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Thanks Alan Cathcart for permission to use this article and photos.

Kenny Roberts’ 1980 YAMAHA OW48R RACER TEST:

Father and Son

The year 2000 saw the famous son of a famous father clinch his first ever 500cc World Championship title, when the Racer Formerly Known as Kenny Roberts Junior, or KR Jr., became King Kenny in his own right - just as his dad had done exactly two decades earlier in 1980, by winning the third of his 500cc world titles for Yamaha.

Quite by chance, my opportunity to test the Y2K title-winning Roberts Suzuki RGV500 at Phillip Island the day after the final race of the 2000 season in Australia, came exactly one month to the day after riding the reverse-cylinder OW48R Yamaha in Holland, on which his dad had clinched the last of his hat-trick of 500cc world crowns 20 years earlier. That came about on an improbably sun-blessed autumn weekend at Assen thanks to Arai Europe boss Ferry Brouwer, who commemorated the 75th anniversary of the historic Dutch circuit by organising a series of demonstrations of the legendary bikes which helped create that history. One of these was the yellow and black Roberts Yamaha, which thanks to the enthusiasm and commitment of British collector Chris Wilson, and the inspired restoration work of former 500cc GP race mechanic Nigel Everett, represents yet another vital ingredient of the early days of 500cc two-stroke GP history saved from the scrapheap and returned to the race track as part of the Wilson collection, for a fortunate friend to be allowed to ride it at Assen, in something approaching anger. It’s nice to have mates like Chris....especially when he also brought along one of the Yamaha’s factory rivals to Assen for a hands-on comparison, in the shape of Kork Ballington’s rotary-valve Kawasaki KR500.

In fact, the Chris Wilson collection of authentically restored 500cc GP racers has a pair of OW48R Yamahas in it, for alongside the 1980 Roberts machine is the 1981 Akai-sponsored version raced by Barry Sheene at the start of that year under Erv Kanemoto’s aegis, before the arrival halfway through the season of his OW54 rotary-valve V4. But the Roberts bike which Wilson found in Holland (the site of Yamaha’s European base) in dissembled guise has a very early chassis number stamping it as a 1980 machine - and since King Kenny was the only Yamaha rider to use an alloy-framed OW48R that year, this puts his name on the bike, quite apart from the various trademark Roberts giveaways on it which he alone used. These include the Morris magalloy wheels (Sheene used Dymags, others Campagnolos); the upsized 320mm front brakes (others came with 280mm rotors) fitted with KR’s special cooling ducts; a front caliper on the rear brake, complete with torque arm rather than a lug on top of the swingarm like on other 500 Yamahas (KR used the brakes very hard, especially the rear, compared to other riders); 40mm Kayaba factory forks with magnesium antidive and the recessed tops retained by circlips; and a tricked out De Carbon rear monoshock with separate gas canister, which only the factory bikes used. Although on its debut in restored form at the Coupes Moto Legende at Montlhery in May 2000, it was ridden by two other factory-supported Yamaha riders of the day, Marc Fontan and Christian Sarron, this bike dates from the year before they first raced one, and is unquestionably the pair of the identical bike which Kenny Roberts has in his own private collection.

“It’s basically a set of TZ500 cassette-gearbox crankcases with a special factory top end on them, with guillotine power-valves, a quite different stud pattern for the separate cylinders, and special porting,” says Nigel Everett of the OW48R motor, whose only weak point seems to have been the extractable gearbox - a fact confirmed by former works Yamaha TT-winner Charlie Williams, who also rode the Roberts bike at Assen and recalled the time he was leading the 1981 Senior TT in the Isle of Man by a comfortable margin on an OW48R, when the gearbox broke flat out in top gear round the fast sweeper at Ballacrie, just after Ballaugh. Ouch! - can’t think of a worse place on the TT Course to lock the back wheel. “Fortunately, it snapped a shaft, otherwise I wouldn’t be here to tell the story,” says Charlie, “but just as I was coasting to a halt I remembered that when I went to pick the bike up in Amsterdam, I’d seen a skip full of junked gearboxes sitting outside the Yamaha race shop! That should have told me something - but I was so thrilled at getting hold of what was a works replica of the current 500cc world champion machine, and undoubtedly the best TT bike I ever rode, I didn’t ask about what I’d seen. Maybe I should have!”

