Day 4

Short Summary-

Today I cut and mitered all of my tubes for the actual bike. The days of practice welding is coming to a close as we begin welding our actual bike.

Frame geometry, frame material, tube diameter and tube thickness all play a role in how the bike handles. For our bikes we'll be using grade A Haynes Titanium Tubing. All tubing has a set of numbers printed on it (i.e. 1.25 x .030 on the top tube). This means it is a tube 31.8 mm in diameter (taken from the outside) that is .030 thick.

Tubing for this bike:

Top tube: 1.25 x .030

Seat tube: 1.25 x .035

Down Tube: 1.375 x .038

Head tube: 1.5 x .087

Tubing was rough cut using a lathe.

Before any mitering was done, I made my very first weld that will be on my bike by welding the seat collar to the seat tube. This seat collar is used to accept the seat post, which is usually 27.2 mm instead of the seat tube dimensions of 31.8 mm. 

Seat tube thickness:

Collar thickness:

Welded piece (raw):

Cleaned up just a little... weld barely even exists.

Tubes were then mitered using a vertical milling machine. Circular drill bits of the tube being mitered to were cut to give correct seat tube dimensions.

closeup...

Down tube with mitered cuts for seatube and bottom bracket-

All tubes mitered and prepped and ready for welding tomorrow:

Long Technical Section- How to design your dream bike...

As promised, today I'll walk you through designing the front triangle of the bicycle- ill try to lead you step through step.

Please refer to yesterday's post for recommended values and an understanding of why choose certain angles over others.

If you don't want to go through these steps, but still want to fool around with frame geometry, one thing you can do is use BIKECAD- a free program that allows you to play with different configurations of angles and tube lengths to get a rough idea of your frame design.

It is available at:

http://www.bikeforest.com/CAD/bcad.php

1. Draw horizontal line to serve as horizontal wheel axis line. From that line draw your Bottom Bracket (BB) drop distance. I have chosen a BB drop of 68mm. The BB diameter is 40mm, which is also drawn. 

2. From BB center, draw another horizontal line perpendicular to your wheel axis. Draw your seat tube angle from the center of the Bottom Bracket (BB). I have chosen a seat tube angle of 73.5, as its the same geometry of both Cannondale road bicycles that I enjoy and will provide a fairly aggressive geometry.

3. Seat Tube Diameter for this bike is 31.8mm, which we shall refer to as 32mm. Using a compass to draw 2 16 mm circles allows you to offset center line to tube dimensions. If you have CAD, you can just use the offset tool. Tentative Seat tube center line is drawn from inseam measurements. I will use 550 mm for now.

3. From top of top tube center line, draw effective top tube length, which is determined by reach. I will be designing around an effective top tube of 550 mm. Although I know my top tube diameter (also 32 mm), I will not draw this in yet, as we are only measuring "effective" top tube length. 

4. Head tube angle is drawn from end of effective top tube centerline. I followed Jim's advice and used the same 73.5 angle as my seat tube. Higher head tube angles will quicken steering and will decrease the trail, which will offer more stability at lower speeds. However, it does this at the expense of shock absorption and will also decrease the wheelbase, which will increase the likelihood of toe overlap. In retrospect, I probably would have chosen a smaller angle.

5. Fork rake- fork rake length is determined by the fork you are using. Default cyclocross rake is 45mm, but its better to know what fork you will be using beforehand. I have chosen to use Easton's EC90x that has the same 45mm rake. A 45 mm line perpendicular to the head tube angle is drawn and slid up and down along the head tube line until its end intersects the wheel axis line. 

Heres a closeup at 6x magnification: (Intersection of horizontal center wheel axis and rake line is your front wheel axis)

6. Length of fork is then drawn in. I couldn't find the length of the easton fork online, so I went with the default 400 m length. 400m line is drawn from front wheel axis and is again rotated until is intersects the head tube angle line again. A line perpendicular to the head tube angle is drawn.  Headtube is 38 mm in diameter, so line should be 19 mm to either side.

7. Lower headstack height is drawn in. Because I know I will be using a Chris King headset, the offset is 13.7 mm. For integrated headsets, you do not need to do this step, but it needs to be accounted in height of head tube. (again at 6x magnification)

8. Head tube length is drawn in, again parallel to head tube angle (duh). I have chosen 120 mm, which is my best guess of the existing head tube lengths minus the height of the integrated headsets (13 mm lower headstack + 13 mm upper headstack). If you are looking for added comfort, you would usually add to the head tube length for a more upright geometry. For every mm added to the head tube, an equivalent distance should be subtracted from the seat tube. For instance, if I added 20 mm to the head tube (effectively giving me the "virtual" headtube of a 57 cm frame), I would subtract 20 mm from the seat tube for a distance of 53 mm.

9. Bottom of Down Tube clearance on the headtube should be between 10-20mm. Im using 11 mm. Down tube diameter being used is 35 mm, so circle of diameter of 35 mm should be drawn in BB shell. Line tangent from bottom of down tube diameter in BB is drawn to down tube clearance. Lower line is then offset 35 mm.

For mountain bikes, this number should be larger to account for the large crown forks used. Because many head tubes on mountain bikes are very small, and fork crowns are very large, they use curved down tubes.

10. Top of Top Tube clearance is drawn in. Ranges are from 10-12mm, so again I will be going with 11 mm. Angle of top tube really depends on aesthetic taste and stand-over height desired.

Stand-over height is usually calculated 250 mm from the seat tube, or halfway just to be on the safe side. Even with a traditional geometry of a level top tube, I have enough stand-over height, so top tube is drawn in (my inseam without shoes is 850 mm, so im within the safe 20-30 mm range. Notice that top tube does not need to be drawn on center of effective top tube length. Top tube diameter can be 32 or 30 mm. Since not much stiffness is derived from the top tube, frame builders will often use 30 mm, as this is an easy place to drop some weight off the frame. 

When it comes to designing frames, you are allowed to pick 2 of the 3: head tube length, top tube length or seat tube length.

11. Lastly, distance of TT at seat tube clearance is drawn in with a range of 20-30 mm. Ill be using 25 mm. You now have the front triangle of your bike drawn in.

Ill cover drawing the rear triangle in in a later post...