2021 Honda CBR1000RR-R Chassis Is Aimed At World Superbike

Going up? Honda takes the production superbike to another level.

Honda CBR1000RR-R
Performance first, passengers second: Of special interest on the Honda CBR1000RR-R is the integration of the outflow from the winglet arrays with the outflow from the coolant radiator. Riders in MotoGP complain that turbulence from winglet upflow disturbs following bikes. More motivation to be up front!Honda

This past year, Ducati came close to overturning World Superbike by producing the strong MotoGP-based 1,000cc V4 R. Because this machine was competing against the old breed of mass-market Japanese literbikes, its short 48.5mm stroke and up-to-the-minute chassis in the hands of ex-MotoGP rider Álvaro Bautista won 11 straight races before Kawasaki's Jonathan Rea and his crew were able to strike back with a hard-won fifth consecutive championship. Despite that, the message is clear: The future of World Superbike will belong to MotoGP replicas.

Honda has replied with its 2021 CBR1000RR-R Fireblade SP, an all-new 999.7cc MotoGP-based design that arises from the specific problems of the previous model with its long-stroke 55mm engine and a chassis designed for tire technologies very different from those in use today.

History does repeat itself, for I remember the words uttered 50 years ago by the late Dick O’Brien, Harley-Davidson’s racing manager from 1957 to ’82. He said, “Just about the time we get the chassis working pretty good, the tire people come up with more grip and our chassis is back jumping and wobbling so we have to do it all over again!”

airbox cover slot
I thought those airbox cover slots looked familiar. I first saw them on the tank cover of Freddie Spencer’s 1984 factory Honda NSR500. Under them were the exhaust pipes, with the fuel tank slung under the engine.Honda

The new Fireblade—the first time Honda has used this name in the US—will add 2 inches to make its wheelbase 57.3 inches. Of this length, the swingarm will now make up 24.5 inches, up 1.2 inches from the previous model. Honda tell us that 18 different material thicknesses are used in this "MotoGP-style" swingarm.

As these changes effectively moved the engine ahead of the rear wheel by that 1.2 inches, it has been moved back from the front wheel by 1.3 inches. It has also been raised by 0.63 inch, despite being narrower than the previous design. A possible reason is the increased grip of tires, making achievable greater in-corner lean angles. During the last years of 500cc Grand Prix bikes, Yamaha’s twin-crank YZR500 engine was raised in its chassis by a total of nearly 2 inches for this very reason.

At the same time, the bars are moved forward and the footpegs are moved upward and rearward; the amounts are not given in the press materials. Riders have been moved forward again and again. When I was first paying attention, a tucked-in rider’s face was on the fuel tank. Today, his or her face is over the steering head, all to keep the front end down during hard acceleration. Fuel tanks used to be long “bread-loaf” affairs; today their shortness lets riders come much farther forward. Nominal static seat height is 32.6 inches.

ducted winglet array
At the upper right, we see the black leading edges of the ducted winglet array. The front fender in optional carbon fiber is designed to deflect airflow around the fork legs and onto the fairing sides. Tires are Pirellis, the spec supplier for World Superbike competition.Honda

Steering geometry of the previous CBR was a steep rake angle of 23 degrees with short trail of 3.7 inches, notionally giving quick steering. Computrack founder Greg McDonald has said for years that when a motorcycle’s rear anti-squat geometry is correct, radically steep rakes and 250cc GP-like short trail become unnecessary. Could that be the case here?

Today, the problem is to provide this lateral springiness without letting braking force flex or twist the steering head.

In the new century, it is understood that when a bike is at full lean—angles above 60 degrees to the vertical in some cases—its normal suspension is getting close to being at a right angle to pavement bump forces, making it ineffective. Since it’s hard to imagine a “sideways suspension” without adding a bunch of complexity, cost, and weight, the present solution is to reduce the lateral stiffness of the chassis and swingarm, making them into “sideways springs” that allow the wheels to flexibly track over small pavement irregularities, which would otherwise cause loss of grip. At Honda, this development began in 1997.

Today, the problem is to provide this lateral springiness without letting braking force flex or twist the steering head. In this CBR1000RR-R, the bending stiffness of its aluminum twin-beam chassis is increased 18 percent over previous, and torsional (twisting) stiffness is up 9 percent. Yet lateral stiffness has been reduced by 11 percent in the interest of better tire hook up in corners. Similar for the swingarm, whose lateral stiffness is reduced by 15 percent.

