Kenny Roberts OW54

Yamaha’s 500 GP bikes – Part 2, Enter King Kenny…

Kenny Roberts OW54Once Kenny arrived on the scene his superb analytical skills quickly revealed some startling errors that had previously gone unnoticed by the factory riders that came before him. The Japanese had always sought power over and above everything else, the paddock call it “Dyno Blindness”, and most riders will go all gooey eyed when told that this years machine has 30 bhp more and its yours, just sign here!

Roberts had a different approach and concentrated in other areas, he immediately identified the deficiencies within the chassis of the 77 bike, the front discs were too small, the swing arm pivot being to high, the rear caliper being transferred its energy to the swing arm instead of the frame and the suspension settings completely unusable. These errors were rectified to Kenny’s liking and the rest as we know is history. Kenny began his 1978 campaign on the previous years OW35 although modified to his own chassis specifications following the winter testing. The bore and stroke again changed back to the over square 56mm x 50.2mm to give Kenny maximum top speed and a peaky power delivery. Originally equipped with standard barrels they were later replaced as the season wore on with the latest power valve models, even then though, Roberts reports his bikes were well down on top speed when compared to the works Suzuki’s of Barry Sheene and Pat Hennen, it is just that the young Californian simply “rode the wheels off of his Yam”.

1979 Yamaha YZR500 0W45The following year saw the OW45 arrive, very little had changed from the OW35 but the adoption of a new “De Carbon” rear shock, utilising nitrogen as a secondary spring, that also ran a little cooler enabling it to still be effective at the end of a long GP. This was also the first full year with he latest in exhaust valve technology. Initially the power valve was going to be implemented using a form of moving cylinder liner that also raised the transfer port height up and down to completely alter the port timing throughout the rev range but technical difficulties meant that just the exhaust would be effected with a sliding shutter type of valve. Strangely enough this valve does not have to be actually touching the piston to have an effect, even when positioned well away from the cylinder wall the engine still “thinks” the port is lower. Kenny once again took the title despite a serious crash during testing that left him with a ruptured spleen, a broken back and similarly damaged foot. That was in late January and Roberts was nowhere near fit by the start of the season in March forcing him to miss the first round, the Venezuelan GP, completely. Kenny returned ready for the start of the European campaign with a stunning win in Austria followed by further wins in Italy, Spain and then Yugoslavia. His performance that year was enough to clinch his world second title.

During the winter of 79 Kenny told the factory he needed at least a further 10-15 bhp to counter the on going development of the RG Suzuki’s. The response he got from the Yamaha technicians was not promising. It was obvious that the piston port design was near the limits of development and all they could do was make the new machine lighter by using exotic metals and new materials like carbon fibre. Looking back, it seems strange that Yamaha, back in 1976, made the decision to drop the reed valve induction when they had so much knowledge of the system, especially when Honda then went on to use exactly that method in their dominant RS 500 triple of 1983. Reed valves have since become the universal method of induction control for every two stroke 500 four of the last 15 years.

1980 YZR500 0W48The 1980 Yamaha, the OW48, was indeed lighter than the 45 by quite some considerable margin bristling as it did with magnesium engine castings, carburettors and other large components like triple clamps and fork sliders. Carbon fibre was used for the first time on a bike chiefly for the fairings and more noticeably the exhaust end cans. I crafty technique used by the Japanese, once again still maintaining their air of secrecy was the painting of the all new aluminium chassis in a shiny black exactly the same as the steel version, from a distance the two types of frame were almost indistinguishable. Initially the ally frame was too flexible and as the season wore on extra sections were welded to the top tubes to aid strength. The engine was mounted 20mm further forward to increase the weight on the front wheel, enabling Roberts to utilise his rear wheel spinning antics to good effect. A new design of brake caliper was used for the first time to grab the oversized 320mm discs on the OW48, once again creating a great saving in weight, up until then the factory bikes had been using the standard iron caliper that could be found on virtually every Yamaha road machine from the RD250 to the XS650.

