Author Topic: Chainring reduction from 54T to 50T  (Read 2730 times)

Chainring reduction from 54T to 50T
« on: 02 January, 2017, 12:15:42 pm »
If I'm going to do the above on a 2 speed, how many (if any) links should I shorten the chain by?
Or is the answer obvious - like 2 x inner + 2 x outer links?

Re: Chainring reduction from 54T to 50T
« Reply #1 on: 02 January, 2017, 01:32:21 pm »
I think it's only 1 of each.

Reasoning is that the chain runs around half the circumference (roughly) of the chainring so will see 25 instead of 27 teeth. The runs from the chainring to the rear won't change by enough to notice.

chrisc

Kim

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Re: Chainring reduction from 54T to 50T
« Reply #2 on: 02 January, 2017, 01:36:24 pm »
I'd guess that since the chain engages with about half the teeth on the sprocket, it'll need shortening by two teeth's worth (ie. one whole link).  In reality, the Brompton's chain tensioner is reasonably forgiving (I found mine could cope with an 11 tooth difference between chainrings while still being able to take up the slack during the fold), and you can probably get away without shortening it.

I prefer a suck-it-and-see approach to chain length.  Keep removing links until it jams, and you know the right length.  (This works best if you've got a length of worn out chain kicking around to experiment with, rather than ruining a good one.)

Re: Chainring reduction from 54T to 50T
« Reply #3 on: 02 January, 2017, 04:09:53 pm »
I've just returned from the Brompton Store on Long Acre, where I visited on a not entirely unrelated matter.
When I explained to the mechanic what I was doing, his considered advice was to reduce the chain length from 102 links to 100 so the same as I was suggesting upthread.
I think that the more prudent mechanic in me might remove one at first and then see how it works.
It is probably one of these things where you cannot go back.

More interestingly, on acquiring the kit for replacing the rear sprockets, I asked him if it was necessary to replace the dork disc, he replied that it was - otherwise the sprockets would remain loose.
I asked him if I could cut away the periphery of the disc, leaving just the centre to do the job of a spacer as all the rest of the disc was doing was touching the chain when it is on the larger sprocket and making an unpleasant noise.
He looked me directly in the eye with that I-know-exactly-what-you-mean look.
Then, without averting his gaze, he reached behind him, opened one of the many drawers and (still without looking) picked out two aluminium spacers and handed them to me with the words 'Here, fit these instead'.
(Who knows, its entirely possible that the drawer contained nothing other than those spacers. It did make him look very dextrous thobut).
And in a fit of festive spirit, he refused to take any money for these from me. :thumbsup:

rogerzilla

  • When n+1 gets out of hand
Re: Chainring reduction from 54T to 50T
« Reply #4 on: 22 January, 2017, 04:38:27 pm »
Dork disc?  There is a whole world of Brompton cool with which I'm not familiar.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

Torslanda

  • Professional Gobshite
  • Just a tart for retro kit . . .
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Re: Chainring reduction from 54T to 50T
« Reply #5 on: 22 January, 2017, 05:58:55 pm »
**OMG! Ima 'bout splain summat to Zilla**

Dork disc -  usually placcy and fitted behind the cassette. Especially useful when yer mate - the one who knows all about bikes - 'adjusts' your gears for better shifting by dicking about with the limit screws . . .
VELOMANCER

Well that's the more blunt way of putting it but as usual he's dead right.

Feanor

  • It's mostly downhill from here.
Re: Chainring reduction from 54T to 50T
« Reply #6 on: 22 January, 2017, 06:09:58 pm »
Yebbut, they normally just clip onto the spokes, with a fairly large central cut-out don't they?
They don't normally go all the way down onto the hub and act like a spacer between the cassette and the hub body, do they?

Kim

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Re: Chainring reduction from 54T to 50T
« Reply #7 on: 22 January, 2017, 06:12:38 pm »
It's Brompton.  Any excuse to do things differently.