Will driving through heavy rain with bike on my car damage the bike?
The prevalent thought among many members of the Cycling Illuminati (trademark) is that the high vehicle speeds will drive rain into your bearings on a bike if it is mounted to the roof rack. Seems kinda plausible, but here is a GCN video about how hard it is to actually drive water past seals and into bearings with a pressure washer:


Should You Jet Wash Your Bike?


  Spoiler: It took them a long time, with the pressure washer jet focused directly on the bearing itself at point blank for 3+ minutes to finally drive some water past the seals.



Unless you are traveling down the Autobahn at top speed with your bike mounted to a roof rack you hyper-car, you will probably be just fine.  Pressure washer water velocity is about 243 mph (391 km/hr). A Bugatti Veyron 16.4 Super Sport is the world's fastest road legal production car with a top speed of 431 km/hr (268 mph).  Technically, if you had a  special rack designed for your Veyron (wouldn't want to upset the handling at 400 km/hr), you might be able to generate enough velocity for this to be a concern. Even then, it is likely only the head set bearings (or maybe bottom bracket if you have external bearing version) that will take the brunt. Even then, these bearings are oriented in a plane perpendicular to the plane of motion (which will greatly reduce the velocity of the water that actually interacts with the bearing seal face).  Plus, don't forget that the front wheel usually rides inside the car and the rear hub is left on the bike, but will be running in disturbed air, which will further reduce the force of the rain water.