Saturday, 2 September 2023

Velocette KTTs 55 and 305: Girder Forks

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.

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