A 50/50 mixture of water and "Beaver Cleaner" from Beaver Research will do the trick. I've used it to clean track grime from our race bikes for years and iIt's by far the best cleaner I've ever used. Be careful not to let it sit for more than a few minutes, as it will etch aluminum, etc.
If you buy it, be sure to get the foaming dispener they offer also.
http://www.beaverresearch.com/solution.htm