By the mid way point of the 1980 season Kenny Roberts had a new twist on the ageing OW across the frame design and the latest, indeed the last, version was labelled the OW48R. The R denoting the turn around of the two outer cylinders enabling a much straighter run for the exhaust pipes and considerably more ground clearance with only two pipes running underneath the engine. But with one major problem, the new engine would not fit into the latest ally chassis so it was back to the steel one to try it out. That didn’t work so a new ally chassis was fabricated to accommodate the reversed engine.

The design of the OW48R’s pipes harked back to the days of the MZ racers when the Kaadenecy effect was first fully understood, basically the top man at MZ in the sixties, Walter Kaaden had developed the formula to work out the exact dimensions to produce the shock waves within an expansion chamber, in the process he unwittingly gave modern motorcycling its greatest gift to date. Consequently the MZ racers always utilised a straight exhaust to reduce the effect of any unwanted waves within the expansion chamber and maintain power levels. The defection, from east to west and straight into Suzuki’s arms, of works MZ rider Ernst Degner gave the Japanese this top secret technology. Suzuki put the knowledge to very good use in the sixties with their successful 125 and 250 machines and then again in 74 with the square four RG500 design. It was the overwhelming pace of the RG that finally led Yamaha to “about turn” the two outer cylinders enabling the straightening of the pipes as they routed under the seat and also freeing up a lot of space for to two remaining pipes to follow a more natural curve under the engine in the bargain. The continuing demand for top power output from the outdated piston port design had led to the need for massive transfer port speeds and this can only be achieved with a fatter mid section to the expansion chamber, more room had to be found.

The side effect of this was extra cooling this provided for the rear shock as in previous Yamaha designs the #1 exhaust had passed within millimetres of the oil damped unit. This gradually heated up the shock and as the long GP races wore on the ability of the suspension to perform correctly became seriously hampered. The #1 pipe also created a pocket of warm air that subsequently fed the four rear facing carburettors, this literally sapped engine power as the intake will perform far better with a fresh charge of cool air. Most of the 7bhp gain claimed for the OW48R over and above previous models would have been from this improvement alone. The one negative of the cylinder reversal was the excessive piston wear for the outer pair as the piston inlet now had to force against the massive inlet port as it made its power stroke. The prevention of this was the very reason why the engine had been designed to run backwards in the first place but 1980 saw desperate times for the Yamaha camp and it was considered a small price to pay until the next generation of GP racers were ready to race. All of these modifications were enough to keep Roberts competitive and take his third and final title.

Roberts remembers…
I had the fantastic opportunity to talk to Kenny about what sort of power he had at his disposal at great length when he rode the OW48R again at Montlhery. His recollections of the time were incredibly sharp and his reply was startling to say the least.

Kenny takes up the story “The most we ever had out of the Reverse cylinder (OW48R) was 104bhp and that was on a good day. Even a private RG would be near the 110 bhp mark and the works ones a few more than that. This was the last roll of the dice and the absolute limit of the across the frame fours abilities.”

1980 YZR500 0W48RWhen asked about riding the OW48R Kenny once again replied “ This was the best one they made (inline fours),I could lay it in to a turn as hard a I liked and just gass it out! After that we had the OW 54 and 60 square fours, they were simple knee jerk reactions to the RG and were a total disaster. About a year after that we had the first of the V fours, which again was not good. There was too much new technology in there like the weird lay down rear shock that never worked and engine components that were too damn light. That thing used to spin up everywhere and burn up tyres. Looking back I would have been better off running the OW48R for the 81 and 82 seasons until we got the V properly sorted. After that we got the Ohlins and Brembo people onboard and in 83 Yamaha gave me the OW70 that was the best race bike they had ever built at that point”

Kenny also claimed that Suzuki “shot themselves in the foot” by supplying so many RG’s to top riders. Kenny again “ Their bike (the RG) was sure good enough to win that year (1980) but their guys kept tripping themselves up. One would win one week and a different guy the next. Now they had four or five guys out their wanting a win I just kept on cruising (oh yeah?) to a win here, a second there and what d,ya know? I won the championship. They ( Suzuki) went out of their way to make it easy for me”.

Yamaha 500 GP Race Bike Gallery

[dmalbum path=”/wp-content/uploads/dm-albums/Yamaha GP Bikes 73-77/”/]