Riding The NRV588, A Modern Iteration Of The Wankel
Rotary-Powered Norton Racers That Once Ruled British Racing
writer: Alan Cathcart
Alternative ideas for improving sportbike performance usually
involve yet another arcane front suspension layout like the Bimota Tesi or
BMW's Duolever/ Telelever designs - but rarely has anyone offered an alternative
engine. Except for the two-stroke, which became a mainstream concept that
eventually has been forced into near extinction by environmental concerns, and
not counting the current fascination with renewable fuels, there haven't been
any truly innovative concepts in gasoline-powered motorcycle engine design.
Well, perhaps except for just one: the Wankel rotary.
The Norton Rotary Reborn
It's been 20 years since the rotary engine invented by German engineer Felix
Wankel back in 1924 made its motorcycle roadracing debut in 1987. Powering the
racebikes of the resurgent Norton factory, the rotary's then-superior
performance led to eight years of competition that resulted in successive
British roadrace titles, points-scoring GP finishes and victory in the Isle of
Man TT against the top superbikes of the day. The fact that each Wankel engine
cylinder has only three moving parts that simply rotate-compared with myriad
moving pieces in a conventional four-stroke piston engine, many of which run
at incredibly high velocities only to stop dead a couple of hundred times a
second-makes this a plausibly more efficient mechanical solution. Admittedly,
the thorny issue of the bike's exact engine capacity was always a matter of
controversy. Because the rotary engine's unique three-sided rotor's power
stroke occurs twice during a revolution of the crankshaft (actually called the
eccentric shaft, basically a camshaft around which the rotor orbits), some
claimed that its displacement should be measured by the single
rotor-face-swept area of each of the Norton Wankel's two triangular rotors
times two. Nonetheless the rules were massaged on an ongoing basis to make
sure the crowd-pleasing Nortons could play their role as British underdogs
taking on the might of Japan Inc.
I was able to test each successive year's variant of the
Norton rotary racer and found the bikes thrilling to ride, with a good
power-to-weight ratio that made them unquestionably fast and with adequate
handling. Although the rotary is a dirtier, thirstier design than a
conventional four-stroke, it's also lighter and more compact-and potentially
more powerful. Just what you need to build a better racebike, as Brian
Crighton proved by conceiving the Norton rotary racers that flourished two
decades ago. Unfortunately the Nortons withered away after their last race in
1994, thanks to the corporate intrigue that eventually downed the parent
company.
But some people never give up, and for 13 years, the
59-year-old Crighton continued to dream of building a Norton rotary racer as
light and powerful as he always knew it had the potential to be. Now, with the
financial backing of dedicated Norton enthusiast Roy Richards - founder and
chairman of Britain's National Motorcycle Museum, where 16 of the 17 Norton
factory rotary racers ever made now reside - Crighton's dream has reached
reality. The Norton NRV588 is the bike he always wanted to build.
Launched a year ago at Britain's Birmingham Show, and after
a season of development held back by the dismal British summer's constant
rain, the NRV588 currently produces 165 horsepower at 11,450 rpm from its
liquid-cooled twin-rotor engine, yet weighs just 289 pounds with all fluids
but fuel. Crighton's ultimate rotary racer offers a power-to-weight ratio
comparable to today's factory superbikes, with performance enhanced by current
technology that includes a ride-by-wire throttle, fuel injection with a range
of usable maps and traction control. "This is the bike I wanted to build for
1995 after we'd beaten the Yamahas and Ducatis plus the RC45 Honda to win the
1994 championship," says Crighton. "I wanted fuel injection for the road bikes
to cope with emissions and ride-by-wire throttle to make it easier to ride in
the wet via traction control. But they changed the rules to get rid of the
Nortons, and anyway the company was fizzling out, so it never got built. But
[a British magazine] published an article about my ideas back in August 1994,
and when Roy Richards hired me three years ago to restore all his rotary
Nortons for the museum, after I'd been working for him for a while I showed
him the article. After reading it he said, 'Well, would you like to build it
to finish the story off?' He said that he'd finance it personally, and that's
how it's come about."
