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The Black Shadow Story

The Black Shadow Story

(from Two Wheels, June/July 1973 - courtesy of 'U.K.Vincenthusiast')

THE GENTLEMAN'S name was Cliff Horn.

That's as much as I know about him. He had scarcely won a race anywhere when he rode in the Clubman's TT on the Isle of Man in 1949. His Black Shadow Vincent recorded a fastest lap of 85.6 mph - pretty fair going considering the fastest lap in the International Service event was set at 89 mph on a. special Guzzi twin by the seasoned campaigner, Bob Foster. Mr Horn wasn't the only rapid Vincent rider - but he was of an era of riders who understood the origins of their machines.

I often wonder if the Vincent victors of later years or the modern-day riders of carefully preserved Vincents understand exactly how difficult it was to produce an entirely new motorcycle in those hard, closing years of World War II.

The Series B Rapide was conceived when the war clouds were lifting and production of munitions lessened, the first one fired in 1946. Two years later the Black Shadow came on the scene. It was similar but faster still.

After five years of rationing, air raids and a six-day, 60-hour regular working week, the whole nation was tired.

Machine tools were worn to shreds.

Materials, especially steel, rubber, and copper, were short, and worse still, only obtainable by officials permits, very unfairly based on pre-war consumption, and issued on the condition that 80 percent of the products would be exported.

The Vincent-HRD Company had a comparatively tiny pre-war production, and our material allocation was pitifully small, except for aluminium, which was plentiful.

All this added up a pretty grim picture, but it had to be borne in mind when deciding the main specification and detail design of our post-war machine.

Not having an extensive chain of overseas agents with specialised service equipment, the bike had to be relatively simple to look after, but we had little hope of competing on a cost basis with the factories which had been making military motor cycles in the 500 and 350 classes.

On the other hand, the pre-war Rapide V-twin, though open to criticism in many respects, had been easily the fastest thing you could buy on two (or four) wheels, and, morever kept its tune better than high-performance singles.

The Design

The decision was almost made for us. We would capitalise on the Rapide's reputation by making an up-dated version which would be shorter in the wheelbase, lighter if possible (it wasn't), have a 50 deg V-twin 1000 cc engine built largely of aluminium, developing around 45 bhp, on 6.8:1 compression ratio to suit the poor quality petrol then available, and a clutch, primary chain and gearbox able to cope with 100 bhp for future power increases. The frame had to have as little steel and steel tube as was possible, but in detail we wanted to incorporate any feature which would appeal to the practical rider without being merely a gimmick.

It must be mentioned that with the exception of the retired engineer who filled the post of managing director, we were all, from P. C. Vincent down, active motorcyclists, riding every day in all weathers to work, or in long distance trials, while racing men like George Brown and C. J. Williams were employed as development testers. The front and rear duo-brakes which we had pioneered in 1934 had been so satisfactory that they could be continued unchanged, except for widening the rear hub to move the chain out far enough to clear a 4 in. tyre. In these hubs, all four drums, the eight shoes, plus the minor bits and the taper roller bearings were identical, thus simplifying the spares situation, while the drums could be detached without disturbing the spokes; in fact, on either wheel you could use two drums, one drum or none at all, according to requirements. The rear wheel, which was reversible, could carry a second sprocket for easy changes of ratio - very handy in long-distance trials combining miles of main road, followed by mountain sections. With a tommy-bar pull-out axle and quickly-detachable torque arms, the wheel could be removed without using any spanners in under a minute, the operation being aided by a rear stand and a hinged section on the mudguard.

Frame and Suspension

The post war machine that rebuilt a legend. This 1947 model was the original Black Shadow of that year. It's pictured here still on its assembly bench in the factory. Note the early-type Brampton forks.

Although they were a little too light for heavy sidecar work, and had no trail adjustment, the pre-war Brampton girder forks were considered to be adequate for the time being, as most of the machines would be used solo. The possibility of a change to something else was kept in mind, but had to be shelved because it was more convenient to use a design for which tools existed, and the steel required came off Brampton's allocation, not ours.

