chain ring compatibility issue

jedster

Sorry if this is a dumb question but here goes:

I bought my bike with 105 triple chainset (50/39/30). Recently I decided it deserved a new chain/cassette/ middle chain ring (as it sees much more use than the other two). I bought a 42T middle chain ring rather than a 39. I'm now having problems with setting up the front mech, either:

a) the cage hits the middle ring as I try to shift onto the outer or
b) the cage is too high above the outer ring to pull the chain back down to the middle ring.

Now I note that shimano label the rings with either 50/39/30 or 52/42/30 - guess this could be my clue!

So do I need to run EITHER 52/42 OR 50/39?
Will my front mech be compatible with 52/42?

Thanks

J

schlepcycling

jedster wrote:

b) the cage is too high above the outer ring to pull the chain back down to the middle ring.

J

How can the cage be too high above the outer ring to stop the chain from dropping back down to the middle ring?. If the chain is in the big ring and you click to change down to the middle ring then the cage will move inwards and unseat the chain from the outer ring and gravity will drop is onto the middle ring.

When the chain is in the 'big' ring the cage should sit a few mm's above the ring to allow it (the cage) to move freely over all 3 chainrings, The fact that Shimano say you can use either 50/39/30 or 52/42/30 means that the front mech will cope with rings upto 52 on the outer and 42 in the middle so you 50/42/30 shouldn't be a problem.

jedster

thanks for the reply

I think the problem is that the ring sizes of 50/42 are too close together. THis means that if the cage is set a couple of mm above the outer (50) ring then one of the "steps" in the cage that would just pass over the top of a 39t ring actually hits a 42t ring Do you see what I mean?

If I then adjust the position of the mech on the seat tube so that this step just clears a 42t ring, the cage sits well clear of the 50t outer (again you have to imagine the steps in the cage) this means that the cage only touches the chain some way behind the ring and does not produce enough deflection to ppull the chain of the outer ring. Sorry if my explanation is not very clear.

J

schlepcycling

I think I get you, maybe it's case of adjusting the outer limit screw so that when you change up from the 42 to the 50 the cage only goes out far enough so that the chain doesn't rub when you're tanking it along in the smallest sprocket. That would mean that the 'step' from 42 to 50 doesn't throw the cage too far past the 50 and this 'should' mean that the return step should be adequate to throw the chain from the 50 down to the 42.

For a much better explanation try here [/url] http://www.parktool.com/repair/readhowto.asp?id=75 [/url]

SDP

had exactly same problem on wifes OCR ...

so i just got a campag front triple mech & bingo ! .....i dont like mixing but it worked..