Motobi racer
This 250cc pushrod four-stroke single, developed from a humble roadster, resulted in a 135mph Historic GP challenger. Just don’t mention ‘Aermacchi…’
Even by the standards of Italy’s Etceterini array of little-known marques back in the 1950s, Motobi is an obscure subplot to the ongoing soap opera that the ever-changing panorama of Italian motorcycling represents.
Characteristically, the company only ever existed because of a family squabble which saw Giuseppe Benelli, eldest of the six brothers whose mother Teresa had set them up in a mechanical repair workshop in
1911 as a prelude to attaching the Benelli badge to their own range of motorcycles from 1921 onwards, walk out of the family factory in 1948 and cross the street to found his own rival marque. Its products were at first merely identified by a large letter ‘B’ on the fuel tank – nothing else, and for over a decade the two companies coexisted in barely suppressed enmity in the city of Pesaro on Italy’s Adriatic Coast, later home of such sporting names as Morbidelli, MBA, Sanvenero and more recently the Benelli QJ marque, now under the control of China’s Qianjiang.
But Giuseppe Benelli passed away in 1957 aged 78, leaving his sons Marco and Luigi to grapple with the task of keeping the firm afloat in the wake of the birth of the Fiat 500 car, which had brought the post Second World War boom in Italian motorcycling to such an abrupt end.
Motobi (as it was known from the mid-1950s onwards) had carved itself a good slice of that era’s booming
Italian bike market, with a range of innovative models. Initially, these were all two-strokes, including from 1953 onwards a 200cc rotary-valve parallel-twin bearing the curious name of the B200 Spring Lasting (it’d take too long to explain!), whose pressed-steel frame and eggshaped engine with its finned horizontal cylinders and vertical downdraught carbs, established the trademark format of future Motobi products.
Late 1955 saw its first four-stroke models appear, a 125/175cc OHV duo bearing the Catria name and designed by freelance engineer Piero Prampolini, whose later credits included the 350/500cc GP fourcylinder Benellis which Jarno Saarinen took to victory on their 1972 debut, and the Benelli 650cc Tornado, a late-60s Italian idea of what a British OHV parallel-twin should have been, but never was. Interestingly, the Catria’s horizontal-cylinder engine layout preceded by one year the later arrival of the similar-format 175cc Aermacchi Chimera, which duly morphed into the Harley-Davidson Sprint.
The Catria family formed the backbone of Motobi’s future four-stroke range, progressively upsized from the 62x57mm 175cc base version, first in 1963 to an overbored 66.5x57mm 200cc model called the Sprite, then in 1966 to a full 74mm-bore 250cc version, whose Sport Special performance kit delivered a heady 90plus mph in street guise. This OHV single stayed in production until 1973, by which time it had been badged as a Benelli to form part of the Pesaro firm’s 1960s annual production peaking at 90,000 units, which had led to Alessandro de Tomaso acquiring it in 1972.
For peace had broken out after Giuseppe Benelli’s passing, and in November 1961 Motobi was absorbed into the family empire, to form the Gruppo Benelli-Motobi. For the rest of the decade, Motobi motorcycles were in fact marketed as the more sporting lightweight products of the Benelli company, aimed at providing customers with a bike they could ride to work one day, then compete with in their local hill-climb or road race, the next.
“The Catria family formed the backbone of Motobi’s future four-stroke range.”
The Motobi Catria was a true production racer, tailor-made for success in Italy’s thriving MSDS (Moto Sportive Derivate dalla Serie, aka Formula 3 Junior) Modified Production category, one step down from National-level racing with purebred racers, and was competitive in both the 125cc and 175cc classes this originally catered for, as well as in the 250cc category that came later. Motobi-mounted riders won 16 MSDS Italian Junior championships all told in the 1960s, providing the stepping stone to a Grand Prix career for famous names like Roberto Gallina and future world champions Eugenio Lazzarini, Paolo Pileri and Pierpaolo Bianchi. But Motobi was a brand largely unknown outside Italy, except in the USA, where its models were imported by Cosmopolitan Motors as part of their Benelli distribution deal – even if many were rebadged as Benellis, or else sold through the giant Montgomery Ward department store chain as Ward Riversides.
