The chain on an off-road bike is susceptible to damage due to the excess wear that dirt, sticks and rocks put on the links. And, a broken chain can quickly ruin a ride and force a long walk home. So, the most valuable tool in your seat pack when you're trail biking might just be the chain tool (it extracts and inserts chain pins to separate and join the chain). As long as you carry this important tool (they're small and inexpensive), should your chain separate or a link fail, you can simply remove the bad link and rejoin the chain to continue riding.
Many chains require special replacement pins for repairs. Be sure to carry a spare or two in your seat bag if they're needed. Also, remember that after the repair, the chain is shorter. So, avoid using your large chainring in combination with the larger cogs on your rear sprockets because this can jam the drivetrain.
Practice Makes Perfect
It's a good idea to practice using your chain tool on a length of used chain to get a feel for what it takes to align the chain tool's pin with the chain pin and drive it out. If you know how to do this properly, you won't struggle when something goes wrong with your chain on a ride.
Check Chain Wear
Chains don't last forever. You can keep track of wear by measuring. When new, you'll be able to measure exactly 12 inches of chain between two pins (just hold a ruler next to the chain, align one pin with the beginning of the rule and see if a pin lines up with the 12-inch mark). If the second pin is at 12 and 1/8 inches or more, it's time for a new chain.
Replace The Cogs With The Chain
If you need a new chain, you'll probably also need new cassette cogs. This is because the cogs wear at approximately the same rate as the chain. If you replace the chain without new cogs you'll usually experience skipping, a disconcerting and possibly dangerous slipping sensation when pedaling hard. What's happening is that as you apply pressure, the new chain links can't grip the worn teeth adequately and as a result, they slip off the teeth and jump forward. New cogs will prevent this aggravating (and unsafe) condition.
Keeping a chain in tip-top condition will prevent premature wear. Lubricate the links before they get dry and squeak. And, use a lube that's appropriate for the riding conditions (we can recommend the best for our area). After applying lube, be sure to wipe off the excess because it'll attract grit and grime if you leave it on. If the chain gets grimy, you can clean it quickly by wiping it with a rag, a few links at a time until the entire chain is down almost to bare metal (there should be a thin layer of lube).