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[Bike] Extended Gear Ratios, Shimano XT M8000 (24/36 Chainring)

From: andrew cooke <andrew@...>

Date: Wed, 25 Oct 2017 10:30:48 -0300

I've been frustrated with the highest gear of my Shimano 2x11 XT M8000
drivetrain, which uses the lowest stock 24/34 chainrings and an 11-42
cassette (with a 2X front pull, high cage derailleur and the long SGS
rear derailleur).  Compared to my old 3x9 setup I spin out
significantly earlier on downhills (the gearing on the new 2x11 was
chosen to match the lowest gear on the 3x9).


So I tried changing the front chainrings to 24/36, and have been very
pleasantly surprised by the results.

First of all, everything works fine - I noticed no difference in the
quality of the gear changes, despite the front derailleur being
specified for a maximum change of 10 teeth.

Second, the difference feels like a BIG improvement, despite the
figures (below).  This was surprising and I have the following
tentative explanations:

 * Maybe Chilean roads, or at least the routes I ride, are built so
   that the steepest parts were just outside my gear range (and are
   now just inside)?

 * Maybe there's a "terminal velocity" where wind resistance (for me,
   on my bike, in my riding position) imposes an upper speed limit
   that was just beyond my original gearing, and that is now
   achievable.

 * Maybe it's just the shock of the new and I am over-estimating its
   significance.

Whatever the reason, this feels like a big improvement: I have new PRs
(without even trying!) and can pedal pretty much constantly on my
common routes.


The numbers, however, are not startlingly different.  Below I give the
log of the gain ratio for various combinations of gears.  You don't
need to know what it means - just that it's a measure of how "fast"
each gear is:

                  Old 24/34     New 24/36

   big top        -0.69         -0.63
   big top-1      -0.86         -0.80
   big top-2      -1.00         -0.94
   big top-3      -1.12         -1.07
   
   small top      -1.04         -1.04

Which shows that:

 * The additional range is small - about 1/3 of a gear (0.06 v 0.17)

 * Previously, the small chainring ratios sat nicely between the big
   chainring ratios, effectively giving very closely spaced gears (if
   you constantly switched between chainrings).  That is no longer the
   case.

Note that the rear derailleur capacity is 47, so there is no problem
there - the only limitation is the front derailleur.


Disclaimers:

 * I made the change at the same time as changing the front derailleur
   (the spring broke on the previous one), rear derailleur (bent) and
   rear cassette (worn).  So the entire drivetrain is new (apart from
   the cranks and shifters - I also changed the cables).

 * My bike has a Boost rear hub but normal front derailleur mount /
   bottom bracket (this is standard for 2017 Cotic Soul).  So my
   chainline is not "correct".

Andrew

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