But the Wise family which owned Cosmo realised there was room to establish the Motobi name through competition, bringing in a handful of factory-developed Formula 3 racers aboard, which established names like Kurt Liebmann and Jess Thomas competed successfully in the USA and Canada.
It was Texan Thomas who delivered the most illustrious sporting moment in Motobi’s history, when, on his pushrod Italian single, he defeated a pair of works Honda RC162 fours on the Daytona infield circuit to win the 1962 US GP – not then yet a world championship event, but nevertheless still a prestigious victory. Indeed, Thomas campaigned his tuned 175cc Motobi bored out to 207cc
via sandcast crankcases, a roller-bearing bottom end and needle-roller gearbox, to 26 race wins that season.
One of the men Benelli sent to America later that decade to help Cosmo put Motobi on the map was a Pesaro factory race mechanic named Eraldo Ferracci, who ended up staying on in the USA and carving out a flourishing reputation for himself as a tuner, eventually winning the 1991 World Superbike Championship with Doug Polen riding his privateer FBF/Fast By Ferracci Ducati 888.
The man Ferracci had worked under in Italy was Primo Zanzani, a former racer from Forlí who’d been enlisted by Luigi and Marco Benelli to take over the Motobi race shop in 1957 on the death of their father. A self-taught tuner and former winner of the gruelling Motogiro’s 100cc class on a Laverda, winning four of the six stages, Zanzani was the man tasked with turning the four-stroke Motobi models into such competitive hardware in both Italy and the USA in the early 1960s.
This feat duly earned him the role of developing the four-cylinder Benelli 250GP racer from 1962 onwards, when the two companies were united. Having helped turn it into a Grand Prix race-winner in the hands of Tarquinio Provini, but in the process fallen out with chief engineer Giovanni Benelli over what he believed (correctly, as it turned out) was a fundamental error in the design of the four-cylinder Benelli 250cc GP motor, Zanzani moved back to Motobi full time in 1966, to work again on modifying production models.
There, Zanzani focused his attention on the new
250cc Sprite model, introduced that year, with the specific purpose of making it a winner in the recentlyintroduced quarter-litre MSDS class for customers of the Pesaro factory. The fact that Motobi won the Italian 250cc Junior title in 1966, 67 and 69 shows how well Zanzani succeeded in the face of intense competition from rival factories like Ducati, Aermacchi, Parilla and Morini – though perhaps the most glorious display of the bike’s performance came at Daytona in 1967, in the 250cc support race for the Daytona 200. In that, an unknown rider named Sam Bertarell aboard an OHV Motobi four-stroke single spent the entire race run on the full 3.81-mile banked circuit battling with Florida’s Kenny Stephens on one of the very fast then-new TD2 Yamaha two-stroke twins, before gapping him in the infield on the final lap to win the race. It turned out that ‘Sam’ was really future 500cc GP podium finisher Silvano Bertarelli, riding under an alias since the Italian Federation wouldn’t grant any of its riders permission to race at Daytona! This highly impressive result certainly helped Cosmo’s Motobi sales drive in the USA.
To make Motobi such a competitive contender for top honours, Zanzani created just nine examples between 1965 and 1973, of a limited edition homologation special known as the 250cc Motobi Sei Tiranti (six-stud). This had six cylinder head studs instead of the street Sprite’s four, to resolve problems with cylinder head sealing when compression and engine speeds were raised in pursuit of power – and considering that by the end of the decade Zanzani had more than doubled the 16bhp output of the street 250 Sprite to 36bhp in MSDS form, this need was hardly surprising. So an extra pair of studs was grafted in to bolt the cylinder head on tight to what on this tricked-out special were sandcast crankcases
“In the US, the Wise family which owned importer Cosmo set about establishing the Motobi name through competition.”
