I replaced an old Craftsman garage door opener with a new one. The replacement matched the hp of the old unit; 1/2 hp chain drive. During installation I noticed the new chain bar was constructed much lighter than the old one. None the less I installed everything new. The door operates as designed but I noticed the bar bends about 3" during the door open cycle. Seems the light weight bar bends under the torque of the motor until the first rollers make the turn at the top of the track run. This flex actually pops the door open initially as the track straightens out. Also, in order to close my door completely I've had to adjust the closing pressure until the bar flexes the opposite way, again about 3". My old unit never did this. I followed the instruction exactly as specified. I've also checked my door to make sure it opens and closes freely. What did I do wrong?