By "driving" rain I mean, I'll be driving. In the rain (according to the forecast). With the bike on top of the car. For about 350 miles.
I'm (a little) worried about grease getting washed out of bearings.
The headset is loose ball, with very thick CV joint grease in there.
Wheel hubs are Shimano cup and cone.
BB is Shimano UN55.
Pedals are Shimano T8000.
I guess the one most susceptible is the headset?
Any suggestions as to how to protect the bearings? Am I worrying unduly?
This well be the highlands of Scotland, West coast.
I could take tools and tubs of grease, and service things when I get there. But I'd rather not.
Thanks.
Your concern is real and rational. Jets of air (wind) can indeed drive water into bearings, this in spite of some sort of seal on the bearing.
In engineering, there's a mantra that a single seal cannot separate two different liquids. All good seals are designed with this in mind and are doubled up. Good components will thus have two seals.
Let's first consider the familiar lip seal. I tis a contact seal that lets the lip ride on the inner axle shaft. It is rubber and like any rubber, it abrades when scraped against a smooth dry surface. Run a pencil eraser over a mirror if you want to understand the effect. if the mirror is moist, the rubber doesn't abrade. It is very easy to demonstrate - try it yourself. Thus, in bearings, lip seals are designed (or it just so happens) that they leak a tiny bit - grease creeps past the lip and escapes the bearing. This is good, because it lubricates the seal. It works so well, that the seal lasts as long as the grease. As soon as the grease is depleted, the seal is ground down and destroyed, making most refill attempts futile. Unless of course, you refill before the bearing starts to squeak and grind. The fact that the seal allows grease to escape means that water can ingress. A single seal cannot separate two liquids.
Now back to the double seal I allured to. A good seal that is intended to isolate rain water and bearing grease, has to be a dual seal with a neutral area in between the two where mixing doesn't matter. The way they do this can be seen on well-designed hubs - Campag, Shimano, Chris King etc. The outer seal is a contactless seal called a labyrinth seal and the inner seal is the well-known contact seal, called a lipped ring.
The contactless/labyrinth seal is two interlocking, non-touching rings. Toi visualise them, make a C shape with both your hands. You'll end up with two Cs facing each other - one proper way, the other reversed. Now offset the one slightly so that the bottom part of the C is in the centre of the hollow part of the other C. Bring them closer and visualise an S in the air-gap between the two. Now imagine that the two Cs are the cut-through profiles of rings. rotate your hands and therefore the two Cs into each other as if the two rings are rotating freely viz a vie each other.
It may help you if i you have a Shimano wheel handy and you can look at the hub. The so-called dust cap is the inner C and the hub itself forms the outer C. Now, if water were to enter the gap, you can see that it just drains out the bottom. That's your outer seal.
Inside there is the more familiar contact seal you know so well. There's your two-seal system - the labyrinth seal keeps the water away from the lip and prevents it from saturating the outside of the lip seal and allowing it to creep inside.
Labyrinth seals have limits. They cannot be submerged - that simply overwhelms the airspace inside. Also, the gap between the two cups has to be larger than the maximum gap that can be bridged by water's surface tension and capillary action.
Back to your bike on the roof. One advantage is that wheels, crank or fork isn't turning whilst the seals are overwhelmed by a jet of water. However, if the jet is powerful enough, it will force water past the lip seal and pump it into the grease. Think power washers. These should ideally not be used on bikes and if they are, the jet should not come near any bearing seals, whether doubled up or not.
A good hub with labyrinth outer seal will probably OK for most journeys in the rain. Poorly designed hubs with only a lip seal will not do well. Water will get in. Hubs that do particularly well in the rain are Shimano MTB hubs with three seals - an outside rubber cap that rotates on the rim (without lubrication), an inner steel or alu labyrinth dust cap and an inner lip seal.
Headsets are generally only protected from water raining down on the bike, not shooting up from the wheel directly into the bottom bearing. Some air dynamics on a car roof rack may or may not mimic an upsteam of water onto the bottom bearing.
BBs are a mixed batch. Hollowtech has a plastic lip that acts as a labyrinth seal - kinda OK but not great, SRAM has rubbish left bearing seals, BB30 has only a lip seal etc.
Grease getting washed out isn't the problem. This doesn't happen. A tiny bit of water that gets in, is the problem. That water then cannot escape easily, emulsifies with the grease (which then cannot evaporate) and rusts the bearing.
One last thing: grease is NOT waterproof, no matter what it says on the tin and grease is not a waterproofing agent. It cannot be smeared over a dynamic gap to keep water out. It won't.
Therefore, is your bike safe on the roof? It depends.
