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I bought a used Mongoose XR-75 and when I got it out to go for a ride my chain started jumping over the sprockets. Doesn’t seem to matter what gear I am in. If I Try to speed up it wants to jump over the sprockets. It’s almost like the sprockets are worn out, but they look good to me. It seems like the rear derailer doesn’t have enough tension on it. It is extremely easy to move.
Any help will be appreciated.
I do not plan to take this bike on any serious offroading rides. So keep the comments about it being junk to a minimum please. -
Does it actually jump into another gear on the rear cassette when you pedal hard or does it just make that gear shifting noise like it’s trying to shift but can’t find the gear?
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It actually slips over the sprockets.
I checked the chain a little while ago and it’s got about 2 inches of slack before the derailer moves. The derailer just doesn’t seem to have enough tension to do its job. That is as far as keeping the chain tensioned. It almost seems like the chain is about 4 or 5 links too long.
I have another bike here with a different make of derailer. Should I be able to use it to work on this bike. It is a 12 speed bike and this one is a 21 speed.
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You’ll probably be able to use the old derailleur. You’ll just need to readjust the H and L screws to the limits of a 7 speed cassette. This is where you MAY run into a problem as it might not have been designed for a range wide enough to include 7 sprockets. The spacing of the 6 speed cassette vs the 7 speed is normally the same though, so at least the amount of cable taken up in a shift should not be an issue for you.
I’m a little confused by what you meant when you said, "I checked the chain a little while ago and it’s got about 2 inches of slack before the derailleur moves." The chain has 2 inches of slack or the CABLE has two inches of slack? Where are you measuring?
Anyways, it sounds like you might want to make sure your chain is the right length 1st. Just put the chain on the big cog up front and the big sprocket in the rear and pull it as tight as possible. Remove the excess links, put her back together and it’s done.
Still, I’m not sure that this alone would cause the chain to skip up into another gear when pedaling hard. I’m wondering if your rear derailleur needs adjustment. Sounds much like the results you get when the cable is too tight and constantly pulling the derailleur into the wheel and into a larger rear sprocket. Try letting off a little cable tension and see if this helps. Some rear derailleurs have a cable tension knob right where the cable is attached to the derailleur. If you don’t have a model with that knob, most shifters will have cable tension adjusters next to the housing of the shifter. Just make sure you are turning it the correct way when you try to adjust it and small increments (1/4 turn) can make a BIG difference.
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It’s not shifting to another gear, it’s slipping over the front sprocket. It’s almost like the sprockets are worn out and won’t grab the chain.
The derailer just don’t seem like it holds the chain tight enough and allows the chain to do as it pleases.
I may have to take it in to someone and get it looked at.
Thank you for helping,
Larry -
OK, it’s a front end crankset issue not a rear end cassette issue…that definitely helps clear things up. I’m used to most people referring to the rear gears as "sprockets" and the front gears as chainrings or "cogs" so I see where the confusion was.
Being a used bike, you could have significant chain stretch. That’ll happen on an old chain. You can check it with chain wear tool. Along with chain stretch, you may have also worn the chainring down enough to alter the shape of the teeth. Most people replace a chain and a cassette at the same time. In this case, it may be your crankset up front that needs the replacement. Just try a new chain first though, as you would need it anyway and it [i:1nlzb948]might[/i:1nlzb948] save you some $$. Sometimes a stiff chain link will also do this to you so hand check each link to make sure it easily moves both directions and falls back into place.
Another cause could be loose components. A loose cassette or front crankset will slip under load and cause the chain to jump. Also check to make sure your rear axle is tightened down correctly.
You didn’t mention any rubbing from the front derailleur so I’d rule out derailleur and cable issues but IF you are getting rubbing these could also be issues.
See if that helps any and get back with us.
Hope you are up and running smoothly soon. -
From an article on Sheldon Brown’s website…
[i:1tsi9fjy]Measuring Chain Wear
The standard way to measure chain wear is with a ruler or steel tape measure. This can be done without removing the chain from the bicycle. The normal technique is to measure a one-foot length, placing an inch mark of the ruler exactly in the middle of one rivet, then looking at the corresponding rivet 12 complete links away. On a new, unworn chain, this rivet will also line up exactly with an inch mark. With a worn chain, the rivet will be past the inch mark.
This gives a direct measurement of the wear to the chain, and an indirect measurement of the wear to the sprockets:
* If the rivet is less than 1/16" past the mark, all is well.
* If the rivet is 1/16" past the mark, you should replace the chain, but the sprockets are probably undamaged.
* If the rivet is 1/8" past the mark, you have left it too long, and the sprockets (at least the favorite ones) will be too badly worn. If you replace a chain at the 1/8" point, without replacing the sprockets, it may run OK and not skip, but the worn sprockets will cause the new chain to wear much faster than it should, until it catches up with the wear state of the sprockets.
