bobc wrote:"handbrake" - my trike back brake lever incorporates a small button which locks the brake on. A trike of course can run away for miles....
"four brake levers" - not necessary, there are several solutions for driving 2 brakes from one lever. I have used brake levers that actually incorporate a balance bar for 2 cables, and there are widely available in line "one cable in, 2 cables out" widgets (these don't balance so adjustment is more important, but in practise not difficult - this solution is used on the front brake(s) of my trike...)
The question is,is it necessary to have two brakes on one wheel ?
On tandems it can be,especially in high mountains with big descents but one of those brakes needs to be a drag which needs to be a drum with a fair bit of heat sink and which can be set at a variable measure,preferably on a friction lever,with the other two brakes used to bring the bike to a halt as per usual.The drag can also be used as a parking brake.
I question the need for two brakes on one wheel especially if they're rim brakes which will generate more heat on one rim.of course if one is a disc and one a rim on the same wheel that problem is solved.
I know trikes use two brakes on one wheel but that's a compromise due to not having rear brakes,though that's not an insurmountable problem and some trike do have rear brakes.
Two cable brakes from one lever needs more power on the lever for the same clamping force per brake and an unequal cable adjustment can lead to trouble.
I know that for some people the problem is insurmountable for a couple of reasons I can think and that is unavoidable,but usually it's undesirable.
IMO if a bike needs three or four brakes to stop it(other than a drag on a tandem)it needs two effective brakes not an extra two brake to make up for the ineffectiveness of the other two.
We've been riding tandems for more than 20 years and outside of long mountainous type descents I've never found the need for more than two brakes even with an all up weight(bike,luggage,riders) of at times 190+kg.
If someone wants a third brake it's up to them,need in normal use is another matter.