Joe Potts Desmo part 2

As information on the Potts desmo filtered out to other racers and the press, much anticipation started to build about the project. In Geoff Duke’s preview to the 1958 season, he wrote the following:

“With rumour upon rumour circulating about the Joe Potts’ stable, and all sorts of items, from the 270 lb “500s” to 90mm bores and desmodromic valve gears being accepted facts, one can hardly discount Bob McIntyre’s chances.”

The Editor of Motorcycling magazine went to visit the team at Bellshill before the start to the season. He concluded his article with the following:

Progress had been slower on the Potts desmo than anticipated, so it was not ready for the start of the season. At this point the cambox had been part-machined, but all the bearings had been fitted. Three sets of rockers for the desmo had also been made (for 250cc, 350cc and 500cc engines).

The rolling chassis for the 350cc and 500cc had been manufactured. So for the start of the 1958 season Bob Mcintyre would race these machines with more conventional Manx Norton engines. The 500cc machine scaled in at a very lightweight 274lbs. It should be noted that for the initial development, conventional Manx Norton wheels were utilised instead of the Gilera wheels.

Whilst Bob raced the lightweight 350cc and 500cc Manxes, the team at Bellshill would continue to develop the desmodromic valve gear so that it could be used later in the season. The more conventional engines that Bob used in his Manxes were still far from standard.

Bob's 270lb 500cc Razorblade Potts Manx

For fitting into Bob Mac’s Razorblade frames the team had been working on new 500cc and 350cc Manx engines. These two engines were completely Potts work using only the crankcases, timing case, cam drive assembly and cambox from a Manx Norton. The 500cc engine was to be 90mm bore and 78mm stroke. The 350cc engine had an 80mm bore and 69.5mm stroke. The crank assembly for both these engines was made at Bellshill. The flywheels were machined by Potts from blank forgings and were oval in shape to allow the use of ultra-short connecting rods. The flywheels had integral mainshafts and a flanged crankpin was pressed into the drive-side wheel before being drawn into the timing-side by a nut that was subsequently cut away. A specially commissioned big-end made by Alpha bearings was used, which incorporated two more rollers, all of a smaller diameter (14 rollers as opposed to 12). This was done to reduce skidding of the rollers at very high engine speeds. The short con-rods were also machined at Bellshill from solid steel billets. Special pistons for the engines were also machined at Bellshill which featured very short skirts to help clear the flywheels and reduce friction. Blank cylinder heads were machined to allow the use of larger valves (2 inch inlet valve on the 500cc) and a more down-draft intake port. Due to the greater down-draft intake port, a specially modified Amal GP1 carburettor had to be used to prevent flooding. The cams in the engines were Bellshill made specials, drawn up by Charlie Bruce.

More to follow

1957 Joe Potts Desmo

With Bob McIntyre primarily riding for Gilera during 1957, the Joe Potts racing team had more time to experiment with their machines. This coupled with Gilera’s withdrawl from racing at the end of 1957 mean that Bob would be back on Joe’s machines fulltime for 1958.

In 1957 Bob had been riding 250cc, 350cc and 500cc Potts Manx Nortons. Bob knew he could beat any other rider who was riding a single cylinder production racing machine, but it would not be possible to match the MV Agustas of John Surtees. So for the 1958 season the Potts team had to come up with something radical in order to take on Surtees and the Italian Fire Engines.

The inspiration for what they would undertake would come from the all conquering Mercedes W196 Formula 1 car. This car had dominated motor racing, with much of its performance attributed to the use of desmodromic valve gear within the engine.  

Looking at current technology the benefits of desmodromic valve operation (positive valve closure) are debatable. However, in the mid 1950’s the situation was different. Two valve per cylinder engines were commonplace. The large diameter and hence heavy valves combined with poor valve spring technology (Hairpin valve springs) meant that valve control was a real problem. This valve gear limited both maximum engine speeds and the valve timings that could be used.

