Honda's Cbr929rr Leads the Way In Y2k

Red Hot & & COOL

Honda's CBR929RR leads the way in Y2K

KEVIN CAMERON

NOW THAT WE HAVE THE FACTS about Honda's new CBR929RR, here is a short menu: 150 crankshaft horsepower at 11,000 rpm from a short-stroke fuel-injected engine, carried in a lighter, stiffer pivotless chassis, adding up to a claimed 375 pounds. Factor in a titanium exhaust system, four-piston front brake calipers and a 43mm inverted fork. Performance is up, but without violating engineer Tadao Baba's stated policy of creating "an ever-widening circle of appeal" for this established classic.

As you can see, the new CBR929RR has racier styling, with a sharper, swoopier fairing. But instead of a souped-up radical sportbike, we find a completely remade motorcycle whose every part has been rethought for improved performance and lighter weight. Motorcycles become harder to ride and their circle of appeal narrows when their engines are tweaked to the stratosphere. Fortunately, there are other paths to performance. You don’t need a lot of new horsepower to improve acceleration if your machine becomes lighter. You don’t need to lean hard on the engine if most of the changes fortify the lesser parts of the powerband to match its best parts. If you work through the whole package in this detailed way, you can come up with a big overall improvement, yet leave the machine’s basic character unchanged. The circle of appeal can continue to grow.

Skeptical about the CBR929RR weighing 375 pounds dry? Even allowing for the traditional

Trans-Pacific Correction Factor of 10 percent, it still weighs 20 pounds less than the original RR.

Whatever the number, weight has come out of a long list of both engine and chassis parts.

Most sportbike inline-Fours use 4into-2-into-l exhaust systems because they cancel a 4-into-l pipe’s natural flat spot at about 75 percent of peak-torque rpm. Yet a 4-into-l generally gives higher peak power. You’d like both? By placing a computercontrolled spool valve in the header junction, Honda has made a dual-mode exhaust system. In 4-into-l mode, the system delivers extra peak power. Switching when necessary to 4-2-1 mode, it cancels the aforementioned flat spot.

Titanium is a nearly ideal material for exhaust systems-40 percent lighter than steel, strong, temperature-tolerant and highly corrosionresistant. Its drawback has been cost, but the end of the Cold War has titanium producers seeking civilian applications. The 929’s header, mode valve and muffler are titanium, saving more than 6 pounds.

On the intake side, the resonant airbox is a proven technology. Just as an empty bottle resonates when you hum the right note into it, so an airbox resonates when engine intakes pulse at the right frequency. Tune the box so the engine takes air only when box pressure peaks and you have a 10-15 percent torque boost, almost for free. What if the engine hums another note? Just as there is a “right” frequency that boosts engine airflow, there is also a “wrong” anti-resonant frequency, at which the engine’s suction pulses take air from the box when its pressure is bottoming. This causes a torque flat spot. Honda’s Grand Prix bikes kill the anti-resonance by opening an airbox valve. The CBR929RR has such a valve system, so its airbox can deliver a boost with no lower-rpm anti-resonance losses.

This is an all-new engine, not an overbore. The old bore and stroke was 71.0 x 58.0mm for a 918.5cc displacement. The new numbers are 74.0 x 54.0mm and 929cc. The original engine peaked at 10,500 rpm, for a piston speed of 4000 feet per minute-a pretty racy number. Four-cylinder engines vibrate because they have unbalanced secondary forces that increase with piston weight and speed. Taming this vibration is part of the reason for the 929’s lighter forged pistons and shorter stroke. At its higher peakpower rpm of 11,000, the shorter stroke keeps piston speed to 3900 fpm.

The new cylinder head features a narrowed 25-degree angle between intake and exhaust valve stems, making the combustion chamber more compact and preventing undesirable growth in its surface area. Valve sizes are increased slightly, and operation/adjustment is by shimunder-bucket tappets.

In place of the RR’s old 38mm carburetors, we now find Honda’s PGM fuel-injection system, with its four injectors delivering fuel via four tiny sprays in 40mm throttle bodies. Fuel injection means excellent throttle response and a fuel mixture that is always corrected for weather and altitude. This is another step in what I have long hoped would happen; that once the development costs were paid, fuel injection would leapfrog from one model to the next, replacing the physical complexity of carburetors

with the lean simplicity of throttle bodies and pumps (and with the intellectual complexity of injection software). As with automotive systems, the 929’s fuel injection automates the cold-starting procedure, eliminating that uncivilized holdover from the past, the manual choke. Turn the key and go.

The growth of computer power has made it possible to treat each cylinder individually, so the 929’s engine-control computer provides two 3-D maps for each cylinder’s fuel injection (the three dimensions are engine rpm, throttle angle and the corresponding injector on-time) and one 3-D ignition map for cylinder pairs. Why should different cylinders need different fueling and spark timing? Because each cylinder really is an individual, receiving its intake air and cooling water in a slightly different way, located at a different point along a flexing crankshaft.

California models carry three-way exhaust catalyzers, with post-combustion air supplied by a pump. Fuel injection keeps the mixture centered in the sweet spot, at which point the catalyzer can most efficiently complete the combustion of hydrocarbons and carbon monoxide, and still convert nitrogen oxides (which result from high-temperature combustion) back into harmless atmospheric nitrogen. This puts to rest fears that future sportbike emissions compliance will require apparatus the size of a tourer’s travel trailer.

CBR 929RR

A new engine deserves a new chassis. This is another in Honda’s series of “pivotless” designs (meaning that the swingarm pivot is part of the engine, not the chassis). One attraction of this scheme is that it avoids duplicated structure-normal uprights, formed as main frame beams turn downward behind the engine to carry the swingarm pivots, are eliminated. Another benefit is that it permits some lateral flexibility to be designed into the engine mounts, possibly resulting in better rear-tire hookup exiting bumpy comers. We know that the original RR had quite a stiff chassis, and that it was later made more flexible in a controversial switch to calm the machine’s “busy” feel. Now, we are told, overall chassis stiffness is increased 11 percent. What does this mean, please? Torsional stiffness? Verticalor horizontal-beam stiffness? Much is made of tuned flexibility, but stiffness remains a potent buzzword nevertheless.

Reflecting what recently has been learned in Superbike racing, the RR gets a massive, braced, RC45-type swingarm. Evidently, one of the places where flex has been found to be undesirable is here. Honda claims overall

swingarm rigidity is increased 21

percent over the previous design.

Honda was the first to build racebikes with extremely long swingarms, finding that they simplify rear-suspension problems. The longer the arm, the smaller the angle through which it

rotates for a given suspension movement. Yamaha has applied this idea to their new R-series sportbikes. On the CBR929RR, Honda has lengthened the swingarm by 21mm-probably as a result of some shortening of the engine. There’s no giving in to bigbike wheelbase growth here. Axle-toaxle, the year-2000 RR measures a quick-turning 54.9 inches, half an inch shorter than previously.

As predicted, the new bike, priced at $9999, also receives a 43mm inverted fork with the usual sportbike adjustability of spring preload and damping in both directions. More performance calls for more braking power, and this comes from a pair of floating 330mm front discs and four-piston calipers.

Improvement in detail is the order of the day, not only to preserve the basic character of a successful design, but also because the big tweaks have already been used. What remains-at least until the next Big Idea-is to increase the sophistication and lightness of every part. That is what has created the new CBR929RR. □