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The 2023 Kawasaki Ninja ZX-4RR, a revolutionary 399cc inline-four sportbike that combines the thrilling performance of a superbike with the affordability and insurance-friendly specifications of a mid-displacement machine, setting a new standard for the lightweight sportbike market.
Superbike Dreams at Mid-Range Prices: The 2023 Kawasaki Ninja ZX-4RR
The 2023 Kawasaki Ninja ZX-4RR’s spec sheet demolishes that of every other small-displacement sportbike on the market. In a field of twins and singles, the ZX-4RR’s four cylinders are an ace in the hand, making it the lightweight sportbike many riders have been dreaming about since the 1980s—complete with rider aids that were barely being imagined back then. The only downside? Price of admission.
The Kawasaki Ninja ZX-4RR is a 399cc inline-four sportbike—a mini pseudo-superbike that hearkens back to the 1980s and ‘90s when the Japanese market was flush with exotic, state-of-the-art 400cc sportbikes. The class was a hotbed of activity in Japan partly because of a licensing system that made bikes of 400cc and below much easier and less expensive to get permits for than larger-displacement machines.
So, how is a 400cc inline-four sportbike relevant to today’s US market? After 2008, manufacturers realized that the combination of high insurance premiums and empty pockets hadn’t changed riders’ taste for performance. The answer was highly capable but simplified motorcycles—many of them twins—at prices people can more easily afford and that raise fewer red flags with insurance companies.
While new-style mid-displacement twins are great, so too is the high sweet song of the inline-four, particularly in full-on sportbike trim. With the ZX-4RR, Kawasaki must be thinking: let’s spec out a bike whose displacement is too small to frighten insurers, and then let’s see how much of the traditional superbike look we can roll out at a price a lot of riders can pay.
In addition to a four-cylinder engine, the ZX-4RR uses radial-mount calipers, adjustable suspension, an assist and slipper clutch, a TFT dash, and electronic rider aids derived from Kawasaki’s top-end sportbikes. The thought of those four small 57mm pistons whirring away at 16,000 rpm is enough to excite the most jaded sportbike aficionado. This is no mere Ninja 400. However, for the sake of affordability, the ZX-4RR veers away from the Japanese-market 400s of yore by using a steel frame and swingarm instead of pricier aluminum numbers.
In certain ways, the ZX-4RR is history in reverse. Take for example the historical trajectory of the 600 supersport: for years, performance-with-economy was the philosophy behind, for instance, Honda’s CBR600F Hurricane and Kawasaki’s GPz600R, which used steel frames and budget-friendly components. In Japan, that whole concept was scrapped in the heat of the performance wars that defined the 400cc sportbike class. Eventually, America and Europe would come to demand bleeding-edge race-replica 600s. Now, it seems, economic realities, tetchy insurance agents, and emissions standards have caught up with us. Three decades later and half a world away, as the popularity of the 600 supersport class wanes, there’s finally a fully modern, albeit less exotic, 400cc four-cylinder sportbike begging to be run at redline and at the edge of its tires.
The ZX-4RR is essentially in a class of one, as Kove is the only other manufacturer adding a 399cc inline-four sportbike to its lineup. In every other way, the ZX-4RR far outpaces similarly displaced machines in terms of spec, number of cylinders, and price. While those budget-oriented machines are largely aimed at beginning riders with a focus on street riding, the 4RR is geared more toward riders who value light weight above all else, and who may wish to take their bike to the track.
Accordingly, 600cc supersports, like Kawasaki’s Ninja ZX-6R ($10,699), the Honda CBR600RR ($12,099), and Suzuki GSX-R600 ($11,699), though a step up in price, will likely make buyers think twice. But as a lightweight, high-rpm alternative, the ZX-4RR is really the only choice. Your local track will probably be full of ‘em.
At the heart of the ZX-4RR is a liquid-cooled DOHC 16-valve inline-four displacing 399cc. Bore and stroke measure 57.0 x 39.2mm, which is almost identical to the 1990 ZXR-400R, which measured 57.0 x 39.0mm and displaced 398cc. From there, however, this new engine is fully modern, benefiting from more than 30 years of technological advancement, including electronics that simply didn’t exist back then. Engine redline is 16,000 rpm. Although the US press kit only quotes torque figures (26.5 lb.-ft. at 11,000 rpm), UK press kits list power at 77 hp (without ram air) and 80 hp (with ram air). At least this provides a ballpark figure.
Feeding the engine are 34mm throttle bodies with Kawasaki’s latest Electronic Throttle Valves (ETV), allowing the high-spec ECU (rider aids are discussed below) to precisely control both the fuel to the injectors and the air entering the intake stream.
Ultralightweight cast-aluminum pistons have a molybdenum coating on the skirts for durability, while a reasonable 12.6:1 compression ratio ensures regular gasoline can be used (for reference, the 1990 model’s compression ratio was 12.1:1). Pistons slide in die-cast aluminum cylinders with an open deck design. The connecting rods have a carburizing treatment to help improve durability, while the crankshaft’s light flywheel mass contributes to the engine’s quick-revving nature.
