Bicycle Mechanics - Rear derailleur pulley rubing against cassette

Can't seem to figure this one out. I've got 105 parts with a 12-25 cassette, shifting works fine but when on the second or third largest sprockets the top of the pulley comes in contact with the largest sprocket. I tried shortening the chain but the chain was too short to reach all gears and still didn't solve the problem. I've tried using the b-limit screw but it did absolutely nothing.

I've done a search and turned up a couple of threads that seemed to end without answer. Any ideas?

EDIT: Also, I've built this bike recently myself (first build) the parts are used but there are no bent or otherwise damaged parts.

Let me get this straight; your RD was rubbing against the cassette and to remedy it you shortened the chain???? 12-25 should be fine even with a short cage RD.

First, read the info here on chain length, and get the chain length right. There's also info on b-screw function and adjustment in the article. http://sheldonbrown.com/derailer-adjustment.html#chain

Read and understand the b-screw adjustment procedure. If your b-screw "does absolutely nothing" then it's not working properly. Research, look at pics, etc. and figure out why it's not working. It should adjust the derailleur's angle to the point that you can run up to around a 30t cog without it hitting the pulley.

Btw, short cage vs. long cage has nothing to do with the large cog capacity. Shimano road derailleurs have an official large cog capacity of 27t, and can usually be used with as much as a 30t cog, whether long or short cage. The length of the cage changes only the chain wrap capacity, which is a separate issue-

What kind of bike and drop-outs? If you have horizontal drop-outs, you can slide the axle either forward or backwards from its current position to increase the distance between the cogs and RD. If the drop-out doesn't have a built-in RD hanger, but uses a bolt-on one instead, you can remove it and get a longer one.

I adjusted the b-screw to it's maximum (all the way in) and although it didn't look like it did anything it's seems to have solved the problem. I also just checked the chain length and according to mr. brown it's at the correct length. If the problem returns I'll see if I can get a different hanger.

Thanks!

Hi guys sorry to revive an old thread again but I'm having the exact same problem only my B screw is all the way in and the top of the derailleur cage is rubbing the largest cog when in the second to largest cog. The rubbing is just barely there but its enough where any back pedal motion snags the chain. The freewheel is a shimano 6 speed 14-28t and the derailleur is an older Shimano 105 short cage rd-1055. I tried a longer B screw to get more clearance but once the screw gets too long the angle changes and doesn't hit the tab. The bike is an 89 cannondale aluminum road bike it no hanger. Any thoughts would be greatly appreciated. I can post pictures as well but it's pretty much exactly as described. My only thoughts now how to remedy this are to buy another freewheel with a slightly smaller diameter cog on the large range like maybe a 24? Thanks very much.

Back pedaling in 1st or 2nd gear often will "snag" the chain because of the chainline angle between the front chainring and the cassette, regardless of other issues.

Back pedaling in 1st or 2nd gear often will "snag" the chain because of the chainline angle between the front chainring and the cassette, regardless of other issues.

Right I know what you're saying but this is happening when coasting when just letting the pedals rest back a smidgeon.

Hi guys sorry to revive an old thread again but I'm having the exact same problem only my B screw is all the way in and the top of the derailleur cage is rubbing the largest cog when in the second to largest cog. The rubbing is just barely there but its enough where any back pedal motion snags the chain. The freewheel is a shimano 6 speed 14-28t and the derailleur is an older Shimano 105 short cage rd-1055. I tried a longer B screw to get more clearance but once the screw gets too long the angle changes and doesn't hit the tab. The bike is an 89 cannondale aluminum road bike it no hanger. Any thoughts would be greatly appreciated. I can post pictures as well but it's pretty much exactly as described. My only thoughts now how to remedy this are to buy another freewheel with a slightly smaller diameter cog on the large range like maybe a 24? Thanks very much.

I should add it's a double chain ring and this is only happening when on the smaller chainring, as it's intended to be ridden.

bump for some advice

Is this something that the has been doing for a while or did it just start? First check to see that your rear wheel is straight and centered; another likely suspect is a bent derailleur hanger.

Flipping the b-screw around will let its head rest on the tab. It'll give you a bit more clearance.

Tried flipping the B screw using the head doesn't clear it enough. I just bought this derailleur and put it on theres no hanger it screws to the frame. this alignment was checked and is fine. :(

Tried flipping the B screw using the head doesn't clear it enough. I just bought this derailleur and put it on theres no hanger it screws to the frame. this alignment was checked and is fine. :(It is normal that a derailleur hangers is part of the frame unless it is very old or very cheap or has a replaceable hanger(the hanger is whatever the derailleur is attached to whether is a bolt on or an integral part of the frame). And it is common that this hanger can get bent. How exactly was the "alignment" checked?

Lets look at what you've told us.
#1 you have an 89 Cannondale; probably a road bike (SR600?), with a Shimano 6 speed freewheel and RD-1055 derailleur 14-28.
The max cog for a RD-1055 is 28T so there is no need for a smaller max cog. But I need for you to confirm is it the pulley hitting the cogs or the derailleur cage hitting the cogs? If it's the cage, I am wondering of the inside cage is on backwards. 253065

BTW most of my multi-speed bikes skip a little when peddling backwards; the drivetrain simply isn't designed to go backwards. Lets see some photos of your bike.

Thanks very much for replying. It is the cage that rubs on the cog not the pulley and it's a good point the cage might have been flipped at some point. I know that you should pedal backwards and that it will skip but this happens when just holding the pedals still and coasting. Also you're correct its an sr600, and I understand what the hanger is but this bike does not have one the just a threaded hole beside the dropout that the deareailleur mounts too and I checked the alignment with the park tool that threads into this hole and then measures the distance between different points of an installed true wheel rim.

One reason this can happen is if the spring inside the pivot has lost a bit of tension.

It's probably not that likely, but if everything else checks out and the issue persists, it may be possible to add some preload to the spring.

Under that plastic trim piece on the inner side of the mounting bolt there's a circlip. Pop that off, and where the end of spring inside engages with the derailleur, you may find another hole to add tension. Or reducing tension (if possible) on the lower pivot should have a similar effect.

It's a bit of a fiddle to reassemble the pivot (you have to wind up the spring and push it together pretty hard, and hold it together to put the circlip back; the sort of job it'd be nice to have three small hands for), but it's not too hard. The lower pivot is another story, look for a small philip head screw hiding near there.

Although I don't think the cage should be touching any cogs if everything is straight... unless maybe you have a really low bailout gear that's much bigger than the cogs next to it. Even so, any contact should just be momentary on shifting.