Since the topic has come up before, both in ON and airhead news - here is a product that is acid based which cleans aluminum very well. Degrease the aluminum before use.

Autozone sells this, and also includes the MSDS:

This is the real deal and will clean aluminum wheels, and/or the aluminum hubs, forks, and engine. However, it is nasty stuff. Heed the warnings and follow the directions carefully.

The primary ingredient is phosphoric acid and a secondary ingredient is hydrofluoric acid, which is extremely hazardous (i.e. it will hurt you). Wear gloves, glasses, use outdoors upwind of the spray, and have a garden hose at the ready when using this product. If you get it on you, rinse immediately with the hose. It does a really good job and requires almost no effort to bring old aluminum back to like new, but must be used carefully.