Uprated rear brakes, wheel keeps locking up.

[Agemax]

cannot understand why, any ideas?

[spchips]

I'd like to see the story behind that

[hampshirehog]

Bet it's still not as bad as a '76 Bonnie....

[bellucci]

At least its not on the front wheel... or maybe there sillyness up there also?
Curt

[Nessism]

Ha, bet they could have squeezed in a forth caliper if they closed up the angle between them. Crappy design work.

[GregT]

Obviously a mastercylinder mismatch...

And someone has too much time on their hands.

[kawfeedave]

I'd like to see the story behind that

It's a 'stunt' bike. Look at the giant rear sprocket behind that giant rear disc. They use the super-low gearing of the sprocket to make it easy to wheelie, and the stronger rear brake to hold it at the right angle/save their ass if they roll it up too quickly/or bring it back down when they're done doing whatever they do.

Contrary to most other forms of riding, stunters use the rear brake more than the fronts (the front wheel is usually off the ground).

Another aspect is that they also have a thumb lever, like the throttle on an ATV, to work the rear brake so they can apply it if they have their right foot off the peg. If you note the brake lines, you can see that two of the calipers are linked together, and one has it's own line, so the dual calipers are probably run off the thumb lever, and the single is probably connected to the foot pedal.

The last clue that it's a stunt bike is the paint peeling off the swingarm and chipping off the edges of the bodywork. Normally, a custom rear brake like that would be on a 'nice' bike. If you were modding a bike that like, wouldn't you at least rattle can the swinger with some epxoy paint, or something? Stunters know they're going to toss their bike down at least a couple times per month (or week), so cosmetics are not a concern.

[Agemax]

that is correct, you can see the single line going direct to the rear m/c, and the other line heading up to the front of the bike.
as you say a lot of stunta's utilise 2 calipers on the rear.
also if you look the calipers are mounted to the bracket using "washers" to get the right spacing. if it were a race bike or a "nice" bike then they would have made up some proper spacers to fit.