How much is too much? Do you have gears on your bicycle that you never have used before? Have you ever been in a situation in which you wished that you had a much smaller (or larger) gear?
The bicycle is an amazingly efficient machine whose transmission hasn't changed appreciably for over one hundred years. In most instances, a bicycle chain, chainring, and rear cogs are 95% efficient in translating your energy into movement. While most riders will carefully consider how to optimize their gears for going up hill through the use of broad cassette ratios or compact cranksets, not too many ask if their chainrings have been maximized for optimal efficiency on the flats or downhill.
Chester Kyle, in High-Tech Cycling (ed. by Edmund Burke), notes that large chainring/large cog combinations are more efficient than small chainring/small cog combinations for the same gear ratio. For instance, a 52/15 chainring/cog combination offers the same ratio as a 42/12 combination; in other words, either combination would enable a rider to travel the same distance for each revolution of the crank.
All else being equal, however, the 52/15 combination will be more efficient because the bend of the chain is less than that of the 42/12 (engineering types: think chordal action); additionally, in the 52/15, the chain will be positioned toward the middle of the rear cogs, which also leads to greater efficiency (the chainline will be straighter). According to Kyle, cogs smaller than 14 teeth are less efficient than cogs 14 teeth or greater. A 1% increase in drivetrain efficiency could yield over 12 seconds improvement in time over 25 miles.
The question then becomes one of how to run large cogs in the back while still having a gear high enough to move quickly on the flats and downhills. The answer is to consider a larger chainring.
Here's a 130 tooth chainring that was used by José Meiffret for derney races on the track, circa 1960:
In 1941, USA's Alfred Letourneur road this bike to 109 mph:
Notice the chainring used by Albert Marquet in his 1937 record breaking ride of 139.90 km/hr:
Here's a 77 tooth chainring used by a British time trialist last season; in back, his most frequently used cogs were 16, 17, and 18:
While most of us never will use a chainring as large as what is seen in the photos above, chainrings larger than the stock 52 or 53 that is most likely on your bike are worth considering if you frequently find yourself in the smallest cogs on your rear wheel for extended periods of time. In order to run a more efficient chainline and chainring/cog combination (and given a propensity of this rider to push large gears), the following bicycle has a 60 tooth chainring:
While a chainring this large may seem excessive, remember that a 53/13 has almost the exact same gear ratio as a 60/15. But because the 60/15 is based on a larger chainring/cog combination than a 53/13, it will be mechanically more efficient, which should yield a slightly faster time, all other factors being roughly equal.
Of course, there still remains the issue of going uphill with a single 60 tooth chainring up front!