You can make a pretty good low shelter by removing the front wheel (quick-release levers help), putting the fork in the ground and staking down the frame. It's more stable than trying to do it upside down. It's also nice because you can leave your light on the handlebars and shine it directly into your tarp.
Best to put a tent peg or something between the front forks to make sure they don't get out of alignment and your wheel no longer fits back in afterwards.
>>2151734 is still a better answer though, unless for some reason you're cycling through some kind of terrain where there are literally no trees or anything else you can tie a tarp to. 
