SF Cyclotouring

Ride reports and other ramblings from a San Francisco cyclist.


UBI Framebuilding Class - Day 3

5/13/2015 - Day 3

UBI frame drawing

Bike Bits

Raking fork blades

UBI measuring fork offset

Paragon Fork Ends
  • Third Lug Practice
  • About Bike Frame Tubing
    • 9/6/9 Butted wall thicknesses
    • Butting lengths
    • Can see butt transitions inside a clean tube
    • Butted lengths vary widely, and can be asymmetric (one butted end is longer for trimming)
    • Seat Tubes
      • 28.6mm is most common, to fit a 27.2mm seat post
      • Single butted for lugged construction
      • Single-butted at bottom and EXTERNALLY butted at top for fillet-brazed construction
    • Choosing Frame Tubing Diameters
      • 1 inch steerer dictates 31.8mm head tube; be sure to choose an appropriate lug
      • 8/5/8 tubing is light
      • 9/6/9 is mid-range
      • 1/7/1 is heavy
      • Tubing choice factors
        • Rider weight
        • Rider strength
        • Cargo load
        • Frame size (larger frame implies stronger tubes)
      • Pick lugs first, since these might limit your tubing options
  • Frame Drawing
    • Work clockwise
    • Use a SHARP pencil for accuracy
    • Choosing the lower head lug is an iterative process due to unknown HT/DT angle
    • Drawing steps (refer to workbook)
      • Draw axle line about 7cm from bottom of paper
      • Draw BB drop
      • Draw seat tube centerline at correct angle from BB point and to desired length
      • Draw TT centerline to desired length
      • Draw HA/HT centerline back to axle line
      • Draw Fork Offset parallel to HT centerline
      • Draw arc set to radius appropriate to desired tire Bead Seat Diameter around front wheel axle center
      • Choose a fork crown
      • Consider caliper brake reach (if using caliper brakes)
      • Draw in perpendicular line for 13mm lower HS stack height
      • Measure lower head lug intercept
      • Draw in headtube width (31.8mm)
      • Add intercept distance
      • BB shell
        • For lugged, use the ID of the shell (typically 35mm)
        • For fillet, use the OD of the shell (measure shell)
    • Forks
      • Dropout styles
        • Tab style vs. socket style
        • Socket style has more limitations for fork blades due to requirement for matching diameters
      • Blade lengths
      • Blade thickness
      • Raking fork blades 
        • Check blade length before bending
        • Iterate/sneak up on desired rake, alternating between both blades
        • Get both blades as even as possible
      • Cutting fork blade dropout slots
        • 6-8mm depth is good rule
        • Clamp using wooden blocks
        • Sight down the bend/ridge of the blade to align the dropout slot
        • Make a Sharpie mark
        • Scribe width of dropout on blade parallel to sharpie mark, and centered with it
        • Scribe slot depth onto blade
        • Clamp and cut
        • Bias the slots inboard for extra hub clearance
        • Rotate the blade 90° and angle it to about 30° with floor
        • Use a large flat file to knock down the corners and bevel the edge
        • Use bevel protractor to gauge bevel against clamping blocks for repeatability with other fork blade
        • Clean out burrs with round file


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