“Owing to the Motobi’s sparse nature, there wasn’t much weight to stop. Well, before you add in the rider, that is...”
for extra stiffness, rather than the diecast ones of the streetbike – for this was a silhouette class, where every little trick was employed to gain an added edge.
Sadly, the global Japanese onslaught meant diminishing sales and no budget for racing, so in 1970 the Motobi factory race department was closed, leaving Zanzani to start his own machine shop operation in Pesaro, where he built another 15 Sei Tiranti racers in the early 1970s.
But in parallel with this, Zanzani developed a specialist expertise in the disc brake technology then new to motorcycles, but which he’d been the first in the world to adopt on any GP racer back in 1965 on the four-cylinder Benelli 250, using US-made Airheart discs rather than the Campagnolo disc brakes then produced in Italy for lightweight 50/125cc GP machines. Developing his own process for plasma-spraying aluminium disc rotors with iron to produce a far lighter disc brake assembly than the steel or cast iron discs then commercially available (with the additional benefits of reducing both unsprung weight, and their gyroscopic effect on handling), Zanzani’s brakes became ubiquitous components on GP racers in the smaller classes, winning no less than 24 world championships in the 50/80/125/250cc GP classes from 1978 on, up to and including Alessandro Gramigni’s 125cc GP Aprilia world crown in 1992.
Thereafter, together with his two sons Athos and Mirko, Primo Zanzani continued to run the trio of high-tech machine shops he owned in Pesaro, producing intricate components for the local woodworking machinery industry – and complete Motobi 250 Sei Tiranti replicas! He died in November 2017, aged 93, after a long and productive, but also highly passionate life on two wheels.
But before that, I’d come to know Primo during my regular visits to Italian race teams’ workshops, as well as track testing current GP racers fitted with his brakes. He was always eager to chastise me for having started my own racing career riding what he always termed the ‘anti-Motobi’ Aermacchis, which had copied the horizontal OHV cylinder format of the Pesaro brand’s singles, albeit without replicating the unique ‘eggcentric’ format of the Prampolini-designed Motobi motor. So when in the late 1990s Zanzani resumed building complete
Sei Tiranti replicas for the flourishing classic racing marketplace, he was eager I should try one out. Hence in the Italian summer of 1998 I took him up on his offer at the tight, 1.56mile Magione circuit near Perugia – an ideal track to sample the merits of these light, nimble four-stroke singles.
“I was asked to make so many parts for Motobi enthusiasts either restoring or racing our original bikes from 30 years ago, that it seemed natural to consider building complete motorcycles again,” recounted Primo, bending to adjust the clutch on the latest of the five Sei Tiranti replicas he’d built so far in his Pesaro workshop, as we sheltered from the torrid summer sun in the shadow of the pits.
“After all, I still had all the jigs and patterns from when I built them in the old days, so restarting production didn’t entail much preparation.” The fact that he also had access to original Motobi parts stocks in both Italy and the USA – where, as proof that what goes around, comes around, Cosmopolitan Motors were marketing the modern Zanzani-built replicas to American historic racing enthusiasts – meant that Primo could deliver a brand-new 250 Sei Tiranti built to order at a price of £11,500/$18,000, back then. Given that these were faithful in every detail to the original Motobi Formula 3 MSDS design, they weren’t so much replicas as the resumption of manufacture of the original model a quarter-century down the line – just like today’s continuazione versions of his dad’s 500cc Patons that Roberto Pattoni has been building for the past two decades.
To produce the modern Sei Tiranti, Zanzani took an original Motobi pressed-steel spine frame (produced in large quantities and so readily available, since it was common to all the marque’s OHV four-stroke models), and reinforced it with his own tubular steel bracing, which just as back then effectively constituted a subframe attached to the main structure.
