You're probably wondering why we didn't include the Kawasaki ZX-6RR in last issue's 600-class shootout. The reason was because Kawasaki officials were originally keeping tight reins on their 6RR test bikes, and specified that we could only test one on the track under their supervision. Since our testing also included street riding (plus the fact that the 6RR is technically a "limited production" motorcycle aimed primarily at homologation for supersport racing), we decided to leave it out of the 600 comparison test in the interest of fairness. After we completed the middleweight shootout, however, Kawasaki in Japan apparently changed its mind and released the ZX-6RR for full road testing by the media
The major technical details of the new ZX-6RR were covered in our first ride feature previously (Late Braking, April '03). To briefly recap, the chassis is identical to the new 6R-save for the adjustable swingarm pivot. The engine, however, is totally different; in order to stay at the 600cc maximum displacement limit for supersport racing worldwide, a highly oversquare bore/stroke configuration (67.0 x 42.5mm, compared to 68.0 x 43.8mm for the ZX-6R, and 66.0 x 43.8mm for the old ZX-6R) is used. Other than the parts associated with that displacement, such as the crankshaft, rods, pistons, cylinders and cams (which feature adjustable cam sprockets on both models), everything else is the same as the new ZX-6R motor, including the cylinder head and fuel injection system which utilizes 38mm throttle bodies (versus the 36mm carbs of the old 6R).
The 6RR's gearbox sports the other big difference between it and its 636cc brother. Although the transmission ratios are identical to the 6R (more closely spaced ratios from third through sixth gear compared to the old 6R), the 6RR has the added benefit of a ramp-type slipper clutch, a first on a production 600. Aimed at eliminating wheel hop on downshifts during the heat of battle, the 6RR's slipper clutch can be adjusted by using optional accessory springs to alter the amount of slip. The shift action itself can also be adjusted via an accessory detent spring.
Since the suspension and brakes are identical to the new 6R, we aren't surprised that the 6RR exhibits the same razor-sharp handling manners. Steering is on the quick side, with light effort needed to initiate turn-in, and the bike holds a tight line with ease and precision. We noticed a very slight improvement in turn-in effort compared to the new 6R, probably due to the lighter crankshaft. Suspension action is very good, though the 6RR exhibits the 6R's slightly nervous handling when pushed to the limit. A bit too much high-speed compression damping, coupled with stiff spring rates (we ended up taking out almost all the preload in the front fork and softening up the rear considerably), ends up feeding a lot of bump energy back into the chassis over rough pavement, and one tester still noted a tendency to tankslap if he wasn't careful powering through transitions. Heavier riders (more than 175 pounds) probably won't notice this as much.
Braking action from the radially mounted four-piston Tokico calipers and 280mm discs is excellent, with stupendous power, progressiveness and feel. We noticed that the 6R and 6RR's brakes on our test units lost a bit of initial bite and progressiveness over time, but their overall power was unaffected; if anything, it helped alleviate some of the oversensitivity that a couple of testers initially complained about (though one felt they were still a little too touchy).
One thing readily apparent when you jump onto the 6RR after riding the 6R or one of the other new middleweights is that the motor is lacking on steam. There just isn't the midrange snap of the 636cc 6R off the corners, and areas where you have to modulate the throttle on the 6R, you can basically hold it wide open on the 6RR. Even compared to the other "real" 600s, the 6RR is down a bit on power, especially in the crucial midrange section of the powerband. Although the power curve is fairly linear and the 6RR revs more quickly than the 6R, real power doesn't begin on the 6RR until 10,000 rpm, requiring you to keep the motor above that point at all times.
Gearing selection is vitally important with this bike; if you enter a corner that has you falling "between gears" (too fast for one gear, too slow for the next), you'll be caught waiting for the motor to catch up to the powerband, resulting in lost time. Granted, this is a racebike, so some tuning with the adjustable cam timing and fuel injection should probably alleviate this problem.
Thankfully, the slipper clutch allows for faster corner entries due to less engine braking, so it's easier to keep momentum up. It is still possible to get the rear wheel moving around a little too much, but this occurs only during extremely hard braking situations where the rear tire is barely in contact with the ground. With MotoGP racebikes employing and perfecting slipper clutch technology, you can bet we'll see more of these units on future sportbikes.
The ZX-6RR uses the 6R's circular LCD bar graph tachometer, and we're growing to hate it. There's not enough contrast to distinguish its reading at a glance, especially in broad daylight. It's a good thing that the shift light is surprisingly bright enough to be visible; the unit has two brightness adjustments (we used the highest one) and can be set in 250 rpm increments up to the 6RR's 15,500 rpm redline.
It's pretty obvious that you'll need to be a serious racer to extract all the potential that the Kawasaki ZX-6RR has to offer. The regular ZX-6R has better power all around, the same suspension and brakes and a $400 cheaper list price. For the average 600 buyer, the 6R is the better purchase, hands down. But racers are not your average buyers, are they?
This article originally appeared in the August, 2003, issue of Sport Rider.