With the hubs away with my wheel builder and the patterns
for various small parts at the foundry it was time to turn attention to another
major part of the chassis, namely the forks.
I am quite accustomed to working on girder forks and apart
from a couple of 1960s Velocettes all of the bikes that I’ve restored and most
that I’ve owned in recent years have girders. However, the restorative work
that I have done has been limited to making new spindles, re-bushing and sorting
out friction dampers.
Luckily, I have 3 good sets of fork blades - 2 that belong
to the KTTs and the other for the cammy special.
and 2 sets of steering columns (part # 12F) and top clip
(part # 8F) parts. “Top clip” is Velocette terminology used in their original
parts book and we would now usually refer to this as the “top yoke”.
but I was missing a steering column, top yoke and links for
the cammy special.
Casting patterns for the links had already been made and
were at the foundry (see here) but I was still missing the other 2
components. These are not easy-to-find parts and so an alternative plan was
needed.
A search on Ebay UK found that fabricators in India are
making these parts …but not for early Velocettes. However, I did find a column
and a top yoke for an Ariel Red Hunter that were listed with their important
dimensions and that I believed could be suitably modified. I had been pleased with
the petrol tanks that I have had from India in the past (see here)
and so I ordered the girder fork bits. This is what arrived a few weeks later.
The manufacture could best be described as “agricultural”,
however the parts appear to be quite strongly made and I found the materials -
steel stem and cast iron top and bottom to be good quality on subsequent
machining.
A lot of modifications were needed to make them fit the
Velo. In no particular order:
- The
stem diameter was slightly too large (ID/OD on the Velo is 0.75”/1” whereas the
Ariel stem is 0.875”/1.125”) and would not fit through the headstock
- The
bottom was too wide for the fork blades and would need reducing. It would also
need bushing.
- The
top yoke was too narrow for the fork blades and would need both extending and
bushing
- The
arrangement for supporting the upper bearing and clamping the top yoke to the
stem would need to be modified.
After grit blasting both parts, the first job on the top
yoke was to make an insert that would support the bearing and would later be
bored to fit a reduced diameter stem.
This was pressed and brazed in and extension pieces were
made to fit the wider Velocette girder blades.
The extension pieces, which had been bored to accept
phosphor-bronze bushes, were pressed and brazed into the ends.
As previously mentioned, the stem diameter was slightly too
large to fit through the Velocette steering head and the entire steering column
was mounted in a 7/8” diameter mandrel through the centre and set up in the
lathe.
The first step was to slightly reduce the diameter over the
whole length
and then to recut the partially “flattened” 24 TPI thread for
the top nut.
This needed to be set up carefully to “pick up” the existing
thread and a new top nut was made to fit the slightly reduced diameter thread.
This is a large nut and I would not normally stock 1 ½” AF
hexagon bar and so the hexagon was first formed from a piece of round bar using
the rotary indexer on the milling machine.
Am I concerned about the reduction in strength from a smaller
OD? No. After reducing the stem diameter by 0.020” from 1.125” to 1.105” the stem is
still over 30% stronger in bending and around 20% stronger in torsion than the
original Velocette stem due to the larger ID and OD (assuming the same material
properties for both).
Finally, the assembly was set up in the dividing head on the
milling machine to reduce the width of the bottom section
and to then bore accurately the internal diameter of the spindle holes for phosphor-bronze bushes.
An advantage of using the dividing head to machine the
internal diameters is that the holes are perfectly orthogonal to the stem and
in line with each other ….and corrected some of the original machining.
There were 2 more parts required to complete this stage of
the girder fork assembly: the bolt that
clamps the top yoke to the stem and the spring holder that is attached to the
top yoke.
Whilst these might sound like trivial items and hardly
worth mentioning it is actually a couple of hours work to make each of these.
This is an original clamp bolt assembly from KTT 55.
Two points to note, both of which are important
for it to function correctly: firstly, the threaded portion is off-centre from
the larger diameter that fits into the top yoke and, secondly, both the bolt
and clamp sleeve are scalloped to clamp around the stem.
The first step in making one of these is to carefully set up
a piece of steel bar (EN24T was used here) in the 4-jaw chuck on the lathe such
that the off-centre distance gives the correct eccentricity
and to machine the diameter for the threaded portion and cut
the thread (5/16” BSF was used here).
After parting off the main body of the bolt and without
removing the steel bar from the chuck the hole in the spacer was drilled to
ensure the same eccentricity for the clamp sleeve.
The second part of the machining is to clamp the assembly to
the milling machine table and machine the scalloped portion. I happen to have a
28mm diameter cutter, which is about the same size as the stem, and this was
used to carefully remove metal from both parts.
….and a new nut to complete the machining of this part.
The final part to be sorted out is the holder for the girder
fork spring. This is what an original part looks like in the KTT 305 top yoke.
The shank in longer than on many other girder fork bikes of the era to avoid interference
with the top steering head bearing.
I did not have a spare one of these but luckily my buddy JT
had an Ariel spring holder that engaged with the spring correctly but was too
short to be easily modified. A substantial extension piece was therefore made to insert
into the Ariel spring holder
and silver soldered in place.
This fits neatly into the 60 taper on the
top yoke. A dome nut to secure it would be more elegant and one will be made in
due course.
Finally, all the bushes were reamed to fit unmachined
spindles and although there is further work when the link castings come back
from the foundry at least the cammy special now has most of the important parts
for a complete set of forks.
Before leaving this section about girder forks, I should
mention that I would not undertake work on girder fork blades that involves
retubing. This is a specialised job and I would leave this to a specialist (and
there aren’t many of them!) that has the appropriate jigs, access to taper
tubing and experience such as Jake Robbins.
