One inch can mean tons

Minimum lift axle spacing can be tricky. Suppliers have become really good at packaging their products so more axles can go under a truck. The catch is, math (isn’t that always the case?). You can get yourself into a scenario using the Federal Bridge Formula where the pusher is too close to the tandems, and it provides NO BENEFIT.

Federal Bridge allows for 34,000 lbs. on a 3-axle group spaced up to 96” and 42,000 lbs. on the same 3-axle group spaced at 97” or more. Since tandem axles spaced from 40-96” are permitted to carry 34,000 lbs., adding a third axle within the same envelope makes no sense.

To illustrate this, see Scenario 1. With 54” tandem axle spacing, and a lift axle set at 42” ahead of the tandem, 34,000 lbs. is the max ground load for the group. With 52” axle spacing (less common) and lift axle set at 44”, or less, you would have the same reduced capacity situation.

Scenario 2 spaces the pusher forward one inch to 43” with 54” tandems (or 45 with 52” tandems) to maximize the advantage added by the auxiliary axle. To calculate minimum spacing always just subtract the tandem spacing from 97”, or 8 feet 1 inch. Never locate your lift axle closer to the forward drive axle than this result.

With multiple lift axles we see the benefit of pusher axle packaging. The group of lift axles can be placed at 80 inches, or perhaps less. This helps with locating other items on the rail, though may not allow you decrease your overall wheelbase, that depends on the GVWR requirement. You might ask why we do not care about the spacing for 3 axles in this instance. Most groups of lift axles positioned like Scenario 3 are individually rated at 8,000-10,000 lbs. The weight combination will never exceed what is permitted with a 40-96” envelope. In fact, after a Federal Bridge evaluation is complete, most light weight lift axles that are grouped cannot legally carry their rated capacity.