Hi.

I lived in the chicago area for 50 years and drove the salty streets daily.

Once a week in the winter and summer I took my car to the carwash [ it was free with a fill up ] and flushed the undercarraige and guess what? It still rusted. That road salt is murder. You can't stop it but you can slow it down by flushing. On new cars, undercoating like Ziebart helps a lot. If you are going the flushing route, wait until it's over freezing just to be on the safe side.

Funny thing, In Wisconsin, they sometimes put cinders on the roads. It helps as far as traction is concerned but plays heck on the paint.

So as you see, there is no easy answer except for what I did.

I retired and moved to Florida where the only salt we get is on our fries! {black}:hi:  

I would rinse just the bottom. I'd rather have ice under there than salt.

Hall, my only experience washing a car at a car wash in freezing temps was in the south one winter and not near -14....basically all I ended up doing was coating the car with ice.  I'd think your results would be the same or worse, and who knows what might happen by ice collecting underneath.

Maybe someone with more experience in that can advise better, but I would wait until it was above freezing.

good idea

Move to somewhere colder where the town gave up for melting snow and ice on roads. 

I would wait until temperature is a couple or three degrees above freezing, … and I wouldn’t use car wash since car washes recycle water, … that is they just filter dirt, not salt.

Anyway, … I wouldn’t bother to wash the car, but just keep rinsing it with a clean tap water whenever possible ( … temperature few degrees above the freezing - enough time to dry the car in a breeze). Otherwise, water gets into crevices, expands as it freezes, damages paint and sealers, … causes cute little places to accelerate rusting!)

F.S.