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PostPosted: Sun Nov 11, 2012 11:56 am 
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Joined: Fri Apr 06, 2012 7:34 am
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Location: Hook, Hants
The Klein I bought earlier in the year came with rapidfires (my first bike with them!) and I noticed the left hand shifter only allowed one click. One thumb press would take you from the granny ring to middle ring but pressing again did nothing... The lever moved as if it wasn't attached to anything.

Now I've replaced them with thumbies I've had a chance to take them apart and see what the issue was. Turns out it's the tension in a tiny spring and made worse by having your shifters at a steep angle. I've seen someone mention the same issue (maybe in an eBay listing) so I thought I'd take pictures in case anyone else has rapidfires that are playing up. The fix is fairly simple and you don't need to fully strip them, sending dozens of tiny parts flying!

This shifter is a MC38 STX-RC but MC37 Alivio/STX shifters are identical (just 7 speed on the right). I believe all rapidfires work on the same principle so this fix probably applies to all, right hand shifters too as they're just a mirror image with more clicks.

So step 1, remove the mount and covers. Straightforward on these as all screws are visible:

Image

Now re-attach the mount as this makes it easier to hold and pull the levers. In this pic you can see the sprung pawl attached to the lower thumb lever. In granny ring position this rests/pushes against a big tooth on the black metal disc so not a lot can go wrong there:

Image

In middle ring position the pawl rests on the edge of the metal disc underneath:

Image

When you go for the big ring the pawl must quickly slip off the lower disc and engage against a tiny edge of the upper disc. Over time the pawl spring weakens and the surfaces are likely covered in dirt/grease meaning the pawl just slips right past. If you have the shifters at a steep angle like me the spring also has to work against gravity making it less likely to engage:

Image

Luckily the circlip holding the pawl and spring is fairly easy to remove:

Image

To add a little more tension to the spring I pulled the end hooks towards each other. You only need to bend the spring a fraction though, too much tension and the pawl will get stuck in the position in pic 4. This is the shape of my spring when I'd finished:

Image

Getting the pawl and spring back on is a bit of a fiddle but not too bad. Grease the pawl pivot to help it rotate smoothly but keep any grease off the face of the pawl and can only make it slip.

The end result is a much quicker and positive shift when going for the big ring. Funny how the tension in this tiny spring can make so much difference! Hope this guide encourages a few people to tackle their misbehaving rapidfires :wink:


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PostPosted: Sun Nov 11, 2012 8:41 pm 
Old School Grand Master
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Joined: Sun Aug 21, 2011 11:57 pm
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Location: Antwerp, Belgium
That might come in handy. Thanks

I have a 7-speed shifter that occasionally doesn't engage one or two clicks when going to the larger rings.
It'll usually get the third one though, and with the right touch it'll shift perfectly fine every single time. (in other words, I'm the only one who knows how to properly work that shifter).
I've been wanting to take it apart, but too scared of seeing half a million parts fly across the room as soon as I undo the first screw. Perhaps now I'll give it a go this winter.


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PostPosted: Sun Nov 11, 2012 8:51 pm 
retrobike rider / Gold Trader
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Joined: Thu Jul 09, 2009 2:12 pm
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Location: Yateley, Hants.
Did you try cleaning the old dry and sticky grease off first with some brake cleaner and re-lubing with something less viscous? It worked for both sets I had that did this, seem to remember it being in the LGF bible.

Carl.


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PostPosted: Sun Nov 11, 2012 10:16 pm 
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Location: Hook, Hants
I did clean a bit of factory grease off first but it made no difference. The 2nd thumb shift worked when I held the shifter at a certain angle (so gravity was assisting) but not in any other position. Hence my theory the spring had lost power after being under tension for 14 years.

On well worn/used shifters it could also be a stiff pawl pivot and worn/greasy pawl face but mine are barely used. At least on these shifters you can easily open them up and get to the all-important pawl/spring without fear of an explosion of springs and ball bearings. Even the circlip isn't that tight and doesn't fly off.

With my spring tweaked and the pawl pivot greased it's now impossible to "beat the pawl" no matter what position the shifter is in or how fast I press the lever :)

Raging_Bulls wrote:
It'll usually get the third one though, and with the right touch it'll shift perfectly fine every single time.

Sounds like the same issue. I bet you're pushing the lever slowly at first, giving time for the lazy pawl to engage, then faster?


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PostPosted: Mon Nov 12, 2012 1:53 am 
Old School Grand Master
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Joined: Sun Aug 21, 2011 11:57 pm
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Location: Antwerp, Belgium
lrh wrote:
Sounds like the same issue. I bet you're pushing the lever slowly at first, giving time for the lazy pawl to engage, then faster?


Indeed. A certain amount of downward pressure will speed up the engagement on mine, any more or less will not help at all.
As soon as I feel the mechanism engage, I can slam the lever forward as fast as I want.

I'm pretty sure there's no more factory grease in there to clog things up. Last year I've sprayed half a can of Brunox in the shifter in an attempt to solve the "problem".
I guess that narrows it down to decreased spring tension or worn cogs. Altus is generally bulletproof, so the cogs won't wear out that quickly. I'm guessing it's spring tension indeed.

I've kinda grown accustomed to it though. It's one of those quirks that gives the bike its character.


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PostPosted: Thu Nov 22, 2012 4:36 pm 
Gold Trader / PoTM Winner
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Just a question about the similarities between these, and XTR M950 sti's/shifters.... as you seem to be The Man for shifter knowledge


Only ask as my right shifter has an intermittent but probably getting worse in hindsight issue. When you push the thumb to go acreoss the cassette (to the left, shifting down) it will slip through where the initial engagement would be, and only pick up the shift right at the front of its rotation/movement

Dont want to take them apart without any pointers or words of experience



Cheers
G


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PostPosted: Thu Nov 22, 2012 5:30 pm 
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Location: Hook, Hants
Haha, I'm not really "The Man" when it comes to Rapidfires... The set above are the only ones I've ever owned!

I've found a picture of an replacement STI shifter which suggests the XTR shifter will come off quite easy and give good access to the mechanism. Looks different but I can see it works on the same principle. The light grey part hiding on the lower left should be the pawl and it does sound like the same issue I had.

http://www.ebay.ca/itm/ws/eBayISAPI.dll?ViewItem&item=221042382121


Attachments:
XTR-M950-Rapidfire.jpg
XTR-M950-Rapidfire.jpg [ 71.42 KiB | Viewed 1472 times ]
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PostPosted: Thu Nov 22, 2012 5:43 pm 
Gold Trader / PoTM Winner
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Ace... guess what I am doing on my day off tomorrow then...... :wink:


G


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