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Incision width calculation

How wide incision we must make for the tube to bend by some desired distance:


W = (H*B)/L
  • B is width of the tube
  • L is distance from the incision to the end of the tube
  • H is how far the end of the tube should bend
  • W is the needed width of the incision

Use any length units you like, they just have to be the same for all four values.

Pivot bearing load calculation

This is greatly simplified calculation for the initial bearing selection. Instead of taking into account the actual geometry of your frame, the equation assumes the worst possible case: the pivot is in the middle of the frame and all the rider's weight sits right atop of it.


F = (G*WB)/(4*D)
  • G is the rider's weight in force units
  • WB is wheelbase
  • D is vertical distance between the pivot bearings
  • F is the resulting horizontal (radial) force on each of the bearings. The vertical (axial) forces are almost negligible.

Use any units you like. The resulting force will be in the same units as the input rider's weight. Usually the bearings are rated in kN, so the easiest way is to take the weight in kg and divide it by 100 (that is *10 to convert kg to N and then /1000 from N to kN).

In reality, the pivot is closer to the front wheel and the rider's weight is distributed over much larger area, so the actual loads will always be lower than the calculated value. But roads can be bumpy and you never know what situations you'll get in, so better to choose unnecessarily strong bearings and be on the safe side than to try to save some little weight and money and risk frame failure.