[towelie]

Hi guys. My rear derailer won't quite move over all the way, so it never makes it to my biggest cog out back. It runs rough when it is in it's second biggest cog too, since it is on the verge of shifting. I tried backing out the limit screw, but the derailer seemed unable to slide inward any further. How do I fix this?

 

[BikeGeek]

Did you try turning the barrel adjuster on the derailleur or shifter? Any places where the cable housing is kinked? How old is the chain/cassette?

 

[towelie]

The bike is pretty new. I got it last summer, but I've been injured most of the time since I got it and am just now starting to put some good wear on it.

I tried messing the the barrel adjusters, but i couldn't see the derailer move when I did. I'll look for kinks when I get home, although it seemd to shift fine at the other edge of the casette.

 

[johnbryanpeters]

Let's start with these questions: Shimano? SRAM? Model?

J

 

[oldfart]

If it used to shift into low gear and now it doesn't, what has probably happened is the cable housings have seated in and the cable is loose. Assuming its a high normal rear derailleur, shift it to the highest gear, turn both barrel adjusters, the one at the derailleur and the one at the shifter all the way in, but for a couple of clicks. Grab the cable at an open run and pull on it hard to make sure the housing is seated. Undo the cable fixing bolt and take any slack out of the cable and retighten. Shift the bike through the gears and adjust the barrel adjusrters accordingly.

Check the Park Tools website or Sheldon Brown's site for better instructions, or ask a shop for the derailleur manual. Most shops have a stack of these in the garbage from replacements. I keep them in a box at home myself. I still have owners manuals for Marzocchi XC500 shocks!

 

[Matt D]

Make sure the derailleur can physically move inboard to hit that gear. To do this, put the bike in a stand and shift all the way to your smallest (highest) gear. While pedalling and NOT shifting the shifter, push the derailleur around the gears and see if it goes into the biggest (lowest) gear. Pushing the derailleur basically simulates the cable pulling on it.

If it does not go, most likely your limit screw is not adjusted properly, your der. is muffed up, or your der. tab is muffed up. If it does go, your cable tension is wrong.

 

[towelie]

Thanks for the input guys. I didn't have time to mess with it last night, but hopefully tonight I will.

It is last years XT with LX shifters.

I'm thinking it probably is cable stretch, so I'll check this out. One other note...when the shifter is in the lowest gear position, you can still push it in a little further...like half a click. When i do this, the derailer just barely rides in the largest rear cog, but as soon as I let go it pops back.

Man...I need to get a stand. I'm tired of working on my bike upside down. Any recomendations on a stand?

 

[towelie]

Hmmm...look at this...my high limit screw is bent. I wonder how that happened?

 

[towelie]

OK...I got it to shift into the big gear. I think my problem is the derailer is getting hung up on the little piece of metal that the "B-tension" screw is attached too.

I'll post a pic, but this doesn't look right to me. Shouldn't that screw be centered on the derailer hanger, and the metal the little screw goes threw not be angled toward the derailer?

You know...I think I'm finding the damage from the crash that laid me up for the past 6 months. Except for the missing gear, it was actually shifting remarkably well considering...

 

[towelie]

OK...I just took the derailer off. That little bracket is in fact cracked. I thought it was just bent, so I tried to bend it back. When I did the crack propegated and now the thing is hanging by a thread!. Damn...I hope I can just replace that little part instead of the whole derailer.

 

[towelie]

Here we go. You know, when I got my first mountain bike, a $350 Schwinn 12 years ago, they replaced the derailer no questions asked after it was ripped clean off, at a little mom & pop LBS. I wonder if Velo Pro in Santa Barbara will do the same on a bike that is almost 10x as expensive?

 

[johnbryanpeters]

Nice diagnosis, Dr.! When you get the new derailleur, be sure to get the instructions with it. Aside from the fine print, they're pretty good.

J

 

[Matt D]

You have to replace the whole derailleur, not just one piece. Unless you have to coolest shop in the world, they will charge you for it. But request they install it and you watch and show you what they're doing. If it's really busy at the shop, don't be surprised if they say no.

Also ask them to check the der. tab allignment, by sight first (because it's free).

 

[towelie]

Hmm...somebody on mtbr.com said that you can get rebuild kits that include the part in question. This kind of pisses me off- the derailer is practically new.

So, is the current XT rear derailer "rapid rise" only? Am I going to have to scrounge for last years XT? Does anybody know of an online shop that still has last years XT on clearance or something?

 

[Matt D]

You might be able to get the rebuild kit, but it will probably be expensive.

You're right though, I forgot you can replace that B tension spring assembly. I'd suggest finding one of the numerous people who have destroyed an XT derailleur by ripping the knuckle appart or bending the cage. They'll probably just give you what you need.

That is a strange break, you sure there isn't anything else wrong with the bike or the derailleur?

 

[towelie]

Well, there was the messed up tension screw. It was either messed up in my crash, or was seriously riddeled with manufacturing defects. Ahh...fugit. I should probably just "bite the bullet" and get a new one. I don't want to spend a lot of time and effort fixing a part that might have other problems. The springs will probably explode when I begin to take it apart, and it'll be a problem. I was looking around and found there are still M750 XT derailers available brand new for about $40-$50. The new rapid rise M760 is about $65-$70.

I think I'll just buy mail order and try to tune it myself though. If I can't get it right, I'll just take it to the LBS for a derailer adjustment.

I'm really suprised how much damage this thing had. It was shifting remarkably crisply except for that last gear.

Well, I'll call the shop I bought it from and see if I have any luck there, but otherwise I'll probably just by the new derailer. I should probably get my hands on a spare anyway before they stop making them- then I'd be stuck with rapid rise only, wouldn't I?

Thanks for the help, guys.

P.S. I just noticed in the last pic, you can see where the little bracket was rubbing on the derailer body. The derailer body has rub marks on it. Yup...I can see how that could mess up my shifting.

 

[oldfart]

That's not uncommon damage. You can get that part separately for XT although I think the kit comes with the bolt and spring too. But. Most shops have a box of broken derailleurs where you can salvage the b tension plate, probably for free. I have a couple in my tool box from derailleurs which have died from other causes. That is crash damage you have or from a stick or something which causes the derailleur to be slammed backwards until the spring bottoms and the only thing left to give is that b tension plate. Better to tear that off than damage the frame although it looks as though you have a replaceable hanger.

If you do buy a new derailleur, save the old one for parts like pulleys springs and cable bolts and unbent limit screws.

 

[towelie]

OK...my new rear der FINALLY arrived from supergo today. Finally, I can shift to my lowest (biggest) rear cog (but only while pedaling). However, the top pully rubs against the cog no matter what I do with the B-tension screw. My cable is VERY taught while in this gear. Would it help to loosen it just a bit? If not, what might help?

Thanks
-Adam

Edit- wait...I think I might have got it.

Sheesh...I've never been good with derailer adjustment, but tonight has been a real learning experience. I think I have it.

 

[Matt D]

If you unscrew your Low limit screw a touch, it might help. Be careful not to do it too much or your chain will overshift into your spokes, which can be very bad.