An Old Dog With New Tricks
The heart of the NRV588 is obviously the twin-rotor engine, one of the spare
'94 Duckhams motors that incredibly hadn't even been opened and serviced since
being built that year. Thus original mechanical spec is unchanged, with 9.6:1
compression and original porting (rotaries control intake and exhaust flow
timing via ports, just like a two-stroke). But it's now fitted with fuel
injection controlled by an Omex ECU using GEMS software, powered by a Yamaha
R6 generator and controlling a single injector per rotor in Crighton's own
dual 36mm throttle bodies. These are located beneath the required bulge in the
front of the fuel tank to allow space for the tall shower-style injectors;
they're necessary to permit usage of the NRV588's variable-intake system
(hence the "V" in its designation). But while the concept is similar to the MV
Agusta and Yamaha systems, the NRVs feature a linear electric servo motor that
controls the movement of the telescopic intake trumpets via a gearbox driving
a screw. This allows an actual progressive system, instead of the MV and
Yamaha systems that only permit two positions. Plus, while the MV/Yamaha
systems only allow a maximum of 75mm of variation in length, the Norton's
fully progressive system extends over a huge 120mm range. The trumpets are
fully extended at low rpm, then begin to contract from 6000 to 8000 rpm.
Another electric motor controlled by the ECU operates the
throttle plates, in a copy of the modular ride-by-wire system on Ducati's
MotoGP bikes. This has a conventional throttle cable running from the grip to
a potentiometer, which translates the action in digital form to the ECU. The
ECU then instructs the electric motor to operate the butterflies according to
a preprogrammed map that can be altered depending on weather, track
conditions, tire wear and so on.
Having ridden both four-cylinder sportbikes fitted with
variable-intake systems, I was interested to see how this would work on the
rotary engine. I remembered from my several previous Norton rides that it had a
broad spread of power but suffered from a lack of low-end torque, especially if
tuned for top-end power like the original bikes were. That always made them even
more like a two-stroke to ride, and the heavier flywheel required to tame the
otherwise explosive powerband also tended to compromise acceleration, only
partially rectified by the bike's light weight.
The NRV588 has no such problem. For example, compared with
Steve Hislop's TT-winning ABUS Norton, which had a nice midrange but was
definitely peaky in terms of power delivery, the new bike feels like it's
powered by an extremely potent electric motor. There's a huge range of seamless,
linear power from 3000 rpm all the way up to the 11,500-rpm rev limiter.
Throttle response is crisp without being abrupt, a welcome trait considering the
NRV's high power output and light weight. The 2D LCD dash is hard to read
because of the lack of contrast in daylight and the smallish numbers, and the
engine is so smooth that you have no sense of feel that it's time to shift. This
caused me to bump into the rev limiter often; a bright shift light would help
here. The excellent low-end torque meant that driving out of the tight turns
could be done in second gear-no need for first gear in the separate six-speed
Yamaha OW01 gearbox with belt primary drive. However, there's no power shifter
fitted to the race-pattern gearshift, because the engine's relatively heavy
flywheel and long gear spacing make it difficult to prevent shock loading while
shifting through the lower gears.
But where the variable-intake system in the NRV588 really
shows is when you get to 8200 rpm, where maximum torque of 82 ft-lb is
delivered, according to Crighton. Here, instead of the midrange flat spot that
plagued the older Nortons, the engine keeps pulling hard and continues to
accelerate strongly right to the 11,500-rpm limiter. Note that the
165-horsepower number is quoted at 11,450 rpm, which incidentally Crighton says
is a conservative figure-he feels 175 horsepower is easily achievable with more
development.
The ECU software allows Crighton to dial in a different engine
map for wet pavement as well as a different ride-by-wire throttle program and
(though it wasn't yet installed for any of my tests) a choice of traction
control programs depending on track conditions. With the continuous and
overlapping power pulses of the rotary engine making the bike especially prone
to wheelspin, this will be a useful feature in the future. It could also allow
riders to run a softer rear tire, enhancing traction even more. As it was, even
without traction-control programs installed, the NRV's smooth throttle response
and linear power made it a confidence-inspiring ride on a wet track-maybe just
not as much fun on a dry one.
