Hey I have a single speed bike with rim brakes. The problem is at a certain position, the wheel gets stuck and after some effort gets going. This happens at almost every rotation. It needs a lot of effort to get it going and causes a problem on slopes. What could be causing this? What should I do?
Check to see where it's rubbing. It will probably be rubbing on the brake pads, or less likely on the chain stays. If it happens once per rotation it means your wheel is probably warped and needs to be straightened. Should be a pretty basic fix if you take it to a bike shop. Straightening a wheel can be done at home with a simple cheap tool, but takes some time to figure out how to do it well.
I'm assuming its the rear wheel getting stuck, and its every rotation of the wheel, not every pedal rotation.
Hang the bike up, coast the wheel gently and let it stop. Eyeball where the wheel passes the chainstay and seatstay parts of the frame, and the brakes if you have brakes on that wheel. You should be able to slip paper between the wheel and the frame/brakes easily.
The wheel should look "still" while spinning too, there's a chance you've put it out of true so one bit of the rim is bent sideways. This can be fixed by using a spoke spanner on the rim end of the spokes to pull the rim back into line. You can use a finger as a rudimentry gauge, but be careful.
If that's all fine, unmount the rear wheel and turn the axles by hand. They should be smooth, with no notchy parts in the rotation. You should have no more than a barely perceptible wiggle in the up/down direction. Anythign else and the wheel's axle/bearings needs a clean and repack. It should cost freely for at least a minute if you hold the wheel by the axle ends.
Less likely is your bottom bracket, between the pedals. - this would cause your jam to be once per pedal rotation though.
Finally if its once per chain rotation, then look for a stiff link in the chain.