Thanks for the responds.

@bikerdoc: that's the thread that made me find ridechina :) Most links are about guided tours though, not about rental places. But i should probably just contact them and ask about renting.
The tripreports are really great, can't stop reading.

@Lao Jia Hou
I'll be in beijing til end of July. February is an option because we got semester break till early march, july would be pre-departure fun.
Didn't know B-plates where that much cheaper. nevertheless, with buying i see too many problems:
-No use to have a bike in beijing in february because of the weather & transportation issues you mentioned
-Hardly any time to sell if i tour in july
-no safe place to park since i live on campus (on which only electric scooters are allowed)

Another question about the vehicle registering: Can i just buy a B-Plate in beijing, take it with me to wherever to buy a bike and then just put the plate on? I just assumed there would be paperwork to be done in beijing to register the vehicle, but then again their system is so diffrent to what im used to :)

ridechina makes a good impression on me, just wanted to know if there are other rental companys around with good reputition. Simply don't want to end up with a bike in bad condition putting me in uneccesary danger or having to pay unrealistic laowei-prices ;)