That settled the wheels and forks, but nothing else. After much discussion it was decided to eliminate the front downtube by employing a hollow sheet-steel backbone which would also act as an oil tank, and bolt this to brackets on each cylinder head. It wasn't actually a new idea; the American Flying Merkel used it, and of course, the inclined cylinder of the P & M had acted as the front downtube for half a century. In effect, our engine would be held up by its ears and by adopting unit construction and pivoting the rear-forks directly on the gearbox tubing was done away with, yet as a frame unit the engine was far more rigid than any tubular construction could be.

With this general layout the wheelbase was reduced to 55 1/2 in. and the weight was brought well forward - the way it should be for good handling but nowadays frequently isn't. Another advantage was that it was easier to detach the whole front end and wheel it away than to juggle a bulky engine out of a loop frame. The design allowed the rear frame pivot to be mounted very close to the final drive sprocket centre to minimise chain tension variation as the forks moved up and down (another design requirement often disregarded today) and the chain line could be widened to clear a big tyre, yet the overhang of both the final drive sprocket and the clutch from their adjacent bearings was reduced to a minimum, a factor helping to prolong chain and bearing life. The rear forks were of P. C. Vincent's extremely rigid triangulated design which he patented in 1928 and in 1931 the Vincent HRD was the only bike in the world produced with rear springing. This fork had never been known to break, and contrary to popular belief the unsprung weight was little, if any, greater than that of the later swinging-arm type which is often lacking in torsional strength. In the interests of standardisation, the same taper roller bearings used in both hubs were also employed for the fork pivot, and were all mounted on hollow spindles so that the adjustment was not disturbed when the forks or wheels were detached. With this system, the entire rear end could be put together as a unit for later assembly to the powerplant, or handled separately if damaged in a crash.

Engine Ideas

This brings us to the focal point of the design - the engine. The prewar bore of 84 mm was retained (being chosen in the first place because it was the smallest size which I could get my hand through for cleaning) and with a 90 mm stroke gave a capacity of 998 cc with the possibility of further enlargement.

The cylinder angle was settled at 50 degrees to suit available magnetoes and the rear pot was offset 1 3/4 in. to the right partly to allow identical conrods to be mounted side by side on one crankpin, partly to provide better cooling, and partly to enable the rear exhaust pipe to clear the front inlet rocker box.

The experimental Rapide showing the brake pedal mounted on the foot-rest hanger and adjustable for height. Irving describes the gearbox filler cap/dipstick combination as "the type of sensible design appreciated by the average motorcyclist". Note the blanked-off hole for the left-hand kickstart shaft.

Coil valve springs were adopted in place of hair-pins because they could be more easily enclosed, but the prewar double-valve guide system with forked rockers thrusting on collars midway along the valve-stems was retained.

This system, which I cannot recall being used on any other engine, provides the equivalent of a very long guide without making the whole engine tall, while the rockers are straight and the pushrods less than 6 in. long. The entire rocker gear was enclosed, with access through four identical inspection caps per head. Both heads were similar except that on the front one the carburettor was angled to the left to clear the rear cylinder, but at the back was over to the right to clear the generator and battery.

The heads were held down by four long bolts to the crankcase. As the bolts also carried frame stresses they were precision-made articles of high tensile steel, at one time costing nearly $4 each (extraordinarily expensive for the times), Centrifugally cast iron liners were shrunk into aluminium jackets and deeply spigotted into the crankcase, which was split vertically and also formed the gearbox shell, the rear of the chain case and the timing gear chamber.

This compartment contained the gear wheels that drove two high camshafts, a timed breather valve and the magneto through an idler gear which was originally a bronze casting. It was later changed to a forging made from the very strong aluminium alloy RR 56. This gave good service, but some local replacements apparently made from melted-down saucepans broke up very quickly, giving the light-alloy wheel an undeservedly bad reputation.

The gear-type oil pumps used in the A series engine are prone to let oil through into the sump when the bike is standing, so this type was replaced by a Pilgrim pump which has a rotating plunger that also reciprocates and acts for both pressure and scavenging.

