rebuilding hubs
 

how often should hubs be rebuilt? after how many miles? talking about a mountain bike, used almost exclusively on road.

How clean is clean?

When you rebuild a hub, what you are essentially doing is cleaning out the old, dirty, contaminated grease and replacing it with new clean grease. Modern sealed mechanism hubs don't allow water and dirt to enter nearly as often as older hubs did so they seldom need rebuilding. Most guys that I know do it annually, but I think that's as much out of winter boredom as it is from need.

This is also a good place to add YMMV. If you ride in the rain a lot or ever ford streams, all bets are off.

Most MTB's use sealed cartridge bearing hubs which are suppose to be lifetime lubed. But the seals really are only effective at keeping out dirt and air, but these seals are not guaranteed to keep water out of the bearings. If not exposed to water you should rebuild the hubs about every 2000 to 3000 miles; if exposed to water then your looking at 800-1000 miles. The reason you need to rebuild more seldom then most think is because the grease still gets contaminated just like car oil even though it's being filtered; and water will get things to rust inside hubs if left unattended.

The cone and cup hubs are actually easier to maintain but still have the same mileage recommendation for overhauls as the Sealed cart hubs. But some cone and cup hubs have a grease injection port that can allow you by squeezing new grease in till it comes out of the side of the hubs clean to go further between rebuilds. I have Suntour Superbe hubs that have never been rebuilt just added grease via the injection port every 500 miles; and now these hubs have over 140,000 miles.

when you say exposed to water, does that mean riding in the rain at all, or riding through streams, or deep puddles and stuff?

Yes to all.

Really??