Detail Design - Part 1

With a general design complete, the next step is to start taking some measurements and laying out the dimensions of the bike.  This is where custom frame building really shines; a custom frame is like buying a tailored suit - it fits perfectly for your body type.

I used two resources for sizing my frame, Richard Talbot's book Designing and Building Your Own Frameset and BikeCAD, a java applet for designing bicycle frames.  Talbot provides some instructions and tables for sizing a frame according to the C.O.N.1 method, which was developed for sizing racing frames by the Italian Central Sports School.  Talbot states that the method is also applicable to touring frames, and the measurements will provide you with the dimensions for your seat tube and top tube.  BikeCAD has the ability to take just about any of your body measurements - you can give it only your height, or all of the C.O.N.1 measurements - and spit out a properly sized frame.

The three measurements required for the C.O.N.1 method are:

• Distance from the floor to your pubis (A)
• Distance from the shoulder joint to the wrist "fold" (B)
• Distance from the pubis to the top of the sternum (C)

 The "lower limb" measurement (A) determines the seat tube length, and the "upper torso" measurements (B+C) determine the top tube length.  Measurement (A) is slightly longer than your inseam, as it is measured directly to the base of the pubis bone - you may need to feel around a little bit to get the right position.  The arm measurement is taken from the back; raise your arm slightly and feel for a depression by your shoulder blade, then measure to the fold of skin made by bending your wrist back (as if you were doing push ups).  The top of the sternum can easily be felt by hand.


Talbot provides a table for determining the tube lengths from these measurements; you can also put all of these measurements into BikeCAD and get the lengths.  I did both and found that BikeCAD suggested a longer seat tube length than the C.O.N.1 method, but roughly equal top tube lengths; if you already own a bicycle that you find comfortable, you may wish to measure its dimensions.  Another alternative is to go down to your local bike shop and get fitted there.

My measurements provide me with a seat tube length of 53.7 cm and a top tube length of 56.8 cm.

The head tube and the down tube are sized based on the wheels you've chosen, the top tube angle you want, and the head and seat tube angles.  If you haven't thought about wheels yet, now would be a good time.  I picked 700c x 28 wheels because they're fairly standard and not too beefy.  I defined the head tube and seat tube angles as 74 degrees each.  I also wanted a flat top tube, so I put all of this into BikeCAD and got a head tube length of 12.7 cm.

Why did I pick those particular head tube and seat tube angles?  Talbot provides some guidance: the seat tube angle "...usually falls between 68 degrees and 75 degrees..." with time-trial bikes going even steeper.  Seat tube angle affects the stiffness of the ride and the center of gravity.  Head tube angle affects the steering qualities of the bike and the front-end stiffness; I want a little snappier handling than my Raleigh provides, so a 74 degree angle with an associated rake on the fork (more on this later) seems like it will provide fairly responsive steering while still having a comfortable ride.  This head tube angle also shortens the wheelbase a little, which affects the center of gravity.  Since I'm expecting to have weight over the back wheels fairly often (loaded panniers on the rack from grocery shopping), this will hopefully minimize the effects of the rearward center of gravity while carting goods home.

Also, lugs with these angles are very easy to obtain.  I don't want to go fiddling around with bending lugs at this point.

Determining the down tube length is left to geometry and bit of math.  The free version of BikeCAD does not provide this dimension, unfortunately.  You can idealize the frame with a set of lines that make a quadrilateral, then split it into a series of triangles and use trigonometry to determine the down tube length; my result is a 60.2 cm down tube.

 

 Main Triangle dimensions provided by BikeCAD

That's the main triangle of the frame; the rest of the frame and the fork are left to draft up.