Power is passed through Kawasaki’s Assist & Slipper clutch into the six-speed transmission that comes with a standard Kawasaki Quick Shifter (KQS) that allows clutchless up- and downshifts.
While the trick 400s of old brought stout aluminum perimeter frames to the public (they’d previously only been used in Grand Prix racing), the ZX-4RR uses a cost-effective steel-trellis frame and a steel banana-style swingarm. Claimed wet weight is 415 pounds, so expect about 391 when weight of the 4.0 gallons of fuel is subtracted. Looking at the spec chart, we see that the wheelbase measures 54.3 inches, which happens to be identical to the Asian-market Kawasaki ZX-25R, that this 4RR is largely based on. Front-end geometry has 23.5 degrees of rake and 3.8 inches of trail, which are actually a bit more aggressive than the 250?s and quite similar to the quick-steering 400s of old.
Like the ZX-25R, the ZX-4RR utilizes a 37mm inverted Showa SFF-BP (Separate Function Fork-Big Piston) fork with provision for spring preload adjustment. The advantages of this fork are that a larger-diameter damping piston is used compared to a cartridge-style fork. This reduces damping pressure which allows the stanchions to move more freely. The rear suspension features a horizontal back-link-mounted Showa BFRC (Balance Free Rear Cushion) Lite shock. The position of the shock and link is above the swingarm which also gets it out of the way of engine and exhaust heat. The shock has provisions for preload, compression, and rebound damping adjustment.
Braking is handled by a pair of radial-mount four-piston calipers up front. The calipers have a pair of 32mm pistons in the upper body and two 30mm pistons in the lower section. Twin 290mm semi-floating discs are used up front while a 220mm disc with a single-piston caliper is used for the rear. The latest Nissin ABS control unit is used, but there is no mention of a six-axis IMU, so that means a traditional ABS system without lean-angle-informed intervention.
The ZX-4RR has slightly more relaxed ergonomics than purebred supersports like the ZX-6R and ZX-10R. Clip-ons have a slight rise for comfort on the street.
The rider interface to access settings is anchored by a 4.3-inch full-color TFT display. Multiple screen modes can be selected, including a circuit mode that prominently displays track-related information such as the lap timer, tach, and gear-position indicator. The display can also be customized with dark or light themes and the brightness level is automatically adjusted based on ambient lighting conditions. Using Rideology the App, the rider can link their smartphone to the bike and access various vehicle info, a riding log, mobile phone notices, and customize settings to preference.
The ZX-4RR features twin LED headlights and taillight, while the front LED turn indicators are integrated into the front lower fairings’ leading edge. Slim LED turn signals are mounted to the license-plate bracket at the rear.
Perhaps the single biggest difference between those 400cc exotics of the late ‘80s and early ‘90s and this modern remake are the electronics. Not only does the ECU (which is similar to the unit in the Z H2), offer a level of engine management that didn’t even exist in Grand Prix racing back in the day, but it opens the door to a ton of rider aids that align the bike with other modern family members like the ZX-6R. The ZX-4RR does not have a six-axis IMU to manage rider aids.
At the center of the system are the Integrated riding modes with four options including Sport, Road, Rain, and Rider (the latter of which allows manual selection of the parameters). The first three options have optimized settings for different types of riding, while the rider mode allows the user to choose between three levels of intervention on the Kawasaki Traction Control (KTRC), and between two power modes: Full or Low with milder response. The TC can also be turned off completely. As mentioned above, the ZX-4RR comes with an up and down quickshifter as standard.
The Kawasaki ZX-4RR features typical Kawasaki build quality. With features one would expect on a 600cc supersport, the ZX-4RR brings a new level of quality to the modern small-displacement sportbike.
2023 Kawasaki ZX-4RR KRT Edition Specs
Engine: DOHC, liquid-cooled inline-4; 4 valves/cyl.
Bore x Stroke: 57.0 x 39.1mm
Compression Ratio: 12.6:1
Transmission/Final Drive: 6-speed/chain
Claimed Horsepower: N/A
Claimed Torque: 26.5 lb.-ft. @ 11,000 rpm
Fuel System: DFI w/ 34mm throttle bodies
Clutch: Assist w/ slipper function
Engine Management/Ignition: TCBI w/ digital advance
Frame: Steel trellis
Front Suspension: 37mm inverted Showa SFF-BP fork, spring preload adjustable; 4.7 in. travel
Rear Suspension: Showa BFRC Lite Shock, fully adjustable; 4.9 in. travel
Front Brake: Dual radial-mount, 4-piston calipers, 290mm discs w/ ABS
Rear Brake: 1-piston caliper, 220mm disc w/ ABS
Wheels, Front/Rear: Cast-aluminum
Tires, Front/Rear: 120/70-17 / 160/60-17
Rake/Trail: 23.5°/3.8 in.
Wheelbase: 54.3 in.
Ground Clearance: 5.3 in.
Seat Height: 31.5 in.
Fuel Capacity: 4.0 gal.
Claimed Wet Weight: 415 lb.
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