Thanks to the attention of his quality team of race mechanics, headed by Kel Carruthers and comprising Nobby Clark and Trevor Tilbury, that wasn’t a problem that Roberts ever encountered in a race in 1980 - and it also didn’t feature on the impeccably restored reverse-cylinder bike at Assen, on which a gearshift linkage which kept going over dead centre before Everett fixed it, and underdamped fork settings which had the front wheel chattering a little on some of Assen’s banked, sweeping turns, were all there was to worry about. With 18-inch slicks now unobtainable, Wilson runs his bikes on treaded Avon race tyres, which probably have at least as much grip as the Goodyears that KR used to race with 20 year ago, and certainly allowed me to appreciate the Yamaha’s key advantage compared to its more powerful rivals, namely its more capable and forgiving handling. The OW48R feels quite modern for a 20 year-old racer, low and compact in build with a balanced setup and tight riding position, especially compared to the taller, rangier stepped-cylinder square-four Suzuki which was its main rival in its championship season. It steers really well, not only on faster turns like at the end of the Assen main straight - where you can maintain an improbably high corner speed after fllicking down four gears in swift succession on the race-pattern gearbox, while squeezing hard on the front brake lever to take advantage of the surprisingly potent brakes for such period stainless steel kit - but also flicking from side to side in the chicane, where the bike’s low build and short 1350mm wheelbase encourage a quick change of direction. In spite of being so short by modern 500 GP standards - but thanks also to the low cee of gee - the Yamaha is also stable over bumps: though the monocross rear end isn’t as compliant as a modern shock would be, it’s still a big improvement over the twin shocks it replaced. Suzuki took a long time to come up with their Full Floater rising rate rear end as an answer to the Yamaha monoshock system, whose long nitrogen-charged shock with its separate gas canister, is fully adjustable for compression and rebound damping. Though the aluminium brake calipers each only have two pistons, the slotted Yamaha steel discs solidly mounted to their meaty aluminium carriers surprised me by their effectiveness, as well as their initial response when you just touch them lightly to scrub off a bit of excess speed in a turn - though Kenny Roberts’ insistence on maxing out front disc sizes surely helps, here. But it was noticeable when braking hard from high speed for the Assen chicane, and again for the National circuit’s tight Turn 1 hairpin, that the brake-operated hydraulic anti-dive on the Kayaba forks really minimises weight transfer, by using caliper reaction to close a valve and increase compression damping, thus slowing front end dive. Though Roberts wasn’t a big fan of the system, frequently opting for non-antidive forks with adjustable damping obtained by changing externally accessible jets, I found it added to the feeling of stability delivered by the accomplished Yamaha chassis, without at the same time resulting in that dead feeling you get from most other anti-dive systems, where you can’t feel what the front tyre is doing because the hydraulics dial out front end suspension response, when trail-braking into a turn. Here, you can.

But the OW48R’s piston-port engine doesn’t feel as powerful as the more explosive disc-valves like the Kawasaki and Suzuki, even if the YPVS power-valve system help max out top end power without doing so at the expense of rideability. Just as KR was later to tell me himself, I found you can’t crack the throttle wide open at low rpm exiting a slow turn like the Assen Nationale Kurve, else the engine will bog down and struggle to get back on the pipe. It’ll pull OK on part throttle from as low as 6500 revs - but get it revving above 8000 rpm, keep up the turn speed, and you’ll be rewarded with good drive by the standards of the day, as well as a smooth transition into where you really want to be, in the strong power band above ten grand. From there to the engine’s peak at 12,000 rpm the power builds pretty strongly, though not in the same eager way as the rotary-valve bikes - engine acceleration is more progressive and user-friendly, but also not as fast. In fact, Everett has fitted a set of later 38mm TZ500J Powerjet carbs to the Roberts bike, on the grounds that they make more power on back-to-back dyno tests than the factory magnesium flatslide carbs which came with the bike, which are also much harder to set up right, as well as to jet properly on the day - but even with these, it’s not as fast as the Kawasaki I was riding the same day. And the narrow real power band would have meant the OW48R’s extractable gearbox was a key element in its championship success, with the art of setting it up just right in terms of the right choice of ratios for every circuit (KR had a choice of four possible ratios for first and second gears, and three each for third and fourth) even more critical than it is nowdays, where though his son’s Suzuki is down on power compared to its Honda competition, it has a much wider spread of strong power, with substantial overrev capability. Here, even on the last of his dad’s title-winning Yamahas of 20 years ago, it’s a different story, with a much narrower powerband, and an engine that’s all done just over twelve grand.

Kenny Roberts really had to work to earn the third of his world titles, and the key element in allowing him to do so was the Yamaha’s more assured handling package - but 20 years on, nothing has changed. Ask any Honda rider or engineer involved with their current 500cc GP race programme, and they’ll tell you that the Yamahas of McCoy and Biaggi - as well as Roberts Junior’s title-winning RGV500 Suzuki - all handle better than their NSR500s, only aren’t as fast. The key element in making the last of the piston port Yamahas into a world champion was the man who rode it: King Kenny, we salute you.

Alan Cathcart

(Picture: Yamaha's official factory portrait of the OW48R's predecessor, the OW48. Note the left hand outer exhaust exiting behind the carbs the same as the 750's. The OW48R had the outer two cylinders reversed to allow the exhausts to exit the cylinders straight off the back of them and through the "ducktail" and out.)

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03/23/08 06:38 AM +1000