Drawing the Frame in CAD

We now have all the info we need to draw up the frame in CAD. To recap we are going to use the following values: (SR = seat reference mark)

SR above ground = 609.6mm (24")
C of G behind SR = 39.7mm
BB center above SR = 228.6mm (9")
BB center in front of SR = 783mm
Heel Clearance = 75mm

FYI the CAD program I'm using is a nice shareware program called DeltaCAD. It's pretty easy to use and I've gotten used to it's quirks. There are more featured apps out there, but familiarity is worth more than features in this case. It's a windows app but runs well under wine on linux.

Step 1:
Draw a horizontal line that is 2000mm long. Then in the center draw a vertical line that is 1000mm tall. The horizontal line is the groundline and the vertical line is the center of gravity.

Step 2:
Draw a vertical line that is 609.6mm tall, 39.7mm in front of the C of G. This represents the SR. Then draw a second vertical line that is 609.60 + 228.6 = 838.20 mm tall, 783mm in front of the SR. This is the BB center.

Step 3:

Now we need to know the wheel dimensions. We are going to use conventional 26" mtb wheels, but with narrow, 25mm tires. To get the outer diameter of the finished wheel, you take the bead seat diameter of the wheel, 559mm for us. Add 13mm for the bead height, then divide by 2 to get the radius and add the tire dimension (25mm). So we get (559+13)/2 + 25 = 311mm.



Draw a horizontal line 311 mm above the ground line.

We now need to find the front axle location. To do this we will need the heel clearance dimension which is 75mm. I've defined this as the distance between the arc defined by a 175mm crank and the top of the tire. But in this case we really want to find the distance that is 175mm + heel clearance + wheel radius = 175 + 75+ 311 = 561mm away from the BB center. Draw a circle with this radius and mark on the axle line where it intersects. This is the front axle location.

Step 4:

Draw in the wheel and measure the distance from the c of g line. In this case it is 630.92mm. We will use this in the next step.

Step 5:

Now that we have the front axle distance from the c of g, we can find the location of the rear axle. But first we need to decide on the weight distribution. I said previously I want between 45 and 50% on the front wheel. So I'm arbitrarily deciding on 48%. Now we need to solve for the wheelbase. 630.92 = (100%-48%)*wheelbase. Wheelbase = 1213.31. Mark out the rear axle location 1213.31mm from the front axle and draw in the wheel.

Step 6:

We are going to braze in the BB for this bike off center so that the top edge of the BB shell is a the same level as the top of the maintube. This give us better front derailleur clearance. So I took a look at Nova cycles and found this BB shell. It has an OD of 31.8mm so we draw a circle of that diameter centered on the BB center.

We then need to ensure that the maintube clears the seat. Conveniently we used the low point of the seat for the SR, so we just need to account for the thickness of the seat and a little extra. The seat I am considering is wood, and is ~3/8" thick. So I gave 15mm of clearance. I draw a circle with r=15 centered on the SR.

Then draw a line that is tangent to both circles and ends at the rear tire (we'll shorten it later). This is the top edge of the maintube.

Step 7:

Finally draw a parallel line that is 50.8mm below the top line of the maintube, then draw a longer parallel line that is halfway between them that extends to the rear axle. Mark the offset from the axle.

So far so good! The wheelbase looks good as does the rear axle offset. Next time I'll talk about steering geometry.