Bicycle Mechanics - Info needed to purchase chain/chain tool

Hi helpful people!

I have a kid with a TREK Jet 20. After a few months of riding, the chain began coming off with increasing frequency, despite regular lubrication and several tension adjustments. The local bike shop was unwilling to do anything at first, and just blamed the problem on abuse. Eventually, they agreed to replace the rear sprocket, but would not replace the chain, which I suspect had been stretched. That helped for a while, but now it's coming off again, and I want to replace the chain. I do not wish to go back to shop, as they were incredibly unhelpful, even though the bike was purchased there and was under warranty! What parameters do I need to know to purchase a new chain? Will a department store have a "standard" chain which will work for 2-sprocket systems? Are there different sizes or types I need to sort through? Will they sell a chain tool as well? Normally, I would not undertake this myself, but as I said, the local shop is unhelpful. I'm gonna give this TREK one last chance before I go back to Wal-Mart. This kid rides hard, but this 150 dollar TREK is the first bike we've had this problem with......the rest were from Wal-Mart, cost less than half of that, were assembled at home, and lasted until he outgrew them.

Do I need a new sprocket, too? If so, where will I buy that? Would that part be TREK specific, or could I just get a sprocket with X number of teeth and the appropriate sized hole from an online store? Thanks for any help or advice!

this is just a single speed bike, correct?

how old/big is the kid riding it? I doubt you need a new chain, I would look to see that the front sprocket isn't bent.

I wouldn't think a child could generate the force needed to stretch a chain. and a stretched chain shouldn't come off all the time anyways, if the chain is aligned correctly, and the sprockets are not bent. (I am running a very stretched chain on my bike right now, stays on fine.)

figure out WHY the chain is coming of. if the rear wheel is in the dropouts correctly(where it bolts to the frame) and the front sprocket is NOT bent, the chain should not be coming off.

If you do want to buy a new chain, any bike specific chain from wal-mart/k-mart should work fine.

It would be unusual to wear out the transmission on a new kids bike, esp if you have been lubing the chain.
Are you setting the chain tension correctly? You use the rear axle position to tension the chain. The bolts need to be fairly tight to prevent movement.

Yes, single speed. It's possible that there was a period during which the chain was not well-lubed, so I'm still gonna replace it, simply to eliminate the chain as a factor. At that point I will inspect, clean, lube everything and try to get it back together right. I'll probably be back asking questions when I get to it! Thanks for your replies!

If the chain is tight,and rear wheel is square, but the chain still keeps jumping off, the chainring may be warped. That can be caused by shipping damage, overtightening of the chain be an overzealous wrench, or manufacture defect. A worn chain won't jump off, just make lots of noise.

Either your wheel isn't straight, or your chainring is bent, chain is twisted, not enough tension, or the freewheel is moving... chains only come off singlespeeds for that reason really.

1. Take the chain off the chainwheel (front gear) and spin the cranks... does the chainwheel have a wobble to it?

2. Carefully look at the chain as you pedal a bit with it upside down/on a stand/etc... look to see if any part looks kind of twisted, where the chain is looking like // vs ||.

3. Wheel alignment is easy to check. Its very hard to get right on some bikes though. I usually just take a wrench and push it against the tire and rest it on the chainstay, and put a finger where the wrench is resting on the stay. I move to the otherside and do the same, if there is more distance on one side or the other adjust accordingly.

4. Chain tension... you should have little movement in the chain, but not so tight that there is noticable resistance to pedaling. If the chain can hit the stay/chainguard its probably too loose.

5. Freewheel... pop the chain off the back gear (freewheel) and try wiggling it side to side, sometimes these wear out and just sort of start falling apart.

Thanks again to everyone for the input. Maybe that you are right about the chainring.....I don't have the proper terminology down for bicycles....is chainring the same thing as the front sprocket? I just took the chain off tonight and inspected everything, and it looks like warping is a good possibility, but I don't know what the tolerance should be here. As I turn the crank, the distance between the outer sprocket edge and the adjacent tube on the frame's rear triangle--don't know what to call it, would be the swingarm on a motorcycle--varies by at least 2-3 millimeters. I'll try to attach a picture to clarify; the variation occurs in the space between the two arrows. I guess it could be free play of the crankshaft in the frame boss, alternatively? I can feel a bit of movement there, but again, don't know what spec is, or how to measure it precisely. What should I do next?

Oh, and by the way, the kid is 10, but small.....maybe 65 lbs. And after looking at the bike tonight, I realize that I've not been entirely responsible about lubrication. Chain tension has been fine, though, I'm confident. That's a procedure I've done many times on motorcycles, and I've also examined posts here, so I understand how axle position in the dropouts tensions the chain and sets the alignment. Yup, I can inflate tires and tension a chain, and I believe that to be the sum total of my competence with bicycle repair, sadly. Which is why I appreciate all the help I can get!

I wonder if improper setup of the bike initially could have eventually led to a bent sprocket and/or excessive play of the crank?

Put the bike in a repair stand, if you don't have a stand, a trunk rack will work, and pedal the bike. Take the chain gaurd off, and if yoe hear a pop at the same place on the chian ring, it's warped. Watch the chain, it will jump slightly or even just fall off the front at the top of the chainring. You can take an adjusable wrench and clamp the chainring and try to striaghten it yourself.

I don't know if it's me but that tooth on the chainring on the right side of that picture looks like it's got a lot of abrasion on it cause I can see a lot of scratched bare metal expose. Your kid quite possibly could've landed on the that part of the chainring on a curb or something and put a warp in it.

