Frame geometry design

In order to design the frame geometry I prepared a very simple spreadsheet, which I show below and explain briefly. Unfortunately, I cannot attach files to this blog. Anyone interested, let me know and I send the spreadsheet.



The input parameters are the following:

- wheel radius:
in my case 342mm as I am using 32 x 622 (700C x 32) wheels. Is not only about the rim radius but about the radius of the outer rolling surface of the tyre. For every other tyre type this information can be found in the following site

http://www.sheldonbrown.com/cyclecomputer-calibration.html

- angle of the chainstays with respect to the horizontal
In the donor bicycle this angle was 8 degrees, but it is possible to modify this angle, say to 6 degrees or less, thus increasing the height of the front bracket center, positioning the pedals higher with respect to the ground and allowing for more clearance of the back stays with the rear wheel

- relative position of the upper node of the frame where the top stays and the down stays are attached together to the head tube.
The selection of this point will largely define your frame geometry, and both the vertical and horizontal coordinate have to keep a certain ratio. By modifying the height of this point the size of the frame can be adjusted to match the desired inseam distance of the prospective cyclist. The vertical coordinate of this point, together with the rake angle and other fork parameters, will define your wheelbase.

- rake angle and fork offset
It is the angle between the steering axis and the horizontal. This is a key parameter of the steering geometry, together with the fork offset. Both these parameters are essential in defining the steering stability and maneuvrability of this bicycle.
Standard bicycles have a rake angle of 73 degrees, along with a curl at the end of the fork in order to position the wheel center a certain perpendicular distance from the steering axis. This distance, called fork offset, is usually about 40 to 45mm in standard bicycles. These two parameters combined, define a certain trail, which is the horizontal distance from where the steering axis intersects the ground to where the front wheel touches the ground. Except exceptional cases the front wheel ground contact point is behind the steering axis intersection with the ground, which is then called positive trail. The trail is directly linked with the steering capabilities of the bicycle. However, for a more detailed analysis of bicycle steering geometry the following article is recommended,

http://www.phys.lsu.edu/faculty/gonzalez/Teaching/Phys7221/vol59no9p51_56.pdf

In the Pedersen design the truss construction of the fork with a central bracket inherently generates a very large fork offset of more than twice as much the standard value of 45 mm. Using this large fork offset in combination with a standard rake angle of 73 degrees would result in a very small or even negative trail which would render the bicycle very unstable or even unridable. That is why for such large fork offsets of a Pedersen fork, rather shallow rake angles of about 65 degrees are needed to get a reasonable positive trail of about 40 to 50mm.

- seat tube angle position
This angle should be chosen so that it allows clearance to the rear wheel and yet get a reasonable position of the anchoring point for the hammock saddle. One important parameter for frame sizing is the inseam distance, which for my purposes I defined from the calculated seat point to the center of the pedals bracket. A feature to calculate the seat position and the inseam distance. included in the spreadsheet. I compared this distance and also the angle to the horizontal with the same parameters in my standard conventional frame bicycle, to get an idea of the ergonomics of my saddle position and size frame. For this specific frame I am constructing, my target is a M size frame with inseam distance adjustable between 640 and 740 mm.