SF Cyclotouring

Ride reports and other ramblings from a San Francisco cyclist.


UBI Framebuilding Class - Day 4

5/14/2015 - Day 4

Fork Blade Jig

Brazing in the fork ends

Centering the Fork Crown

Brazing the Fork Crown

My Fork

Chainstay Overview Drawing & Choosing Chainstays
  • Measuring Fork Crown Blade Seat
    • Check catalog specs OR use the back side of the caliper to measure the blade socket depth
    • Measure each side in case they're different
    • Mark line across outside of fork crown
    • Measure to crown race seat
    • Mark on your drawing -- line perpendicular to HT between fork crown race seat and underside of fork crown/tire clearance
    • Draw 11/32-inch circle on frame drawing at front axle center
  • Fork Dropout Jig
    • Ensure symmetry in slot at dropout dummy axle
    • Position blades & dropouts in jig
    • May need to refile slots to adjust blade angle to get ovals parallel
    • Measure inside width of fork crown sockets (95.6mm)
    • Flip calipers over & measure outside width of fork blades at crown height
    • Check slot fit
    • Check blade ovals for parallel
  • Brazing fork dropouts
    • Use 2-4 inches of brass rod per dropout
    • Tack corner points first
    • Seal tab sides
    • Then fill plug
    • Braze one dropout, then the other
  • Chainstays
    • Overview drawing (see worksheet)
      • Dropouts
      • Tire Clearance
      • Figure out seatstay angle to help select dropouts (70°)
      • Chainstay length
      • Axle spacing
      • BB Shell width: 68mm
      • BB Shell diameter: 35mm for lugged
      • Axle to back of BB length (should equal CS length minus (35mm/2))
      • Draw overview on frame drawing
  • Choosing BB shell
    • Round vs. Oval chainstays
    • Seattube & downtube diameters
    • Tire clearance
    • BB edge to chainstays
    • Chainstay to dropout angle
      • Draw OLD of hub axle
      • Add 1/2 thickness of dropout -- watch for thick/raised dropout faces
      • Measure dropout tab length -- axle center to end of tab
      • Draw tab on drawing
      • Mark tab depth (6-8mm)
      • Mark slot depth (6-8mm)
      • Determine overall chainstay length
    • Draw in tire diameter
  • Choosing Chainstays
    • Straight vs. S-Bend
    • Round vs. Round-Oval-Round
  • Draw Line along Chainstay
  • Measure Chainstay Angle with Protractor
  • Chainstay Fixture
    • Dummy Axle
      • 137mm for 135mm OLD rear
      • 132mm for 130mm OLD rear
      • 122mm for 120mm OLD rear
      • 102mm for 100mm OLD front
    • Dummy axle slop -- keep in phase
    • Mark chainstay length for back of BB off of frame drawing
    • Check BB shell CS socket width with calipers
    • Mount stays & drops in chainstay fixture -- watch for any gaps at tabs
    • Adjust CS clamp fore/aft to set proper CS width at BB sockets
  • Fork Blades
    • Mark each fork blade length off drawing -- add 1mm to length
    • Cut to line -- ensure cut is perpendicular to blade
    • Clean up cut with flat file and level
    • Deburr inside and outside of cut blade
    • Check for square cut with machinist's square
  • Fork Crown
    • Mark left & right on crown for orientation and keep this orientation for fork leg insertion
    • Clamp blades in dummy wheel
    • Check alignment
    • Tweak as needed
    • Sight down through empty steerer hole in crown to check for center
    • Side with more rim clearance is the shorter side -- LIGHTLY file down other side 
    • Iterate to get alignment perfect
    • Squish oval ends of blades in vice as needed to fit crown
  • Fork Vent Holes
    • 1-2 inches above dropout; 1/16 inch hole only
    • 1-2 inches below fork crown; 1/16 inch hole only
    • Deburr any holes
    • When drilling, clamp dropouts, not fork blade
  • Brazing Fork Crown
    • Anvil Fork Jig
      • NOTE different offset scales for 1-inch and 1-1/8-inch steerer tubes
    • Sand & wipe down with alcohol
    • Check for close fit at fork crown tangs & points
    • Add Silver flux everywhere -- inside and outside of crown; use LOTS of flux
    • Fork blades should evenly touch alignment bar on jig
    • Use a hotter/bigger flame
    • Braze fork in jig upside down
    • Heat steerer tube first
    • Start at fork crown front and work around
    • Heat from top to bottom
    • Load silver on top (underside of crown) and pull it to bottom of crown (top side of crown)


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