An exterior wash makes the fleet look right. An undercarriage wash is what keeps frame rails, crossmembers, air tanks, brake lines and fifth-wheel plates from rusting out early. In the Northland, where MnDOT and the counties treat roads with salt brine that clings far longer than rock salt did, it is the add-on we recommend most for anything that runs all winter.
What an undercarriage wash is
With the exterior done, the crew go under the unit with a wand on a long lance and hot, high-pressure water, working from front to back along the frame rails, crossmembers, suspension, tanks, battery boxes and the inside of the wheels. On trailers that includes the landing gear, the slider and the rear bogie. The goal is not shine, it is to get the chloride off the steel before it has a week to work.
Why brine is worse than it looks
Salt brine, the liquid magnesium and calcium chloride mix sprayed on highways before a storm, is hygroscopic: it pulls moisture out of the air and stays wet on the metal long after the road is dry. A dry white crust on a frame rail is still corroding underneath. Rinsing it with cold water helps a little; hot water with the two-step pre-soak takes it off. What road salt and brine actually do to a truck.
What it protects
- Frame rails and crossmembers, where rust means an out-of-service frame
- Brake lines, air tanks and valves, which fail inspections when they corrode
- Fuel tank straps, battery boxes and steps
- Trailer landing gear, sliders and ABS wiring
- Electrical connectors and lighting harnesses on the back of the cab
How it is priced and scheduled
Undercarriage washing is an add-on priced per unit on top of the regular wash, with a lower recurring price once the unit is on a cycle, because a frame that is rinsed every two weeks takes a fraction of the time of one that is done once in March. Most fleets add it to every wash from first snow to the spring thaw and drop it in summer. You can also book it as a one-off before an annual inspection or a sale.
Do you use an undercarriage flood bar or a wand?
A wand on a lance. A flood bar on a mat only reaches what is directly above it; a crew member with a wand reaches inside the frame rails, behind the tanks and up into the wheel wells, and can see what came off.
Can you do the undercarriage on equipment?
Yes. Belly pans, tracks and the underside of a loader or excavator are part of a heavy equipment wash, and a good undercarriage rinse is the first thing a mechanic wants before a service.
Is it worth it on trailers, not just tractors?
More so. Trailers sit for years and get the least attention, and a rusted-out crossmember or slider on a 53-footer is a trailer off the road. The ABS harness along the frame is also the first thing brine eats.