Newberry

Going over everything on my new bike, I noticed a little too much noise on the rear derailleur when on the largest (28t) cog. I notice that it is bouncing as I pedal. To stop the contact, I had to turn the B-screw almost to its max. I am running a triple up front, but I get the same contact on all three chainrings.
I also noticed that the small rear cable puts quite a bit of pressure on the derailleur, and it tends to counteract the spring that the Bscrew controls.

Is there a standard size for that rear cable housing?
Does this adjustment seem a bit extreme?

Thanks for any help!

Oakie

The B-tension screw controls how much of the chain wraps around the rear cogs. the closer you can get the deraileur to hug or "wrap" the cogs without hitting the cogs, the quicker and more responsive your shifting will be.

You say that your deraileur cog bounces on the 28t cog on all three of your front chainrings and that indicates 2 possibilities:

1) your chain is wrong length. You need enough chain to go from "big to big" , from your largest chainring in front and your 28t cog in back. Add one or two extra links to that amount and it's just about perfect.

2) once you have the chain length correct, then if it still hits the largest rear cog, then your rear deraileur is probably a "short cage" model. You need a long cage mountain bike or touring style deraileur. Long cage deraileurs are usually rated to have a capacity of 30 or 32 teeth.

Sounds to me like you are using a short cage deraileur with a capacity of 26 teeth at this point.

sydney

1) actually, it's big/big plus 1"....2) That's total nonsense. Cage length is about wrap capacity. Big cog capacity is in the parallelogram.....FWIW, he may have nailed it with the short length of casing at the RD. He could also try a longer B screw.

Newberry

Thanks for the replies guys.
I completely forgot to attach a pic. I'm using a long cage that was stock on the bike.
When I give the rear cable housing a little tug, it really lifts the der away from the cog.

I took the cables apart today to shorten the front shifter cables so maybe I didn't get the "bend" right in the rear casing. It seems pretty taught. Its working fine right now, but I just thought that the adjustment was excessive.
Here's a pic:

Oakie

center to center on the chain pins is 1/2 inch. put a male and female together and you have a link of 1". I said one or two links extra would do it. that would be 1 or 2 inches. You simply repeated what I said in your own way.






2) B-Tension screws control "Chain Wrap" not wrap capacity. Cage length controls wrap capacity that is why I said to go with a long cage deraileur. You misunderstood the my meaning of chain wrap. The more the chain wraps the cogs, the sooner it contacts the teeth during a shift, hence improved shifting.

sydney

I understand perfectly,and what you said about long cage to shift bigger cogs doesn't fly. A short cage shimano road RD has the same large cog spec as the road long cage. Big cog capability is in the paralelogram.....AFWIW, you might also want to check Sheldon Browns daffynition of b tension at www.harriscyclery.com

Peek the Geek

Newberry,

There was a recent thread regarding this a week or so ago. The poster had the same problem and fixed it by shortening the cable housing running to the rear der. (which you alluded to). Sounds like the excess housing is shoving your rear derailleur into the cogs.

Sydney,

I believe you had a post in the thread I mentioned. Do you think this might be what's going on?