Velocette KTTs 55 and 305: Wheels – Part 3 – Rear Wheel Hub

As I mentioned in a previous blog posting, whilst I had rear hubs for both KTT 55 and KTT 305 I was missing a hub for the Mk 1 OHC special . Specifically, one of these:

I’m not sure of the correct part number for this: the 1925 – 1931 parts book list this as K7 whilst the later 1932 to 1934 book has K7/4. Is there a difference between them? This hub is from KTT 55 and so mine is most probably the earlier component.

The mating brake drum part numbers are KS2 and KS2/3 respectively, the difference being the size of brake shoes that will fit. Grove Classics lists both of the brake drums (see eg here) and I had already purchased one of these

in the hope that I could find the hub. Unfortunately my efforts to find a hub were in vain -  I put a “Wanted” advert in Fishtail, scoured the autojumbles that I attend and put the word out among my buddies but there was nothing readily available. Not wanting to be held up in the project I decided that I would make one (note: if you need one look at the end of this blog....)

The first step was to make a drawing based on the hub that I had. However, there is one problem with this, namely that the taper roller bearings that were used originally are no longer available and the internal design would need to be changed.

By chance, I had a couple of new Timken taper roller bearings in my box of bearings, still in their original wrappings, that would be perfect for the job. The picture below shows one of the bearings together with an original (upper part of the picture - Velocette part # KS-18) from one of my other rear hubs.

I bought these over 50 years ago when still an engineering student with the intention of building an Aspin rotary valve Engine (I won’t digress into that here ….see eg http://www.aspin.info/) but that project never happened ….but I still had the Timken bearings. These bearings are still available today and after checking their load-carrying capacity decided that these would be an excellent substitute for the originals.

Timken provide a lot of information about their bearings; the detailed specification of this particular bearing is summarized below:

Apart from the dimensions, one of the most important pieces of information here is C₉₀, which is a measure of the radial load capacity of the bearing. The value of 1100 lbf means that a pair of bearings mounted opposite each other on the shaft will have a life of 90 million revolutions if subjected to a radial load of 1100 lbf. What does that mean in practice? Using 21” wheels (rolling diameter with a tyre is approximately 24”), 90 million revolutions equate to over 100,000 miles. How big a load is 1100 lbf? It’s half a tonne.

In simple terms, if the little Velocette and rider weigh one tonne (remembering that the weight of the bike + rider is distributed approximately 50/50 between the front and rear wheels) then they should be able to ride 4 times around the planet without worrying that the rear wheel bearing would fail. So, even going for a long ride after a heavy lunch shouldn’t be a problem.

The main dimensions of my existing hub were measured and modified internally to use the new bearing.

What material to use? I found that EN3B steel, which is quite adequate, could be obtained in 110mm diameter and a length of this was ordered.

In the above picture, the steel for the new hub is on the left, the piece of steel in the centre was also bought as I believed I would need to make a mandrel to hold the hub during machining and the existing hub is on the right. It turned out that I didn’t need a mandrel to support the machining.

The first step was to machine the larger (4.06”) and smaller (2.85”) spoke flange diameters.

This is a fairly large lump of metal to have spinning round and the surface speed of the cutter at these larger diameters is relatively high so the lathe is run at one of the slower speeds. In addition to producing large quantities of swarf (more of that later) the smoke from the cutting oil fills the workshop. In fact, it was so bad that I would initiate a cut and then leave the workshop whilst watching progress through the window from outside!

The next step was to bore the inner diameter for the bearing.

The workpiece was then turned around and, after carefully checking that it was on-centre with a dial gauge (I replaced the chuck on the lathe some years ago and it is pretty accurate) the larger diameter end was bored and the wheel and spoke flanges machined.

The workpiece was again turned around and the smaller diameter finished to size and the centre portion “carved out” using left and right hand tools alternately.

At both ends of the centre section, the corner was left unmachined

to leave sufficient material for a radius to be blended in.

The final operation on the lathe was to put a radius on the spoke hole flanges.

The lathe work is now completed and it was time to set up the hub in the dividing head on the milling machine.

The first operations were to drill and tap the 5/16" BSCY threads that secure the brake hub/sprocket and to drill 20x 0.15” holes for the spokes.

The hub was then turned round and the spoke holes on the smaller flange were drilled. It is important to note that these are staggered with respect to the spoke holes on the other end.

Apart from a ¼” BSF threaded hole for a grease nipple, machining of the hub is now complete.

 

A couple of statistics: the cost of the steel was 80 GBP; total machining time was 5 days in the workshop – around 28 hours.

And I have one full dustbin of steel swarf to dispose of!

Last, but my no means least, if you need one of these and either don't have the time, the equipment to make one or don't want to spend a week being asphyxiated by cutting oil smoke then please contact Peter Miles at mr.petermiles@ntlworld.com as he is making a batch - get your order in now! Peter is the K Series Velocette specialist in the Velo Owners Club.