What Gearing to Use - Grampies on the Go - CycleBlaze

April 10, 2011

What Gearing to Use

Few CGOAB writers seem to describe the gearing on their bikes. Perhaps for most people having three rings on the front and some reasonable selection on the rear is good enough. The kind of fine tuning that led rear cassettes to move from 7 to 8 to up to 11 speeds perhaps only concerns bike racers. Yet having ratios that are low enough while also having a good selection of medium gears can make a big difference in touring comfort too.

Dodie started out in training almost always having to use her two lowest gears. Her bike came with 48/40/28 on the front and 14 to 28 on the rear (seven speeds there). The effective gear ratio is the number of teeth on the front divided by the number of teeth on the back times the wheel diameter. So the three lowest gears (using a 27 inch diameter) were 34.4/29.1/27.0

Our first step was to swap out the front crank in favour of 42/32/22. This made the three lowest 27.0/22.8/21.2, effectively giving two ratios below her former lowest. Dodie still felt a need for even lower gearing, so we popped in a unit in the rear that had 'mega low' which was a 34 tooth ring as the largest one. This gave a low low gear in conjunction with the new front of 17.5!

Only thing, 17.5 is really too low. Spinning your little legs wildly you will move along at 3.5 kph. Dodie correctly pointed out she might as well walk, plus the jump from the second lowest gear into 'mega low' was too abrupt. So out went the 'mega low' and in came a compromise with the largest rear rings being 24, 28, and 32. This gave the three lowest gears as 24.8/21.2/18.6.

The 18.6 is only 69% of the 27.0 that was Dodie's lowest gear when the bike came from the store. So in the end we did change her gearing really a lot.

Other things to consider as one fiddles with gear ratios are the size of jumps between rings on the rear, and the possibility of too much duplication between gear ratios produced by various combinations of the front and rear rings.

In practice now for general riding Dodie is normally on her middle front ring. Any kind of up hill sends her to the first ring and often to those carefully chosen lowest three gears.

With the loaded bike I too normally cruise in the second range, and will only consider the first range (smallest front ring) on a tough hill. The toughest hills, or course, do send me fleeing into the lowest ratios, with my low low being 19.8.

Cast off parts in the quest for the best gear ratios
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