Everything expensive on a 4WD hangs underneath it. The sump, transmission, transfer case, fuel tank, diffs, brake lines and steering parts all sit a few hundred millimetres off the ground, and whichever one gets hit is the one that ends the trip. Underbody protection is what stands between those parts and the rock you did not see coming.

The two mistakes we see most are buying the pieces in the wrong order, and buying side steps in the belief they are rock sliders. Both are easy to avoid once you know what each piece is for.

Bash plates and the parts they guard

Sump or front plate. The first one to buy, every time. A holed sump on a track means an engine with no oil, and the recovery bill makes the plate look cheap.

Transmission and transfer case plates. These cover the next two components back, and on many vehicles they bolt to the same chassis points as the sump plate as a matched set.

Fuel tank protection. Increasingly worth having on long-range tanks, which hang lower and hold a lot more expensive liquid.

Diff and lower control arm guards. Useful on vehicles that see rock steps and ledges regularly.

Steel or aluminium bash plates?

Steel plates are the strongest, and they can be panel-beaten back into shape after a hard hit. They are also heavy, commonly adding 30kg to 60kg for a full set, although that weight at least sits low, where it does the least harm. Aluminium plates save meaningful weight and resist corrosion, but they cost more and cope less well with repeated heavy strikes.

So are aluminium bash plates any good? For a touring 4WD that meets rock now and then, alloy is a fair trade. For a vehicle working hard in rough country week in, week out, steel earns its extra weight.

Two details separate good plates from ornaments. They should bolt to the chassis or engineered mounting points, not to thin sheet metal. And they should give you service access, through drain holes or quick-release sections, so an oil change does not turn into a two-hour job. Ask about both before you buy.

Our underbody protection range covers steel sump, transmission and transfer case guards for a long list of popular 4WDs, fitted in the Brookvale workshop, where we check clearances against the rest of the build while the vehicle is on the hoist.

Rock sliders are not side steps

If you take one thing from this page, make it this one.

Side steps bolt to the body or sill and exist to help people climb in and out of a tall vehicle. They are light and neat, and if you land the vehicle's weight on one against a rock, it will fold and take the sill with it.

Rock sliders, sometimes sold as side rails, bolt to the chassis rails and are engineered to carry the full weight of the vehicle. They protect the sills and door panels from rock strikes, let you slide off an obstacle instead of crushing the bodywork, and on a well-built set you can jack the vehicle from the slider itself.

Many sliders include a step section, so you can have both, and that is what most touring builds should run if the vehicle sees rocky tracks. If your driving is beach, gravel and formed trails, a set of side steps does the job it was designed for, and saves weight and money.

Where protection fits in the build order

Sequencing saves money. Front protection and a sump plate usually come first, because they cover the damage with the biggest consequences. Sliders follow once the vehicle starts seeing rock. Rear protection and rated recovery points come with the towing and recovery side of the build.

Plan for two flow-on effects. All that steel adds weight, typically 50kg to 120kg across a full protection package, and it comes out of the same payload as everything else. It is one of the usual reasons a touring build ends up needing heavier suspension or a GVM upgrade. Protection also changes clearance and departure angles, sometimes for the better with a well-shaped plate and sometimes for the worse with a poorly designed one, so the design matters as much as the material.

What it is worth

Next to a cracked sump, a punctured fuel tank or a caved-in sill, underbody protection is cheap insurance. It is also the category where we most often see people spend nothing and then spend a great deal. On a vehicle that tours, a sump plate is closer to maintenance paid in advance than an accessory.

Frequently asked questions

Want to know what is exposed on your vehicle? Call the Brookvale workshop on 1300 856 888 or book an inspection, and we will get it on the hoist and look underneath with you.