Hi, I have a new Cannondale Trail 5 mountain bike with Altus shifters and Alivio derailer. I've put about 12 hours on the bike, and when I shift from 4th gear to 3rd gear going uphill, the chain moves immediately to the correct gear, and I can feel the pedaling get easier, but 4-20 cranks afterward, the chain slips on the gear for a half-crank or so, then drops back onto the gear, and stays where it's supposed to be until the next time I shift from 4th gear to 3rd gear. This occurs almost immediately under heavy load; It can take up to 20 cranks if under very light load. (like if I don't really need to be in 3rd gear, but downshifting a little early in anticipation of needing to be in 3rd)

Some other information / observations:

I weigh 200 lbs.

I cannot reproduce this behavior on flat pavement or even on a decent sized hill on pavement - It only happens going uphill on a trail, and it happens 95% of the time or more

I have not experienced a single shifting malfunction shifting into or out of any other gear

This happens regardless of which front ring I am using

LBS where I purchased the bike suggest user error. An employee, about 150 lbs tested the bike on relatively flat pavement and was not able to reproduce

I am not an experienced biker but logic would dictate that if I have 0% issues in all other gears and problems every time I shift from 4th to 3rd, it's not user error

Any opinions on what could be causing this?

minor cable drag... a bent hanger... a loose derailleur... improper adjustment of the derailleur... bent or loose cassette sprocket... sticky or defective shifter... etc.....

check for cable drag first... since it's a new bike(Right?), then there might be a defect in the altus shifter, or that cable MIGHT be dragging some...

and i'dbe inspectingthe cassete REAL close, too... look for abent tooth... spin the wheel so that cassette spins, too... WATCH that offending cog REAL CLOSE, ok?

we in my day use tooth ratio numbers what combination is 3rd, 3rd from the lowest?

problem may be riding technique ... you cross chaining..

middle rear gear... no difference when riding in any front gear... not cross chaining, bob.

Opinions. Everyone has one so why not..? Trail versus Pavement. Smooth versus Rough. Light versus Heavy. Immediate Heavy 4 to 20 light.

Check how much the chain is wrapped around the offending sprocket compared to the others. I can't guess/say how much it should be but if it is not enough then Heavy load or a, later, bounce/shock on the Trail might cause the chain to slip.

I experienced similar on an old Golden Arrow rear dérailleur where the plastic had decayed so, wrong words, the upper axis did not rotate freely or rather be pulled forward to wrap the chain by the spring. I had to carve the plastic out and fit a couple of copper washers to shim things away from the bush so it would.

Maddog -- The cog looks good, as far I can tell, and the first thing I did was put some grease on the derailler cables underneath the BB. I don't have the experience to tell if something is wrong with the shifter or derailleur, so I took the bike to a different LBS, and they told me the rear derailleur is incorrectly installed -- they are going to re-install and adjust it.

It kind of sucks that I paid $800 for a brand new bike and the bike shop that sold it to me doesn't want to entertain the possibility that something is wrong with the bike, but if the derailleur re-install fixes the problem, it will be the best $28 I've ever spent, and I'll have a new LBS. Keeping my fingers crossed. Thanks for the input!!

Chorlton - Did not read your post until after I dropped the bike off at a different bike shop -- Seems plausible that a derailleur that's not installed correctly could cause a condition like you mention above. Thanks for the response -- Hopefully everything will be working properly in few days.