As Phil Sieg found out, import duty can be levied on all imports. To see
whether the item that you want to import is subject to duty, you can go
to the harmonized tariff schedule. Most bikes and parts are categorized
with the first 4 numbers 8712 through 8714. You can check out the exact
duty rates here:
http://hotdocs.usitc.gov/
The tariff is levied not only on the 'fair' value of the goods, but also on the cost of shipping and insurance. Generally the customs service will levy duty based upon the declared value, but if you declare what they think is a ridiculously low value, they are completely within their rights to determine the 'fair' value. Many bike parts, such as Q/R hubs, cotterless cranks, freewheels, brakes, alloy stems and derailleurs are free of duty when shipped by themselves, but when fitted to a 'bike' they are charged at the duty rate of the whole bike.
Steven Maasland
Moorestown, NJ