BMX Bikes / G / Geometry

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  • "Head Angle" greatly effects steering quickness. Imagine a straight line straight up from your front hub. That line would be 90 degrees from zero. BMX frames have employed a range of "Head angles"..ranging 68 to 78. Less angle means slower steering, better tracking and stability. More angle, over 74 and you have a very fast steering bicycle..72 to 75 is the average range..
  • "Seat angle" is the next element. Same deal with a straight line up from the cranks. The amount of angle of the seat tube also effects steering and center of gravity. Low seat angles put you over the back wheel and work good for tucking and wheelies. I recall a proline I had with a 68 degree seat tube that I could wheelie for blocks" A High seat angle moves your weight forward, and is usually used in racing bikes.. Hutch and Diamond backs have steep seat angles..74 or more..
  • "Frame tube" Also "top tube" the length is another important factor, mainly so you dont hit your knees. Also to how the bike feels in corners and when cranking.Affects the feel of the frame, shorter for ramps/freestyle.
  • "Chainstay" this is the tube from the crank to the back wheel. If you increase the chainstay length the bike will handle slower, and be more stable at high speeds. This technique is often seen in touring bikes. Redline and Patterson used some longer chainstay lengths in the 16" to 17" in range.
  • "Bottom Bracket height" The range is usually from 10 3/4" to 13" This is measured from the center of your cranks to the ground. Low and you may hit your pedal in the corners, lower center of gravity. High and more room to peddle, less stable at higher speeds.
  • "Fork rake"is the amount of angle your fork has above the head angle. The type of fork greatly affects this usually. motocross bikes often have alot of "Rake" as the wheel is often mounted to the front of the fork to allow more travel.
    The length from the center of your front wheel to the center of your back wheel is the
  • "Wheel base" This is the length from the front axle to the rear axle. I prefer shorter wheel base bicycles for ramp riding and jumping. A longer wheel base is more stable at speed, that why the downhill bikes are so long..

frontfork.jpg

frontfork.jpg

 

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