dvroberts

Hi Folks,

First time posting here. I have a Bridgestone RB-T (1994 touring bike with Shimano RX100 derailleurs) that I am having trouble adjusting the gearing while the chain is on the crankset's largest chainwheel. I have 3 chainwheels on the crankset, and with the chain in either the smallest or the middle chainwheel I have no problem getting the rear derailleur to go through all 9 sprockets on the freewheel. But in the largest chainwheel, I can't get the rear derailleur to shift to either of the two largest sprockets. In fact, when I shift from the third-largest sprocket to the second-largest, the chain often gets stuck/jammed. When it's on the third-largest sprocket, the rear derailleur is almost horizontal, and looking from the back of the bike, the rear derailleur looks too far to the right. I am at a loss for what to try. Any advice would be much appreciated. I'm about to throw in the towel, admit defeat, and take it to my friendly bike shop.

Thanks,

Doug

echo

Maybe this will help: https://www.parktool.com/blog/repair-...illeur-systems

FBinNY 

Take a good look at the chain. It's possible that it's too short to loop the larger rear sprockets along with the largest chainring. If the RD pulley cage is laid out almost straight when on the largest sprocket it'll shift to, that's a pretty good indicator that the chain is too short.

To be sure, pull the lower loop forward to see how much more chain slack is available. Figure 1/2" per sprocket tooth to see if there's enough extra length to wrap the largest 2 sprockets.

IMPORTANT. If I'm right and your chain is too short, it's a risky proposition. Should you accidentally get it to shift into one of those impossible sprockets, serious damage will happen. Anything from damaged chainrings, trashed derailleur, bent axle, or the gear hanger broken off the frame, or more than one of the es.

gmouchawar

Sounds like your chain is too short for your gears. Follow Sheldon's directions for adjusting chain length.
Also you are not supposed to use the largest chainring with the largest freewheel sprocket. The chain line is suboptimal and it will lead to accelerated chain wear.

Al1943

The chain may be too short. This sounds like the most likely cause.
The rear shift cable may need a bit more tension.

FastJake

Sounds like your chain is too short. Read this: https://www.sheldonbrown.com/derailer...ent.html#chain

Whatever you do, don't try shifting into the large sprockets when riding in the largest chainring until you verify the correct chain length.

dvroberts

Thanks for the very helpful suggestions. A too-short chain would not have occurred to me. Looking at the chain, I see it's stamped with Shimano Narrow IG51 (one of the links also has '4F' stamped on it). The crankset is a Shimano SG 'For Narrow Chains'. I have several follow-on questions:

(1) How can I tell if this is the right chain for this set-up?
(2) After reading your helpful reference to Sheldon, I want to push out a link and measure the chain: is this the kind of chain where I need a special replacement pin if I push an existing pin out? Can I push any of the pins out on this chain, or is there a special 'magical' pin that I have to find to push out? I did notice one of the pins was dark colored and has a round stamp in its little pin head whereas all the other pins have a square stamp in the middle of their heads.
(3) If you advise me to get a new chain, how do I know which one to get? This bike has a 9 speed freewheel.

Thanks again,

Doug

dvroberts

Poo....I just see that I wrote that my cassette is 9 speed. It is not: it is a 7 speed cassette. I also did some poking around in Google and see that the Shimano IG51 chain is compatible with 7 and 8 speed cassettes, so I am assuming it's the right chain for my drivetrain. But my previous questions about whether I can push out any of the pins or if there's a special pin I must use: I still can't find an answer to that.

FastJake

If it's a Hyperglide chain, which is what it sounds like, I've heard you're not supposed to break it because it won't go back together securely enough.

You can do Sheldon's test without breaking the chain though. Just un-bolt your rear derailer pulleys and free the chain from the derailer. Then try running it through the big sprockets as described in the link.

rccardr 

It will go back fine if you break it, just get a replacement pin from your LBS.

FBinNY 

There's no need to cut the chain, or unthread it from the derailleur to see. if it's long enough.

Follow the instructions in my earlier post. Shift to the large chainring and the largest sprocket it'll now shift to (the 3rd largest?). Look at the lower loop and the RD cage. If the lower loop is already almost straight, your chain is definitely too short. If there's a decent amount of chain making an "S" bend in the RD cage, pull the lower loop forward while holding the cranks to rotate the RD cage and see how much more slack you can create. Coiunt how many links you can pull forward out of the RD cage as it rotates. You need more links than the number of added teeth from this sprocket to the largest.

For example if it's now on a 22t and your largest sprocket is 28t, you'll need to be able to pull at least 6 more links forward out of the RD cage.

dddd

"For example if it's now on a 22t and your largest sprocket is 28t, you'll need to be able to pull at least 6 more links forward out of the RD cage. "

That's an oopsy. It only requires 3 more links to allow a 6-tooth larger sprocket.

Remembering that the chain only wraps half-way around each sprocket.

Having said that, it really does sound like the chain might be too short, or something impossibly catawampus with the chain routing.

FBinNY 

Absolutely right, posted before having enough coffee. My real point was that he could confirm whether chain length was the problem, without dismantling anything. fb

dddd

"That's an oopsy. It only requires 3 more links to allow a 6-tooth larger sprocket."

Then again, you can't add 3 links, has to be even # like 4

I was advising a budding LBS mech recently as he was setting up a bike on the stand. He knew me and asked about how many links to cut out of the new chain, as it was hanging down with the smallest sprockets engaged.
I looked it over and did the full-tight-plus-two measurement, and advised him that he needed to take out 6 links.
A bit later, he again asked about a new problem of the chain not wanting to shift into the bigger sprockets. I immediately saw that the chain was waaay to short, pulling full-tight on only the 3rd-largest cog.
So what happened? He took out 12 links, counting only the "outer" links, and now I'm telling him he needs to use a 2nd Powerlink to splice 6 links back in. He didn't want to hear that and i believe he went ahead and re-riveted one of the pushed links back in.
Oh well.

Drew Eckhardt 

Nope. A complete roller chain link consists of two half-links. Things only get interesting when you need to add 3 half links.

He did exactly what you asked, removing 6 links totaling 12 half links.

Some 9 speed chains have reusable pins (ex: Campagnolo) and should be fine. Some do not (ex: Shimano) but have special pins for re-joining.

FBinNY 

This is one of those " a pair of pant's are one garment debates". Normal nomenclature in the bike world is to count links by the half pair. ie a 108 link chain is 54 inches. Which makes sense since it's 1/2" pitch, so whether we count pitches or links we're describing the same length chain.

Is it any wonder that early on ranchers decided to count heads of cattle, rather than go on forums to argue whether they meant hooves or some other part.

dddd

Damn, those ranchers were smart!

I can still see that look on the mech's face, no way did he want to put his "signature" (that extra PowerLink) on that mis-mangled chain!