You can find the current tariff rates for everything, including bicycles and bicycle parts in the US Customs Harmonized Tariff Schedule. Here's how to find it:
go to http://www.customs.gov
at the top of the welcome screen click on Imports
On the right side column of the import screen click on 2003 HTS by Chapter
Scroll down to Chapter 87 and click on 87
Rates for all bicycle things begin at section 8712.00 and go to section 8714.99.