If you go back to the mid-seventies and have a look at motorcycle frame design the first thing you notice is the pre-eminence of the Norton Featherbed style cradle frame. This originated in the early 1950’s and twin rear shock absorbers generally supported the swingarm. Apart from the Monoshock Yamaha RD 400, virtually every street bike of the 1970s followed this principle. While there were variations from the full cradle design, twin rear shocks were universal. Then Bimota came along and released the astonishing SB2 at the 1976 Bologna Motorcycle Show — powered by an engine from the newly released Suzuki GS750 — and it was totally unlike any street bike before.
Bimotas origins went back to 1972 when three brilliant Italian engineers Bianchi, Morri, and Tamburini created Bimota as a hobby. By taking the first two letters of each name, Bimota was born.
Bianchi departed Bimota in 1972, followed later by Tamburini. Tamburini went to Ducati where he designed the 916 and headed the Cagiva Research Centre in San Marino.
When Cagiva sold Ducati in 1997 Tamburini stayed on, later designing the MV Agusta F4. In 1975 Bimota designed the chassis for Johnny Cecotto’s 350cc World Championship winning Yamaha.
They followed this in 1976 and 1977 with the chassis for Walter Villa’s winning Aermacchi Harley-Davidsons. These, and other early designs like the KB1 for a Kawasaki 900/1000 four, were quite conventional in concept, but were superbly designed and constructed.
The designation SB2 indicated S for Suzuki, B for Bimota, and 2 for the second series of Bimota with a Suzuki engine. The first SB1 was a racing bike built on the TR500 two-stroke twin.
With the appearance of the Bimota SB2, Bimota showed they were unsurpassed innovators. Taking a bike that was already much lauded by the motorcycle press, Suzuki’s GS750, Bimota made it 27kg lighter and gave it a 108mm shorter wheelbase at 1392mm. In those days four-cylinder Japanese engines hung alternators and points systems on the end of their crankshafts, and the engines were wide.
The SB2 carried the engine 25mm higher (for improved ground clearance) but was otherwise tiny, with GP-like in dimensions. Compared to the GS750 the seat was 45mm lower and the front fork assembly 75mm shorter. But there was much more to the SB2 than mere numbers.
The frame design threw all conventions aside, and topped it off with bodywork that still looks unique. The design was simply decades ahead of its time.
The minimalist frame weighed only nine kilograms, and large aircraft style self-aligning conical joints connected the front and rear sections. With the engine as the principal load-carrying member, twelve tubes located the steering head and eight tubes comprised the rear section, most straight for increased strength. While the main frame was unusual, the swingarm and steering head designs were more so.
Although the 24-degree steering head angle is not unusual by modern standards, it was quite radical in a day when 28-30-degrees was the norm. However, in an effort to reduce trail change under braking, the fork tubes were set at 28-degrees. This method of minimising trail change with suspension deflection was quite popular with Italian manufacturers during the 1950s and 1960s.
Trail was also adjustable by rotating eccentrics held in the triple clamps, much as Tamburini later included in the Ducati 916 and MV Agusta F4. Trail could be set short for tight, twisty roads, or long for more stable steering on faster roads. The Speedline magnesium wheels were 18-inch; the rear a massive, for the day, WM6.
Bimota was the most innovative in the swingarm and rear suspension layout. A single Monoshock unit and rising rate linkage provided 140mm of travel, much more than was usual with a twin shock system. The box section steel swingarm was very long at 610mm, and almost half the length of the entire motorcycle.
It widened appreciably towards the pivot that was supported by tapered roller bearings mounted concentrically with the countershaft sprocket. Thus the final drive chain operated with constant tension, and the chassis was isolated from any chain influences. It’s a solution that has only caught on more recently but Bimota had it forty years ago.
The fibreglass bodywork was also unique. Detachable in seconds, four rubber bushes and two rubber straps supported the fuel tank and seat unit and revealed a bank of Mikuni carburettors and the heavily gusseted steering head.
The thin fibreglass seat had a suede leather insert with the Bimota logo imprinted, and the frame fitted the engine so closely that there wasn’t even room for the engine’s four chromed camshaft covers.
The instruments and wiring were standard Suzuki GS750, but the front fork was a higher spec 38mm Ceriani providing 115mm of travel. Needless to say the brakes were state-of-the-art for 1977; Brembo 08 calipers with 280mm cast-iron discs.
Indicative of Bimota’s attention to detail was the front brake hose splitter that was cast integrally with the milled aluminium triple clamp. Other bespoke details included a tiny cam on the mounting plate directly above the rear brake lever that allowed individual tailoring of the lever angle.
The SB2 created quite a stir when it was released, but it was a very expensive kit and (in Australia at least) you had to provide your own engine and electrical system. In Italy Bimota offered the option of a Yoshimura 850cc engine with Yoshimura 69mm high compression pistons, Yoshimura Road and Track camshafts were installed with race valve springs, and a Yoshimura ported cylinder head fed through a bank of Mikuni 29mm carburettors replacing the standard 26mm items. This upgrade brought power up to 100 horsepower compared to the stock 75.
The shape of the tank/seat unit still looks outrageous and the SB2 prophetically anticipated the move towards fully enclosed bodywork for street bikes. If it wasn’t for the wide engine cases protruding through the side of the fairing and the skinny 18-inch wheels, the SB2 could easily be mistaken for something more contemporary.
Set beside late 1970s Italian sports bike such as a Ducati 900SS, Laverda 1000, or Moto Guzzi Le Mans, the Bimota SB2 makes them look antiquated. Of the 200 frames manufactured a total of 140 SB2 kits were built. Of the 60 remaining frames 30 were used for the successive SB280 and the remaining 30 destroyed. Only two kits were sold in Australia.
The SB2 charted new territories in motorcycle frame and suspension design and the extremely rare SB2 was one of the most significant motorcycles of its time. Combining the finest in Italian engineering with Japanese power and reliability the SB2 struggled to overcome its “kit bike” categorisation and was overlooked for decades. But now the SB2 is enjoying its deserved status as a highly collectable classic and an example of Bimota’s amazing foresightedness.
Australia’s go-to-guy for classic motorcycle wisdom. Ian has written nearly 45 books on motorcycles that each offer unparalleled insight on the historical and technical development of particular makes and models.
