Rear mech alignment: EUREKA! Perfect shifting!!!

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Rear mech alignment: EUREKA! Perfect shifting!!!

Postby esuhl » 27 Oct 2020, 7:58pm

Maybe I'm too much of a perfectionist... or maybe I'm a rubbish mechanic... but I've always had problems in aligning the rear mech hanger for "perfect" shifting.

Even with a brand-new rear mech hanger (and having replaced an old rear mech), I needed to use my Park Tools "hanger alignment gauge"... and even then, using it as-per the instructions, I couldn't quite get it perfect. Until now!!! :D

I'm not sure if you will find this tip helpful, or if you'll roll your eyes like I'm teaching you to suck eggs! But in case it helps... (and assuming you've trued the rear wheel, and got the rear mech hanger roughly straightened and want to fine-tune it), here goes...

What I hadn't realised is that the rear mech doesn't keep the sprockets parallel with the cassette as it moves between every gear -- it adjusts for the chain path. So, the top sprocket should be angled slightly inwards when aligned with the smallest gear of the cassette, and vice versa.

With the chain on the central front chainring (or the "main" one), there should be a rear gear you can select where any sideways movement ("crossing the chain") is minimised or eliminated. The top sprocket of the rear mech SHOULD align perfectly with THAT gear.

To help, I got a length of cotton thread and weighted it at both ends with clothes-pegs. I removed the chain, temporarily slackened off the B-tension screw (so the top mech sprocket was touching the cassette), and hung the cotton thread between the front chainrings, and to one side of THAT rear gear.

Moving the rear mech to THAT gear position (by hand) I visually gauged how well aligned the sprocket was the line of cotton thread. (When bending the mech hanger, it helps to do so along only two perpendicular axes -- parallel and perpendicular to the line of cotton thread.) I made a mental note of how out-of-alignment each axis was, and used the Park Tool alignment gauge to correspondingly bend the hanger. Then re-checked the alignment, repeating as necessary.

A quick re-calibration of the rear mech (limit screws, barrel adjusters) and... bingo bango! The best shifting I've ever had on any bike everrrrr!!! :shock:


I'd love to hear what you all think. Is this obvious stuff? Am I making a mountain out of a molehill?! Has this actually helped anyone?!

Posts: 41357
Joined: 4 Jan 2012, 6:25pm

Re: Rear mech alignment: EUREKA! Perfect shifting!!!

Postby Brucey » 27 Oct 2020, 8:07pm

In an ideal world you straighten the hanger (using the alignment tool) and then you bolt a nice straight mech to it, and all is lovely.

In the real world, used mechs are often slightly worn/bent and this means bad shifting unless you do something violent to the mech itself or tweak the mech hanger to compensate.

When aligning everything, it can help to temporarily attach some kind of straight edge to the mech cage, and another which is parallel to the biggest sprocket.


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Re: Rear mech alignment: EUREKA! Perfect shifting!!!

Postby fredN4 » 28 Oct 2020, 7:51pm

lost me mate :P

De Sisti
Posts: 1018
Joined: 17 Jun 2007, 6:03pm

Re: Rear mech alignment: EUREKA! Perfect shifting!!!

Postby De Sisti » 29 Oct 2020, 8:03am

fredN4 wrote:lost me mate :P

Me too. :?