I wonder if improper setup of the bike initially could have eventually led to a bent sprocket and/or excessive play of the crank? The latter is a strong possibility. You shouldn't feel any play in the crank. In fact, that might account for the abrasion slvoid noticed; maybe it's rubbing on the chainstay (that's the frame member you're wondering about, BTW). And it may be a contributing factor to the chain jumping off. If the bike is that new, that's what I'd assert at the LBS. Now, it may have come from the factory with excessive play in the crank, but the LBS should have corrected that before issuing the bike to a customer.

Is there any paint nicked off the chainstay at the point where the chainring goes by?

OK, I will do a more in-depth inspection of the sprocket and the chainstay this evening. It seems like there should be a factory specification for freeplay in the crank....it can't be zero, right? But I think you're saying it's so small that I should not be able to detect it by feel, is that correct? I assume this type of spec might be identical or very similar across all brands of bikes, or at least one-speeds. Does any one have an idea what it might be? How is it measured accurately? Would it be useful to contact TREK tech support about this? I believe the bike is out of warranty at this point....what if there is excessive freeplay here? I'm guessing that the entire crankset should be replaced in this case. Can anyone confirm this, and give me some gross estimate of cost, parts and labor? Replacing a crankset (if this turns out to be necessary) would be something which a novice should not undertake, am I right here?

If this is what happens, I would lean toward buying a new bike right away, then complaining to TREK and/or the shop in hopes of getting something done without cost to me.

There certainly shouldn't be any freeplay you can detect just by grabbing one of the crank arms with your hand. There may be a specification for instrument-detectable freeplay, but if you're able to "jiggle" the crank, it's way beyond that.

How old is the bike? How many miles on it? If it's less than a year, I would presume it to be still under warranty for the components.

the bearings MIGHT be smashed or something in the bottom bracket (where the cranks go in) but more then likely they are just loose. get a wrench and tighten them.

the bearings MIGHT be smashed or something in the bottom bracket (where the cranks go in) but more then likely they are just loose. get a wrench and tighten them.
Not so fast. You don't want to tighten them too much as the crank will bind. http://www.parktool.com/repair_help/FAQindex.shtml go and learn my child

the bearings MIGHT be smashed or something in the bottom bracket (where the cranks go in) but more then likely they are just loose. get a wrench and tighten them.

Where, exactly, would I tighten them? What size wrench, roughly, would be required? Am I grabbing onto the....wait....I need to get some terminology straight. And I'll take a close look when I get home in a few hours, and it will probably be obvious......thanks!

Where, exactly, would I tighten them? What size wrench, roughly, would be required? Am I grabbing onto the....wait....I need to get some terminology straight. And I'll take a close look when I get home in a few hours, and it will probably be obvious......thanks!
Read my last post and click on the link - all will be revealed

Since it's a one-piece crank, the Barnett's manual pages at http://www.bikeforums.net/barnettes/barnetts_ch22.pdf might be more germane.

Not so fast. You don't want to tighten them too much as the crank will bind. http://www.parktool.com/repair_help/FAQindex.shtml go and learn my child


do you work in a shop or something? or are you a roadie?

you come across like an as$ sometimes.

do you work in a shop or something? or are you a roadie?

you come across like an as$ sometimes.

Ah, now I see!! Those who work in shops, or are road bikers are a$$es!! :D :D

I'm just being amused, that's all....no offense intended to anyone. You've all been VERY helpful to a guy who doesn't really know what's going on, and none of you had to spend ANY time helping me. So I consider it a big favor. I still need to go home and have a look around, but I'm 99% certain now of the following things:

1. Something about the crankset is damaged (sprocket, arm, bearing, etc.)
2. I have a one-piece crank.
3. While there is a possibility for adjustment, the crank most probably needs to be replaced.
4. To do this, I will need to buy a few tools, and spend some time reading repair stuff and asking more Q's on this board, and then, since I'm inexperienced, I may still not be entirely successful.
5. The damage could have due to a hard spill (bent crank arm), or lack of bearing maintenance; so factory or local shop error are not the only possibilities.
6. The hundred bucks or so it would cost me to get a new bike are probably cheaper, in the end, than the price of tools, time, and potential frustration.

If anyone cares to, off the top of their head, make a guess at total cost of tools, if I were to buy what I need; also, what a reasonable repair shop might charge for the job; and lastly, if I do decide on repair (by me or the shop), I should probably just replace the chain and both sprockets as well, right? Give me any commentary to which you feel inclined. If not, thank you all so much for the education!

Todd

You may only need to to tighten the Bottom Bracket before buying any parts or a new bike. The pin spanner (Park tool SPA-4) is $5.99 and a double sided (32mm and 15mm) spanner wrench (Park tool HCW-6) is $12.99.

You could find a different local bike shop and just have them tighten the Bottom Bracket for cheap. If that doesn't eliminate the 2-3mm wobble then a new chainring shouldn't cost too much to buy and install either (at your new bike shop).

naw roadies and shoppies aren't all asses, they just tend to be more snobbish.. no offense to the ones that aren't..:D (snobs, that is..)

as for the correct tools, you can do the work on a one piece crank with a large adjustable wrench and a screwdriver. sure, the snobs will tell you "no way dude!", just ignore them.

simply tighten the bottom bracket so that the cranks are not loose, as they seem to be now. they should spin freely, but not have side to side play. of course you don't want to tighten it like the Hulk and ruin the bearings. but you knew that part already. :)