bicycle derailleur gear replacement

My son’s bike needs a replacement derailleur gear assembly. It came loose, and got caught in the spokes. I put it back in place, and it kind of works, but one of the gears broke off a few teeth. The brand is Shimano Tourney, but there is no indication of the model number. One of the derailleur gears says 15T on it.

Looking on Amazon, there are TX35, TX55, and TX75 models. I don’t know if I need the correct model number, or if those are all OK, and the numbers mean something else.

He doesn’t mountain bike with it, although there is a park with trails nearby. Nothing too severe, though. Tell me what to buy!

Here’s a related song on YouTube you can listen to while composing your excellent replies.

I’m assuming I can reuse the existing cables from the gear shift knobs. Just remove the old assembly, replace it with the new, and adjust a few screws.

This is easily resolved if you’re used to working on bicycles, however, if you’re not I recommend finding a good bike shop and start from there. Depending on the age of the bike and possibly other things, your sons bike will either have a free-wheel or a free-hub. The cluster of gears (one of which has some broken teeth) is known as a cog-set. If the bike has a free-wheel then you will be replacing the entire cog-set. If the bike has a free-hub then you can individually replace cogs. One thing to consider is that the chain, the cogs, and the chainrings (the front “gears”) wear in concert with each other so when it’s time to replace one of those items you should simply replace them all.

If you were to give me the bike and the parts I could do the whole thing for you in about half an hour. But if you don’t have the proper tools it will become a significant experience in frustration and you probably won’t achieve satisfactory results the first few times. If its your intent to maintain his bike into the future then you should do some research, buy the tools and go to it. I recommend, however, that you should find a good bike shop and let them look after it.

By “bike shop” I mean a bike shop that deals exclusively, or almost exclusively, in bikes. This may require some research and referrals but the right shop will provide you the best they can for the best price, that is, they won’t try to rip you off or sell you something you don’t need.

On the web, check out this site to help get you started. The late Sheldon Brown was a significant guru and his site remains as a very useful resource.

Best of luck with this.

Agree with velomont. I swap out gears on my bikes all the time, and if you know what you’re doing, and have the right tools it’s a piece of cake. If not, you might find it daunting. A bike shop shouldn’t charge much to do this. If you have a friend who rides a lot, ask him or her and they might do it for you, too.

BTW, the derailleur is the mechanism that switches the gears. The gears are just “the gears” or “cogs”. There really isn’t anything called a “derailleur gear”.

The sprockets/cog-set are real easy to replace, but you need the right tools. I routinely remove mine as it makes them easier to clean. You would need a Cassette Lockring Tool and a Chain Whip - though you could improvise one of those yourself. Only you can decide if using a mechanic makes more sense than purchasing the tools.

Replacing the rear derailleur mechanism is more challenging as it requires more careful adjustment.

Park Tool, one of the better bike tool manufacturers, also have helpful articles on repair. Be aware that they are in the business of selling tools so will recommend a special tool for near every task!

I disagree, chains wear much faster than the chainring or sprockets. Serious wear of sprockets is usually due to riding with a worn chain. I reckon a sprocket cassette will last at least 3 chains and chainrings even longer.

Do you mean some of the teeth broke off from the derailleur, or from the rear cassette?

IMHO you’re writing about the former, but the replies so far seem to be about the latter.

If the derailleur got caught in the wheel it’s probably bent and should be replaced - not an expensive repair as your links show. To do that you need to split the chain - easily done but requires a particular tool. If you fancy doing that, just check how many cogs are on the back wheel. Those derailleurs you link to are for 6 or 7 cogs. If there’s more (ie 7 or 8 etc) then you’d need a derailleur for 7 or 8 speed etc.
Taking the der off and fitting a new one will just need v standard tools. As mentioned, setting it up right can take a bit of fiddling.

The OP mentions one gear with 15T on it. I assume from that it’s a 15 tooth gear wheel, which would be on the cassette, but talking about the Shimano Tourney TX3 5/55/75 sounds like the derailleur mechanism, as I see when I follow the links.

Cassette replacement is easy and there are many youtube videos showing it done, just look for “bike cassette replacement”.

Upon re-reading the OP, I think you’re right.

Zen: You don’t have to replace the unit with the same model. Keep it to Shimano, but maybe get something a little more higher end. You can mix and match the components without much problem. But for this job, go to a bike shop. This is more complicated than just replacing one of the cogs.

If you are insistent on repairing it yourself and/or want to learn how to repair bicycles get the book “Zinn and the Art of Road (or Mountain) Bike Maintenance”.

Then you should get the proper tools which is another investment. If this is a one time fix then take it to the bike shop. If it is a long time ambition to be able to do the maintenance then get the book and the tools. You are also going to need a chain tool and the pins to put the chain back together. Just replacing parts on a bike is the easy part. Proper adjustment borders on an art. The Zinn book will explain it all.

Since Zenbeam linked to derailleurs, I assume he means derailleurs. The Shimano Tourney is an entry-level product line for mountain bikes, and the various derailleurs in that line are, I think, different sizes. (don’t trust the pictures – go to a bike shop. Don’t trust me, either.)

I think the “15T” on the gear refers to the number of teeth, since that sounds about right.

I presume you mean the pulley wheel broke some teeth.

If the thing works otherwise, you could just replace the wheel and not the whole thing. If it “sort of works” that could must mean you need it adjusted just right and/or the broken teeth are a problem.

