Originally Posted by
kbarch
All the engineers here will probably scoff, but has anyone else been surprised to notice how shifting on a bike is sometimes done the opposite way to shifting gears while driving a car?
I was spinning along on the greenway this morning at a good 100 or so cadence, and as I approached a curb cut where there was some traffic, I felt compelled to shift to a smaller cog, i.e., "higher gear," to slow down as I feathered the brakes. Totally the opposite to what one does for engine braking in a car - shifting to lower gear. Similarly, when wanting to "drop the hammer" on someone, I also always shift to a smaller cog right away so I can mash a bit, unlike when in a car, I would shift to a lower gear in order to get a better kick of acceleration.
What's up with that, mechanically speaking?
You haven't contemplated physics, taken measurements, or read enough about sports physiology.
Power isn't just pushing hard - it's the product of speed and RPM. As every American gear-head knows horsepower = torque in ft-lbs * RPM / 5250. When cycling you're unlikely to put more than your body weight on a pedal and need more speed to make more power.
Although it feels like you're being "powerful" pushing a bigger gear at a lower cadence, you actually measure lower power output sprinting.
Higher power outputs require more RPMs to get enough muscle fibers involved in the effort, with the peak around 120 RPM.
If I want to "drop the hammer" I'll shift at 120-130 RPM until getting to 50x16, 50x17, or 39x14 netting 30 MPH which is the limit of my puny legs on flat ground.