By Norman Ford · Updated October 2026
Truck axle guide: steer, drive and trailer axles explained
Axles carry everything: the truck, the load and the forces of braking, turning and bumps. They also decide how much a truck can legally carry. Knowing what each axle does, what wears on it and how weight is shared between axles helps owners spot problems early and load legally.
Inside a truck's axles
Tap each part for what it does and what to check. The wheel end, where most axle maintenance happens, has its own detailed diagram in our truck axle repair guide.
Tap a numbered balloon for the part's job, check and spec.
| ITEM | PART | WHAT IT DOES | CHECK |
|---|---|---|---|
| 1 | Steer axle | A non-driven front beam with kingpins that let the wheels turn. Carries the engine and cab weight. | Kingpin play, tie rod ends and steer tire wear. |
| 2 | Drive axle housing and gears | Holds the differential and gears that turn driveshaft power into wheel rotation. | Gear oil level and leaks at seals and covers. |
| 3 | Inter-axle differential | On tandem drives, lets the two drive axles turn at slightly different speeds; a lock engages it for traction. | Use the inter-axle lock only as the maker directs, never while wheels are spinning. |
| 4 | Wheel ends | Hubs, bearings, seals and brakes at each end of the axle. | Hub oil level, seal leaks, bearing play and brake condition. |
| 5 | Suspension | Air springs or leaf springs connecting each axle to the frame, plus torque rods and shock absorbers. | Air bag condition, leaks, torque rod bushings and ride height. |
How many axles does a semi truck have?
The standard five-axle tractor-trailer has:
- Axle 1: the steer axle on the tractor.
- Axles 2 and 3: the tandem drive axles on the tractor.
- Axles 4 and 5: the tandem axles on the trailer.
Some tractors have a single drive axle, which saves weight for lighter loads. Box trucks usually have two axles, a steer and a single drive. Heavy haul tractors, dump trucks and some specialized trailers add axles, including lift axles that are lowered when loaded, to spread weight and stay legal.
Steer, drive and trailer axles
Steer axle. The front axle turns the truck. It is a solid beam with kingpins at each end. It carries the weight of the engine and cab, and its rating often limits how much weight can sit on the front.
Drive axles. The rear tractor axles contain gears, called the differential, that turn power from the driveshaft into wheel rotation and let the wheels on each side turn at different speeds in turns. On a tandem, power goes to the forward drive axle and through an inter-axle differential to the rear one. The axle ratio, the number of driveshaft turns per wheel turn, affects fuel economy and pulling power.
Trailer axles. Trailer axles are not powered. They carry the load and brakes. Many trailers have a sliding tandem, so the axles can be moved to shift weight between the trailer and the tractor.
Axle weight limits
On the Interstate System, federal rules in 23 CFR 658.17 limit weight to 20,000 pounds on a single axle and 34,000 pounds on a tandem, within an 80,000 pound gross limit and the bridge formula for axle groups.
Source: 23 CFR 658.17, checked October 2026
Every axle also has a weight rating from its maker, shown on the vehicle's certification label. You must stay within the lower of the legal limit and the rating. To check where your weight sits, weigh on a certified scale; our guide to truck axle scales explains how scale tickets show each axle group.
The fifth wheel position on the tractor moves weight between the steer and drive axles. Our guide to the fifth wheel on a truck explains how sliding it works.
What wears on a truck axle
- Wheel bearings and seals. Bearings wear and seals leak. A leaking seal lets oil or grease out and dirt in, and a failing bearing can overheat.
- Brakes. Linings, drums or rotors, and slack adjusters sit at every wheel end.
- Kingpins and bushings on the steer axle wear and cause loose steering and uneven tire wear.
- Differential gears and bearings wear slowly and are affected by gear oil condition.
- Suspension parts: air springs, shocks, torque rods and bushings.
Signs of axle problems
- Oil on the inside of a wheel or tire, a sign of a leaking hub seal.
- Uneven or rapid tire wear on one axle.
- Noise from the drive axles: whining, humming or clunking.
- A wheel end that is much hotter than the others after a run.
- Steering wander or vibration.
- The truck leaning or sitting lower on one side.
Axle ratios and gearing
Drive axles contain gears with a ratio, such as 3.36 or 3.73, that tells you how many times the driveshaft turns for each turn of the wheels. A lower numerical ratio lets the engine turn slower at highway speed, which usually helps fuel economy. A higher ratio gives more pulling power, which helps with heavy loads, hills and frequent starts. The right ratio depends on the engine, transmission, tire size, typical speed and load. It is set when the truck is specified, so check it when buying a used truck for a particular kind of work.
Lift axles and tag or pusher axles
Some trucks add an extra axle to carry more weight legally. A pusher axle sits ahead of the drive axles and a tag axle behind them. Many of these are lift axles, raised when the truck is empty to save tire wear and lowered when loaded. Lowering the axle spreads the weight and can raise the legal limit under state rules and the bridge formula. Lift axles need the same care as any other axle: tires, brakes, wheel ends and the air system that raises and lowers them.
Box truck axles
Most box trucks have two axles: a steer axle and a single drive axle. The rear axle carries most of the cargo weight, so its rating, shown on the certification label, often decides how much the truck can carry. Overloading a single rear axle wears tires, brakes and bearings quickly and can put the truck over its legal limit even when the gross weight looks fine.
Keeping axles healthy
- Check hub oil levels and look for leaks at every PM service.
- Change drive axle gear oil on the OEM schedule; it is often a long interval.
- Inspect kingpins, tie rods and steering joints regularly.
- Watch tire wear patterns for alignment and suspension problems.
- Keep axles within weight limits; overloading wears every part faster.
The preventive maintenance schedule generator puts these intervals on your calendar.
Axles and the electrical system
Axles do not need electricity, but the anti-lock braking system sensors at each wheel end do. A damaged ABS sensor wire or a weak electrical system can trigger ABS warnings. Our truck battery guide covers the electrical basics.
Where we fit
We are a dispatch desk, not a shop. We dispatch 26 ft box trucks and larger, from two-axle box trucks to heavy haul rigs, and we check axle weights and equipment fit on every load. Learn more about our dispatch service.