Pressure oil goes through one of the largest felt filters ever used in motorcycle, housed below the front-mounted magneto (cowled for weather protection) and then through drillings in the timing cover to the cams and cylinder bases. Scavenge oil goes back to the tank by an external pipe which passes over the rocker boxes and supplies oil to them through jets, the idea being to use the accumulated oil in the sump to lubricate the valve gear immediately the engine is started.

Driveline Development

Devising a clutch to handle perhaps 100 bhp and yet light enough to lift with one finger wasn't easy. Several designs of servo-clutch were built before we hit on the idea of using a drum type clutch with the shoes expanded by toggles connected to a conventional single-plate primary clutch which needed only light springs and actually could be lifted with one finger. This device later proved to be able to handle the 120 bhp obtained from supercharged engines, but was not entirely without its bad points.

This is the later Series C Black Shadow which used the famous Girdraulic forks and an hydraulic damper at the rear. Power on this model was considerably up on the original model due to a variety of engine modifications.

It tended to be snatchy at take-off until the starting technique was acquired, and it was very sensitive to oil on the shoe linings; puzzling, because all development tests had been done on a prewar model with the entire clutch exposed to oil. No production clutch would grip under these circumstances, so the layout on the new power unit had to be redesigned to isolate the clutch from oil, unavoidably increasing the width by half an inch to 22 in.

The spinning weight was also rather too great, making the gear change a rather leisurely process, but this clutch stood up well to violent abuse, even on speedway outfits and hillclimb cars without much trouble.

This is the Series B Rapide, with the small external indicator lever which could also be used for gear selection. Note that the lefthand propstand is down and the righthand side up. This model also has the dual seat with toolbox beneath.

The fourspeed box had three close ratios of 1, 1.19, and 1.6 with a lowish bottom of 2.6 to 1. This gave overall ratios of 3.5, 4.2, 5.5 and 9.1, so that the engine speed was only 4600 at 100 mph, and 3700 when cruising at 80, yet by running up to 5800, 80 mph was possible in second.

The change was effected by a camplate with two snakey grooves, bevel driven from the positive-stop ratchet operated by the foot-lever. The ratchet mechanism had adjustments which had to be made in the correct sequence to get a precise change, but many riders failed to grasp this and then blamed the box for bad changing.

The change pedal was originally pivotted on the footrest hanger, but this was not always satisfactory so eventually the pedal was moved back to the box.

Finding neutral with a foot change is always a bit of a nuisance, so a small external lever was provided to indicate what gear you were in. This could also be used to change gear manually.

For the convenience of Continental chair-men or someone with a gammy leg, the kickstarter shaft was positioned so that it could extend right through the box to take a special left-hand pedal, and although this provision complicated the design somewhat, it was often greatly appreciated. So was the ability to check and top up the gearbox with a dip stick underneath a knurled filler cap.

Aluminium was used extensively for mudguards, engine plates and so forth, and stainless steel was preferred instead of chrome-plated mild steel for brake rods, banjo bolts, tommy-bar axles and similar parts, which still polish up well after 25 years' exposure. A dual seat with tool tray beneath it was specially moulded by Ferdax, this being probably the first production dual seat, although the idea was originated by Velocette for racing many years before.

Altogether The Fastest!

The result of all this work and thought was a machine scaling 453 lb with full equipment, tools and oil, but no fuel. This weight was disappointingly a little more than was expected, but was still 100 lb or so less than contemporary big machines.

With small carburettors, 6.8:1 compression ratio, and mild cams providing 45 bhp at 5600 rpm the prototype attained 80, 104 and 112 mph in the three upper ratios, and subsequent production bikes equalled these figures in road tests by the motorcycling Press.

We didn't boast about it though - just adopted the slogan "The world's fastest standard motor-cycle!"

Impressive on paper and in prototype production, the post-War Vincent now faced the acid test -factory and consumer evaluation. In part two of his story PHIL IRVING details the highlights and development of "the world's fastest standard motorcycle."

THE DEEP ditch episode was probably the most memorable of our early factory test incidents.

It was part of a tremendous amount of road testing done before the Black Shadow was released for sale, most of it by C. J. Williams.

On this occasion a 6 ft deep drain collected both rider and bike due to the designed ground clearance being insufficient for high speed cornering, but all episodes of this type provided an opportunity to learn more about the bike. This one proved that the frame construction could withstand rough treatment without bending anywhere except the Brampton forks, which were wrecked.

