Bead Seater Tire Tool (Tire O-Ring/Tire Doughnut)
If you have ever done tire mounting you learn a lot by each style of tire and how they mount. For example, regular sized car tires are the easiest to mount and seat. The harder ones are low profile and racing tires, they usually take a bit of coaxing to get them to seat, but they almost always do. The real problem area is truck tires. Truck tires are large floppy sidewalls that sometimes will just not seat no matter how much air blasts you try. You have a few options from dangerous to expensive, but the one that did the job for me was a $20 giant O-Ring Bead Sealer. These are available in a number of sizes to fit your rim. When I got it I though that it would be a waste, but worked as advertised and have used them a few times with great results. Simple, cheap and effective adds this to the list of keeper tools. The "Tire Doughnut" was purchased by Prier Tier Supply.
UPDATE 06/02/2015 -
Finally the price of the Cheetah Style Bead Seat tanks have come down I had to pick one up. I got one called the 'Rambo DSR-7' which was an aluminum tank version. It was plagued, like many of the imports with poor fit and finish. The nozzle was too short and threads were fitted with a huge amount of Teflon tape to mate incompatible threads. After fixing some of the crap up I gave it a try and was unsuccessful to making it work. After re-reading and watching a video on youtube I had figured it out and the floppy sidewalled Kumho's for my truck popped right on. I think pricing is pretty good on these now that their are many competitors to the original Cheetah version (they can't be too happy about that). I will offer this advice before using it, watch videos on how it works, wear gloves and hearing protection!
If you have ever had to mount tires you run into three types of tires, the kind that mount easy, the kind that mount with high difficulty and the kind that you can't seem to mount. Regular car tires usually are easy, racing tires and low profile tires are a bit harder, but the worst one are larger truck tires that you can't get the bead to seat.
Dangerous Way to Seat Tire
A can of Quickstart (starting fluid) and a match. Spray a few shots into the inside of the tire, hold a match and KABOOM, tire will seat. This has the drawback of a tire exploding in your face or buring a tire to the ground. This is a NOT RECOMMENDED approach after having tried it out.
Cheetah Bead Seater
This is a great invention, essentially its a small air tank with a ball valve that can dump a lot of air to pop the bead. These work well, yet could be somewhat dangerous but not near as bad as the Starting Fluid technique. The downside of this tool is the cost, which was in the $300 range. If you do this for a living this is the tool to get. Their are other similar tools (Bazooka) that perform a similar task. I was on the path to get one but a simpler solution was presented by an old tire guy that recommended the Tire "O-Ring" or "Tire Doughnut".
UPDATE : I have purchased a similar tool and after learning how it works, it does do a good job at blowing up the tire to pop the bead. Not fool proof, but gets it done and unlike the O-Ring's below it will work on any size rim diameter!
Tire Bead Seating O-Ring
I found these from a tire mounting equipment supplier. They were sort of buried in the listings of various tire tools, but I gave them a call and mentioned my problem and was assured these simple rubber rings would do the job. He was right! They come in a few different rim sizes and were about $20-$50 bucks each depending on how large. You simply place them on the rim pushing down the unseated bead with it to form a seal. You then proceeded with a normal bead seat with air pressure. As the tire begins to seat it (mostly) pushes out the O-Ring. Pretty simple. In a few cases you have to give it a tug. Also a good trick is keep it and the rim lubricated with "Tire Mounting Goo" (Technical Term).