Belts slipping (street setup 83mm)

So when im either at a high speed and go to brake my belts are slipping same as if im going down a decent sized hill anybody know if its that my belts need to b tighter or does it sometimes happen to you guys?????????

 

Check the tension of the belts dude, If they seem ok have a look at the condition of the belt and the motor cog plus the drive wheel.

 

i had to clean the dirt off the screw holes on the motor plate because it kept slipping which made the belt too loose. just took a qtip and dipped it in some alcohol.