Thursday, March 25, 2010

Changing from triple to double chainring

Gear ratio table 1: An original setup of triple chainring

Gear ratio table 2: The new setup of double chainring
Prior migrating from triple chainring to double chainring (and lost 9 gear ratio) I did this table for comparison basis.  It turn out that (expectedly) some gear ratios are 'redundant'.  For example, 1 revolution of crank using 32/16 (front chainring over rear cog) will give same output as 22/11, i.e 2 revolution of wheel.  But again, the use 22/11 (smallest chainring to smallest cassette)  combo is not a good choice since it put an extreme sideways load to the chain.

The reasons I am switching from triple to double chainring were :-
1. less shifting for front derailleur
2. I need an extra 44t and 32t chainring to be fitted on idle crankarm for my touring bike.

In deciding on how many teeth I need for middle chainring (apparently become outer), I need to figure out what were those gear ratios I'll be 'losing' and whether that lost will be very much regretted or not.  In this case, in very rare occasion I used 44/11 and 44/12 combination.  This rare occasion was, downhilling tarmac from Genting Sempah back to Hospital Orang Asli in Gombak.  In some other occurrence, I was on shallow knobbie tires on night ride. flat terrain.  Since this setup intentionally for my ultimate MTB bike i.e. Kona, it will not be an issue.

My choice of 36t chainring was due to it is the best number of teeth for my riding style.  At 36/11 (3.27) , it caters top ratio of slightly over 44/14 combination (3.14).

The Installation
It was quite straight forward if you are familiar with installing and removing chainring for cleaning purposes.  However, I need to emphasize that you need a short stack bolt to tie the chainring to the crank arm.  Other things that matter: -
1. The need to readjust (read: lower) the FD's clamp on seatpost.  And it comes with PIA of FD tuning.
2. Protruding bottle cage's tab may obstruct FD's clamp.
3. FD's cage only can gets lower to the point it hits the chainstay.  This cause top of the cage may not have an ideal clearance on largest chainring.
4. Optionally, the chain may be shorten by 1 link to compensate chain slack.
5. And again, FD requires precise, perfect and painstaking adjustment to avoid chain derailed outward (it did happened to me). Alternately, fitting a bash guard may prevent this mishaps.

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