From: smee@bristol.ac.uk (Paul Smee)
Newsgroups: rec.bicycles
Subject: Re: Stupid Question from novice on gears
Date: 21 Jul 92 17:20:00 GMT
Reply-To: P.Smee@bristol.ac.uk (Paul Smee)
Organization: University of Bristol

In article <1992Jul20.182428.7348@acuson.com> lawson@acuson.com (Drew Lawson) writes:
>In article <NICKH.92Jul17171516@VOILA.VENARI.CS.CMU.EDU>
>	nickh@CS.CMU.EDU (Nick Haines) writes:
>>
>>"When you want to go faster without spinning the pedals too quickly,
>>you need a big chainring and a small cog. When you need to go slowly
>>without inching the pedals around, you need a small chainring and a
>>big cog."
>
>This sets me up for another ignorant novice question thta I have been
>too embarassed to ask before.  Just how do you work with the multiple
>gears?  I mean what is the sequence?  With, for example, an 18 speed
>bike, when do you shift from the lowest chainring?  After you get to
>the smallest cog?

There are lots of different ways of gearing a bike, and your shift
pattern depends on how yours is set up, so there's no quick and easy
answer.  What you have to do is to calculate your gearing for each
combination, and see what order they sort into.

So, first you need to count how many teeth you have on each (front)
chainring and on each (back) sprocket.  Then, calculate the gear ratio
for each combination of chainring and sprocket.  This is simply the
number of teeth on the front ring divided by the number of teeth on the
back sprocket, for each combination.  Do this in a little table.  The
bigger the number, the higher the gear.  (Remembering, lower gears are
for working under load, e.g. uphills; higher gears are for cruising and
downhill.)  

Now you've got enough info to work out what the REAL order is for your
bike.  You'll probably find that there is overlap between the gears
available on each front ring; you'll probably also find some
combinations which are close enough to each other that there's no real
point considering them to be different.

Now, a digression.  There are actually 2 standard ways of measuring the
'gearing' of a bike, used in different parts of the world.  For both of
these, you start with the ratios you calculated above.  If you multiply
each ratio by the diameter of your rear wheel, you've got 'wheelsize';
this is basically based on one of those old high-wheeler bikes with the
pedals fixed to your wheel, so that one turn of the pedals turns the
wheel once.  E.g. if you've got a 52 tooth front, a 13 tooth rear, and
a 27" wheel, that's equivalent to a 108" 'wheel'.  (Again, obviously,
bigger numbers mean higher gears.)  

The other standard is 'progression', where you take the gear ratio and
multiply by the circumference of your wheel; that tells you how far you
travel for one turn of the wheels.  (And, it's just pi times the
'wheelsize'.)  So, if you've got, say, a 50 tooth front, 13 tooth rear,
and 27" wheels, you've got a gear ratio of 3.85, a 'wheelsize' of
103.85", and a progression of 326.24".

Anyhow, as I said, the ordering varies from bike to bike.  My own is
set up with a lot of overlap.  3 ring front, 6 ring back, but I've only
really got about 10 speeds, because it's set up so that a single shift
on the front is almost the same as a 2-ring shift on the back.  For my
style of riding, and local terrain, this works pretty well.  Our area
has a lot of short sharp hills, and often a 2-gear shift down for going
up hill and a 2-gear shift up for the next down is reasonable, and I
can often just shift using the front changer, and simply fine-tune with
the rear.  

For more moderately changing terrain, I tend to think of my gears as
having 3 ranges.  Put the front into whatever is handy, adjust the back
for whatever is happening right now, and shift the front only when I
get near the end of the range on the back.  (And, especially, if it
looks like I'm going to want to shift even further in that direction in
the near future -- got to plan ahead.)

-- 
Paul Smee, Computing Service, University of Bristol, Bristol BS8 1UD, UK
 P.Smee@bristol.ac.uk - ..!uunet!uknet!bsmail!p.smee - Tel +44 272 303132