Generally disc sytems are heavier and hazards do no come at you as quick on the road. Maybe the number one reason though is performance in the wet and especially mud. I have never used discs, conventional pads work good for my local terrain, however in severe mountainous areas I would get a disc equipped bike.
Discs on a road bike are kind of overkill for the tires. You have less than a square-inch(rough guess) of contact patch with the road. Anything stronger than the typicaly "sissy" brakes of a road bike, and you'll just lock the brakes up and slide. You're better off with more of a anti-lock brake system.
I've seen a couple touring bikes with disk brake systems but for most riding that kind of stopping power is not needed, plus I imagine some bad things might happen if you tried to stop on a dime while going 25-30 mph.
You have to think also about the riders of the different styles of bikes. Mountain Bikes are ridden by big manly men. We need more power to stop all of the huge muscle and manliness we have on a bike. Road riders are scrawny little waifs, they are the starving supermodels of the biking world. Road bikes are light weight and weak, combined with the road riders who are light weight and weak there is no reason for real powerful stopping power. Heck if a road rider were to pull the lever on a disc brake equipped bike it would fling them OTB like a dog shaking a washcloth. Not a pretty sight.
So to answer the questions road bikes don't have disc brakes to protect the road bike riders.
The advantage of the disk is that the disk is farther away from the water that can reduce the efficiency of the braking system. Second reason for disk is to prevent sticks and other foriegn objects from getting jambed in the wheel and frame.
Touring bikes and tandems typically will have the disk brake available on the rear wheel to better handle higher capacity loads and heat problems with long down hills that a road bike does not have.
Your comparing sports cars (road) to tractors (off road), Cadillac (touring), and Bus (tandems).
Interesting arguments, I recently read a Tweet from LA, while he was training for Leadville, he was wondering when we would have discs on road bikes, hmmm? I just ain't smart enough to figure this one out, but admit I just don't seem to need the stopping power so I have pads.
I tend be that one guy that likes to stop over there, instead of stopping a few feet back where I should have.
Or taking a corner a bit too fast and scaring myself into a panic brake to slow it down real quick.
To my knowledge there is no such thing as a disk brake system for road bikes. I've sometimes wondered if they will appear some day but so far.... no. When you think about it having a caliper squeeze the rim is a bit primitive but I guess the rim is already there for the squeezing as opposed to a disk which has to be added, thereby heavier, more stuff, wider, less aero. Naw, let the gorrillas on the MTBs keep there disks.
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