What are they for? They're handy for doing tricks on a bike, they're essential for "Street Cred". There are two main purposes for axle pegs:Free style or Flatland Riding - They're for standing on. Freestyle pegs usually have a serrated grip. Street or Ramp Riding - They're used for grinding. That is, they're used to scrape along railings, benches, ramp tops. Grind pegs usually don't have a serrated grip on them.
Which Pegs fit which bikes? Pegs are either threaded (they replace the axle nuts), or the socket type; they slide over the axle and are held in place by the axle nuts (inside them; a socket and extension will be needed.) Threaded Pegs: You'll need to get the correct thread. Most threaded pegs are made for a standard rear or BMX front axle (3/8" x 26tpi). These will not suit a wheel with a back pedal brake. Pedal brake axles use a 3/8" x 24tpi thread. We've been unable recently to locate a treaded peg to suit back pedal brakes. If you have oversized axles, you'll need to use a socket type of axle peg; threaded pegs are not available for oversized (14mm) axles. Socket Pegs come with either a 10mm hole for standard axles or a 14mm hole for oversized axles. To use a socket type axle you'll need some spare thread on your axles. The thickness of the peg under the axle peg is generally around 5mm to 8mm, so you'll need this much extra thread at each end of the axle.
Axle and Bearing Damage - A fairly common result of using axle pegs. The bigger the contact area is between the peg and the frame, the less tendency there will be to bend the axle and damage the bearings. A large contact area means the peg will be pulling on the axle more than twisting it. So a thing peg (cheap) will never be strong. Generally, socket type pegs have a bigger contact area with the frame. For heavy or rough riders, we recommend against using threaded pegs.