22 Gap On 20 Tooth Front Mech

womack

So when I was building my bike I was really struggling to get hold of a 34.9 front mech for a triple. The only one I could get was a Claris which stipulated it could handle the 52 big ring but also stipulated it had a 20 tooth capacity.
My set up is 52-42-30 so a 22 tooth range.

Whilst I know Shimano and other manufacturers are often over cautious with their capacities so fitted it anyway.

Now most of the time it works just fine (middle and big no problems) but every so often going down into the small ring it will ship the chain.

I am not in small front / rear when this happens so not a chain length issue as far as I am concerned.

Do we think they were serious about the tooth capacity or any other thoughts?

lesfirth

I do not think your problem is caused by the front mech capacity. I have used a Shimano triple front mech with a 22 tooth gap. I always found triple front mechs a bit fiddly to get set up properly.
A chain catcher will help. If you have a round seat tube a little gadget called a " dog fang" works well, otherwise one that attaches to the front mech mounting .

womack

lesfirth said:

I do not think your problem is caused by the front mech capacity. I have used a Shimano triple front mech with a 22 tooth gap. I always found triple front mechs a bit fiddly to get set up properly.
A chain catcher will help. If you have a round seat tube a little gadget called a " dog fang" works well, otherwise one that attaches to the front mech mounting .

Yes Les a chain catcher would be a simple fix, can't believe I didn't think of it as I have one on my other bike!!!...old age setting in!!

pilot_pete

Isn’t it just the low adjustment? If correctly set it shouldn’t drop the chain inside. Are you following the dealer manual guidance?

https://si.shimano.com/pdfs/dm/DM-RBFD001-01-ENG.pdf

womack

Thanks Pete / oxoman. Have ordered one of those dog fang things anyway (band on mech) but will give the L adjustment a slight tweak in the meantime.

It's hard to replicate as it doesn't happen in the bike stand and only happens every so often when riding.

pilot_pete

Often the case. Ensure there is no play in the crank bearings and no ‘slop’ or endfloat (such as a required missing spacer/ plastic washer behind the crank/ non-drive side crank arm as required).

Also check the chainrings bolts are all tight and that none of the teeth are damaged/ bent, especially your middle and little ring.

Make sure the front derailleur is set exactly as per the set up guidance in terms of height above the big chainring (as close as possible) and that the cage is aligned with the ring. Also ensure the inner cage plate isn’t bent.

How new/ old is the chain? Are the chainrings excessively worn? Check the chain for wear with a simple chain checker and inspect the chainring teeth for ‘shark fin’ shape, where the ‘U’ shape between the teeth is elongated on the back edge from wear. These can lead to bad shifting/ dropped chains.

Assuming the shifter itself and the cable inner/ outer and routing/ clamping are all in good working order and correctly set, these items should get to the bottom of it.

PP

womack

PP. It is a bike I have built from a new frameset utilising where possible bits off my old donor bike.

I put a brand new BB in it and new chainrings. When removing from the old frame didn't come across a plastic washer on the non drive side, this is an old square taper type is that washer on more modern technology?

Using the existing cranks I replaced the chain rings with new ones as they were shot, tightened them up in the correct way.

Cables are brand new, whilst ordering the dog fang thingy I have ordered a chain checker to get to the free postage amount so will check chain when it arrives.

I am of the mindset that between a little low adjustment and the dog fang thingy we will sort it.