Sunday, 20 October 2013

A Few Notes

I'm posting this more for my benefit than anyone else's but it does give you a bit of an insight into some of the technicalities involved when building a frame from scratch.

Bottom Bracket Shell
  • Identify right hand (drive) side by screwing in a spare bearing cup / crank cartridge etc.
  • Measure outer diameter
Seat Tube
  • Locate top of it by sliding a seat post into it.  Mark top and bottom. Check a second time to make sure you got it right.
  • Mark Centreline
  • Mark and cut 90 degree mitre in the bottom end so it butts up to the bottom bracket shell and so that the centreline passes through the larger parts (or ears, tabs etc.) of the mitre
  • When placed in the jig, ensure that the centreline is facing into the frame as you'll be using this to position the bottle cage mounts. These are 65mm apart.
Down Tube
  • Identify long and short butts by looking down the middle
  • Mark centreline
  • The short butt is the bottom end
  • Mark and Cut a 90 degree mitre in the bottom end so that it butts up against the bottom bracket shell.  Ensure that the centreline passes through the larger part just as with the seat tube, for the same reason
  •  When placed in the jig, ensure that the centreline is facing into the frame (upwards) as you'll be using this to position the bottle cage mounts.
Seat Tube and Downtube: bottle cages.
  •  Check location for bottle cages either by measuring an existing bike (note that the frame angles might be different or by getting two bottle cages and fitting them to your frame.
  • Note that the seatpost won't go below the top bottle cage mount so don't put it too high
  • Ensure that the top tube doesn't get in the way or either bottle.
Set up the Jig
  • Set up the seat tube angle as per the CAD plan
  • Set up the head tube angle
  • Set the bottom lug height at the front
  • Set the front axle height in relation to the bottom bracket and fork rake
  • Put bottom bracket shell into jig, along with seat tube at correct angle
Down Tube (again)
  • Measure required length (NB ensure that you understand if you're measuring the outer dimensions (i.e. ear to ear) or inner dimensions (i.e. between tubes at the lowest points of the mitre)
  • Attach mitre template at correct location + 5mm for adjustment (biggest cut out goes at the bottom)
  • Trial fit into jig to ensure that it'll be long enough(or just too long) once cut to length.
  • Once satisfied, hacksaw to length +5mm
  • File out mitre
  • Trial fit and reduce tube length gradually until it fits at the correct angle
Top Tube
  • Find long and short butt ends
  • Ensure that you shorten the tube by cutting metal off the long butt end
  • Mark Centreline
  • Mark top of tube
  • Mark seat tube to top tube mitre (the bigger cut out goes at the bottom of the tube)
  • Measure tube length based on CAD drawing then add 5mm
  • Mark top tube to head tube mitre (the bigger cut goes at the top)
  • Cut mitres and trial fit
Seatpost clamp
  • Either use a lug or cut a bit of sleeve to the correct dimensions
  • ensure that the sleeve has an internal diameter very close to the seat tube outer diameter
  • Measure length
  • Check to see how much of the dropout will insert into chainstay if using a tabbed dropout
  • Cut to length by removing metal from big end.  Required length of chainstay= distance bottom bracket to rear axle centres, minus half of BB diameter and the remaining length of the dropout once inserted.
  • Cut mitre as per CAD template, position mitre template so that centreline is at top of oval and largest parts of the mitre are top/bottom
  • Place in jig and use dropout in jig to mark centre line at back of chainstay
  • hacksaw slot a little narrower than dropout then file to size
  • Place in jig
  • Check that the taper occurs after the point where you are going to need to cut a chunk off the fat end.
  • Cut slot in smallest diameter end as above for the chainstays.
  • Insert into jig and measure length by eye
  • Cut to length +15mm
  • File to length
  • Place in jig.

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