The Grand Prix Fantasy
Twenty years ago, the legendary Giacomo Agostini and his then-unconventional "gill-valved," water-cooled Yamaha YZR500 two-stroke dethroned the mighty MV-Agusta Gran Prix juggernaut, ending MV's unprecedented 17-year win streak. Several years later Kenny Roberts gave the marque three consecutive GP titles, and fans around the world dreamt of what riding a 500cc Yamaha racebike would be like.
Skip forward to 1984, and Yamaha brought the dream one step closer to reality when they introduced the 500cc V-Four RZ500.
Inspired by the YZR500 factory racer ridden by Roberts during the 1983 GP season, the twin-crank V4 was the closest thing to a Gran Prix bike (with lights) that you could get your hands on. From it's GP-style full fairing to the water-cooled, four-cylinder two-stroke engine nestled in a perimeter-style box section frame, the RZ500 was a dream-come-true for race enthusiasts world wide, and naturally, it became a highly lusted-after machine.
Problem was, due to Environmental Protection Agency anti-pollution mandates, the RZ500 was never sold in the United States (at least not legally), but you can bet a few regulations didn't stop enterprising two-stroke lovers from jumping on the contraband express and importing an RZ or four.
Three different versions of the red and white bike were available worldwide. The RZ500, which came equipped with Yokohama OEM tires, was sold in Australia, New Zealand and Canada. Its European counterpart, the RD500LC , while mechanically identical, sported Michelin tires and a different paint scheme (photo at right). Japan received a limited-edition RZV500R, replete with a lighter aluminum frame (the others used steel) and other trick goodies, but a lso suffered from a de-tuned engine.
Kenny Roberts' 1983 OW-70 GP bike used a very unique 50-degree, twin crank V-four motor equipped with Yamaha's YPVS exhaust power valves, and this same basic design configuration was shared by the RZ: With it's twin cranks, the engine actually functions as a pair of 180-degree parallel twins geared together. Both Roberts' YZR and the RZ500/RD500LCs had removable cassette-style six-speed gearboxes. But this is where the similarities end.
The RZ's design had many concessions to both street-bike reliability and maintenance. While the racer had it's four in-line, big-throat carburetors nestled in the V of the motor, close to the cranks, Yamaha mounted it's YPVS hardware in this same location on the RZ necessitating that the smallish 26mm Mikuni's be relocated off to the side of the V. Additionally, the original OW-70 racer had rotary valves to control intake. The RZ utilized reed valves to achieve a broader, more street-oriented powerband. But, again due to limited space in the V, the reed valves used two different intake arrangements. The lower cylinders were fed through crankcase reeds; the upper pair used cylinder reeds, with 90 degree intake manifolds connecting each reed block to it's side-mounted carburetor.
The dual primary drives were geared directly to a TZ750-sized clutch, while the front crankshaft also spun a counter balancer that is mounted deep inside the "V" between the two cranks. The counter balancer, unusual on a two-stroke, helped dampen engine vibration. A servo-controlled YPVS system -similar to the system used on Yamaha's RZ350 - bolstered low and mid-range power, and let gasses flow into four individual expansion chambers. The lower exhaust pipes exit in traditional two-stroke fashion, but the upper pipes cross over each other just aft of the exhaust ports to help maintain proper tuned length without having the pipes protrude out of the back of the bike. (Due to mandatory horsepower restrictions, the Japanese RZV was de-tuned slightly through the use of smaller-diameter pipes and leaner jetting.)
An unusual engine, indeed. And one which produces an enormous ear-to-ear grin every time it's wound up to it's 10,000 rpm redline. Two-stroke power, by nature, is explosive. Non-linear. Peaky. And the RZ500 is no exception. A distinct lack of power below 6000 rpm coupled with a tall first gear make the RZ rather difficult to get away from a stop - lots of clutch slippage is the rule. But once underway the close-ratio gearbox plays into your hand. As the spins up, you can feel a sense of urgency building from within. The engine starts to buzz, coming alive above 6500 rpm, and hitting the pipe at seven grand. From there to redline it's a rush of two-stroke acceleration. The front wheel goes light, and you dance on the close-ratio transmission to keep the engine in that 3000 rpm sweet-spot.
We recently had the opportunity to test an RZ500 on Erion Racing's DynoJet dyno, and were pleasantly surprised to find it producing almost 80 horsepower. In fac t, if it weren't for a badly worn drive chain, the DynoJet technician said we may very well have seen around 84 bhp. And all this from a motor that Yamaha engineers said was mildly stressed, with plenty of headroom for modifications.
What's really noticeable during spirited riding is how torquey the motor is. For instance, in a chase up the tricky slopes of Mount Palomar, USA, the RZ easily out-ran and out-handled even the latest crop of 600cc sportbikes. Don't believe us? Video clips don't lie! Here's a Windows (IBM-PC) .avi file and here's an MPEG version for UNIX and Macintosh users.
The dyno revealed a very rapid build in torque around 6000 RPM, peaking at 48.3 ft-lb at 8500 rpm, with the curve remaining quite flat all the way up to the 10,000 RPM redline, meaning this bike simply squirts out of corners and pulls strongly until the expansion chambers turn off the fun at around 10,500 RPM.
As if the unique motor weren't enough, the chassis also incorporates several uncommon features. The frame itself is really quite basic by today's standards - mild steel box-section rails in a perimeter layout . But the rear shock, which on most bikes normally resides in an upright position under the seat, had to be placed horizontally under the engine. You see, the under-seat area is already packed full of the upper cylinders' pipes, the YPVS servo motor, and the battery. So, under the motor it went, with the added benefit of consolidating mass to improve the RZ's center of gravity and roll polar moment (meaning it's flickable).
The 'forward' end of the rebound-adjustable shock mounts to a bracket connected to both the frame and the engine - engines are naturally, quite rigid, so overall chassis rigidity can be gained by such amount. The rear of the shock connects to an extruded aluminum swingarm via a beautiful forged-aluminum rocker; a small forged link attached to the rocker's center pivot positions it relative to the frame.
A few other chassis features incorporated on the 500 were innovative back in 1984. For instance, the bike's non-adjustable forks use spindly 37mm stanchion tubes, and the front of each lower fork leg carries an adjustable anti-dive valve. Remember them?