Hi silverbear. The more weight you pull with a belt drive, the more attention you need to pay to the engineering and fabrication. First, belt selection is more critical. For instance, for just a simple belt drive bike (fractional horsepower) belts, like the 4L or 2000 series, are just fine. When you start to add horsepower and/or weight, then you need to get into the more heavy duty industrial types, like the A and AX series. For more extreme service you may consider automotive belts like 17000 and 9000 series. All these will fit our 1/2'' wide pulleys. I like the AX belts for smaller diameter pulleys because they're cogged and and don't take as much hp to turn. That brings up another factor to consider. I always try to use the largest pulleys while still achieving the desired ratio and asthetics. A larger pulley has more surface area, therefore can get the job done with a little less tension. Which means your bearings will last longer. When you have more weight, you have to gear down so you don't over tax the motor. So, with the proper pulley and belt choices, I don't see any reason for your motor not to pull the weight. And, yes, rain does make the belts slip a little.