Mechanical Trail

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What is Mechanical Trail

Mechanical Trail (or Real Trail) refers to the length of the lever that, when combined with the contact patch, produces a self righting effect on the bicycle or motorcycle. The longer the lever, the more the is held true to the geometry of the cycle and influenced less by ground conditions.

Trail views.gif

Here is how the front geometry looks on my 2005 CBR600RR:

600rr-front-end.gif

What is Ground Trail

Ground Trail is makes reference to an imaginary line on the ground. It means nothing. The only geometry feature that runs along the ground on a bicycle or motorcycle is contact patch.

Only ignorant people make reference to ground trail. Sorry folks.

Also, ground trail differences change with head angle change. This adds an additional layer of confusion since the rates of actual change to explained change are moving away from each other when using ground trail.

Groundtomechanicaltrail.gif

The Math

  • Offset = Wheel Radius * Sin (90 - Head Angle) - Mechanical Trail
  • Mechanical Trail = Wheel Radius * Sin (90 - Head Angle) - Offset
  • Rear Wheel Trail = Wheelbase * Sin (Head Angle) + Mechanical Trail



  • Ground Trail = Wheel Radius / Tan (Head Agle) - Offset / Sin(Head Angle)
  • Rear Ground Trail = Wheelbase + Mechanical Trail / Sin (Head Angle)
  • Fork Projection = Rear Wheel Trail * Cot (Head Angle)


  • Mechanical Trail = Ground Trail * sin(Head Angle)
  • Ground Trail = Mechanical Trail / sin(Head Angle)

Charts

70mm-mtrail.gif

80mm-mtrail.gif

What Good is All This?

So you can have a bike built right and choose the right fork. silly. See: Suspension Fork Geometry Info



Links

Tony Foale

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