Fork construction continued

The process of fabricating the fork took nearly a month of patience, miting and brazing. Once both blades were ready the lugs were brazed in place, after careful alignment of the wheel axis.
Here are both fork blades put together with the wheel. The lugs are solidly fixed to the fork and I was satisfied with the resistance of the whole assembly. Note the radial lacing of the front wheel spokes. Anyway, this is a topic for a future post.



The next point was to align the fork and prepare to braze both fork blades together. The upper part of both blades was carefully mited to fit together at a precise angle. This is was can be seen in the next picture. The wheel is assembled in place and centered. Both fork blades are hold together with clamps, cleaned and fluxed.



Neat and solid brazing joint.



This is how the bare fork construction looked like. No bracket or stem yet. I put the mudguard in place to measure the bracket position and dimensions.



For the bracket I used a steel plate with a thickness of 2mm, conveniently bent at 90 degrees, that I found at my standard source place for scrap metal. I started from the very basic specifications for the bracket: I need to hold together 4 fork tubes, to support the articulated joint that connects with the frame and to support the caliper brake and mudguard.

Once the functionality of those features is secured, I proceed with cutting bracket with a design-as-you-go approach unleashing the best of my plastic artist's creativity. In the next picture it can be seen how I carved the holder to braze the nut to fix the articulated joint. The slot for assembling the brakes and mudguard is already done.



Here is the nut in place, fluxed and ready for brazing. Note the carvings I did in the bracket. It took some time, but was a rewarding experience.



Here is the bracket aligned, fluxed and ready for brazing. Below you can see my Sievert torch with a 70 g/h nozzle and a 0,4 kg camping-type butane bottle.



The idea to fix the stem was originally just to mite the inner stem tube to match the four tubes of the upper fork blades. I was concerned that this point where the stem is brazed to both blades, would become the weakest point of the whole construction. After some discussion with my work colleage Peter, he came up with the suggestion of a bolt with cylindrical head brazed to the tip of the fork blades that fits the inner diameter of the stem. This contraption was named the "Boldten" piece, and really reinforced the whole stem assembly considerably. Thanks, Peter!



The stem was recycled from the donor bike. It was necessary to cut out the recessed part where the headset lower bearing was fixed, since the new head tube will have shorter length and different positioning. A new ring for fitting the lower headset bearing will be needed.
Careful miting of the stem inner diameter to match the four tubes of both fork blades can be seen in the next picture.



The stem was solidly brazed in place. Note the small holes in the stem. Their purpose is to access the cylindrical head of the "Boldten" piece for brazing. Now it is evident that the stem is attached to the blades fork construction in two points, increasing the rigidity of the whole structure.



Next step was to manufacture the ring that supports the lower bearing of the headset. This was done in my old trusty Unimat 3 from Austria, as shown below.




Coincidentally, the ring needed to be placed over the holes used for brazing the "Boldten" piece, and therefore, no traces of such holes were left after the ring was brazed in place.

Fork construction

The fork is the key element in a Pedersen design, where the paradigm of the diamond frame is really shattered. Instead of the classical fork with two blades hold together to the stem by a bracket, the Pedersen fork consists of a truss construction out of basically four triangles united at the top and reinforced with a central bracket. Instead of a stem supported by bearings in a 10 cm head tube as standard bicycles, the Pedersen fork has two separated fixation points about 35 cm apart, so that the fork is an integral part of the frame structure.




Here is a picture of the Dursley Pedersen factory workers showing the advantages and resistance of a truss construction for the fork. Impressive resistance, and yet lightweight. This picture is a really convincing example of marketing and advertising techniques of the early XXth century.



The construction of the fork and the upper head tube with the frame fixation point is rather complicated and time consuming. It took me more than half of the whole project duration, as I did all tubing miting by hand and using no power tools. Just old fashioned hand filing.

For my fork design I recycled the top stays of the donor bicycle, as you can see in the following picture. This recycled bent tubes were used for the rear fork tubing, along with straight tubing 12mm diameter with 1mm wall thickness for the front tubes. This tubing was of regular quality and I bought it in the nearest hardware store. I did not use for this bike any specialized Columbus tubing, chromoly or anything like that. In the picture below also a prototypical bracket can be seen, along with a recycled lug, the shackle and the lower fixation head.



The first step was to manufacture two fork blades, left and right, with a triangular shape, out of one straight and one bent tube. For the top joint I had to mite the tubing very carefully to an exact match before brazing. The miting can be seen here, and it took much more time than what could be guessed from the picture.



Prepared for brazing, fluxed and ready to go.



A brazing professional would probably describe this as a butcher job rather than a clean joint. However, a solid joint it certainly is.



Here is after cleaning and miting for the upper joint between the two blades. Notice that the miting revealed a very nice layer of brazing filler material at the joining surfaces in the tip. There it is, a sound brazing joint after all.



For the lower side of the triangle I deviced another type of design for joining both tubes, as the lug has to be solidly attached. I turned in the lathe a steel bolt which I brazed to the tip of the rear bent tube. In the tip of the straight tube I filed and shaped the connection to fit the lug. In the picture below, all this can be seen, with the raw brazed joint.



Finally I did mite the steel point at the tip of the bent tube, to meet the straight tube precisely at an angle and prepared the connection for the lug. I did not yet braze the whole assembly with the lug, as I need first to check proper alignment of the wheel within the complete fork.

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.

Slaughtering the donor frame

The first thing before starting with the detailed design of the frame geometry was to slaughter the old frame and see which parts I can re-use, so as to design a frame around these parts.

The frame was a classical woman type with these upper bent stays from the rear dropouts to the head tube. Here it is, after disassembly.



The bent stays and the chainstays were cut, as well as the head tube and front dropouts. All these parts will be re-used. Here are the results...








I left the down tube and the seat tube uncut for the moment, as I have the idea of producing a bolted design but I am not yet sure about how to fabricate the coupling points in the existing frame. There will be a lot of time to consider this problem as I am busy with the fork construction.

I measured carefully the distance between the center of the bottom bracket and the point in the rear dropouts where the center of the wheel will be normally fixed. This distance I will take as a fixed parameter in my spreadsheet for frame geometry design.