It is true that the closer to the centre the greater the force required, but the efficiency of the disc brake is such stopping is not a problem.

If you have a heavily loaded bicycle (or tricycle), such as a touring bike with lots of heavy bags or a cargo bike, then you can opt for disc brakes which have additional pistons forming the caliper.

Standard disc brakes have only one piston per brake, but brakes such as the Hope M4 http://www.hopetech.com/page.aspx?itemID=SPG336 have 2 pistons per brake. Which in turn greatly increases the amount of power available to stop the wheel as the pad size is also increased.

Other factors are, as you imply, the rotor size. Small rotors have less stopping power as the same power is transferred to the disc but yet the decreased size requires an increase in power to be equally effective. Small discs are in the order of 160mm in diameter, and the largest are 203mm in diameter.

For the greatest stopping power one would go for 203mm diameter rotors and use the "4-pot" (4 pistons in total, 2 in the front brake, 2 in the rear brake) brake calipers.

Other factors in disc brakes are that the discs can get very hot. Effects of this is that they can deform the shape of the disc.

So also on the market are floating discs, where the braking area is not physically attached to the internal fixing area, hence the term floating... so only the outer area deforms.

And you can also get vented rotors, which are made of two braking areas separated by air to increase the surface area by which the rotors can cool. They tend to also be floating.

So disc types:

1) Standard single piece of metal, can deform with extreme heat

2) Floating disc, less deformation with heat: http://www.hopetech.com/page.aspx?itemID=SPG312

3) Vented and floating discs, less deformation and reduced cooling time: http://www.hopetech.com/page.aspx?itemID=SPG313

The last factor in disc brakes is the method by which one would brake. Hydraulic gives by far the greater performance over cabled, but there is an outside chance that if you haven't managed the temperature of the disc that the hydraulics can overheat and freeze. It's something you manage and I've not heard it happen in years, even as a myth. Also, cable is easier to maintain.

The result of that though, is that touring bikes tend to go for cabled brakes and of the cabled brakes the Avid BB7 stands out. Everyone else just goes with the hydraulics because it's unlikely they're going to be doing a 25km descent with luggage that could cause such extreme overheating.

What else whilst I dump knowledge...

Why not put the discs on the rims? You're correct that it would be better performance but wheels themselves deform on impact (potholes) and the disc would fight against that and cause problems. Also... rim brakes are actually good enough already, much cheaper and require less of a high tolerance in engineering. That said, the Buell motorcycle company did this and it works well if you can afford it: http://www.totalmotorcycle.com/photos/2006models/2006models-... although you now cannot easily get commodity replacement parts. There is also the issue of weight, and that a large ring of metal isn't just additional weight, but rotational weight which requires greater energy to get moving.

Why do Tour de France style road bikes not have disc brakes if they are so good? Basically the UCI made them illegal, and manufacturers of ultra-light bikes prefer rim brakes as disc brakes require them to strengthen the fork and crown to deal with the extra force involved, and both the calipers and fork adjustments would add weight. That said, you will find discs on a lot of consumer road bikes now thanks to cyclocross pushing the technology.

In summary: disc brakes are awesome and will always stop you, even when it's wet. But only consumer bikes have them, and to make them simple and cheap larger rotors are used with plain discs. Performance discs are vented and floating rotors for long and hard braking periods, and touring people tend to use cables over hydraulic.

And one final thing... mix and match. You'll see professional off-road riders putting large 2-pot calipers up-front on large floating rotors, but then only put a single pot on the rear with a small rotor. The vast majority of braking power goes through the front wheel, and people still obsess over weight to the point of wanting to save a few grams where they can.