Airing up and putting it all back together
Okay. Tube's seated, tire's back on the rim, bead's tucked in all the way around. This is the last stretch and it's honestly the part people rush because they think the hard work's done. It mostly is. But there's a step here that'll bite you if you skip it.
Before you air it up all the way
Give the tire a little air first. Not full pressure, just enough to give it shape, maybe 10-15 psi. Then stop and look at it.
Spin the wheel slowly and check both sides of the tire where it meets the rim. You're looking for the bead sitting evenly all the way around, and you're looking for the tube. If the tube's pinched between the tire and the rim anywhere, you'll usually see a little bulge or an uneven spot, sometimes you'll hear a faint hiss if it's bad enough already.
This is the step I skipped on the very first flat I ever fixed for my kids. Pinched the tube with the tire lever going back on and didn't check anything before I aired it up. Watched it go flat again right there in the driveway, thirty seconds, maybe less. Jackson thought that was the funniest thing he'd seen all week. He still brings it up. I deserve that, honestly, because it was a dumb mistake and a completely avoidable one. Check before you commit to full pressure. That's the whole lesson right there.
Airing up for real
Once you've checked it and it looks clean, go ahead and air it up to the pressure printed on the sidewall of the tire. It's usually a range, like 40-65 psi. For a casual trail bike or a commuter, I lean toward the lower-middle of that range, it rides smoother and it's more forgiving on our gravel and chip-seal roads. If you're riding actual pavement fast, or you're heavier, go toward the higher end.
Use a pump with a gauge. The little bike pumps with no gauge are fine for topping off but you're guessing at pressure, and guessing is how you either underinflate and pinch-flat again on the next pothole or overinflate and blow the bead off the rim, which is loud and startling and not great for the tire.
Once it's at pressure, spin the wheel again. Same check as before, both sides, all the way around. Takes ten seconds and it's the cheapest insurance you'll ever do.
Putting the wheel back on
Front wheel, this is usually simple, drop it into the dropouts, make sure it's seated evenly on both sides, close the skewer or tighten the axle nuts. Rear wheel's the one people fumble because the chain's in the way. Shift to the smallest cog before you took the wheel off if you can remember to, it makes getting the chain back around the cog way easier. If you didn't, no big deal, just guide the chain onto the smallest cog as you drop the wheel in.
Once the wheel's in, spin it and make sure it's centered, same distance from the brake pads or the frame on both sides. If it's rubbing, it's not seated right, pull it back out and try again rather than forcing it.
Reconnect the brakes
If you had to open the brake to get the wheel out, this is where you close it back up. Rim brakes, squeeze the pads to the rim and reconnect whatever cable or clip you released. Disc brakes, just slide the rotor back between the pads carefully, don't force it if it's not lining up, the rotor's thin and it'll bend if you're rough with it.
Spin the wheel one more time here too. I know that's a lot of spinning the wheel in this lesson but that's genuinely the check. If something's wrong, the wheel spinning slowly will tell you before a hard stop on the trail does.
The final test
Squeeze both brakes, make sure they grab. Push the bike a few feet, make sure nothing's rubbing or catching. Then, if you've got the driveway or the street for it, get on and ride it a little ways before you call it done. Small wobbles or noises show up under actual weight that you won't catch just spinning the wheel by hand.
Here's the thing, this whole flat-fixing process, start to finish, is maybe fifteen minutes once you've done it a few times. The first time you do it, give yourself half an hour and don't rush it. Rushing is how tubes get pinched and bolts get rounded off, and both of those cost you more time than just going slow the first time would've.
Before next time
Bring a bike with a flat if you've got one sitting around, or just a spare tube. Next time we're troubleshooting the stuff that goes wrong even when you did everything right.