Drafting Tutorial: How to Draw a Traditional Bicycle Frame. Step Two.

Here we'll move into finishing the skeleton of the frame. After this section you'll have a good idea of what your finished frame will look like. Remember that all markings and measurements are made center-to-center.

Let's continue on.

  1. At this point you'll draw in your seatstays. Whether you're doing Hellenic stays, a fastback on the seat binder, or a mono-stay setup, you should start by drawing the seatstay from the ST/TT intersection to the rear axle center. Once you have the actual dropout you're going to use you can trace it and get a more accurate line after filling in the rest of the frame.
  2. Since you've drawn the TT line far beyond the needed length, mark it for the headtube (HT) line. Now draw the HT center line all the way to the bottom of the paper. This line represents the center of the steerer tube and also the steering axis. If you have the same HT angle as your ST angle you can easily check the accuracy of your angle. To do this, measure from the center of your BB to the line parallel to the TT and you should have the same length as the TT measurement.
  3. The next step is to draw in your front axle. To know where you need to draw it you need to know what your trail measurement should be. The general consensus seems to be that 56 mm provides the most neutral steering. With this number, wheel diameter, and your HT angle you can calculate your fork rake. An Excel spreadsheet to calculate this can be found at Anvil Bikeworks page. Once you have your rake distance, in my case 43 mm, you make a mark that distance forward from the steering axis along the wheelbase line. Finally, draw a 9 mm circle to represent your front axle at this mark.

    NOTE: You will notice that I draw a line to help represent the actual curve of the fork. This is not necessary but it helps me visualize the complete frame better.
  4. Up until this point things have been pretty straightforward and simple; now we're going to get a bit complicated, so pay attention. First measure FROM THE AXLE CENTER, not the wheelbase/steering axis intersect, and mark the rim radius and wheel radius on the steering axis. If you don't measure from the front axle center all the rest of your measurements will be incorrect.

    Next, if using caliper brakes you need to measure your brake reach from your rim diameter marking. Example: with standard 48mm to 58mm brake reach calipers you can use 52mm or 54mm, depending on how much room you want for fenders. This measurement will give you the brake center hole marking on the steering axis.

    At this point you'll need to measure the actual fork crown you're going to use. Measure the distance from the center of the brake hole to the base of the crown seat for the headset. Now mark this. Make one more mark above this one representing the lower headset stack height. As you can see, I write the actual measurements next to the markings so things don't get too confusing.

    NOTE: If you're going to be using cantilever, "V", or disc brakes you need to measure your wheel radius with the largest tire you're run on the bike. Now add the amount of space you want between the fork and the tire radius and mark this as the bottom of the fork crown. Now you need to measure the height of your crown from the bottom to the crown-race seat. Mark this height as your crown and, as above, add in the lower stack height marking, too.
  5. Just to make your life easier it's time to draw in the headtube, and the steerer tube, too, if you like. In this case I started at the crown-race marking and made a 1"-wide (25.4mm) line perpendicular to and centered on the steerer line. Now I marked spots moving upwards 25.4 mm wide centered on the steerer line. Now you can just connect the dots and have your steerer tube drawn. I extend it far above where it needs to be and will finish it in the next step.

    Now you will repeat the process for the HT. The base line should be the marking for the top of the headset lower crown race that you made in step #4. If you're not sure how to make the correct width-markings, skip forward to step #1 in the next section.
  6. Lastly we'll draw in the downtube (DT). First you need to calculate how much of the headtube you'll want extending below the DT. This distance is usually between 5 mm and 10 mm. In my case it's 5.7 mm. Add this number to half the diameter (i.e. radius) of the size DT you're going to use. In my case I'm using a 1.125" (28.6 mm) tube, so the radius is 14.3 mm. Whatever this sum is (20 mm in my case) make a marking from the lower headset mark in the inner headtube wall. From this final mark draw a line from it to the center of the BB shell. This is your DT centerline.
  7. Your final step is to check and mark all your frame angles. The one quality tool that helps at this step is a very good protractor. In theory your ST and HT angles should be correct, but measure them again to be sure. Now measure your lower HT angle. Note that the HT angles are actually obtuse angles (i.e. greater than 90 degrees) measured on the inside of the frame. Frame components are not measured this way so make sure to mark the corresponding acute angle (i.e. less than 90 degrees): for example, 72 degrees corresponds to 108 degrees in the obtuse. If your protractor isn't marked with both angles, just subtract the angle you know from 180, and that's the corresponding angle.

    Once you have the lower head angle and bottom bracket angle measured and marked, add all the numbers together. The two acute angles (seat tube and bottom bracket) added to the two obtuse angles (both headtube angles) should equal 360 degrees. Isn't math neat?

    Your last step is to do the same with the rear triangle, but in the end the three acute angles should equal 180 degrees. If all your angles are correct and measure up to what they should you've now completed the major part of drafting your frame.

    This is the point where if you have any real design problems it'll be easy to change. If you've decided to use a sloping TT or different seatstay connection this is the point where you want to make the changes.

Forward to step three of the tutorial!