Undercarriage car wash - GIF

[Frank S]

But why wash under a car anyway?

One good reason is here in the US the Highway departments seem to love to spray brine on the motorways every time there is even the slightest hint of possible freezing precipitation, if you drive on a sprayed road soon afterwards if it happens to rain sleet or snow the spray from your tires coats the underside of your vehicle with instant corrosive, If it happens to snow the snow plow trucks will distribute sand &salt mix as they plow adding to more corrosive causing materials, Regular washing of the underside may help to stave off your vehicle rotting away from under you. In the coastal areas where the humidity is always high it is more important to have the cavities in the underside of a vehicle coated with a waxlike substance than simply spraying an under coat especially on the newer pickups which all now have boxed frames/