Why the underbody of your car matters

What is down there

Lie on the ground beside a car and look under the doors and you will see most of the parts that keep it safe to drive. On a typical vehicle that includes:

• The frame rails or unibody structure, plus the front and rear subframes bolted to them
• Brake lines, often steel, running most of the length of the car
• Fuel lines and the straps holding the fuel tank up
• Control arms, springs, sway bar links and their bushings
• The exhaust pipe, hangers and heat shields
• Floor pan seams and the inside edges of the rocker panels
• On electric vehicles, the battery enclosure, its fasteners and cooling lines

Why it gets the worst of it

Your own tires throw slush straight up into the wheel arches and along the floor, and the car ahead adds more. Brine hits these parts at highway speed and packs into every gap between brackets and seams.

The paint on top at least gets rained on in summer and dries in the sun. The underside never does. It stays shaded and damp, and it goes through heat cycles from the exhaust and brakes that keep salty moisture active. A car can look spotless from the driveway and carry a full winter of crust underneath.

What to look for

Most Ontario drivers swap between winter and summer tires twice a year, and that is the best time to see the underside with the car on a hoist. Ask whoever does the swap to tell you if they notice anything, or take a look yourself.

Watch for brake lines with a swollen or flaking surface, scale that comes off the subframe in flakes, bolts with heavy rust around the threads, heat shields that rattle at idle, and bubbling along the bottom of the rocker panels. A clunk over bumps can point to a corroded sway bar link. Catching these early is the difference between a small repair and a big one.

How to keep it clean

A garden hose on the driveway mostly reaches the outer edges. To get into the floor and the middle of the car you need spray from below, which is what an underbody spray in an automatic wash is for. Through the salt months, aim to have the underside rinsed every week or two, then give it a thorough flush in spring once the roads stay clean.

If you have an annual oil-based rust-proofing spray done, which is common in Ontario, ask the shop how long to wait before an underbody wash. A fresh oil coat is still settling for a while, and high-pressure water can remove some of it.

Which wash covers it

At Fast Lane, the under-car spray is part of the Full Wash and the Luxury Wash. The Luxury Wash follows it with a rust inhibitor. The Quick Wash cleans the body but does not include the underbody step, so it is better suited to summer dust than January salt.