Spring weights – Springs are stocked at M-3 to handle doors with springing weights of 45 lbs through 300 lbs. 205 lbs through 300 lbs systems have a center bearing plate and weld bracket assembly for added support.
Drum sizes – Standard M-3 drums are 4” diameter. Both 3” diameter and 6” diameter drums are available. The 3” drum is often used in trailers with less than 6” of headroom clearance above the door opening. The 3” drum will carry only 150 inches of cable and requires 10 inches to remain on the drum as the safety wrap. The 6” drum is often used in extra tall trailers. The 6” drum will carry 240 inches of cable and requires 29 inches to remain on the drum as the safety wrap.
Trailers with extra heavy doors – If the springing weight of the door exceeds 315 lbs, M-3 does not recommend using a spring assist. We recommend that doors exceeding 315 lbs be raised and lowered using an electric winch of the type used in loading boat trailers. The cost of a winch is generally similar to the cost of a spring assist for these extra heavy doors, and the safety factor is greater with a winch system.