A British thermal unit (Btu) is a standard unit of energy that is used in the United States and sometimes in the U.K.
There is another defect in tires that can cause a vibration called loaded road force variance. The tires and pass a visual inspection, a radial and lateral runout test, be perfectly balanced, but still cause a vibration. This is due to the internal defects in the tire.
This is one of the most common causes for engine vibration; this is especially true in front wheel drive vehicles. Many of today's engines have a balance shaft to keep engine vibrations to a minimum. They work like a counter weight to the engines crankshaft.