What can you do, if anything, when you're driving with bikes on your car rack and it starts to rain? Rain is forecast here for Thurs, our travel day. Try to cover the headtube and seatpost somehow? I'm thinking duct tape (maybe electrical tape = less residue) around those joints?

Pile them in the back of the car?

Cover leather saddle with two plastic bags, and tape well with packing tape or Duck tape to keep it from flying apart.

Be ready to lube chain, pivots, and cables when you can dry the bikes off.

My bike just gets wet.. but I'm usually riding it not transporting it.

Truly! Bikes will withstand rain and washing and all kinds of things. You sound like you are a concerned bike owner and likely one who takes care of your bike. Water is OK! It will survive.

When I have my wife's bike and my bike on the trunk rack and there is a chance of rain I throw a Trader Joe reusable shopping bag over each seat. The bags are fairly long and cover both seat and the joint where the seat tube goes into the frame. Then tie them snug with some cord.

I found that I did not like water getting down around the seat tube clamp. Simply promotes corrosion. Plastic bags will not take 60 to 65 mph on the highway. If you have "stock" brake and shifter cables be prepared for problems if rain water gets into the cable housings. On all four of our bikes I have replaced the stock cables with stainless steel cables. The stock cables corroded from water getting into the housings. The more expensive stainless cables seem to last forever.

I usually just cover leather parts with plastic bags or shower caps.
Then I give the bike a bounce, and the wheels a spin to get excess water off the bike.
I sometimes lube pivot points and exposed shifter/brake cables.
I use Chain-L No.5 chain lube, so I don't bother with relubing the chain after rain.
I am trying to see how it does with no relubing.

People say:
Clear silicone caulk the headset where stem enters (if threaded headset). Caulk top of the seat tube (where seatpost enters).

If the bike has a leather saddle, I'll put a plastic bag over the saddle. Otherwise nothing.

A bike bra will do the job best, and waaayyy better than electrical tape!

Here's a link to a good one, Scicon, at REI: https://www.rei.com/product/723020/sci-con-bike-defender



As noted upthread, the idea is not to keep the bike dry, per se, as they can take the water, but rather to prevent caked on bugs and grime, and to ameliorate the driven rain at high road speeds.

You are in LA .. Gaffer's tape is what they use in 'the Business', There.
it also leaves less adhesive residue than cheap generic duct tape.

This. Seriously. You bought a piece of outdoor equipment and you're afraid to take it outside? (I know, I used to live in Santa Monica, and a quarter inch of rain paralyzed the whole southland...).
Bikes' vital parts are pretty well protected from rain. I would cover a leather saddle, like a Brooks, if you have one, and after the ride, bounce the bike to shake off water, then lube the chain and pivot points on the brakes and derailleurs. But it's in no sense a threat to the bike.
Do watch out for oily spots on the pavement, though. You don't get much rain down there, and the first storm floats up all the crud on the road.

WTF?!?!

Seriously people Errr... kalifornians... always concerned.

So many vicious a-holes abound. Just asked a frickin' question as I was concerned about water seeping into the BB via the seat tube/post joint or into the headset bearings. Sorry I appear to care more about my ride than you or that I may not know as much mechanically. I bet I could out climb you kooks though. Neener-neener.

BTW, detractors, I'm talking about making a 6 hr drive up the coast through constant rain, not some passing shower, the latter I could care less about.

Nice thought but car is full already.

Anyhow I'm packing my rain gear in the bottom of 1 pannier, just in case..
the weather systems are flipping, summer's tailwinds are winters headwinds , up here.

Pleather covered saddles, are a + ...

Plan ahead - a simple jar of Vasoline provides pretty good waterproofing if you don't have access to a transparent grease like Michlin. Smear around the headtube, seat-tube,BB and wheel bearings. Be generous. Take the excess off with a rag when you get where you're going. Rubber seat covers are available for riding in the rain with or without you on the seat.

I'd recommend against using a colored grease as it can actually stain the paint.

A couple layers of Saran Wrap(tm) secured with plastic packing tape around the headset. Everything else is probably OK.

kitten poo!

Take the saddle and post off. Like said before, Saran wrap the tube and headset . Tape the Saran wrap down with a good sticky tape like duct tape. You dont need to put tape on the painted surface, just tightly around the wrap to keep it in place and prevent the wind from tearing it apart.
You could do the same for the brake levers etc.