A look at the image of the bare chassis shows there is no upper rear crossmember, only a hefty lower with its central suspension link pivot. Where does the rest of the suspension attach? Suspension forces are highly leveraged, and aluminum has only one-third the Young’s modulus—the inherent “spring constant” of a material—of steel. Suspension forces have been known to deflect that upper crossmember enough to pull the uprights (vertical rear parts of the frame) together, pinching and jamming the swingarm! Can’t have that, so, on RR-R, the suspension attaches to a bracket that bolts to the rear of the crankcase.

swingarm transition
This is a time of swingarm transition. The one on the CBR1000RR-R is aluminum, but as metal arms are given more flex to keep the rubber gripping at high lean angle, a point may be reached beyond which only carbon fiber will work.Honda

Öhlins suspension provides 4.9 inches of front travel and 5.6 inches of rear travel. The fork is the pressurized 43mm NPX with second-generation electronic control and the Öhlins Objective-Based Tuning interface (OBTi) that “allows finer adjustment” with three preset modes. Why “pressurized”? Damping force is generated as a fork or rear unit piston accelerates oil through orifices, but on the reverse stroke, something has to push fluid back into the damper cylinder to refill it. If only atmospheric pressure is pushing on that fluid, as in unpressurized dampers, the refill process can be incomplete during rapid suspension motions, causing a chassis-upsetting “ka-thunk” when the damper piston next reverses direction. And so modern dampers tend to be pressurized.

Honda shocks
The smooth organic shape of the swingarm has resulted from the same process that brings us the grace of trees, whose structure is constantly flexing in wind. Because sharp edges and sudden changes of section invite cracking, let’s replace them with gentle curvature.Honda

Brembo Stylema brake calipers, same as on the V-4 RC213V-S, and a Brembo master cylinder convert kinetic energy into heat in the 10mm-larger 330mm steel front discs, each 5mm thick.

Chassis beams are made of 2mm-thick aluminum, and the engine attaches in six places to be a stressed member. Engine charge air enters a central fairing opening (same dimensions as on Marc Márquez’s MotoGP winner) and passes through the front of the frame into the intake airbox. The steering stem lives inside a wing-section tube, allowing airflow to divide and flow around it.

Back when resonant airboxes first appeared on production bikes, their action was explained as being like the hum produced when you blow across the neck of a bottle. The compressible air in the bottle acts as a spring, the slug of air in the neck of the bottle is the mass that vibrates back and forth, and the force driving the oscillation is the air you blow, which alternately enters, then blows across the opening. In an engine airbox, the “hum” is generated by the repeating intake pulses, and the neck is the tube or tubes connecting the airbox to the atmosphere. Get that hum going strongly and time it so the engine takes in its air from the peaks of this pressure cycle, and you can boost torque by 10 percent.

But there’s another refinement. If you don’t slow the entering air down gradually, the conversion of its kinetic energy into pressure loses efficiency; 200 mph is 293 feet per second, which, at 100 percent conversion efficiency, promises a useful 4.7 percent boost in airbox air density. To prevent this loss, engineers are now providing an intake diffuser, a passage that gradually widens.

Clearly visible to all are the right and left arrays of MotoGP-style winglets on the sides of the CBR1000RR-R's fairing nose. MotoGP riders, when at very high speed, began to notice the front end acting light and dancing around in a manner not at all reassuring. The first response from engineering was to add stubby winglets to press the front tire down against the pavement. In some cases, riders and data reported that front ends were lifting enough to trigger anti-wheelie systems, which work by reducing power. Can't have that! So, winglets to the rescue. When Dani Pedrosa and others noted that winglets could be called "blades," possibly causing grievous bodily harm, they were enclosed by strips of material that capped their ends. The result was what you see here: arrays of stubby wings in a duct that a gas-turbine engineer might call a "cascade." It looks like Honda has combined the outflow of hot radiator air with the upflow behind the winglet arrays. Hmm...

How about aero drag itself? Honda says this bike has the lowest coefficient of drag in its class. That means its shape is slippery, but drag is the product of Cd times frontal area, about which the press materials say nothing.

I’m not going to list all of this machine’s modern electronic aids, multiple programmable levels of each, their descriptive acronyms, or the ease with which you can access system data on your smartphone should you happen to be in Kuala Lumpur. Honda has been a little slow in catching the big CBR up in these areas, but now it’s done it. Good. What really excites me about this bike is its take-it-to-a-new-level hardware.

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