* If the rivet is past the 1/8" mark, a new chain will almost certainly skip on the worn sprockets, especially the smaller ones.[/i:1tsi9fjy]
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Thanx GoldenGoose, I will check this stuff tomorrow. Hopefully I can get it up and running for the weekend.
I’ll let you know what I find out.
Larry -
OK, I checked the chain and it is dead on as far as the measurement. It was 12′ from center to center of the pins. I think I am going to try the derailer off the other bike I have and see if that solves the problem.
If that works, I may buy a cheap Shimano from bikepartsusa.com so I can still sell one or the other or both of these bikes. -
I went out to change the derailer over from one bike to the other and discovered 2 things.
1. The chain may be good as far as stretch, but there are a few places where the links and stiff.
2. While I had the bike upside down I noticed that at least 2 of the chainrings have some teeth that are worn down to almost nothing. -
I’d be willing to bet money that you just found your culprit. The teeth aren’t going to help matters out any but the stiff links are probably your main cause for the chain jump. It’s normally an easy fix, but you might be better off buying a new chain if the one on there looks like it hasn’t been maintained well. You can put lateral pressure on the plates of the stiff chain link and it will normally loosen the link enough to allow it to move freely again. The easiest way to describe the process is to liken it to breaking a stick with two hands. The place where the stick would break in two is the place that you want the stiff link. Each normal link will move up and down but you want to apply horizontal (in and out) pressure to the stiff link in order to strech the plates apart at the pins. Good luck.
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"LarryGScott" wrote
I went out to change the derailer over from one bike to the other and discovered 2 things.
1. The chain may be good as far as stretch, but there are a few places where the links and stiff.
2. While I had the bike upside down I noticed that at least 2 of the chainrings have some teeth that are worn down to almost nothing.Ok, I’m late jumping in, but I was going to blame a rusty chain.
I would go to the LBS and buy a cheap new chain, cut it to the length of the old one and see what it does. If it works, keep that one oiled and you’ll be good to go for a while.
As for the chainrings, you may need to go to the LBS for that, or steal from the other bike…it depends on if it’s a name-brand crankset or not: if it is a standardized name-brand crank, you can also look online at either jensenUSA or pricepoint.
Either way, start with the chain and then go from there.
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Thanks, it is original Mongoose. I checked wit a couple of LBS’s and neither had what I need. I will have to replace the whole crank assembly. I am going to see what I may do at a place nearby that sells used bikes.
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"LarryGScott" wrote
Thanks, it is original Mongoose. I checked wit a couple of LBS’s and neither had what I need. I will have to replace the whole crank assembly. I am going to see what I may do at a place nearby that sells used bikes.
Yeah, that’s your best bet.
I’d still start with the new chain, a missing chainring tooth or two is usually ok (not ideal, but ok) for the casual riding you described.
What I mean is, you could probably ride it while your shopping for a chainring or crank assembly.
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Just buy a cheap replacement and slap it on there. You’ll need to know what size your BB before you make the purchase. You’ll probably need a special wrench to get the existing cranks off but you could probably borrow one from your LBS. If they shoot you down, you can either pay them to put it on for you or just get one yourself. I have two different wrenches for the two types of cranksets I have on my bikes and neither cost more than about $15.
BTW, I’ve had to use some serious elbow grease to get certain chains to loosen up. If a little more force doesn’t work you’ll need to get a new chain anyway. A cheapo will run you about $10-15.
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First, what is BB?
I already have the crank apart. Wasn’t anything special needed. It just had a bolt holding it to the shaft that runs through the frame. I then took a leather mallet and tapped it off. The crank arm and chainrings came off in one piece. -
BB is bottom bracket.
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"LarryGScott" wrote
First, what is BB?
I already have the crank apart. Wasn’t anything special needed. It just had a bolt holding it to the shaft that runs through the frame. I then took a leather mallet and tapped it off. The crank arm and chainrings came off in one piece.Sounds like you have one of the cartridge style bottom brackets so you probably won’t have to replace that part. Just find a crankset that accepts your style of cartridge BB, install and you’re good to go. Some have square shaped ends and some are shaped like a circle with teeth(ISIS and Octalink) so pay attention to what you are getting.
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Mines square.
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Most anything that is listed as a square taper crankset should work for you.
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New cheap chain seems to have fixed it.
Thank you very much for all that you helped me with. -
Of course now I have fixed the whole bike by selling it and I bought me a GT Avalanche 3.0. I don’t know what year it is, but it is a whole lot better and lighter bike.
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I’ll get one tomorrow.
Larry -
Well here it is tomorrow(today?)
And here is a link to my post with pics: http://www.singletracks.com/forum2/view … 3786#13786
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