You could try a new pulley wheel, then go learn how to properly adjust the derailleur (stupid French spelling of derailer!) here or at many other websites or videos before buying a whole new one. But it may have been bent or damaged and need replacing.

Thanks everyone for your comments.

Looking at the picture in the TX35 link, that whole thing is what I want to replace. In that picture, the bottom gear is the one that has 15T on it. The top gear is the one that has several teeth broken off it, but I want to replace the entire assembly, not just the gear. This is based on it having got caught in the spokes, although visibly it looks OK.

The “kind of works” comment is due to the broken teeth. With the chain lined up with that gear, it stays on and the bike is ride-able, but trying to shift it, as you might expect, the chain jumps off that gear. I hadn’t considered replacing just the gear, since the whole assembly is just 10 to 15 dollars. ETA: although, if the gear weren’t broken, I’d have him try it for a while to see how well it still works.

The rear wheel has seven gears (is this the “cassette”?).

I guess I’m surprised this is that hard. It’s held on by a single Allen wrench screw, and the gear shift cable. I already took the screw all the way off, and re-assembled it. Maybe threading the chain is hard? I do have a gadget for taking apart the special link in the chain, if that would make it easier. I realize there are several adjustment screws.

This isn’t a high-end bike, it’s a Schwinn SX 2000 mountain bike, possibly bought at Target. He’ll probably outgrow it in a year or two.

I searched for “Schwinn SX2000”, but they all seem to be women’s bikes. Here’s one at Amazon, and if you zoom way in on the back wheel, that’s what our derailleur looks like, and what the attachment looks like: one screw holding it in place. There’s a bit that catches on the frame, to keep the whole assembly from rotating. That bit is still there, it didn’t get wiped off. I suspect that screw came loose, causing the accident.

I’m still a bit confused by the OP

This is a cassette and is really easy to replace with the right tools. A 5 minute job I have performed in a muddy field.

This is a rear derailleur and needs careful adjustment to get it working right. Get it wrong and you might find you cannot easily reach all gears, or you might find that the chain shifts past the last gear on the cassette which means the chain can fall off or, worse, get jammed between the cassette and the spokes possibly damaging the wheel.

It is the derailleur. OK then.

You’re right; taking off and putting on the derailleur is not hard.

This isn’t that hard, either, though it can’t hurt to have someone show you; it’s just a hair more complicated than turning a bolt.

Now, this, it can be fairly easy or very hard, depending. Old-school friction shifting (where you move the shift lever to a different spot for each gear) is pretty straightforward to adjust (though you’ll probably take a couple test-rides to get it right) and even pretty easy to look at and figure out what you need to do (though I recommend a book to help). But index-shifting (where you just push the shift lever once and it automatically adjusts to the next gear) can be very fiddly to adjust. If it were me and the bike had index shifting, I’d try replacing it myself, but be ready to take it to a good shop if I can’t get the shifting right.

There is nothing wrong with the cassette.

I want to replace the derailleur. It has a broken gear, and possibly other damage.

I’m not afraid of making futzy little adjustments on the derailleur. I’m not sure what, if any, specialized tools I’d need. (It doesn’t look like I’d need anything more than a small screwdriver, an Allen wrench, and other basic tools. As I said, I have a chain link splitter.) I appreciate the links to websites with information on that.

I want to know if I can buy any of the derailleurs linked in the OP. I don’t know if all derailleurs are interchangeable, or not. I don’t know if any of them will work, or none of them, or if I have to select the correct one. I don’t understand what the difference is between them. I don’t need a high-end one, just one that will work for my son’s bike for at least a couple years.

If there were an easy way to get a replacement gear instead of an entire new derailleur, I could try that. I had assumed it would be easier to replace the entire derailleur assembly, than to try to remove and replace a gear, but maybe that’s wrong. I don’t know if I can buy just a new gear and replace that.

So: Which derailleur should I buy?
Or: Where can I buy a replacement derailleur gear?

It rotates to each position, although it clicks for each one. It sounds like this is old-school friction shifting. (But not old-old-school, where it was smoothly varying.)

Meh don’t let the non-mechanical “it’s all too hard, take it to the shop” unadventurous types put you off. It isn’t hard, if you have a little mechanical nous and can follow some instructions. There are any number of good guides on the net to adjusting. Even just the instructions from Shimano that will (should) come with the new derailleur are very well thought through and clear as a whistle.

They are online here:

It will have index shifting and it’s quite easy. You just adjust to line up one gear and it will do the rest by design.

I can’t help you on which one though: there seem to be a plethora of tourney models and there is little information on which is which. I suspect any will work.

In that case, yeah, you could just replace the whole thing. On the other hand, replacing the whole thing would require a little more work - removing the chain and the cable. Sounds like you can handle it though.

That probably means you need to adjust the thing to line up properly.

First you need to find out if the shifter on the handlebar is indexed or friction. If it clicks to go between gears, it’s indexed. You’ll need to adjust to line up the chain to each gear, after you adjust the maximum outer reach of the derailer on each side. (Improper adjustment of the inner reach may be what caused this accident in the first place - the derailer was allowed to shift so far in that it made contact with the spokes).

A friction shifter will just need the inner and outer limits adjusted; the shifter must be lined up manually to the gear to stay on it.

If it clicks, it’s almost certainly index shifting.

P.S. My screen name is bike-related, so you have to listen to me! :wink: