Senior moment. Again!

[bigjim]

I've been and done it again. Three times in the last couple of months I've had one of those stupid senior moments when I've overshifted and dropped the chain off the big ring. I've done it again this afternoon. Maybe it's a case of frozen fingers, but whatever. It's rare but nothing new and I usually just trim it back and catch it by pedalling on. However this bike is nine speed. Every time on this bike the chain has wedged itself between the ring and crankarm and been a nightmare to release. I know I should be adjusting the limit screws which I have done but think I'm suffering some cable stretch which I will look at but I'm wondering if this is a nine speed chain thing as they are narrower than eight. Where as an eight speed would not fit in that space. Would it?
Anybody?

[pwa]

I think the difference in the width of 8/9 speed chains is too tiny (less than 1mm) to account for your problems. Probably just the limit screws, as you say.

[reohn2]

You have my sympathy it can be a real BA.
There's not enough difference between 8 and 9sp chains to save that from happening,it seems to me there's a fault with the set up and it all points to the limit screw that needs tweaking IMHO.

[bigjim]

Thanks.
Yea I realise the limit screw thing. You think you have that sweet spot when you set it up at home and when you go out it's a different thing. I'm just puzzled as in all my years I've never had one tangle up between the crankarm like this. Guess Imust have lucky in the past.
I wonder if it's that 9 speed chainsets have a bigger gap? I must go out and check when I pluck up the courage to enter my frozen garage.

[bryce]

Could it be the chainrings are getting worn? I had a similar problem and resolved it by changing the chainrings.

Possibly the worn teeth fling the chain outward on the upshift rather than grip it properly.

[Valbrona]

And you can get chainrings with pegs so that if the chain unships it doesn't jam where yours has been jamming.

[531colin]

The wee peg thingy will stop the chain jamming between crank and ring if it overshifts, but ideally you want to stop the overshift.
The limit stop is the first thing to look at, but the angle of the mech. on the seatpost also has an effect....I like the outer plate of the chain guide to angle in slightly towards the centre....if it angles slightly out towards the outside, it won't keep the chain on the ring, the chain can climb straight over the ring, instead of up and onto the ring. In the old days before profiled cage plates, we used to give them a little bend....

[LollyKat]

I like the outer plate of the chain guide to angle in slightly towards the centre....

Do you mean the front end angles in or the back end? Sorry if it's a dumb question .

[Brucey]

I think he means back end out (to the right slightly).

The 'tweak' for lots of mechs (back in the day) was to angle (bend) the front part of the outer plate inwards (to the left) slightly. This was specifically intended to avoid unshipping the chain.

cheers

[bigjim]

I need to check the mech. It'sthe original 1985 mech with a modern Tiagra chainset. Will it bend?

[Brucey]

It is difficult to be sure from this one shot but I'd look to do a few things;

a) see if you can lower the front mech at all

b) if necessary widen the rear of the mech using a couple of M4 washers and a longer screw.

c) move the chainline inwards if possible.

d) tweak the nose of the mech. (maybe both sides)

e) twist the mech so that the nose can be lowered without the tail touching the big ring (at the moment the height gap is biggest at the front). warning; you may break something.

Now that '105 'Golden Arrow' mech is pretty wide; it is designed for running full-fat 3/32" chains as originally used on 5s and 6s systems. It is also designed for a ~42mm chainline or something; if the chainline is wider than that, the mech is moving upwards as well as outwards as you go onto the big ring and this will certainly help the chain to overshoot the big ring too.

hth

cheers

[mercalia]

re a) lower the mechanism. How far from the teeth should the front part of the cage be approx? I seem to remember new front changers came with a tab to set that properly?

[Brucey]

most front mechs need to clear the big ring teeth by ~2mm or something (and you need to be shifting to see what clearance you have; the mechs don't always move in the way you might expect and the clearance varies through the shift). Higher than this doesn't guarantee disaster, but it can be one of the culprits for this kind of overshift.

Commonly on a triple (or a close-ratio double) the front mech must be set a little 'high' in order that the inner plate clears the middle ring correctly. Then it is time for a bit of 'suck it and see' or perhaps a different mech.

cheers

[reohn2]

TBH IMO the problem could be solved with a Tiagra front mech.£15? what price piece of mind and you'd probably sell the old one on Ebay for a £5+

[LollyKat]

I think he means back end out (to the right slightly).

Thanks, Brucey!