As a result of this prang the footrests were re-positioned and the clutch cover was re-designed with a flat at its base.

Later, when the Brampton forks were replaced with our Girdraulic design, another prang proved the new unit's strength. On this occasion a tester hit an Austin A30 side-on, hard enough to bend the car in the middle, pull the steering column nearly through the head lug, flatten the rim down to the hub and collapse the lower link - but the blades remained dead true.

On the first Shadow we detected a tendency to oil plugs occasionally after much pottering about in town and this was cured by restricting the rocker-feed jets. Overall, very few modifications were necessary before deliveries began, and almost all the first models went overseas.

Unfortunately - but as usual - the public encountered troubles which did not beset the factory testers.

Many were due simply to misuse, poor servicing or ham-fisted efforts to improve the design, but some bikes did have bad gear selection and an annoying habit of jumping out of second.

This was due partly to an error in design (mine) and partly to the makers of the camplates. The first few hand-made camplate samples worked well, but those off the production line were inconsistent and often accentuated my mistake. We scrapped about 400 plates and the offending tools, modified the cam-slots, and sent out replacement plates free of charge for every machine sent abroad.

This is the Vincent Rapide, the model on which the Black Shadow was based. Note that this is a late version produced after the "HRD" insignia was dropped and the Girdraulic forks introduced.

As far as I can discover no plates were ever changed in Australia, but they were in South America, which was a very active market. We also sent out half a dozen complete flywheel assemblies at factory cost to each main dealer to establish a service exchange scheme for worn big-ends, but the reaction of some dealers was to tell customers they couldn't buy a new big-end, but had to buy the whole assembly!

None of the machines sent out in 1947 was prepared for racing, but it wasn't long before some reached the starting grids, and quite a few got the checkered flag.

The Argentineians were particularly keen on racing, and one 12-hour race on a one-mile dirt circuit in high summer was won at 77 mph by a man who borrowed a friend's Rapide for the day but omitted to tell the owner what he was going to do with it! Rapides also filled all the minor placings, and the police were so impressed that they ordered a number for their own use. These had only minor variations from standard, including valanced steel guards 4-inch tyres both ends, and built-in radio. The late Frank Pratt, of Geelong, was the first to win a sidecar race in Australia; in fact he won nine in a row, but was quickly followed by Les Warton, of Sydney, Frank Sinclair, Lloyd Hurst, and many others.

As the numbers in service increased, a demand arose for something a bit quicker and even more exclusive. This was met by introducing the Series B Black Shadow which was basically the same as the Rapide, but all main castings were matched up, serial numbered, and stove enamelled black over a special undercoat. Conrods and rockers for the motors were selected for maximum tensile strength and polished, and cams were selected for "long" timing (all production camshafts, whoever makes them, vary by a couple of degrees). Compression ratio was upped to 7.3, ports were polished and the inlets opened out to l l/8 in. to take carburettors of the same size. This brought the power up to 56 bhp at around 5800 rpm.

Other changes to the new model included ribbed, cast iron brake drums, bottom gear being raised, from 9.6 to 7.2, and the crowning touch was a 5-inch 150 mph speedo.

The whole appearance was striking without being flashy; top speed was in excess of 120 mph, and the speeds in the indirects went up in proportion.

At cruising speeds the consumption was even better than the Rapide's, due mainly to the higher compression, and it was possible to cover 60 miles hi the hour on the roads and use less than a gallon of juice in the process.

The road-test graph published by "Motorcycling" in 1947 gives a good idea of performance. The fuel consumption is quoted at 51 mpg but this included a lot of riding at very high speeds.

In 1948 we decided that the Brampton forks had had their day, so we were faced with choosing a successor. Telescopic forks were becoming popular, but all the examples we tried were woefully weak in torsion and tied themselves into knots when cornered hard with a sidecar. Also, there was no provision for trail adjustment or altering spring strengths.

The Gilera, and in fact almost all Continental racers except BMW, retained their girder forks and we followed suit with out own design, coining the term "Girdraulic" because it had a hydraulic damper in place of a central spring.

