So long story short, I have a truck thats being built as a single axle but with 2 10K drop axles on a 40' straight truck. The first drop will be a pusher spaced at 96" ahead of the drive axle. Bridge law states that a single axle is good for 20K lbs. Thats not a problem since that axle will carry at most 9K lbs. The second drop axle will be right behind the drive axle in a tag configuration. it will be as close as it can to that drive axle which is about 56" from hub to hub. Now The single drive axle has a 23K lb capacity and the tag axle at 10K capacity. With the axle up, I will only be legal for 20K on that drive axle. But with the tag axle down, will that axle group technically be considered a tandem group at 34K permissible? I ask this because if the truck is weighted down to max capacity on the rear, the drive axle may be 21 or 22K while the tag axle sits close to its max capacity (estimated 8 - 9K). So would this be legal since it is technically under 34K lbs for tandem bridge law? Or will DOT still see that drive axle as a single axle and can not weigh more than 20K even though it is rated to 23K.
It would be rare that the truck sees a max weight like this but should I come across it, I obviously want to know my limits.
Lift Axle weights - single axle with drop axle bridge law
Anything 8 feet and under is defined by the Bridge Formula as a tandem.
So in your case, both the drive and pusher as well as the drive and tag combos would be considered tandems when calculating bridge weights. And the 3 together would need to be under 45,000 pounds.
Last edited: Aug 26, 2020
magoo68 Thanks this.
In most states that drive axle would be limited to 20K regardless if considered a single axle or part of a tandem. This is info for Tennessee which specifically mentions no axle in a tandem group can exceed 20K
Tennessee Axle Weight Limits
Maximum Legal Weight Allowed
Single 20,000 pounds
Tandem 34,000 pounds
Gross Vehicle Weight
80,000 pounds (except that freight motor vehicles operating on interstate highway system shall not exceed the lesser of 80,000 pounds or the weight produced by application of the formula set in T.C.A.)
Maximum Permit Weight Allowed
Conditions for Permitting Overweight Movements
In general, the maximum allowable axle weights by special permit are as follows:
Mach Thanks this.
- Single Axle: 20,000 pounds;
- Tandem Axle: 40,000 pounds; and
- In no case shall a single axle in a tandem group exceed 20,000 pounds.