RChung

For the same gear ratio and same cadence, the small chainring-small cog combo chain travels slower than a big chainring-big cog chain. For example, a 39-13 and a 54-18 are the same gear ratio, so if you're pedaling at 60 rpm you're going at the same speed and producing the same power at the crank. However, in the 54-18 the chain travels 54 teeth per revolution, so 54 teeth per second, while in the 39-13 the chain travels 39 teeth/sec. You're traveling at the same speed, so the same power is going through the drivetrain. But the chain is traveling 38% faster in 54-18 than in 39-13 so the chain tension must be 38% lower. So in the small-small combo, not only does the chain bend more, it's also under more tension. That's why the old microdrive MTB drivetrains wore out faster.