The girders or blades were forgings in RR56 aluminium alloy, as were the top and bottom links, which were U-shaped forgings with self-lubricating bronze brushes and ample surfaces to carry cornering loads. The long, supple springs ran from near the axle to eccentrics on which the lower link was pivotted. Rotating both eccentrics about half a turn moved the link pivot centre forward, thereby shortening the trail and simultaneously moving the springs into a more vertical position, making them effectively stiffer. This change from solo to sidecar trim could be made in a few minutes and did not call for any alternative parts. I think the idea is unique to the Stevenage product.

The Girdraulic set-up has since become almost a standard fitting on speedway sidecars, and this is about as tough an assignment as you could wish for.

Models fitted with Girdraulic forks and a hydraulic damper at the rear were designated Series C and at about this time the letters "HRD" were dropped from the name, because American riders confused it with the Harley-Davidson initials.

Although there was not much 1000 cc class racing in England, the Clubman's TT provided a chance to demonstrate the Rapide's handling and reliability. The race consisted of four laps of the 37 3/4-mile course, and machines had to be standard in all respects (including the fuel tank) except for the removal of lamp glasses and rear stands. Riders who had competed in any International TT were banned, so that most were entirely new to the circuit.

In the 1948 Senior event (350-1000 cc) 10 Rapides started, and much to the general surprise, nine of them finished, occupying first and second places, and most of the positions down to llth. George Brown put up the fastest lap at 82.6 mph.

In 1949, the Black Shadow,with Girdraulic forks became eligible for the Clubman's, with just about the same result as the year before except that C. Horn, who had scarcely ever won a road race anywhere, recorded the fastest lap of 85.6 mph, which was pretty fair going considering that A. R. Foster's fastest lap in the real Senior on a Guzzi twin was 89 mph.

In this year refuelling was not permitted and probably the most remarkable thing was that the Shadow, which won at 76.3 mph, covered the 151 miles on less than 3.6 gallons of poor quality (80 octane) petrol; a consumption bettering 42 mpg.

The Black Shadow's drum-type clutch was used on all models and later proved to be able to handle in the region of 120 bhp. The circular shoes were expanded by toggles connected to a conventional single-plate primary clutch, their grip increasing with the torque. The pilot friction plate needed only light springs, and Irving reports that it could be lifted with one finger.

There are plenty of modern two-strokes which do not do much better at touring speeds.

In 1948, we introduced the 85 bhp "Black Lightning", a stripped racing model with the usual speed modifications such as high compression pistons, big Amal carburettors, different cams, and a considerable amount of lightened and magnesium parts which brought the weight down to under 380 lb, but the basic specification . was still the same as that of the Rapide and Shadow.

About 20 were built, but they made their mark by capturing many national and world speed records, the first one taking the American record at just over 150 mph at its first attempt and later going much faster. In 1955 two New Zealanders, Burns and Wright, captured both solo and sidecar world records at 185 and 155 mph with models modified to Lightning specifications but fully streamlined.

Roughly, that is the "Shadow" story.

The selling price 20 years ago was 400 Pounds. Today they are fetching three times that amount and spare parts are still available.

There must be a moral there somewhere.

From the Black Shadow came the ultra-impressive "Black Lightning", a rare, 85 hp racer introduced in 1948 and used by a number of motorcycle land speed record setters over the next decade. The bike is pictured here at Earls Court with actor Alan Ladd aboard. Irving is standing at front. The tommy-bar rear axle nuts are clearly visible here. In a timed test with a similar set-up on a Rapide, rear chain adjustment (achieved without using spanners) took 35 seconds.
The Vincent in Australian competition! This historic shot is of the Late Frank Pratt, of Geelong, who piloted the first Vincent-based outfit in the country to nine successive wins. Frank led a parade of successful Vincents to prominence in local competition.

The torrid post-war competition of South America provided racing outings for many of the Stevenage machines. Phil reports that the Argentinians were particularly keen on them, as apparently were the Cubans. This shot was taken in Havana and the machines are Rapides. Note that this "around the houses" apparently has no barriers or marshalls.

Source: www.vincent-hrd.co.uk