to that of the jockey wheel of your original derailleur. Feature, not bug.

alo

Do you really believe none of them ever malfunction or fail?

I am amazed when people assume that others are too stupid to look into things like this themselves, and correct it if necessary. Or do you think I am like you?

I will give you a tip. Don't imply that people you don't know are stupid.

I have never had a derailleur fail before, and did not inspect every minute detail of it. In future I will be looking to see if it is perfectly straight, and whether the chain could possibly catch on the derailleur.

70sSanO

I’ve only pulled a derailleur into the spokes once in my life. It was because I was a complete idiot and just forced a shift under load. Got caught in a bad situation, couldn’t, or wouldn’t, back off and wouldn’t concede and stop. It was a nice XT RD and I just ripped it apart. It is a great learning experience.

Your theory of a bad, decades old, design is unfounded.

John

alo

I was not shifting at the time, but it obviously failed.

shelbyfv

This is uncalled for, especially since folks are trying to help with your problem. Hints as to our mechanical competence are apparent on BF, there may be no assumption involved.

3alarmer 

alo

But it is true. He said:

A smart approach would be to say, check the dropout/derailleur mounting tab is not bent, or else the problem might recur.

Anyone with minor mechanical ability could look at it themself, and see if it is straight or bent.

Maybe he needs to take his bike to a mechanic, but many others have mechanical skills to do this themselves.

It has never happened before, and not something I ever expected, so not something I investigated in detail. Now that it has happened, It is something I will be conscious of, and check.

I had someone recently tell me I should learn how to patch a tube. I suggest that most people on this forum know how to patch a tube. I believe it is really bad on their part to imply that others are stupid.

I am not saying this because of myself. I believe those that imply that others are stupid, will continue to upset people, and display their own stupidity. I am trying to help him.

I know that some come on this forum, and say things that indicate that they have little mechanical understanding. But most people on this have a reasonable understanding of mechanics.

I take my bike to places where many others would not go. I stress it much more than those who only go on gentle rides. In the future, things will continue to break, and I will continue to need to repair them. But it would be boring if I only ever went on gentle rides.

shelbyfv

CliffordK

Do we have the OP's derailleur model?

I'm not one to claim that Shimano Tourney (TZ30/TZ31) represents the pinnacle of Shimano engineering.

There is a reason that TDF riders tend to choose Shimano Dura Ace over Tourney.

Even competitive MTB riders will choose XTR over Tourney.

The OP's failure was odd, likely due to a number of things. For example, older style chains had much more prominent pins, while new chains (9,10,11,12s) have flatter pins. No bushings in the jockey wheels. And, potentially a lot of flex in whole system.

My guess is the typical kid's bike gets maybe 500 miles tops. And, perhaps gets passed down once or so before getting junked. In fact, there may be a level of planned obsolescence in them.

An adult that is heavily into exercise or commuting riding could put thousands of miles a year on the bike, and can easily exceed the expectations of the average Walmart Special within a matter of months.

Trakhak

I ran service departments in bike stores for over 15 years, training many mechanics along the way. My favorite memory of a service customer was of a highly experienced auto mechanic who brought in his bike for repair. When our mechanic asked him why he didn't simply fix the problem himself, he said, "I know enough about cars to know that I don't know enough about bikes."

3alarmer 

...just to reiterate, your original description of your bicycle indicates that it might be a bargain fatbike. The least expensive ones are not designed with robust adventure in mind, as far as durability of the components. To some extent, these bicycles are designed for different levels of use, all the way from just riding on the street, to full on downhill, go anywhere bikes. the fact that it has fat tyres and wheels does not, in and of itself, guarantee any sort of off road use durability or life expectancy.

So you might want to consider saving some money to upgrade. Some good quality fatbikes show up from time to time, used, on my local Craigslist. In the meantime, you will certainly learn a lot from continuing to ride the one you have, and fixing stuff when it breaks, if you plan on fixing it yourself. I had a '65 VW bug that was like that. I learned more about auto mechanics, driving that car, than any other one I've owned.

CliffordK

I'm not quite sure what I'd choose to upgrade with.

Ok, after a bit of hunting, a few models are available for Shimano 9s.

Alivio T4000 or M4000 are pretty cheap. I think the M4000 is the Shadow.

Shimano Deore M591 and M592. I think the M592 is the Shadow.

There are a few higher levels of Deore 9s including M662 and M772, but they are generally not available new, and NOS ones may be a fortune.

If you get the Deore M592 tuned in right, it should give you years of good service.