Chain rubbing on outer ring puzzle

[ishaw]

I was trying to progress my dynatech build this evening when I stumbled on a problem, well two if you count I discovered the first outer ring was bent (a big thanks to whoever sold me that btw). The main problem after swapping out the bent ring with a straight one is that when the chain is on the middle ring at the front and smallest sprocket at the rear, the chain is rubbing against the outer ring slightly.

I'm running a middleburn rs7 crank, middleburn rings and what I believe to be the correct bb length at 113mm. Chain ring clearance with frame seems fine, as is crank arms, so what's the problem/cause? I've not had this type of set up issue before, so a little stumped. Anyone have any ideas?

Cheers

 

[drystonepaul]

Do you need some spacers between the spider and middle ring perhaps? I think I've had this with some XT cranks. A set of 0.6mm spacers cured it.

 

[ishaw]

I did wonder about that, was hoping it would be something simple rather than a new bb to solve the problem.

 

[jimo746]

as above, you'll often find (very) small washers/spacers betwwen the spider & middle or outer ring, if you're not using some then try that, or even double them up to move the outer ring out very slightly.

Or just don't use the middle ring/11T at the rear combination! It's not very often that you'll find yourself in that gear?? :?

 

[Carge]

Funnily enough, I have a similar issue with the lift pin thingies on the outer ring snagging the chain when on the middle ring and lower down the cassette.
Short of filing down the pins a little - I had wondered about spacers...
So where do we snaffle 0.6mm spacers? (at a sensible price)

 

[Explosif Phil]

I had the same problem recently. I ended up having to use a 2mm spacer on the crank side of the BB to allow room to stop chain rub. Amazing what a couple of mm will do!

 

[marc two tone]

There is no tech issue here at all.

By rights you are meant to keep your chain as straight as possible. i.e on a 7 x 3 try get into the habit of using 1,2,3 with the inner ring, 2,3,4,5 on the middle and 5,6,7 on the outer.

You could go a little longer on the bb but, that will equally transfer the problem to the other side, should you be on the middle and shift to gear 1 on the cassette, see what i mean?

 

[hamster]

I have encountered this - what are your chain ring sizes?

Running a 44T, I had exactly the same problem. Either use a 2mm longer BB or a 42T ring...or put up with it!

 

[ishaw]

26/36/46 and 48z

I think spacers on the spider is the answer as the bb is the correct length unless someone can confirm otherwise?

 

[Carge]

Do the Middleburn rings have lifter ramps?

I was running the same BB, with the same cranks - but with ramp-less outer ring - and had no issue.
Now running with ramps on the outer - I have the issue.