I'll be driving for 4 hours with my bike on the rack through heavy rain. Will this 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.
If you're driving on the freeway, then you might be going 75 mph (120km/h) with a headwind adding another 15mph (25kph) so a total of 90mph (145kph).
That's fast enough to drive rain into the bearings in the headset, which is oriented right into the wind. Rain can also get into the bowden cables for your derailleurs and brakes, which will hasten rusting and sticking.
You can get rack bike covers (condoms). The main purpose of these is to reduce the amount of bugs that collect on your handlebars and seat, but they'll also protect your headset -- as well as your brifters and brake/derailleur cables.
NB: Your bottom bracket and axles are oriented perpendicular to the wind when on a roof rack so they'll get less water driven into them.
It will be about equivalent to exposing it to maybe 4-10 times the rain standing still. And the higher the speed the worse the effect will be.
The problem is that the forward motion will tend to drive rainwater into the bearings, and, to a lesser degree, into cables and frame members.
But 4 hours is probably not long enough to be especially concerning, if the bike is well-maintained to begin with, and if it has a good chance to dry out afterwards (vs, say, being immediately locked away in a damp shed).
I'm trying to think if there's any good reason to mount frontwards vs backwards, but can't think of any. Sideways would be worse, in addition to the obvious wind resistance problem.
There are many people who commute or compete in inclement weather with out issues. just prepare for the conditions. I would apply a coat of wax to the frame and any chromed parts. Look for a chain lube designed for wet conditions. Liberally apply the lube to the derailleur pivot points, exposed cables and the chain. Do not get lube on the brake surface of the rims or rotors. When you arrive at your destination wipe off any standing water and relube if you feel you need to. Avoid covering the bike with a tarp or plastic cover. The wind will cause the cover to flap and buff the paint off the frame.