cpf_carrot

I have a Trek 4300 MTB with a problem that I cannot seem to narrow down completely. It is a fairly new bike, as I purchased it recently this summer while I was in SoCal and I had it shipped to NYC when I went back home, but I ride it every day, except when it is raining very badly. It is rarely left outside. I know it's not very expensive but I love riding it and I'd like it to work at its best.

It says "Shimano Alivio" on my rear derailleur. (picture)

With greater and greater frequency, when pedaling hard or when just starting to pedal after leaving the bike outside for a bit, I notice that something is slipping. It sounds like a metal against metal contact slips off as if something is worn down and today this happened in such a way that alarmed me greatly -- it didn't feel like it slipped just once, but multiple times and sounded like I was grinding something.

I thought at first the chain was slipping on the gears but examining the gears it doesn't look damaged in any way (I could be wrong). In my understanding of the way all of the stuff in the back works there is some kind of ratchet mechanism that lets the pedals spin freely when you pedal backwards (or freewheel) and catches when you pedal forwards. I believe that this ratchet mechanism is worn down but I'm not sure if that is it or if something else is the problem. I looked on the SheldonBrown site for information regarding this but couldn't find anything pertinent.

I would appreciate any and all help in diagnosing my problem, and possibly help repairing it.

FlatFender

I would bet that the cassette is worn out. Replace the cassette/chain, and you will probably be good to go.

kramnnim

Yeah, it does sound like a worn out cassette...does it happen mainly when the chain is on the small cogs? Could also be a stiff link in the chain. (pedal backwards with the chain in one of the smaller rear cogs, and watch to see if the rear derailleur jumps around a little when a certain part of the chain feeds through it...)

When you hear the slipping, do you also feel the pedals lurch forward in a slipping sort of way?

ExMachina

incorrect cable tension can also cause repeated half-shifts and skips and can even be perceived as the grinding that you describe--to that end, try increasing the tension slightly using the barrel adjuster on your read dérailleur.

cpf_carrot

Yes, the pedals do lurch forward a bit.

I'll check to see if there's any stiff links too.

cpf_carrot

I hope this is the case. However, I'm somewhat unfamiliar with the workings of the derailleur -- I see two screws that I can use to adjust the position of the derailleur arm, is that it, or is it something else?

MnIceBiker

Those are the limit screws. They limit the outward and inward movement of the derailleur, so that the derailleur doesn't fall off the cogs or go into the spokes. If they are improperly adjusted, you'll only notice it in the largest or smallest cog in the rear.

ExMachina

no screw driver needed! just adjust the cable tension with the barrel adjuster that's on the derailleur (right where the cable enters it). simply shift it into the second smallest cog and give the adjuster some turns counter clock wise (to tighten cable) or clockwise.

count the turns so that you know where you are relative to where you started.

this may or may not solve your problem (a solid prediction, wouldn't you say? )

raleighrider75

If you make that adjustment on an indexed shifting derailleur it will put the adjustment way out of whack.Counting the turns is a good idea.
It sounds like a stuck(or sticky)link in the chain or the spring in the derailleur is getting weak.Does it happen more in high gears than in the lower ones?

Al1943

This sounds like a common case of cable tension adjustment. Try adding 1/2 counter-clockwise turn at a time to the shift cable barrel adjuster found where the cable enters the rear derailleur. You may need more than 1/2 turn so try again after riding. It's very common for shift cables to stretch or actually "settle in" on new bikes. The original LBS should adjust this at no cost but since you have moved the bike across the country I guess you won't be taking it back to the dealer.
Detailed instructions for derailleur adjustments can be found at https://www.parktool.com. It is important to do all of the adjustments in sequence. But if you have not messed with the limit screws you can probably fix your problem with a simple cable adjustment. Your bike is way too new to have a worn out cassette.
Checking for a stiff link is a good idea but I don't think it's your problem. The bike is too new to have a weak derailleur spring.

Al

cpf_carrot

Had something very interesting happen today on my way back from class. Just after unlocking my bike from the bench it was attached to, I tried peddling and had no resistance at all -- it just spun forward without me getting any torque transferred to the wheel. This convinces me that it can't be a problem with the derailleur, unless there's a part of the derailleur design that I'm overlooking that might cause this. I'm not sure what I did to make the wheels spin again by peddling but after a bit of fiddling with the pedals it worked.

raleighrider75

Well there you go.The bike has answered the question for you.The cassette needs an overhaul.The pawls are sticking causing it to freewheel on you.There's no way the derailleur can cause THAT.

cpf_carrot

Is this something I can fix myself or should I just head over to the bike shop?

kramnnim

There's not much you can do without a chain whip and lockring remover...and even then, the freehub body isn't easily taken apart. So yeah, take it to the shop.

Retro Grouch 

Was this a brand new from the bike shop bike when you got it?

Sticky freehub prawls would be a very uncommon problem on a relatively new bike. Besides, sticky freehub prawls usually stick the other way. The pedals turn or the chain goes slack when you try to coast. Has that ever happened to you?

An out-of-adjustment derailleur, on the other hand is ultra common on relatively new bikes. Usually turning the barrel adjuster by the rear derailleur 1/2 turn counter clockwise if all that it takes to fix it.

If it was my bike I'd try the barrel adjuster trick first on my own. It doesn't take any tools. If turning the barrel adjuster 1/2 turn seems to make it better, you can fiddle with it until you get it spot-on. If that doesn't seem to help, you can always try the bike shop.

sknhgy 

Is it cold out? My Raleigh does that when it gets below freezing. The lube in the freehub/cassette gets hard and it does just what you are describing. I'm fixin' to get around to cleaning and relubing it with a lighter lube.

Vissthew

Does this happen when you shift?

I've had a similar sensation before, but what happened to me was that my bike only half-shifted, so the chain was between gears and making a griding noise; little to no resistance in the pedals.

Next time this happens to you, glance back and see if the cassette is moving or not. That will answer your question. If the cassette is moving too, something is definitely wrong with it. If the chain is skipping over it, you've got a couple different things to check.

cpf_carrot

Yes, it was brand new. I have never had the pedals turn or the chain go slack, however.

It is in fact cold out. However this problem was somewhat intermittent even before the winter rolled around so I assume the cold weather only made the problem worse. Is it possible that the lube used in my bike, which was purchased in California, was intended only for warm weather usage?

I have not tried shifting while this occurs, and when it occurs I am not shifting. I'm not 100% sure, but I thought the cassette was moving when I pedaled.

kramnnim

It wasn't the lube, the freehub mech is sealed too well for chain lube to get in, unless you purposely tried to drip some in there. (and even then, it would be hard)