[Yak] inner chainring

[Geof Gee]

The winter weather led to a bit of boredom.  I decided to borrow the
appropriate chaining and test out the 26t hypothesis.  Here are the
results ...

http://www.youtube.com/watch?v=pgpzQTmsiYQ

I have STI shifting on the NWT, so the big jump between the 39t
chaingring and the 26t chainring can result in the chain falling off
the inside the granny gear if shifting without a slight hesitation in
the pedal stroke.  I took the bike for a quick spin and it seems to do
fine.  

Next question ... if I show up to the Mountains of Misery with a 26t
granny, am I cheating?   ;-)

-G

--- Geof Gee <geoffreygee at yahoo.com> wrote:

> 
> Hey Alex, thanks for taking a look.  It made me curious and I did the
> same this afternoon.
> 
> I put the NWT on a stand and took a look at the chain during a 30
> front
> 13t rear combination.  Moving the chain to a bigger rear cog raises
> the
> chain (and keeps it over the brake mount).  Moving the derailer to a
> smaller cog seems to move the chain off the apex of the brake mount. 
> However, cyclists will generally not use that combination.  So I
> think
> that this is the minimum distance between the two.  The chain is
> still
> over the brake and there is approximately 1/2" - 3/4" of clearance.  
> 
> I don't have a 30t chainring handy ... and there is definitely no 26t
> chainring in this apartment.  But I measured the distance between
> teeth
> at 13 mm (I assume that it must be close to 25.4mm/2 ... probably
> depends on whether you measure it at the tip or the base).  
> 
> So the radius of --- will be --- 
> 
> 30t ~  62 mm
> 28t ~  58 mm
> 26t ~  54 mm
> 
> Of course the drop in the chain will be less than the change in the
> radius.  If my calculations are accurate ... or if the error is
> strongly correlated from one calculation to another ... then the
> chain
> drop should be less than one centimeter.  So I think that it should
> fit.  If one is willing to stay on cogs bigger than 13t, then the
> clearance will even be larger.  
> 
> Then again, someone at Bike Friday must have attempted this
> combination.  
> 
> -G
> 
> --- alex wetmore <alex at phred.org> wrote:
> 
> > On Fri, 30 Nov 2007, Geof Gee wrote:
> > > That is interesting Hugh.  That never crossed my mind.  Is it not
> > an
> > > issue for the NWT and other Bike Friday models?
> > 
> > On my NWT the chain would come pretty close to the rear brake with
> a
> > 30t inner chainring.  I think 26t would have made it rub.
> > 
> 
> 
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> 
> Washington, DC  20037
> 
> "Growing up is a terribly hard thing to do.  It is much easier to
> skip it and go from one childhood to another."  F.S.F.
> 
> 
>      
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Washington, DC  20037

"Growing up is a terribly hard thing to do.  It is much easier to skip it and go from one childhood to another."  F.S.F.


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