I have a hard tail with all new drivetrain. Cassette, chain, cable, chainrings, crank, cable housing, and the hanger was checked for alignment. Temporarily I was using a spare derailleur, a lower end Altus. I had some money issues (yay, student loans) and kept the Altus longer than planned. I was having gear slip problems, despite the new drivetrain. This left me confused on what to do, since everything was new and even the LBS said things look good, are set up right, and nothing is worn, etc.

I got my new derailleur in, an LX, and the problems are gone. I've put 70 hard miles on that thing already without a single issue.

Upon visual inspection, the pulleys are not worn at all (they were simply a little dirty, which even after cleaning them didn't help) and the spring tension felt good. The spring tension on my Altus was better than my brother's Shimano Tourney, yet that Tourney won't slip for nothing, yet my Altus does. There is also no damage to the Altus and the cage appears to be very straight. It is set up correctly with H/L limit screws as well as B tension.

This is just a question with trying to understand exactly what *could* be wrong with the Altus. No matter what, a derailleur shouldn't just allow users to torque down on the pedals and it slips. Even lower quality ones.

Keep in mind, this slippage I experienced wasn't every 5 seconds, but it was common enough to drive me crazy. Plus, I was able to re-produce the slipping on flat ground without any rough terrain.

What could have been up with the Altus?

Low end part.

How is it that I can beat the snot out of a Shimano Tourney and it acts fine, despite the weaker spring, yet my Altus can't get me up the street (albeit, it's a bit of a hill) without slipping once? I'm just curious if there's any "saving" it as an emergency backup derailleur. Along with the simple fact I'd like to know exactly *what* could have gone wrong.

Cable stretch.

Nope. Tried 2 cables. The original cable and I also threw on a new cable from the shop just to cross it off the list that I tried it. No dice.

Keep in mind, the LX derailleur acts JUST FINE on both the old and new cable, while the Altus acts up with both cables.

It's not a cable issue. New drivetrain. Straight hanger. No obvious wear to derailleur. Decent tension, set properly, etc.

It's an issue directly isolated to the rear derailleur itself, nothing else. I'm just trying to understand from a mechanical standpoint what with the rear derailleur could have happened to cause those issues.

The parallelogram on the rear deraillieur is twisted on the altus.

The... parallelogram? I googled it quick but I still found myself a little confused over what that is exactly. Could you go into a little more detail about what the parallelogram is on the derailleur and how it works?

Thank you for your suggestions so far.

EDIT - The only other thing is, I also had an older Alivio rear derailleur I put on the bike, and it too acted up. I have to wonder if that shifter has different issues, because on the Altus, it only slips under load, whether it's me torquing up a hill slowly OR speeding up to go down a hill. As long as I'm applying high torque, the Altus has a chance to skip. The Alivio on the other hand ghost shifts quite a bit, causing similar headaches.

Do you think the Alivio vs Altus problems are unrelated? Like I said, with the new LX derailleur in place, all issues are gone...

The two plates that are parallel.(Not the jockey arm)

Quite an abstract scenario, but it could also be that the bushing in the top pulley was too wide. This would cause excess side-to-side movement in the top pulley, causing the chain to slip, as the pulley is basically undecided as to which gear it's directing the chain on to.

Like I said, very abstract, odd scenario, but it happened to me. Pulled a pulley apart to clean it up, dropped the little metal bushing and couldn't find it, so I used one out of my spares box. Spare was too wide, hence the above slipping.

You had the EXACT same scenario happen, where you had skipping under load despite every other part on the bike being okay?

Your answers to people seem to be short and cut dry. If you want to help people, and I know you know your stuff, why don't you be a little more detailed and elaborate so they can actually take care of the problem?

The OP is trying to diagnose the problem. Dan is throwing out perfectly legit diagnoses. The two obvious fixes are either bend it back if it's not straight or replace the part.

Someone else did say at one point perhaps it was the parallelogram on the derailleur that was bent. Okay, fine. But I don't know exactly what that is and I don't know how it operates. I google image searched it and did not find any entries of it, which left me still confused.

How does a twisted parallelogram to the derailleur happen? A crash? I never crashed once with the Altus... I just find it so hard to believe I can have ALL of these compatible, brand new parts, and the derailleur (I'll admit, a low end used one) exhibits this problem when it appears to be in fine condition, whereas a newer derailleur solves the issue. Don't get me wrong, I'm happy with my new derailleur and it shifts 100% smoother than the Altus... I'm just trying to learn WHAT exactly happened...

Mystery is a part of life.

Low end parts do not have a consistency from one part to the other. I have learned the hard way that when you buy cheap you got cheap and life is full of unwanted surprises.

When it comes to EXACTLY, confine that to balancing your checkbook. The rest will drive you bonkers.

Why worry about the old derailleur? Suffice it to say it wasn't working properly and move on. Hard to diagnose over the internet, too. I don't think I've seen an article that may explain to your satisfaction the parallelogram...did you try Sheldon Brown?

I know, and I understand that it's pretty pointless to be asking questions about a problem that is fixed. I just enjoy bike mechanics and have a hard time letting go of this issue without understanding what went wrong. After all, it looks fine, seems straight, and operates well with the exception of the fact it slips. A few days ago, prior to me asking here, I put the Altus back on. Sure enough, problem came back. Put the LX back on, mashed the living crap out of that bike, not a single issue.

Like I said, just trying to learn as much as possible. I know it's hard to diagnose over the internet, but I figured I'd take my chances.

Yep.

The bike in question I bought second hand. I used an old wheelset and driveline, which was fairly worn. Anyway, I was never happy with the shifting and driveline from day 1.

Bought a new cassette and chain, and it did help to fix the problem, but the derailleur still had troubles finding and staying in gear every so often. Tweaked the hanger a bit and got it set up and aligned the best I could, but still wasn't 100%

Replaced the derailleur hanger and tuned it up from scratch. Bit better, but still not perfect.

Pulled apart the derailleur pulleys to clean them up one day, and that was the final straw. The derailleur did not like that one bit. It was fairly old and worn to begin with, a heavily used 03/04 model Deore XT, but replacing that top bushing with a spare which was slightly too big just threw it all out of whack. Add that in to the aforementioned wear factor, and it just would not stay in gear.

So I got sick of it, forked out the cash and got myself a new SLX.

There's a chance that your Altus may just have been a faulty unit.

If the problem derailleur wasn't new, there's a big pile of things that could have put it out of whack. "Altus" and "used" are never things I want in an investment. Just because you never crashed it doesn't mean the original user didn't do something to bonk it out of line.