Recently I've gotten re-acquainted with riding a bike. Re-acquainted is the nice way of saying that I rode a bike with training wheels when I was 6 for a few weeks and never touched a bike again.
Since I'm a techie, my approach here is largely similar to my approach in photography: aka practice a bit and learn all about the various technology and physics involved.
So here are the first puzzle of those I hit so far:
When I was young, I remember my bike had something that today I learned it's called "coaster brake" (or a torpedo brake, in ye olde days). Basically when you back-pedal there's a tiny knob on the back hub which locks it, thus stopping the wheel. It's good for stopping and very easy to us, like my girlfriend keeps remind me.
However, not many modern bikes have this brake type. Many of the cheaper models do still have it though and of course you can always change your back hub to one that has this mechanism. Question is: what happened?
The explanation I got over coffee today is also quite simple: the coaster brake is good for stopping. It does only this, firmly and better than a hand brake, where you need a bit of force to obtain the same stopping effect while not being nearly as instant.
Then again, in practice there aren't many moments when you do need a full halt. A full halt is also heavy on the tyres, dangerous on wet ground where a locked wheel reduces manoeuvrability. In fact, the risk of locking is much greater when using any kind of rear-brake and the coaster brake works exclusively on the rear wheel (the is higher because when braking the rider's weight is moved forward by the inertia, therefore there is less weight pressing on the rear wheel to keep contact with the ground, this means less friction to keep the wheel moving, this means that applying the same braking power will now make the wheel stop).
Many modern bikes use hydraulic disk brakes to support hand brakes which can successfully replace coaster brakes. Hydraulic brakes multiply the force applied to the brake and it doesn't take a lot of it to obtain the same stopping effect. At the same time, a gradual application of force allows a reliable slow down without locking the wheels, being easier on the tyres as well.
Generally, bikes with hydraulic hand brakes don't need a coaster brake as well, although many people and bike types still benefit from its presence, still useful in many particular cases.
Downside of hydraulic brakes? They need brake fluid and anything that results in damage is a reason for concern for the loss of brake fluid. It also needs periodic replacing. see here. and more info here