Chain/derailler weirdness

father_jack

I'm doing some maintenence on a MTB. Triple on the front. Front derailler is missing so just going to set to medium or largest to make it usable. On the rear derailler end stops were set so only roughly 3 gears in the middle of the cassette were usable. I then adjusted the screws so all the gears were usable. But the following occured

1) When I set to the larger gears, the chain on the front chainset plopped off onto the middle gear. I'm pretty sure my bikes don't do this. I know not supposed to use small/high combo but even so.

2) When I use the smallest gear on the cassette, the rear derailler is bent back so far the chain is rubbing on the jockeys wheels, also the chain plops off the derailer and wraps around the rear derailler cage. Nasty. The bottom jockey wheel is worn.

Because of this, I set the set screws back so only middle gears of the cassette were usable, especially since the derailler could cause a crash. I also shorted the chain. How do I determine the correct length of chain/tightness? Also why is the front plopping off since pretty sure front derailler does not hold the chain on.

bails87

1: What chainring were you on to start with?

2: Is the chain the right length?

nicklouse

1 yes without the mech that happens..

2 in the small front ring this will happen.

father_jack

2: Is the chain the right length?

That's what I'm asking. How do you work out the correct length?

Skonk

There is a "rule of thumb" which I'm sure Nick will tell you cos I cant remember it

It's something like big ring to big ring + 3 extra links (with the chain NOT running through the rear mech).

Thats probably wrong but Like I said, I'm sure Nick will tell you the correct way.

Shatner

Father Jack wrote:

IWhen I set to the larger gears, the chain on the front chainset plopped off onto the middle gear. I'm pretty sure my bikes don't do this. I know not supposed to use small/high combo but even so.

Big-big isnt an ideal combo, so theres a chance it will always drop, especially if the chain is worn.

When I'm fitting a new chain I use the alternative way by putting it in the large front chainring and smallest rear sprocket, then pulling the chain ends together overlapping them until the 2 rear mech sprocket wheels are lined up vertically - maybe a tiny bit more tension depending on the setup.

supersonic

But if you accidentally change into the big big combo, and the chain is long enough - catastrophic drivetrain failure.

For this reason I always measure the chain big/big, bypassing the mechs, and adding two full links. Also take into account chain extension with full suss.

Muddy1182

Don't want to highjack this forum, but need some advice along the same lines.

For this reason I always measure the chain big/big, bypassing the mechs, and adding two full links. Also take into account chain extension with full suss.

What is a full link? Inner section and Outer section = one link? Or Inner section = 1 link and outer section = 2nd link.

Just putting old bike back together and was going to ask same question about chain length.

Cheers guys.

supersonic

Inner and outer together - measures 1 inch.

Muddy1182

Thanks very much.

Will look again tomorrow.