Editing Vehicle Design Guide (section)

=== Armor Facings ===
You should aim to put on at least 70-80% of the maximum [[Armor|armor]] points that the chassis can take. Bottom and top armor are less important as under normal circumstances those won't take damage so 4-6 points there is enough, more than 8 points would generally be too much.

Facings with weapons on will need more armor as they'll be facing the enemy and thus also very likely exposed to enemy fire more than other sides of your car will be. So if you have front mounted weapons, put less focus on the rear armor.

The sides of your car are also quite important as it's usually the easiest part of your car to hit so don't go light on those.