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PostPosted: Mon Jul 25, 2016 4:37 pm 
Dirt Disciple

Joined: Tue Apr 05, 2016 9:56 pm
Posts: 46
Location: Bristol, UK
I want to check the b tension screw on my mid 80s British Eagle MTB. It has 18 gears - 6 Uniglide rear and 3 elliptical Biopace chainrings.

Which gears on the rear and the chainrings is it best to be in when checking and adjusting the b screw? Thanks for any advice.


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PostPosted: Mon Jul 25, 2016 4:49 pm 
Retro Guru

Joined: Fri May 15, 2015 4:57 pm
Posts: 1667
there's a gazillion quick 'how-to' videos about that on-line, covers everything you might need to know (just google "adjusting a b-screw - bicycles")


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PostPosted: Mon Jul 25, 2016 4:51 pm 
Retro Guru
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Joined: Thu Jun 23, 2011 11:25 pm
Posts: 1849
Location: It's not easy being a dolphin.
Small sprocket and small chainring.

You want enough chain wrap around the bottom portion of the small sprocket to stop chain slip,
but not too much either that the chain starts to rise up again from a too high upper jockey wheel
(which will impede shifting).

Hope that makes sense.


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PostPosted: Mon Jul 25, 2016 9:03 pm 
Dirt Disciple

Joined: Tue Apr 05, 2016 9:56 pm
Posts: 46
Location: Bristol, UK
Thanks to both of you for your answers. To k-rod, the trouble with both written guides with photos or videos on the internet is that whilst they usually show the rear sprockets during the adjustment process, the don't show or say what's happening with the chainrings. Hence my question.

Confusingly, they always show the adjustment being made with the chain on the largest/lowest rear sprocket. I will try Woz's method.


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PostPosted: Tue Jul 26, 2016 12:20 am 
Devout Dirtbag

Joined: Sun May 09, 2010 12:53 pm
Posts: 145
Location: Great Britain
Aye, that's right - big sprocket, wee chainring. I set Shimano's so there's just enough spring tension to keep the pulley from forcing the chain up against the big sprocket. A few mm are sufficient.


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PostPosted: Tue Jul 26, 2016 8:24 am 
retrobike rider
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Joined: Fri Aug 08, 2008 2:36 pm
Posts: 17263
Location: Yorkshire, England
Woz wrote:
Small sprocket and small chainring.

You want enough chain wrap around the bottom portion of the small sprocket to stop chain slip,
but not too much either that the chain starts to rise up again from a too high upper jockey wheel
(which will impede shifting).

Hope that makes sense.



No, the b-tension is to adjust the distance of the guide pulley (top) from the cogs.
The one cog it gets closest to normally is the largest cog, so you set it up against that.
Small ring at the front, turn the cranks backward to do the settings.
Once you have it set you check on the bottom cogs and all the way up.

You adjust so it's as close as it doesn't rattle. Tweak as needed as you ride for perfection.

Check indexing can be improved afterwards too.


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PostPosted: Tue Jul 26, 2016 11:57 pm 
Retro Guru
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Joined: Thu Jun 23, 2011 11:25 pm
Posts: 1849
Location: It's not easy being a dolphin.
* QUICKLY RUNS OFF UNDER THE COVER OF DARKNESS TO SHED ARMED WITH MAGLITE, PH1, PH2, PZ1 and PZ2
FOR A B-TENSION ORGY MASSACRE OF ALL BIKES *

True enough, the Shimano blurb states that.....but.....I routinely use road cassettes with a 26T or 28T sprocket,
and my experience leads me to faff around with the 12T/11T sprocket and the slanted parallelogram design takes care of
the rest. I think the only time where I really had to swear, fiddle and fudge the B-tension was exceeding the
rear mech rated max sprocket size of an old road mech by 2T.


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PostPosted: Wed Jul 27, 2016 6:33 am 
retrobike rider
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Location: Yorkshire, England
Road with 28T, are these modern bikes ?


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PostPosted: Wed Jul 27, 2016 8:23 am 
Retro Guru
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Joined: Thu Jun 23, 2011 11:25 pm
Posts: 1849
Location: It's not easy being a dolphin.
I have no "retro" cassettes....all worn out years ago. This is what I use:
SRAM 9 speed PG950 12-26, 11-28 (probably the favorites, a more linear meters development than Shimano)

Shimano 9 speed HG50 13-25, 12-27 & 11-25
(usually I drill and knock the three rivets out to build other options depending on the season and how fat I am)

For the MTB drop-bar and CX with brifters
Shimano 10 speed 5700 11-28, 12-25
The three largest sprockets are on a AL spider cluster that can be interchanged, usefull when
changing tyres to semi-slicks and/or from 26" to 700C wheels)

.....ask a simple question :facepalm:


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PostPosted: Wed Jul 27, 2016 12:42 pm 
Dirt Disciple

Joined: Tue Apr 05, 2016 9:56 pm
Posts: 46
Location: Bristol, UK
Thanks chaps. Got it now - large sprocket, small chainring. Haven't got around to it yet, as bike is off the road til tomorrow when I can pick up a new tyre. Found some canvas threads poking out of the front tyre's sidewall and the inner tube bulging out of the hole. Must have clipped the kerb, but the tyre is ancient so maybe the rubber is a bit ripe. At least the inner tube is still good.


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