The bodywork was designed by Harris Performance and painted in
colors that recall Norton's classic-era Manx GP singles. The chassis is a
Spondon aluminum twin-spar frame based on the '94 Duckhams British Superbike
title winner, but because one of Crighton's key objectives in designing the new
bike was to maximize the benefits of the rotary engine's compact build, the main
spars are 25mm longer to allow for a front-mounted Radtech custom radiator
rather than the previous twin side-mounted radiators. This helps reduce the
NRV's overall width to just 450mm, more than 50 percent narrower than the old
Nortons and not much wider than a 250GP racer. But the rotary engine also has
some heat issues caused by the continuous power pulses that can result in
exhaust temperatures reaching more than 1000 degrees C. This required a new
exhaust system made from costly Inconel alloy, which is more resistant to heat
than titanium and is even lighter. Also, to bring cooling air between the rotor
housings Crighton borrowed an idea from one of his hobbies. "I fly model
aircraft, and they use high-performance duct fans that run at very high speeds
and generate lots of airflow," he explains. "There's an electrically driven
extractor fan in the duct beneath the seat linked to the ECU, so that as the
temperature rises it runs quicker and pulls the heat out of the middle of the
engine. The forward ducts on the fairing give a positive effect there, and
there's a negative effect at the rear, so you end up with a good cooling
airflow. It's the same variable system as the engine's electric water pump,
which has no thermostat, just a sensor that monitors engine temperature. As this
rises the water pump speeds up, but then it slows down as it cools. It
self-regulates so you don't need to tape up the radiator [on colder days]."
There's just a single Öhlins rear shock on the NRV, offset to
the right and operating without any link. "A monoshock with linkage is made very
progressive, so it only moves a little when the wheel hits a small bump," says
Crighton. "This makes it hard to control the damping accurately, as we proved
with back-to-back tests on the Roton GP racer, which became Ian Simpson's
title-winning Duckhams Superbike. Our system is better because we have more
damping control and better consistency, and the braced swingarm is stiff enough
we can move the single shock off to the side to help it run cooler without any
flex." Rear-wheel travel is only 3.5 inches, while the wheelbase is extended
slightly to 56.1 inches with an increased 54/46 percent front-end bias. The
fully adjustable 43mm Öhlins fork is set at a 23.7-degree head angle (adjustable
via eccentrics) with 102mm of trail.
This Norton was also the first one I rode with Dymag carbon
wheels. As a result the pair of chicanes at Mallory Park, where I tested,
suddenly became amazingly easy to flick through. However, I found that the
Norton likes to take wide, sweeping lines to maximize turn speed rather than
stop, turn and accelerate hard, as you might expect with such a slim, light bike
wearing wide, 16.5-inch Dunlop slicks. Edge grip was obviously good on my one
dry test day, although one handling aspect I didn't care for was the way the NRV
liked to understeer under hard acceleration, perhaps because of the engine power
compressing the rear shock without a progressive linkage.
But otherwise the Norton tracked straight and true, and I
welcomed the exceptional feel of the radially mounted twin 320mm AP front discs
and monoblock four-pad calipers. On a bike this fast on such a tight track you
need to have absolute faith in your brake package, and the AP brakes provided
it, countering the rotary motor's near-total absence of engine braking. The
slipper clutch sourced from a Kawasaki parts bin helped keep the Norton stable
as it stopped hard, although it was surely aided by the high 2000-rpm idle
speed. The gear-change was a little notchy but nothing oppressive.
"It would be nice to run in that kind of racing because of the
interest it would create for people who like things that are out of the
ordinary," says Crighton. "Whatever the argument is about the capacity, it's a
165-horsepower bike and we're thinking of running it against the 1000cc
superbikes producing up to 200 horsepower, so it's a tall order. It'd be
incredible if it could be competitive at all, but we're willing to give it a try
by using the power-to-weight ratio to our advantage if it's possible. At the
moment we've only got the one bike, so if we're going to do any sort of racing
we'll need to get a second one built. We do have some spares available, though
it is getting a bit difficult with the engine parts now. But we'd like to wave
the Norton flag again to show people how a rotary ought to be done right."
Even more to the point, though: How great would a Norton
NRV588 street replica be, complete with side ports and fuel injection taking
care of emissions (just as it does for Mazda with its RX-8 sports car) and the
variable-length intake setup offering superb rideability in a 170-horsepower,
300-pound package?