▲ A brave design exercise, but one that lacked performance in an age when the need for speed was everything. For others, however
it's not speed that matters but style, and in this area the Sunbeam S7 is a hard act to beat.
Sunbeam S7 & S8
Erling Poppe and Gerry Bayliss
Sunbeam S7 worm final drive
Sunbeam S7 handling
Sunbeam S7 production numbers
Sunbeam S7 and S8 buying/ownership tips
Top Sunbeam S7 & S8 specialists and links
1950 Sunbeam S7 specifications
Type: Air-cooled OHC parallel twin
Capacity: 487cc (500cc)
Bore & Stroke: 70mm x 63.5mm
BHP: 25bhp (plus) @ 5800rpm
Compression ratio: 6.5:1
single plate dry clutch
Brakes: 8-inch single leading
shoe drum front and rear
Front suspension: Telescopic,
one way damped
Rear suspension: Plunger
Wheels/Tyres: 4.50 x 16-inch front,
4.75x 16-inch rear
Weight: 430lb (dry)
Maximum speed: 75mph
▲ This rough and rusty Sunbeam S7 recently fetched around £2000 (2009). If you see it being restored, report the owner immediately. All it needs is a change of oil, a tune, and a wipe with an oily rag.
▲ Sunbeam's controversial worm gear drive box. It's not the weak link it's said to be. Just use the correct AG140 oil.
▲ Not the ideal location for an ammeter, or an ignition switch. But if you can get used to those balloon tyres, you can get used to pretty much anything.
▲ Neat design touches abound with stylish castings and covers. Spares availability is surprisingly good which means you can keep one of these moving without too much trouble. Or expense. But having a carburettor between two hot header pipes? Not ideal, but how many bikes have actually got up in smoke?
Sunbeam S7 on YouTube
When you've read the feature, you can watch the movie. Here are a few frames for anyone (like us) who just can't get enough Sunbeam S7 stuff.
WHEN THE 1946 SUNBEAM S7 was unveiled to the post-war biking public, it was one of the most technically advanced motorcycles of its age. With its all-aluminium, "unit-construction", single overhead-camshaft, horizontally split engine, it also boasted shaft drive, a smooth power delivery and numerous other technical refinements that ensured it was going to be a massive hit.
Or so felt BSA which had acquired Sunbeam in 1943.
In fact, the bike turned out to be something of a sales flop and was (arguably) one of the most woefully under-supported acts in the history of British motorcycling. It could have been great. But instead, it was merely adequate.
▲ Black isn't the obvious colour for the Sunbeam S7. Mist Green is how most remember them. However, Sunbeam offered these bikes in a range of colours. History has a funny way of tinkering with the truth.
Designed by Erling Poppe and Gerry Bayliss, the S7 was intended to be the (BSA) company flagship, a machine to rival (and largely inspired by) the high-specification wartime Wehrmacht BMW R75—which was offered to the firm as part of the post WW2 reparations deal.
But BSA, already well aware of the R75 (largely through inspection of captured machines), stuck with its own design with all the development problems that that entailed.
Originally, the engine was solidly mounted in the frame, and a batch of such machines was sent to South Africa for use as police escort bikes—and were promptly shipped home again as "unusable" (none are thought to have survived to the present day). And BSA, who was well aware of an inherent vibration issue, initiated a hasty revision and quickly introduced bonded rubber engine mountings (and "snubber" dampers) that minimised the impact of the vibes—but did nothing to address the underlying problem, which was nothing other than the inevitable product of two pistons rising and falling together on a common crankshaft.
A flexible exhaust coupling was also added along with a stiffened frame cross member as a substitute for the now absent engine support.
Other problems with the S7 prototypes included stripping of the underslung worm gear final-drive—that in turn is said to have led to swingeing cuts in engine power, the concomitant of which was a fairly mediocre (and struggling) 75-mph performer in an age when riders were pushing hard for the magic ton.
However, according to David Holyoake of Sunbeam specialist Stewart Engineering, there was never anything wrong with the worm final drive which, over five decades, has proved to be very reliable in service and maintenance light; a claim borne out by the direct testimony of many other Sunbeam S7 and S8 owners who have had no trouble whatsoever in tens of thousand of miles. However, the correct AG140 oil is crucial (contact Stewarts or Holden Vintage and Classic Products.)
Meanwhile, oil capacity for the wet-sump engine was, at just three pints, always marginal leading to overheating with the very real risk of engine casting distortion and/or seizure (an aftermarket extended oil sump is available from Stewarts and is advised).
