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Laverda 750 Prototype

1969 sohc Three-Cylinder Prototype

Having decided to advance even further into the big-capacity league, Laverda chose a three-cylinder layout for such sound reasons as good engine balance, reasonable overall dimensions, acceptable costs and the fact that a triple was still a rare species and, therefore, much more appealing than a twin (or even a four). Laverda's chief engineer Luciano Zen also wanted to keep production costs to a minimum, so he tried to incorporate as many existing parts and features from Laverda's existing twin-cylinder range. These included the timing system, electric starter, five-speed gearbox, chain primary drive and even the cylinder bore size.

However, there was a big departure from the twins in that the triple had a full loop duplex frame, in classic Norton Featherbed style. Zen realized that this would not only provide increased strength, but also give a better riding position, and easier maintenance and accessibility to the upper parts of the engines.

Heads and cylinders were each a single casting, in light alloy, with 'square' fining as on the 650/750 twins, steel valve seats and austenitic cylinder liners (inclined at 25 degrees from vertical) to reduce weight and height, and assist cooling. In an attempt to keep the engine temperature down, Zen and his development team provided generous air ducts around the cylinders, paying particular attention to the unfortunate central 'pot'.

The valves, with double helical springs, had a diameter of 38mm (inlet) and 34mm (exhaust). These were inclined by 32 degrees (inlet) and 37mm (exhaust).

At 75 x 74mm, the bore and stroke were those of the six-fifty twin, itself superseded by the larger seven-fifty twin.

The crankcase was split horizontally and the massive, built-up crankshaft, with 120-degree crankpins, one on either side, plus another on the left (nearside), next to the primary drive. All were of the roller type, the big ends each rotating on two roller cages,, whilst the small ends were on plain bushes. The pistons, with the skirt cut off at the sides to reduce weight, employed two compression and two oil scraper rings, one of the latter below the gudgeon pin.

The duplex chain for the single overhead cam drive, with a slipper tensioner that was adjustable from the outside, was not between the cylinders as on the 650/750 twins. This, Zen reasoned, would reduce crankshaft width and manufacturing complications, and also ensure a symmetric layout for the two central bearings. Also, in a further attempt to reduce transverse dimensions, the three sets of contract breaker points were located at the left (nearside) end of the camshaft, and not at the end of the crankshaft, as on the twins.

On the right was the chain-driven electric starter and the belt-driven 12-volt, 150-watt dynamo, which was set in front of the crankcase, as on the twin-cylinder models.

On the left side of the engine was the triplex chain primary drive, with slipper tensioner (again with external access for adjustment).

The clutch, with a total of 12 plates, had the drum reversed towards the inside of the crankcase, again to limit width.

Carburation was taken care of by three 30mm Dell'Orto VHB square slide instruments, with a metal gauze-type filler in a common, square box.

The five-speed gearbox shafts both rotated over ball-race bearings and employed the same ratios as the recently released 750S (2.619; 1.883; 1.374 and 1.173:1) while the primary drive ratio was altered to provide 'taller' gearing. Final drive, by chain, was on the right (offside).

Running on a 9:1 compression ratio and with the same timing as the 750GT (35-62 degrees inlet; 68-37 degrees exhaust), the 1969 Laverda 1000 triple prototype put out around 75bhp at 6,700rpm, with a maximum speed of around 200km/h (125mph).

The tyres posed a particular problem, a 4.00 x 18 rear not being considered up to the task. Italian tyre giant Pirelli therefore undertook development of a wider 4.50 specially for the machine.

Other details of the prototype's specification included Ceriani front forks (with rubber gaiters). Both four leading-shoe (double-sided) and twin leading-shoe front brakes were tried. The rear was only a single cam unit. The speedo drive was situated on the offside of the front hub. Two exhaust pipes (and silencers) exited on the right, the remaining pipe and silencer on the left. The whole system was finished in bright chromium plate. Borroni alloy rims, tank-top parcel grid and dual seat with passenger grab rail completed the picture.

Although the definitive design used dohc instead of single overhead cam, many of the other features from the 1969 prototype were to be carried through to the 3C production model. This was finally offered for sale in 1973, after being officially presented to the public at the end of the previous year.

Source Laverda Twins & Triples

 

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