[kipps]

Originally Posted by 1976gmc20

My folks had a 1983 C20 Suburban and as I recall it only had a semi-float rear axle. That seemed like a cheat to me.

The semi-float 9.5" 14-bolt axle was not a light vs. heavy gvwr thing. All the 1980's c20 and k20 trucks got the 9.5" regardless of weight. The truck in my signature came with a 9.5 axle, 3.42 gears, and a 8600-lb rating.

The only thing that changed that was if a c20 was equipped with a 454 engine. It seemed that GM figured a 454-equipped c20 was being purchased for heavy towing, so they upgraded those to the 10.5 full float axle. In other words, the 9.5 was plenty strong enough for vertical loads, but the smaller ring gear wasn't considered adequate for the constant heavy towing loads.

This bears out in the modern axles as well. During the early 2000's, GM had two axles under the 2500hd trucks. One was a AAM 10.5 axle that was a near twin to the 1970's/80's 14bff axle. The other was a AAM 11.5 axle. Both axles had the same wheel bearings and hub assemblies. The difference was in the center section only. The 6.0 trucks got the 10.5, and the 8.1 and Duramax trucks got the 11.5. So again, the vertical load was not the issue, but the trucks that were more likely to be pulling heavy loads needed the larger ring gear size.