▲ Technically challenging, but aesthetically pleasing. The real shame was that BSA-Sunbeam never really developed this bike. The idea was right, but it needed a lot of debugging, and the will wasn't there.
But the handling, with its one-way damped telescopic front fork (no damping on early models), plunger rear suspension and 16-inch balloon tyres, left much to be desired. And words such as ‘wallow’ and ‘weave’ were, with some justification, quickly associated with the mount.
Meanwhile, the 8-inch single leading shoe brakes, although not bad for the performance, would never be described as particularly impressive. Moreover, a bulky machine like this was always likely to find both an ally, and an enemy, in ordinary wind resistance.
The mid-Atlantic, laid-back, overblown styling wasn’t to everyone’s tastes either. But BSA, having committed itself in terms of both cash and prestige, persevered with the design (now without Erling Poppe). After the first run of all black finished machines was sold, a much improved 25bhp S7 was offered, which, in 1949, was improved further with the introduction of the De Luxe model.
A new colour—Mist Green—was introduced. Based on an Austin Devon shade (see Sump's BSA D1 Bantam page), Mist Green wasn't universally popular, but by then the S8 was in the showrooms, a "sportier" bike aimed at the European rider. This machine was leaner, lighter, slightly faster—and a generally a more stylistically toned-down/conservative machine.
Also, it was black or silver grey.
It was offered with a 19-inch front wheel (with 3.25 rubber) and an 18-inch rear wheel (with 4.00 rubber). A BSA front fork was also fitted, and a high compression engine was optional.
But the S7/S8 design was always expensive to manufacture, and as unit sales declined, so the bikes were becoming increasingly expensive to justify at boardroom (and for that matter, showroom) level—especially when company already had the well-respected and successful BSA A7 (and later A10) in its stable.
Throughout its production run, the S7 and S8 (which were sold side by side) were continually improved. But the fundamental design was never developed as far as it might have been, and the accumulation of crisis-management fixes coupled with falling sales led in 1956 to its discontinuation—although bikes were still available from stock until 1957.
▲ All-aluminium, horizontally split, in-line 487cc twin. One of the best looking engine the British motorcycle industry ever built? We think so.
In total, 7658 Sunbeam S7s were built—which includes 5554 De Luxe models (some sources say that the original S7 production figure is actually less at around 1200 bikes). For the S8, the total production number stands at 8530.
What remains are widely misunderstood and heavily mythologised machines. And because less is demanded today of these beautiful Sunbeams in this plodding age of classic nostalgia and rose-tinted goggles, the bikes are correspondingly that much more appealing and acceptable than they ever were to the riders of yesteryear. In short, the Sunbeam S7 and S8 don’t perform as well as was intended. But nobody cares too much about that any more.
Better yet, not only is the S7/S8 still comparatively affordable, but it’s also one of the better-served marques of the post war years and is therefore relatively easy to keep mobile.
As a BMW beater, the concept failed. But it was a creditable and highly sophisticated attempt at breaking away from a traditional air-cooled, parallel twin, pre-unit, chain-drive platform that was, even then, heading toward a technical dead end.
It would be almost 60 years before the British motorcycle industry, under the auspices of (Hinckley) Triumph, would come even close to such an engine configuration with the introduction of the awesome 2004 Rocket Three.
1. Early S7s, 1946-1948 are different in many respect to later S7s, with many parts NOT interchangeable.
2. Spares for S7s and S8s are generally in good supply. But original tinware is rare, and many bikes are fitted with glass fibre mudguards and toolboxes.
3. Check that the bike hasn't been fitted with "square" car tyres (S7 models).
4. Only the S8 was fitted with a cast silencer.
5. Sunbeam S7s and S8s were produced in black, silver and mist green, plus various other ICI colours as specified by customers.
6. A sidecar was manufactured specifically for the S7, but the combination (no pun intended) was never popular.
7. The S8 was always intended as a sports version and designed to persuade potential customers that the bike was a lot more lithe and agile than its earlier, bulky S7 looks suggested.
8. S7s were sold for police in various Commonwealth countries.
The Sunbeam Owners Fellowship
International Shaftdrive Sunbeam Motorcycle Community
The home of the Sunbeam S7 and S8
Telephone: 01926 612589
Sunbeam electrical specialist and 120 watt
12-volt alternator conversions
Vintage Motor Cycle Club
Holden Vintage and Classic Products
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