So the Potts team’s solution for an MV beating machine was to make an engine with desmodromic valve operation. The design of the engine was driven by Joe Potts and Charlie Bruce. Charlie Bruce was a gifted racer himself – winning the Scottish 250cc championship on more than one occasion. He was also a good mathematician and would draw out the cam profiles for the Potts machines.

A desmodromic conversion would be made by Potts for the 250cc, 350cc and 500cc Manx engines. The desmodromic cambox was cast at Rolls Royce, East Kilbride. The investment casting method was utilised for production. The cambox was to be a three shaft design, with both pairs of opening/closing cams sitting on the same shaft. This design was chosen because it eliminates the issues of changing clearances between the opening/closing cams as the engine warms up. Four rockers would be used (two on the intake, two on exhaust) to operate the valves.

The desmodromic conversion was not the only piece of the jigsaw to beat the MVs. The rest of Bob’s machines were also to be radical. New frames were made for all three machines. The frame maker at Bellshill was Alec Crummie, who was known as the “Wizard with a welding torch”. The new frames were to be known as the Razorblade.

The Razorblade frames came about from Bob McIntyre’s desire to make a very light and narrow (hence Razorblade name) motorcycle, which would have the same handling characteristics with either a 250c, 350cc, or 500cc Potts Manx engine installed. The most notable feature of the Razorblade frames was the Reynolds Tapering front down tube.  The new frames used the single tapering front down-tube from the steering head and joined separate tubes that ran either side of the engine. Twin top tubes acting as a seat and tank support, then returned to the steering head. These tubes were made in 16 gauge Reynolds 531 with a diameter of 1⅛ inch. All the tubes in the frame were Sif-bronze welded together. The whole Manx engine assembly was not held in the frame by the conventional 11 mounting bolts but by only four and one of these was the swinging arm pivot. There were no front engine plates, as the crankcase fitted directly into a bracket brazed onto the front tapering down-tube. The engine/gearbox unit was designed to act as a stiffening unit for the frames. The Razorblade frames were much lighter, narrower, lower, and had a shorter wheelbase than a standard Manx Norton Featherbed frame. Alec Crummie produced three Razorblade frames that were to be used by Bob McIntyre. The 250cc and 350cc frames were very similar in design, but the 500cc differed in two ways. Firstly it had the timing side tank tube bowed to clear space for the cambox. The kink on this rail had to be large, so the Potts Desmo could be fitted into the frame. The second difference is that the bottom of the front down-tube is bent forward to change the weight distribution.  

When Bob McIntyre left Gilera, he took with him two of the 4LS front brakes and two of the rear brakes from the 125cc twin. The 500cc and 350cc bikes were initially built up with these Gilera wheels fitted.

The suspension to be used on the Razorblade bikes were shortened Norton front forks and short double damped Girling rear suspension units. Initially Bob’s 500cc and 350cc Razorblades were built up using the brakes he had taken from Gilera.

500cc Razorblade chassis as it is now

All the bikes were built onto 19 inch rims. The tyres used on the Razorblade bikes were narrower than standard, to reduce rolling resistance and weight. Bob used only a 3.00in rear tyre on the lightweight Manxes (when standard was 3.50in), and a 2.75in front (3.00in standard). The fuel tanks for the Razorblades were made by Lyta. The tank was then modified, so the oil tank was incorporated into the front timing-side section. This was done to save weight and also to clear the area around the carburettor intake from the hot oil tank. This was always a feature of Potts design as they always wanted cool and therefore dense air around the carburettor. The petrol/oil tank modifications on the 500cc were not done by Alec Crummie and he did not approve of the quality of the welding. For this reason he refused to allow the tank into his workshop, meaning the tanks could always be found on the floor just by the door in Alec’s workshop! The gearbox used in Bob’s Razorblades was to be a Norton Laydown type that was inclined in order to fit it in the frame.

To be continued.