mara777

I may finally take my bike to the LBS to ask my questions and see if anything needs fixing/adjusting (problem is there are none locally and I've been stressing myself out for weeks trying to decide where to go).

I am confused about the RD B-screw. I've read it's purpose is the keep the guide pulley from touching the cog. Thankfully, my guide pulley doesn't touch the cog, but the B-screw doesn't appear to be limiting anything. I'm checking this while on the smallest chainring/largest cog as online guides indicate. I have the B-screw as far out as possible, screwing it in would just push the limiter even further away from the little piece it's suppose to limit (sorry I don't know the name for this?). I can manually push the RD so the pulley is in the cogs (because there is nothing limiting it), as shown in the second image.

Am I missing something? Is something wrong with the way my RD is set up? I am so confused 😐


The reason I'm even wondering about this is I feel a lot of vibration in my pedals while pedaling at high speeds on the large chainring/smallest two cogs. This is something I want to ask the LBS about. I do not believe it is related to the pedals or BB. I think it is something to do with the RD because if I maintain the same speed but move one cog larger, I don't feel the same vibration.

Barry2 

Barry

Andrew R Stewart 

After setting chain length by whatever method you chose the B screw is done but in the low (large cog) rear and inner front. This is the cog combo that will suffer from pulley knock if the B spring can't swing the parallelogram/body down/rearward enough, although all but this combo will likely work well.

My first thought was that the B pivot is not freely moving. Removing the rear wheel and pulling on the der body should confirm that the B spring and pivot are smoothly working. My next thought is that of chain length. I wonder if the knocking you hear in the hi/smallest couple of cogs could be the cage over wrapping and the chain is rubbing on the cage or itself.

While not those last two small cogs, I see a fair amount of wear on the rest of the cog teeth. I would not be surprised if there's not more that will need to be repaired or replaced... Andy

FBinNY 

I suspect that there MAY be a mismatch between the RD and hanger. And/or that the RD has a counter spring which controls it's angle rather than relying on the chain to pull it forward until the B-screw engages the stop tab. See the image below showing the stop tab at the bottom, and compare to yours. In any case, actually working trumps all theoretic discussions of how it's "supposed" to work, so don't fret over it.

Crankycrank

In your photo, to my eyes, it looks like the pully is either touching or too close to the cassette cog. You would need to screw in the B-screw until there is a little more room between them. B-Tension adjustment is always a little foggy to see the change while adjusting. Make an adjustment and run the bike through the gears while pedaling and check again.

maddog34

ummm, Why does the Rear Axle look like it's NOT FULLY SEATED in the Frame? Let's get the Basics sorted out before trying to adjust things........................

And, from what i'm seeing in your photos, you're worried about something that isn't a problem.. the Stops you imaged are to prevent the der. from unwrapping it's knuckle spring and look to be just fine.

Invert the bike, Loosen the QR lever, PUSH the Wheel down into the Frame, retighten the QR. 1/3 to 1/2 lever swing to the fully seated position is a good tension on a QR... adding extra tightness will only ruin the axle bearings.

Kontact 

What you are missing is that the B screw is not a limit at all. On the older design Shimano derailleurs like this one (and copies), the B screw changes the internal spring tension of the rear pivot where it mount to the bike. When you screw it in, the spring force makes the derailleur attempt to swing down and back. This is in opposition to the spring in the pulley arm that is attempting to pull the derailleur forward and up by tugging on the chain.

Shimano has moved away from this design, but the idea was to get the derailleur to self adjust to shifts on the crankset. If your cassette is not particularly large in the low cog, the derailleur may not need any added B tension to keep it away. But if you want to know what it sounds like when the derailleur is too close, pedal the bike in the workstand and push the derailleur forward until the jockey pulley rumbles against the cogs.

There is no such thing as a "mismatch" between a modern Campy style hooked hanger and a modern hanger mounted derailleur. That's a normal hanger with a normal derailleur.

The axle looks perfectly normal in this replaceable hanger dropout.



This whole thread is about a problem that isn't a problem - how all Shimano, Campy, Sachs, Mavic, Microshift double spring derailleurs work and have worked since the introduction of the 1971 Shimano Crane derailleur and its "Servo Pantograph" dual spring design. Surely I am not the only person on this thread that knows what B tension is?

@mara777, if you have a function problem or a noise, let's talk about that. But your B screw sounds like it is working as designed.