“We lengthened the wheelbase slightly by 30mm from standard to 1320mm in the 1960s with a special swingarm,” said Zanzani. “This was to improve stability on fast turns, and increase weight on the front wheel for extra grip in slower ones.” The new bikes had the same mod, with twin Works Performance gas shocks sourced from the USA and available with a choice of springs to suit 70, 75 or 85kg rider weights. (Fortunately, I rode the bike at Magione before lunch!) The 35mm Ceriani fork was ubiquitous period hardware for a bike like this, with braking provided by a 210mm Fontana 4LS magnesium front drum and 160mm Grimeca 2LS rear, though with the complete bike weighing just 96kg (212lbs) complete with trademark ‘double-bubble’ fairing, and the fuel tank with its distinctive upward sweep to the steering head empty of fuel, there wasn’t so much to stop. Well, before you add in the rider, that is ....
Actually, I fitted the Motobi better than I expected, and that’s not just because I spent so many seasons at the outset of my racing career riding its Aermacchi rival of similar slim, low-slung stature and cloned engine layout. For that had been some years earlier, and though I must admit that folding myself with some success around a Ducati Supermono of similar horizontal cylinder layout for the previous six years might have helped, the fact was that, as on so many Italian bikes, the ergonometrics of the Motobi were such that a much taller rider than the star midgets it was designed for, could actually fit aboard quite well. Italian designers have always been masters of packaging, and that was the case again here. And on the freshly constructed silver factory Motobi racer, the choice of a wider WM2 Akront rim for the rear 110/80 Avon AM23 tyre prevented it feeling too nervous or light-steering.
But a customer’s 1970s original bike I also sampled at Magione (which moreover had the footrests 50mm further forward, in streetbike mode, whereas the Sei Tirantis were all built as race bikes, with rearsets, said Zanzani) had the same tyre on a WM1 rim, with the result that it felt like riding my old Aermacchis on Dunlop triangulars – as in, distinctly twitchy! But the new racer felt more stable and steered well, just the nimble side of nervous, and great in Magione’s tighter turns, while adequately stable in the track’s one fast sweeper. But I did catch myself holding my breath as I cranked through that turn at speed – not because I frightened myself, but because you’re conscious that this is an ultra-agile motorcycle with not a lot of weight on the front wheel thanks to the 47/53 forward weight bias, which you want to avoid upsetting with any undue movement. Such as a sharp intake of breath, for example! Seriously, as I learnt from my Aermacchi days it pays to stay crouched down over the fuel tank on such a bike, to try help load up the front wheel via your body weight, for extra grip.
The American rear shocks worked really well, helping deliver the ideal grip of the Avon tyres that are the
benchmark products for today’s classic racers, without any chatter or slides even in the torrid 35°C heat of our test day. But the 35mm Ceriani fork was too softly damped, producing a bounciness that could have been improved with heavier weight fork oil and/or more of it. The 4LS Fontana front drum had quite a bit of bite, but only if you squeezed really hard, whereas the same brake on the customer bike I rode worked much better – probably because it wasn’t so new, and the linings had bedded in, but possibly because it had a different ratio on the lever pivot, as well as a longer lever. In any case, you needed to also use the small 160mm rear 2LS Grimeca to the max to make the bike stop well from any speed – just as on my series of Aermacchis, which riding the Motobi reminded me so strongly of.
The 36bhp at 10,200rpm at the gearbox which the best of these produced compared favourably with the 38bhp at 10,500rpm at the rear wheel which Primo had extracted from the born-again Motobi Sei Tiranti’s 74 x 58mm motor. But in the years after my test, Primo continued to wave his magic wand at the egg-shaped engine, so that by the time he passed away, he had extracted an amazing 42bhp at the rear wheel from the 250cc version, while by dint of rearranging the architecture of the motor, he’d managed to find space to bore it out by 10mm to 84x58mm. In this 316cc guise it delivered 52bhp – again at the rear wheel, still at 11,000 rpm – and apparently with a linear torque curve from 5000 to 11,000rpm. That’s quite a wand!
