subframe

I have 2008 Chorus on my bike. The chain rubs against the inner plate of the front derailleur when I'm in the 3 lowest cogs on my cassette. This occurs even with inside limit screw set all the way 'in'. I have finally realized this is because the BB width is too small.

Is it possible to put washers of some kind between the BB shell face and the BB cups to effectively increase the BB width, moving the front rings out a bit?

Thanks

subframe

bump

cranky old road 

I'd considered answering before your bump but didn't because my answer, no, seemed so obvious that it must be wrong. On reflection, if the bottom bracket is really too narrow (why would this be?) the UT crankset would have side to side play. If you push on the non-drive side of the crankset, do the chainrings move away from the chain stay? What kind of bike is this?

subframe

I'ts a Kestrel 200sc. The UT bottom bracket is not too narrow, per se, since it consists of two seperate external cups; it is the BB shell (and the distance between the two cups, therefore) that is too narrow.

(There is no play in the crankset as currently installed.)

Cheers

tzracer

The UT crank is set up to have some play in the system to account for the tolerance of the BB size.
English 67.2 to 68.8 mm
Italian 69.2 to 70.8 mm

I suppose you could spacer the drive side but you cannot exceed the maximum width for the BB. Measure the width of you BB. Subtract from the maximum allowed for you BB type. This would give the maximum spacer. Since the alignment of the bearings can be affected by the squareness (2 sides parallel and the faces perpendicular to the axis of the BB) of the BB, the spacer would have to be very well made to not affect the squareness.

cranky old road 

I'm curious about why, if there is no play in the crankset, you'd think the BB shell is too narrow. Did you measure it? There is a spring washer on the nondrive side that will take up a little play, but I'm thinking if there is no play now and you put in a spacer, the Hirth joint will not be fully engaged when you tighten the two halves of the crankset together. Please keep us posted if you find a solution.

I've used a spacer on the fixed cup of a bottom bracket when my square taper NR crankset stretched and rubbed the chainstay, so appropriate spacers are available at LBS's.

Rethinking this, perhaps the spring clip on the outside of the drive side cup would prevent play. I wonder if there is play if you remove the clip? But this just about HAS to be a front derailleur rather than BB problem IMHO. Did you loosen the cable tension in addition to unscrewing the limit screw?

subframe

cranky old road - I want to agree, I spent the past year thinking it MUST be a derailleur problem. But with the limit screw set to allow it to move inward as far as possible and no cable attached, it still rubs. For a while I was thought the braze-on mount stuck out too far from the downtube, but every other one I could find measured exactly the same, so I don't think that's it.

The BB shell is just a touch over 68mm, which is right in line with Campy's spec. That also means I'd need a very thin spacer indeed to not exceed the maximum allowable distance....

Such a strange problem. If I ever figure out a cause, or a solution, I'll update...

andr0id

It's not your BB that's in the wrong place. It's your derailleur hanger. Leave the BB alone. Fix the FD hanger.

This is not uncommon with newer bikes with over sized tubing. Frame manufacturers don't leave quite enough room for a FD or they only test the brand that will be stock on the bike. Some Shimanos hit the seat tube on the return.

Either bend of file on the hanger, or try a different FD that has more clearance.

cranky old road 

^+1
Also, the oversized seat tube on my carbon frame actually has a flat depression cut into it so that the derailleur body can move in without interference. I wondered what the heck it was for when I built up the bike. Does your derailleur body hit the seat tube when it is all the way in?

Don't Campagnolo and Shimano derailleur hangers have different specs? I know the clamp on adapters for braze-ons are different between the two brands. IIRC, the radius of the derailleur attachment area is different.

Talking out my butt: if you want to use Campagnolo, you might have to remove the braze on hanger and replace it with a Campagnolo adapter.

subframe

oooh, this is getting interesting.

1st, the derailleur doesn't hit the downtube when it's farthest inward, it just hits itself so it can't move anymore.

2nd, I didn't know there were different specs for Campy and Shimano FD hangers. I spoke to a couple of bike shops about this, they apparently didn't know either. I'll see if I can't get a Campy-specific hanger on the bike and see if that solves the problem. I'd be very very happy if that were so.

Thanks

cranky old road 

Don't take my word for the FD hangers being different.

Spinz

Are you mixing QS with non QS Chorus? Lp

andr0id

Campy stuff is pretty precise, but it might be worth a try to see if another FD of the same make and model fits. There could be some manufacturing variation in the stops.

Another possibility would be to grind or file the stop where the screw hits. Of course that's if you can disassemble or reach it with a tool. Probably would only have to remove a tiny bit of material to allow another mm of inward movement.

Talon

How's the angle of the FD look? Sometimes you can fudge it and make it work acceptably by angling the rear of the cage slightly more inward than the front of the cage. Use the big ring as a reference point. The max difference I'd use is to have the rear of the cage inboard by 1 -2mm .

cbchess

what if you just filed the screw?

andr0id

Well, he said it was backed out all the way already, so that wouldn't help.

subframe

It's all 2008 QS.

i've tried angling the derailleur in a bit, but shifiting was impaired more than the chainrub was helped.

Grinding bits away is not off the table, but would probably be my last last last resort.