From: pardo@cs.washington.edu (David Keppel)
Subject: Re: Are 8sp Really Needed?
Organization: Computer Science & Engineering, U. of Washington, Seattle
Date: Sat, 10 Oct 92 20:50:16 GMT

ehouse@Apple.COM (Eric House) writes:
>[Why do folks like 8-cog freewheels but not triple chainrings?]

I'm guessing that the reason is riding styles, rather than what the
system is/isn't technically capable of doing.  Perhaps some other
people would care to correct me...

Factor 1: how hard is it to shift?

I think a lot of the shifting demands of road bikes are driven by
racers.  Racers often ride in close pack formation, etc., where
there's very little time to contemplate what gear you're in and how to
get to the next one.  They just need to shift *now*.  Furthermore,
downtube shifters are nearly universal and they are far harder to
double-shift than single shift.  Even the newer STI/Ergopower setups
are still more complicated to doubleshift, though it's not as hard as
with downtube shifters.

For MTBing, I think that shifting covers the low-to-high range much
faster than on road bikes.  The alpine triple's ability to make shifts
from lowest to highest in helter-skelter order is a much bigger win
than on road bikes.

Factor 2: how fast does it shift?

The time it takes to derail a chain is partly a function of the
absolute change in diameter.  Changing from a 46 to a 52 is a 13%
increase that displaces the chain vertically much more than 1cm.  The
same ratio increase in the back is, say, 17 to 15, which displaces the
chain much less than 1cm.  Compounding that, rear shifts can be done
with the chain under much more tension than front shifts.  So front
shifts are generally a bigger interruption.

Factor 3: how much does it abuse the equipment?

Spacing is also an issue.  If you pretend for a moment that front
chainrings have the same spacing as freewheel cogs, then the worst
crossover combination on a 3x6 (18-speed) is 3.5 chain widths, only
half a chain width better than a 2x8 (16-speed), which goes 4 chain
widths.  In practice, chainrings need to be further apart than
freewheel cogs, so the worst combinations of the two setups are really
about the same.  Avoiding the worst combinations has the same effect
for either setup.

Factor 4: what gears are available with narrow-range equipment?

Here's a pretend typical gearing setup from a racer's bike:

        22      20      18      16      15      14      13      12
        ----    ----    ----    ----    ----    ----    ----    ----
39 |    47      52      58      65      70      75      81      87
52 |    63      70      78      87      93      100     108     116

Here's an ``equivalent'' setup using triple cranks and a 6-cog
freewheel:

        17      16      15      14      13      12
        ----    ----    ----    ----    ----    ----
30 |    47      50      54      57      62      67
40 |    63      67      72      77      83      90
52 |    82      87      93      100     108     116

The gear ratios are the same extremes and the gap between gears is a
tiny bit less.  But the range of chainrings is really pretty big.  The
most extreme small-small combination to most extreme large-large goes
from ((52+22) - (39+12)) = 23 to ((52+17) - (30+12)) = 27.  Overall,
the rear derailleur has to take up a bit more slack and the front
derailleur has to shift over a much wider range.

On an MTB:

        32      26      21      18      16      13
        ----    ----    ----    ----    ----    ----
26 |    21      26      32      37      42      52
36 |    29      36      44      52      58      72
46 |    37      46      56      66      74      92

versus perhaps:

        28      24      21      18      16      15      13      12
        ----    ----    ----    ----    ----    ----    ----    ----
24 |    22      26      29      34      39      41      48      52
42 |    39      45      52      60      68      72      84      91

Other thoughts?

	;-D on  ( Gearhead does doubles )  Pardo
Newsgroups: rec.bicycles.tech