Inflating the Tire
Once the tire is fully installed on the rim, you can inflate it, but it my not be as round as it should be. If it isn't, it is usualy because the tire needs to be "seated" so that it sits at the same depth in the rim all the way around.
You are less likely to have a problem seating your tire if you have the wheel off of the ground before you start. If the tire is completely flat and is sitting on the ground with the weight of the bike on it, the part that is at the bottom is likely to seat incorrectly.
Generally, if your bike has quick release brakes, it's best to inflate the tire before putting the wheel back on the bike. If you don't have quick release brakes, though, it's easier to
install the wheel before pumping up the tire.
It's best to start by inflating the tire just enough that it takes shape, maybe 20-30 psi, and to check that it is
seated properly before full inflation. Check the seating by spinning the wheel and watching the tire. Once you're sure the tire is properly seated, inflate it to full
pressure.
Once the tire is inflated, you may see that it is not as round as the rim is. Usually this will take the form of having most of the circumference of the tire in the correct place, but there will likely be one place where the tire either bulges out too far, or dips inward toward the rim. It may do this on only one side.
Most tires have a "witness line" moulded into each sidewall. This is a narrow ridge of rubber running around the side of the tire, just outside of the rim. Spinning the wheel and observing the witness line will help you locate the place where the seating might be off. Note, it could be OK on one side of the tire but not the other.
Seating A Bulging Tire
If one part of the tire bulges out farther than the rest, deflate it right away or it may explode with a loud bang! Manually re-arrange the tire to get it centered on the rim before re-inflating it. Make sure the tire bead isn't sitting on top of part of the inner tube.
If the bulge is right at the valve, this usually indicates that the tire is sitting on the reinforcing patch at the base of the valve. Completely deflate the tire, and push the valve up into the tire with your thumb, while pressing the tire down around it, then pull the valve back down before inflating. Seating a Tire that Dips Inward
If your tire dips inward at one spot, it is usually a sign of an unusually tight fit. This may make it a bit of a struggle to install the tire, but it also means that you can get away with considerable overinflation with no risk of blowing the tire off the rim. Indeed, the best way to seat a "dipping" tire is by temporariliy overinflating it until it "pops" into position. In some cases it may be beneficial to lubricate the side of the tire. This can be done with soapy water, but I usually use spray window cleaner for this, because it doesn't leave a soapy residue on the braking surface of the rim.
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