There’s so much you can do with tires and wheels:
1. Let some air out of their tires and watch them get off their bike and inspect their tires for holes.
2. Fill their tires with just enough water to slow them down, using a pump filled with water. Don’t put too much water, though, unless you want them to know right away that something’s wrong!
3. Put a small bike spoke bead onto one of their rear-wheel spokes, and enjoy how confused they look when they’re being followed around by a strange sound.
Seat tubes and handlebars are also great for pranks:
4. Put some food that will go rancid into their seat tubes and/or handlebars, and let their bike get stinkier over time. For a more immediate effect, use a smelly food of your choice, some Liquid Ass Spray or the liquid from a Stink Bomb.
5. Insert some metallic balls or little metal bells down their seat tube and/or into handlebars, and let the mysterious noise drive them crazy.
Other bicycle parts can also be useful:
6. Lower their saddle a little to make them lose power, without knowing why, or raise their saddle a little to make them uncomfortable.
7. Put some mascara or other slimy substance onto their handle grips. A little spray of Liquid Ass Spray will make them imagine the worst when they feel your surprise.
8. Wedge a pebble or small stick into their pedals so that they can’t clip in. Another option would be to put some chewing gum on their pedal so that they’ll find the sticky surprise upon clipping out.
9. Grab their helmet and shorten their strap a little. That should throw them off a bit, make them feel that something isn’t quite right.
10. Replace their energy drink with water, another energy drink, or beer. For a more subtle effect, pour some of their drink out and add more water.