New Chain Rattling.

[nmfeb70]

Hi all, I recently changed the chain on my old GT MTB, I measured it to the exact length of the last two but a rattle occurs when the drive side pedal reaches the downward position. Thinking the new chain wasn't catching on the old freewheel, I changed the freewheel but it still rattles. The gears shift perfectly and the RD hanger is straight, so my next step(s) would be to replace the chain wheel or jockey wheels. Any thoughts? Thanks.

 

[cougie uk]

Pictures might help.
You have routed it round the jockey wheels correctly ?

 

[Ian H]

My first guess would be that it's touching the front derailleur.

 

[Big John]

Daft question but has your rear mech got a clutch? I fitted a rear mech a few months back for a friend and it had a clutch. I'd never fitted one of these before. There's a 'switch' on it and if you have one of these that switch may have accidentally been clicked over. Potentially that could produce a rattle but only if ridden over uneven ground.

Edit: just seen the comment about the front mech. That could be a more likely cause 👍

 

[Ajax Bay]

a rattle occurs when the drive side pedal reaches the downward position

The symptom described suggests a slightly misaligned chainring. As well as checking that, I'd check that all the chainring bolts are tight.

 

[nmfeb70]

My first guess would be that it's touching the front derailleur.

Definitely not the front mech. Here are some photos..

 

[Bazzer]

My first inclination would be to put it on a bike stand, (assuming you have one) and turn the pedals by hand. That way you ears can get closer to the noise.
Does the rattle appear if you pedal using only your left leg?

 

[Mike_P]

The fourth photo looks like the rear mech may be slightly out of alignment - had a similar problem which transpired to a slightly twisted rear mech cage

 

[nmfeb70]

My first inclination would be to put it on a bike stand, (assuming you have one) and turn the pedals by hand. That way you ears can get closer to the noise.
Does the rattle appear if you pedal using only your left leg?

There are no unusual sounds when it's up on the stand, it only happens under weight.

 

[Supersuperleeds]

Going to show my lack of knowledge and why my bikes go to the LBS for everything here.

The chain doesn't have a stiff link does it, causing it to jump when under load?

 

[Big John]

Above point worth checking. Just watch the chain as it very slowly goes round the jockey wheels on the rear mech. The photos have proved inconclusive so is there any chance of a video along with the rattling sound? Diagnosis is a bit hit and miss from a distance. There's nothing beats being there and seeing it first hand. I like a good mystery 🤔

 

[boydj]

Is the chain routed correctly under the tab in the jockey wheel cage?

 

[nmfeb70]

The fourth photo looks like the rear mech may be slightly out of alignment - had a similar problem which transpired to a slightly twisted rear mech cage

Above point worth checking. Just watch the chain as it very slowly goes round the jockey wheels on the rear mech. The photos have proved inconclusive so is there any chance of a video along with the rattling sound? Diagnosis is a bit hit and miss from a distance. There's nothing beats being there and seeing it first hand. I like a good mystery 🤔

The problem only occurs when riding so taking a video would be very difficult. I will go through all the suggestions posted here over the next few days and report back.

 

[DaveReading]

Is the chain routed correctly under the tab in the jockey wheel cage?

Photos 2 and 3 suggest it's routed correctly.

 

[the snail]

Given that you say it occurs with the pedal in a particular position, that suggests the problem is with the chainrings, or something at the front. Possibly bent / misaligned chainrings as above, maybe in conjunction with poor adjustment of the front ferailleur, or some other combination of factors. Or maybe something else entirely like a knackered pedal or something.