After completing an initial frame building course I did not really have much idea about how to design a frame and have muddled along by trial and error. What does become apparent is that conventional designs work which is why they are still around. A conventional rear triangle is always stiff enough and strong enough for most things you throw at it and therefore most attention needs to be paid to the front end design. Always check you are likely to have enough mudguard clearance, toe clearance unless you are not bothered about toe overlap and heel clearance if you are going to fit panniers. If you are going to be fitting large chainrings make sure they will clear the chain stay. This issue has caused me most problems and close shaves when building the frames up after the fact.
The Paterek manual teaches the use full sized drawings, offering up completed sections of the frame to the drawings to check accuracy and even brazing sections over the top of the drawings on a large table. Perhaps most amateur builders will have the same problems as I have had in not having a large enough space to accommodate said table or being able to find paper large enough! I sometimes do full size drawings of lug sections which helps when sanding out lugs to adjust the angle. I guess building without any form of jig would require the use of full size drawings.
I started with A3 graph paper to plan frames on, which is the largest you seem to be able to obtain from most stationers. This will accommodate a 1:3 scale plan of front wheel, forks and front triangle with a separate sheet for the rear triangle which also includes seat tube and bottom bracket. Although it is tedious converting all measurements by a factor of three
I have never had any problems with accuracy when transferring measurements taken from the drawing.
I have since managed to find some larger A1 graph paper which enables a half scale drawing of a complete frame, minus a bit of the front and back wheel.
The following instructions only require basic drawing tools you can obtain from a stationer or your child’s pencil case.
Essential measurements required (always use centre-centre measurements):
1. Bottom bracket drop (distance from wheel axle line to centre of bottom bracket), or bottom bracket height (distance from the floor to the centre of the bottom bracket).
2. Top tube length (always assume a horizontal top tube for drawing, often known as effective top tube length), you can always add a sloping top tube to the drawing later.
3. Seat tube length (centre bottom bracket to centre of a horizontal top tube)
4. Head tube angle(from horizontal)
5. Seat tube angle (from horizontal)
6. Fork blade length (centre of dropout to top of fork crown, ignoring any bend)
7. Fork blade rake
8. Lower crown race thickness (usually 14mm for an external bearing headset)
9. Chain stay length
10. Whilst not essential for the initial drawing you will need to know the rear axle width
11. It can be helpful to work out the trail of the fork for your selected head angle before commencing the drawing (see http://yojimg.net/bike/web_tools/trailcalc.php ). You will be able to take the trail from the drawing but if it is not suitable you will then have to re do the drawing with a different head angle.
Start the drawing with the front wheel drawn to scale draw a horizontal line through the front wheel axle, wheel centre line. Remember the diameter of the wheel includes the proposed tyres. common wheel diameters are 700c 622mm, 26 inch 597mm. Tyres are as thick as the description i.e. 28mm are 28mm, 1 1/2 inches are 1 1/2 inches. More information on tyre sizes at : http://sheldonbrown.com/tyre-sizing.html
Using a compass set at the diameter of the desired fork rake draw a circle centred around the front wheel axle. Draw a line upwards from the wheel axle/centre at the desired head angle using a protractor . Further up this line repeat the circle equal to the fork rake. Then draw the steering/head tube axis by joining the two circles as shown i.e transferring the axis. Alternatively use a rolling ruler to transfer the line (shown in red).
Now draw in the fork blade length from the wheel/axle centre to join the head tube line at the exact length of the fork blade (axle to top of crown). Mark the crown race depth above this intersection. This will then be the bottom of the actual head tube or head tube lug.
Now create a temporary seat tube line. Measure the desired seat tube angle at the bottom bracket line and extend the line upwards. Mark the desired seat tube length on the line then draw a horizontal line to meet the head tube line
.Now mark the desired top tube length from the head tube line and redraw the seat tube line down to meet the bottom bracket line again at the desired seat tube angle. This now sets up the seat tube in the correct position. If you are using parallel seat and head tube angles e.g. 73/73 deg, you will not need to transfer the seat tube line, simply measure the top tube length from the head tube and the seat tube will be in the correct place.
Draw in a sloping top tube if you wish but leave the horizontal top tube line in as a virtual measurement.
If you are using lugs it is best to obtain the lower head tube lug before completing the drawing so you can know where the centre of the lug is going to be on the drawings. This enables you to draw in the down tube to the correct point on the head tube. The critical measurement to take from the drawing is the vertical distance from the bottom centre of the head tube or lower head tube lug, to the centre of the bottom bracket (shown in green) which has to be used in any jig set up, or any other method of construction. I usually deduct 1mm from this measurement when setting up a jig, i.e. effectively lengthen the bottom of the head tube by 1 mm to take account for facing the bottom of the head tube which will shorten it, but do not show this on the drawing.
Desired chain stay length is drawn from centre of bottom bracket to intersect the axle line at the desired length. Some jigs require virtual or effective chain stay length, which is the horizontal distance from the centre of the bottom bracket to a vertical line through the centre of the rear axle which can be taken from the drawing. Of course the chain stay length will include the dropouts.
I tend to decide on seat stay length and position when I have already built most of the frame and fitted the chain stays. However you need to check the possible positions so you can decide on the correct rear dropouts as they may have different angles between the chain and seat stay component, or use articulated rear dropouts.
The drawing is now essentially complete. You can add tube diameters, saddle etc and calculate from the drawing as many measurements as you will need to commence your build, or decide that it will not work and start again!
Essentially you can draw a frame starting at several other points rather than the front wheel and fork. The advantage of commencing with the front wheel in the bottom corner is you know your frame will end up fitting on the page rather than finding you are off the page before you can finish.