Turtle Speed

OK, so I'm messing around with a rear derailer. I've unscrewed the B-tension screw a lot, and I want to unscrew it some more... but I don't know how much farther I can go without it falling out. If that happened, would there be anything with the derailer body that comes apart? Or could anything else annoying happen?

Assume the user is a complete numbskull.

I just don't want to goof anything up before I know what I'm doing. I'm looking at a Shimano diagram, and it doesn't seem like the B-screw holds anything together. If I understand right, the B-screw just adds tension to a spring, and no B-screw would mean no additional tension on said spring.

So is it harmless to unscrew the B-screw all the way on accident? I.e. all I would have to do is place it in the hole and start screwing it in again?

andychrist

No the B-Tension screw does not hold anything together, it just keeps things apart, like the L and H limit screws do.

Turtle Speed

Alright, thanks! Off to have more fun with my B-screw adjustments!

Bill Kapaun

B screw is just to keep DER guide pulley from contacting the cogs.
You might want to do a bit of a test ride between adjustments, lest you find out you backed it out too far at an inopportune time. You have to check it against the largest AND smallest cogs.

With some cog combinations, you may find there is no "real good" adjustment. Example: putting a "road" cassette on a "mountain" DER will result in a lot of clearance between the largest cog & guide pulley. The DER is "expecting" a 30+ tooth cog, while you may have a largest cog with 10 or so fewer teeth. Set the guide pulley close to that largest cog and it'll hit the smallest cogs.

rydabent

Other brands may vary, but Sram says the "B" screw on my RD should be adjusted so that the jockey wheel misses the largest cog by 6mm. That is an easy measurement to make just us a 6mm allen wrench to check the adjustment.

FBinNY 

There's no harm in backing off and/or removing the B-screw, but there's no reason to do so. Once the business end of th screw is recessed into the body of the RD it's no longer reaching the stop and backing it farther out won't change anything.

rydabent

If the RD and frame geometry is correct, by that I mean the B screw is somewhere in mid range it is very effective. It affects the shifts. It can mean the difference between a quiet shift and one that clatters.

Totally removing the screw is not recommended. It could cause the jocket wheel to interfer with the large cogs on the cluster. Best case it causes the chain to ram into the larger cogs and make noisy shifts.

The B screw is probably one of the least understood adjustments on a bike. In the case of bikes with low gear clusters, you have to pick a RD with the proper ability to shift to the lowest cog, and then adj the B screw accordingly.

dddd

One more thing that affects a proper b-tension adjustment is the cable housing loop, which acts like a spring.

Depending on the diameter of the housing, and it's length and angle of bend, the b-tension screw is there to compensate this and keep everything fine-tuned regardless.