Originally Posted by
terrymorse
To minimize drive train friction, bigger is better. The reason: big cogs and chainrings require the chain to bend less.
Let's say you want a 1:1 gear ratio. A 50 chainring with a 50 cog will be more efficient than a 34 chainring with a 34 cog.
An advantage of bigger gears of the same ratio as that of smaller ones is less chain tension for an equal power output. Total chain "angular displacement" (sum of the angles of the individual links) will be the same.