Author Topic: 3 speed Brom tensioner and 15T cog: OK? Is derailleur tensioner OK without one?  (Read 4072 times)

I fitted a 15T sprocket (46T chainwheel) to my Brompton (with no plastic chainguard fitted):

1) The chain jumped off on a bumpy road:
I suspect the tensioner's spring has lost springyness (when compared to a spare).
No other explanation needed I presume.

EDIT The spare tensioner was dubious in another way (and the springs now seem equivalent) so I replaced the current one's long-arm's idler for one with slightly deeper teeth:  so far it has survived a bumpy road...

2) If replacing the tensioner, can the derailleur one be used on a 3 speed when no chain pusher fitted? Any advantage?

EDIT surely having a guard disc on both sides of idler is advantage even for 3 speeds and if an official way of locking idlers horizontally existed (if needed), eg spacers, then only one type of tensioner would need manufactured:  saving Brompton money/simplifying production/stock etc?

No one knows?

Kim

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The Kinetics SA 8-speed conversion uses a dérailleur tensioner modified for single sprocket use (in order to give clearance for a 20T sprocket and a non-standard chain-line).  I've never had an opportunity to get up close and personal with a Brompton dérailleur or unmodified tensioner, though, so I can't tell you how it differs.  It looks like it's had non-standard bolts and some spacers added.  Therefore:  Yes, with a bit of bodging you can lock it in place, no, I've no idea if it'll work for a 15T sprocket.

I assume the dérailleur tensioner gains you some additional take-up capacity over the single-speed one, but I don't have one to compare so I can't say how much.  I've got a double chainring with an 11T difference, and that's right on the limit of what the tensioner can cope with (assuming you want to maintain correct folding behaviour in either gear).


The dérailleur tensioner's jockey wheels have moulded plastic ring around both sides of the teeth (like some designs of recumbent idler), where the single-speed one has an exposed wheel (like that of a normal derailleur) and a guard disc.  I'm guessing they both achieve the same function of keeping the chain from wandering, though the dérailleur ones are surely superior at collecting leaves and small pebbles.

rogerzilla

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I ran 54 x 15 for ages on an S3L.  It's fine.
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

Quote from: kim
  I've got a double chainring with an 11T difference

Really? On a Brommie? Tell me more, please.
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Kim

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Quote from: kim
  I've got a double chainring with an 11T difference

Really? On a Brommie? Tell me more, please.

Side effect of my quest for 160mm cranks and a 28T chainring, mostly:

Spa XD2 triple crankset, 118mm bottom bracket with 1mm drive-side spacer (the Brompton 119mm BB probably works without), 28T and 39T rings, 42T bash ring in the outer position so your trousers don't get oily.  Foot'n'finger shifting.  8-speed hub, so front shifts are a pick-your-terrain sort of deal. 

If you don't need such low gearing the XD2 compact double with a 113mm bottom bracket is the way to go.  Or if you've got an old-style Brompton crankset, you can bodge a granny ring on the inside with a bit of work.

If you're happy to always shift to the big ring before folding, you can have a *much* bigger difference between chainrings.  Don't ask me how much.

Front dérailleurs are possible, but require proper engineering.  The fold complicates things.  Most people running multiple chainrings on Brommies don't bother.  Alternatively: Schlumph Speed Drive.

Biggsy

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2) If replacing the tensioner, can the derailleur one be used on a 3 speed when no chain pusher fitted?

I can't see why not.  Greater chain take-up capacity won't help you, though, so it comes down to how the spring strength compares, which I don't know.
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Quote from: kim
Foot'n'finger shifting.

I take it this isn't a euphemism, you change gears at the front by literally shifting the chain by hand?
I am often asked, what does YOAV stand for? It stands for Yoav On A Velo

Kim

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Quote from: kim
Foot'n'finger shifting.

I take it this isn't a euphemism, you change gears at the front by literally shifting the chain by hand?

Indeed.  It's perfectly viable as long as the chain's inclined to stay put, and you don't need to do the shift in normal riding.  An occasionally useful bodge to extent the gear range on bikes where a front mech would be difficult to add (typically folders and recumbents without a dérailleur post).  With a bit of practice you can do the down-shift while in motion by using your shoe as a derailleur, so it's only the up-shift that requires dismounting and manual fiddling (ideally you'd have a cunning implement secreted about the bike to stop your fingers getting oily).

With my Brompton, the 8-speed hub gives a good spread of gears for rolling terrain in the smaller chainring.  The main advantage of the bigger ring is that it gives access to the missing ratio between 7th and 8th on the XRF8 when riding in flat places, the higher top gear is a bonus.  Effectively, I've got the Birmingham Gears and the London Gears, so only doing the front shift when I take the bike somewhere new.

Most people who add double chainrings to a Brompton are coming at it from the other end: They're adding a granny ring to a conventionally geared Brompton to give lower gears for steep hills and loaded touring.  Again, manual shifting is acceptable as it's infrequently used, and not having a dérailleur saves weight.  Typically, these setups require the chain be shifted back to the big ring before folding the bike, but allow for a very large size difference between chainrings.

Interesting. Thanks. I might experiment converting my 3 speed to 6 this way.
I am often asked, what does YOAV stand for? It stands for Yoav On A Velo

Kinda reviving this thread to ask what the standard sprocket is on the 2016 6-speed M6L?

I want to lower the gearing and it strikes me that replacing the rear sprocket might be the easiest/cheapest option as long as it doesn't bugger up the rear derailleur set up and/or the sprocket is a standard SA one...

I know there's also a 44t front chainwheel option.
Haggerty F, Haggerty R, Tomkins, Noble, Carrick, Robson, Crapper, Dewhurst, Macintyre, Treadmore, Davitt.

rogerzilla

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Standard sprockets are usually 13T.  The easiest way to lower the gearing is to swap it for a 14T although you will probably need another inch of chain (it gets a bit tight if you don't - I can't remember if it actually straightens the tensioner completely).
Hard work sometimes pays off in the end, but laziness ALWAYS pays off NOW.

I am a dullard. I have a twin derailleur set up!

I'll have to go the front chainwheel way...
Haggerty F, Haggerty R, Tomkins, Noble, Carrick, Robson, Crapper, Dewhurst, Macintyre, Treadmore, Davitt.