To achieve this, Zanzani took new sandcast crankcases with his name cast into them, and fitted a lightened and rebalanced roller-bearing crankshaft with external flywheel, a Carrillo conrod carrying a forged Asso piston delivering 11.1:1 compression, his own special C8 camshaft actuating competition aluminium pushrods, and peened and polished race rockers of a new design.
The new cylinder carrying a cast iron sleeve was also sandcast, and carried one more fin on each of its four sides than before, for enhanced cooling. The high compression O-ring cylinder head was ported and flowed, then fitted with oversize sodium-filled 40mm inlet and 34mm exhaust Zanzani valves made to aircraft quality, running in bronze valve guides, each fitted with dual US-made RG springs, and set at a 10-degree wider included angle of 58° compared to the road 250, because of their larger diameter (stock: 35mm/32mm).
America also provided the Crane Cams electronic ignition running 44° of advance, which required a healthy 12v battery, while carburation came from a period 38mm Dell’Orto SS1 with remote float, and no idle jet. Add in a close-ratio five-speed Zanzani gearbox (there was a choice of alternative ratios available, such as a longer second) which however retained the road bike’s oil-bath clutch as was required under period MSDS rules – and go. I did.
Well, only after finding a battery that worked properly and a hard enough plug to let the motor rev hard – but once that was sorted out, the Motobi Sei Tiranti really delivered. It pulled pretty strongly from as low as
4000rpm, which was a bit of a surprise, but then came a patch of megaphonitis which meant it was best to keep it turning above 7000rpm even exiting tight turns, which could be achieved with a slight touch of the quite sensitive clutch lever to coax the revs out of it if you’d let it bog down. The carburettor slide and cable on this brand-new bike were still rather sticky, meaning that in the absence of any graphite grease to free up the throttle, getting on the gas to drive hard out of a turn meant a rather jerky pickup via the Tomaselli quick turn GP throttle, as well as a stiff twistgrip action.
But the customer bike I also rode, fitted with a
Zanzani Sei Tiranti motor, showed how well the package turned out, with an eager appetite for revs on this pushrod motor that I only ever remember seeing safely on my works-spec short circuit ultra short-stroke 350cc Aermacchi, not the longer-stroke standard ones I rode in the Isle of Man TT. The best compliment I could pay the Zanzani Motobi was that riding it reminded me of that: judging by my Magione outing, his customers could enjoy 11,000rpm power deliveries reliably, and with a surprising turn of speed for a 250cc pushrod single of 135mph at Monza, according to Primo. But you had to rev it hard to get competitive power, making the fact that Zanzani held ample stocks of all spare parts for engine and chassis (except Akront wheel rims: don’t crash!) a good reason for buying a new bike from the man who made them back then, if you wanted to go classic racing today with peace of mind.
And that’s what many Zanzani customers have done since then, with considerable success, like Martin Hudson who won the UK’s CRMC 250cc Championship on his Zanzani-built Motobi, or Australian Jonathan Huston, winner of countless 250cc classic races and titles on his similar bike since purchasing it in 2013. Other bikes have ended up in Japan, the USA (four of them), Ireland, Germany and of course, Italy. Indeed, a copy of ‘my’ factory test bike which I rode at Magione would fly to Daytona the following March, where local classic ace Dave Roper would set a 250cc GP pole time two seconds faster than the opposition – only to have the gearbox disintegrate in the next session, resulting in a DNS. Pity – but perhaps the 42bhp Primo had by then extracted from the Motobi engine was just too much for the transmission to handle!
Really, the Zanzani-built 250cc Motobi was the lightweight version of the replica Manx Nortons and 7R/ G50s built today for the bigger classes, which dominate the grids of classic races around the world. In delivering the same option to enthusiasts of the smaller capacity classes, Primo Zanzani was providing a service that others have followed since for other marques – albeit few of them with the same enthusiasm and unbroken line of heritage that his modern continuazione Motobi Sei Tiranti racers undoubtedly represented.
“Zanzani’s customers could enjoy 11,000rpm power deliveries reliably, and with a surprising turn of speed.”