Top swing or Bottom swing?

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fastpedaller
Posts: 2037
Joined: 10 Jul 2014, 1:12pm
Location: Norfolk

Top swing or Bottom swing?

Postby fastpedaller » 11 Jan 2019, 9:16pm

I have a spa touring frame with 28/38/48 triple spa chainset. I'm interested in buying a derailleur I've seen for sale which is a bottom swing (but is bottom also bottom pull so ok) - is this better (or indeed not as good ) as a topswing one? I've never used a bottom swing derailleur. :oops:

Brucey
Posts: 35161
Joined: 4 Jan 2012, 6:25pm

Re: Top swing or Bottom swing?

Postby Brucey » 11 Jan 2019, 9:41pm

bottom swing is the traditional sort; however a lot of MTB mechs have been top swing ones for the last 25 years or so. Just to be clear about terminology

Image

I have heard folk use the terms the other way around. BTW there are also 'side swing' FDs which appear to have been invented just to make sure that no-one wants a FD at all on their MTB.... :wink:

IME Bottom swing FDs are to be preferred on a touring bike because

a) they last longer (which is to do with how the springs load up the pivots)
b) they run a wider range of chainlines (eg top swing ones don't allow triple chainsets and road chainlines)
c) they better suit bottom pull cable routing

However if you are committed to a MTB chainline, a top swing mech may offer very slightly better shifting, because the cage swings in an arc that better matches the profile of a set of triple chainrings.

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~

fastpedaller
Posts: 2037
Joined: 10 Jul 2014, 1:12pm
Location: Norfolk

Re: Top swing or Bottom swing?

Postby fastpedaller » 11 Jan 2019, 10:24pm

Thanks for that Brucey. I have indeed got the description reversed...... I have only used bottom swing.


PT1029
Posts: 653
Joined: 16 Apr 2012, 9:20pm

Re: Top swing or Bottom swing?

Postby PT1029 » 12 Jan 2019, 2:34pm

If you have no mud guards and are slack with you oiling, from what I see at work top swing ones are far more prone to rusting up stiff/solid, especially if they don't shift gear that often.
On a tourer either swing is probably OK subject to previous advice. I have known top swing ones be unable to mount low enough to work due small chain rings and the clamp fouling thick down tubes/chuncky BB joints, and bottom swing ones not mount in the correct place due to seat tube bottle bosses.
The side swing mechs are just for the indecisive!

bgnukem
Posts: 176
Joined: 20 Dec 2010, 5:21pm

Re: Top swing or Bottom swing?

Postby bgnukem » 14 Jan 2019, 12:57pm

I wore out three different top swing mechs in the space of a couple of years commuting, starting with a basic one them moving up to Deore LX and finally XT. Finally replaced them with an XT bottom swing type about 12 years ago and no problems since.

The bottom swing mech (Deore XT) on my winter bike has lasted around 20 years.
Definitely get a bottom swing design if you can!

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Sweep
Posts: 5509
Joined: 20 Oct 2011, 4:57pm
Location: London

Re: Top swing or Bottom swing?

Postby Sweep » 17 Jan 2019, 7:47am

I prefer bottom swing. Seem tougher as brucey implies. Yes they can interfere with bottle cage mounting, the mech clamp getting in the way of the underside of the bottle cage. But I have always managed to sort this by using washers to raise the bottle cage a bit. The rings from presta valves are usually ideal, handy thickness.
Sweep