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JPS norton Monocoque Race Results 1973 n Top scorer TransaTlanic n MaTch race series n 13 inTernaTional wins n 1sT ForMula 750 TT The Norton “Monocoque” was an attempt to reduce the size of the bike by using one component to do several things. The fundamental aim was to reduce the frontal area, but I also expected a reduction in weight from the multiple uses of one component. The so-called “monocoque” principle is the use of the bodywork of a vehicle as a stressed part of it. It requires the inputs of loads to be applied in a more subtle way than in a tubular frame. On a monocoque loads have to be applied along the surface of the structure, whereas on a tubular frame loads can be applied at an angle, and almost anywhere. Tubes work best in tension but they are able to take bending loads because of their cross section area. Resistance to bending increases as the square of the tube’s diameter; double the diameter and it’s four times harder to bend. The same proportions apply for resistance to buckling. A good demonstration of this is to use a piece of paper. Roll a piece of A4 into a tube. Stand it upright on the table and put a table mat on top. You can put quite a large weight on top if you keep it central. Put the weight off-centre and the tube collapses. The paper is over-stressed. Now try putting a force into the side of the paper tube (poke it in the ribs). If the tube is about 3inches diameter it takes very little force to distort the paper. The large diameter paper tube is a very good model for the principle of the monocoque. Loads must be fed into this
kind of structure along – tangential to – its surfaces and they must be distributed over large areas. The 1972 JP Norton had a tubular spine frame and pannier tanks, similar to those used on the Mk2 Arter Special, and before that by my father for the “works” AJSs in 1954. The 1972 season was well under way and it would have been easy in the heat of racing to leave preparation for the next season too late. But the 1973 season was the one our sponsors would expect us to be really prepared for. We had to be competitive. One night I went to bed with my mind full of thoughts of what might be possible for our 1973 JPN. I began to think about the pannier tanks on the 1972 Norton and wondered if these two boxes could be
essentially just a box, triangular in crosssection. The engine hangs from it; the front of the bike hangs from it; the rear suspension and the rider’s seat are connected to it. In our case it was originally planned as an integral oil tank but we had to put a separate one in front of the engine to keep the oil cool. That was disappointing because it added weight and changed the weight distribution but we could not think of a better remedy for the over-heating of the oil. The front was more difficult to visualise but it was a triangular cross-section box, too, with the steering head tube at an apex. It also had to be the fuel header-tank because we had to have a pump to raise the fuel in the side tanks to above the carburettor level.
stressed and used structurally. But how could you join the front of the boxes to the steering head and the rear to the swinging arm pivot? By the morning I knew the answer. And, to his credit, team manager Frank Perris saw the potential of the sketches I showed him. In the end we created four boxes instead of two. The ones each side of the engine were joined by a third box at the front holding the steering head and the fourth behind the engine. This held the rear engine mounting – rather than the swingarm pivot because we felt that we should use the Norton Commando Isolastic engine/gearbox/swingarm/rear wheel mounting system in which the swing arm pivot is contained in the gearbox plates. The rear box is what I now think of as the most influential part of the motorcycle. I call it the ‘central bracket’ but it was
The engine would really suffer if it was starved of cold air and the boxes either side of the engine acted as ducting for cooling air to reach the engine. I already knew from my father’s and my own experience that we must allow cold air to reach the carburettors so we put a splitter in the air intake to direct over the engine to the carburettors. The chassis could not have been made without John McLaren. For me, it was the culmination of all the experiments I had made in the preceding years of my racing career. The 1973 John Player Norton was at some times that year the best racing motorcycle in the world. It won 12 International races and the Isle Man Formula 750 TT at record race and lap speeds. From the first test ride of the fabricated mild-steel prototype on a cold grey day at Thruxton Race Circuit I had known it was a winner.
“It was the only bike I ever rode that could be drifted like a car – front and rear wheels sliding”
MY Best Ride ...
The JPN was the only bike I ever rode that could be drifted like a car – front and rear wheels sliding. To do this round Woodcote you have to be brave but the Monocoque gave great confidence and I drifted round on every lap of the John Player Grand Prix. This is the race I am most proud of. It was hard work at about 145mph to lay the bike over more and more until the rear tyre was slipping and the front was going light. But I was ‘in the zone’. We equalled the Jarno Saarinen lap record and it could have been the 14th win of 1973, had I not run out of petrol on the second-last lap. The JPN Norton was the incarnation of my vision of a racing motorcycle – in that race it made it possible for me to ride as close to the perfect as my perfect can be. It vindicated my engineering approach.
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Above: Craig Davies has 14 years of experience with the GPZ900R. Far left: Automatic Variable Damping System – Kawasaki’s name for anti-dive. If it seized it could overload fork seals. Left: Removing the o-ring seal from the unit
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