bike skips and i slam my knee

Question
I have a Next 26" 18 speed bicycle that shifts fine but when i am crossing a street from a stop or when i need to accelerate from a stop fast the gears slip and i slam my knee into the handlebars which is extremely painful. I know its just a cheap bicycle but i was wondering if there is a way to fix this or even just make it happen less often

Answer
Keith,

Getting rid of this problem depends on how well you can tune your bike.  It sounds like the problem is that the rear derailleur is not placing the chain squarely on the gear in the back, so when you apply more torque than usual, the chain tries to shift to the next gear.  On the cable that goes into the rear derailleur, there is a barrel adjuster (a knob around the cable housing that has grip ridges.  Depending on which direction you turn the adjuster, you can adjust the rear derailleur pulleys (the little twin gear wheels below the gears) either right or left to center the chain on the rear gear.

If you don't quite understand what I am talking about, it would probably help if you had a friend who knows bikes show you the barrel adjuster.  Short of that, a friendly bike shop employee can show you what it is.  

That is only part of the solution, though.  If the rear wheel is on straight and you have tried adjusting the barrel adjuster and it still won't stay solidly on the gear, then your problem may be more serious.  One of the problems with less expensive bikes is that the components like the derailleurs and the parts of the frame, such as the "derailleur hanger" (the part of the frame that the derailleur screws into) are not as strong and get bent from every day use.  If you just can't seem to get the chain adjusted, then take the bike to your local shop and have them look at it.  The skipping chain may mean you need a new derailleur or you need to have them straighten your derailleur hanger.

Good luck and let me know how it turns out.

Graeme