Originally Posted by Paul Eastham
When I give my main wheels a spin, there seems to be a flat spot on each tire, perhaps 1/8" out of round. It's not huge, but definitely noticable if you spin the wheel and look at the tire from above.
Don't pitch your tire in the trash. In order to remove the flat spotting you have to use your tires. They will have a very large first harmonic for a while but after you get some heat in them, they should clear up.
A little back ground:
All tires will flat spot. In fact it starts as soon as you park your plane, bike, car, whatever. There are many factors that effect the flat spotting, construction material and the position the tire was left have the biggest effect.
Some tire materials will actually cold flow and cause very extreme flat spotting. Nylon is one of the worst for flowing and causing flat spotting. Unfortuanely it's cheap and does very well at high speeds and loads, so it's in most aviation tires along with higher performance car tires. Heat and time is the only way to get most of the flat spot out. It won't come all the way out but over time it will get better.
It is not unusual for a truck or high performance tire to take over 30 miles at 60 mph to reduce a flat spot to an acceptable level.
One way to cut down on flat spotting in to not run the tires against a sharp object, like a wheel chock. Also if you park a hot tire on a cold hanger floor, you may want to roll the plane around a little bit if you happen to still be at the hanger. A cooling tire is a bad case for flat spotting so if you can move the tire deflection around while it cools, you will end up with less flatspotting.
One final note: Most tires are built with a 5 year shelf life. I'm not sure about all aviation tires but all DOT tires will have a date code on them. There will be the letters DOT then a few letters and numbers, at the end will be 4 numbers. That is the week and year the tire was built. 0107 would be the first week of 2007. The date code before 2000 was only three numbers. 017 would be the first week of 1997, 1987, 1977...