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Wednesday, December 1, 2010

Not Just any Shaft Drive

A lot of development went into building the single-sided shaft drive for the VFR1200FA.

Here is an article from Honda:

Progress – the VFR Drive Shaft Up Close

It's a first in the history of the VFR for Honda to equip the VFR1200F with a shaft drive. But not all drive shafts are created equal. The design effort that went into ensuring extremely low-reaction operation for this component is more than remarkable and warrants detailed examination.

The shaft drive on the Honda VFR1200F is something special. Integrated into the single-sided swing arm, its design impresses for its unusually low-reaction operation. Motor cycle riders who are used to machines with chain drive to the rear-wheel and test ride a VFR1200F for the first time often discover that there is little or no noticeable difference. And motor cycle riders who are used to other shaft-drive machines are astonished on their first VFR ride by the smooth transmission of power and the absence of the typical accompanying traits.

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Did the Honda technicians perform some kind of magic? Let's resist the temptation to answer "Yes". Let's say instead that innovative development and design went into providing a shaft drive that sets new standards from a functional point of view. Several factors contribute to the impressive result.

First the basics: The drive shaft transmits engine power to the rear wheel and rotates on the left side, sealed in the one-piece, cast aluminium single-sided swing arm in the drive shaft tunnel. The final drive housing is flanged to the single-sided swing arm.

One special feature that distinguishes the VFR design is offset axes. The drive shaft and the swing arm in which the shaft rotates do not run on a parallel axis but at a pointed V-angle to each other. The mount for the swing arm on the frame is offset upward, so that the drive shaft can run below it directly for deflection at the transmission output.

This design refinement offers two benefits: First, a continuous swing axis can be used, which benefits the overall stability of chassis and swing arm mount (shaft-drive bikes mostly have to put up with short pins on the left and right to carry the swing arm in order to make room for the drive shaft running behind it along with a front universal joint). Second, Honda's design with offset axes does away with an additional torque bracing using lever systems against the frame, which not only saves weight but helps to achieve a clean look.

Anyone looking at the drive shaft assembly of the VFR1200F from the side who draws an imaginary line from the centre point of the wheel to the transmission output and then observes the position of the swing arm bearing on the frame can clearly recognize the upward offset. The benefit of this design is that reactions to load changes are successfully suppressed. Normally, with shaft drive the motorcycle lifts when the throttle is opened and it drops when the throttle is closed, an effect that is more or less pronounced depending on the model. This idiosyncrasy is foreign to the VFR1200F.

The shaft drive of the VFR1200F is astonishing in all riding situations for its smooth transmission of power. No shaft jacking, no stiffening of the springs under acceleration load, no scrunching when shifting through the gears, no clunking from the rear end in energetic braking maneuvers when the foot brake is used as well.

The exemplary manners of the secondary drive are helped out by the elaborate design of the drive shaft. At the rear end, it is fitted with a constant-velocity joint (instead of the usual universal joint) to transmit rotary motion consistently and smoothly even at large deflection angles. A plunging function additionally ensures length compensation as the rear wheel goes through jounce and rebound. The constant-velocity joint, which is about half the size of a Red Bull can, is completely sealed, filled with special grease and requires no maintenance.

Hint: If you enter the term "constant-velocity joint" in Wikipedia on the Internet, you will find animation that clearly visualises the ability to articulate forces smoothly.

Constant velocity joints are widely used in automobile design, for example on drive shafts on front-wheel drive cars. Their use on bikes, on the other hand, is not usual. The Honda VFR1200 is currently the only motorcycle on which this technology is used in the drive shaft area – although it is not the first. As a marginal comment for history buffs let us note: Only the shaft drive on the Van Veen OCR 1000, a Wankel-engined machine produced in Holland in the seventies, was also equipped with a constant-velocity joint.

But let's get back to the drive shaft on the VFR1200F. This component is fitted with the aforesaid constant velocity joint at the rear and with a universal joint at the front. Between them, a rubber damper is vulcanised in a tubular sleeve on the shaft that permits a certain amount of torsion. The drive shaft is meshed to the pinion that transmits the power to a ring gear and finally to the rear wheel mount. Three more damping elements can be found in the engine in addition to the rubber damper on the drive shaft. So the VFR1200F is equipped with a total of four dampers in the drive train, which contributes to its smoothness and the untroubled pleasure of riding with shaft drive.

The final drive housing with ring gear and pinion is designed as a closed system, which is also something new. During operation, the oil in the final drive heats up and expands; in order to equalize pressure in the housing previous Honda shaft drives were fitted with a cast-in vent cap. That is different now. Special shaft seal rings are used on the VFR that can withstand higher pressures. The pressure provided by these seal lips provide reliable sealing without the need to provide extra pressure compensation at the housing. The drive shaft housing could be made smoother and more modern looking on the outside without the vent cap.

It is perhaps also worth noting that ring gear and pinion are shot-peened in the manufacturing process at the Honda Kumamoto factory. As a result, the surface of the material is smoothed again and compressed after milling, being hardened only afterwards. The surfaces are made particularly resistant to wear as a result. The rear axle housing is filled with SAE 80 transmission fluid that is particularly stable under pressure. Because of the special mesh on the ring gear and pinion, very high flank pressures are created on both sides of the gear. The molecules of the hypoid gear oil are not crushed between the metal surfaces so that lubrication remains constant.

The drive shaft of the VFR1200 is visually more compact, the housing of the final drive smaller in diameter and more compressed than is usual. This is also a consequence of the design because the bearing for the rear-wheel carrier that sits internally in the usual construction is now placed outside. The diameter of the ring gear could be reduced accordingly and the pinion moved further inboard. The overall structure ends up saving more space. Note for technology mavens: The tooth flank clearance of the ring gear and pinion can be adjusted axially (by means of adjusting shims), the specification is 0.05 to 0.15 mm.

As a technology showcase, the VFR1200F has a lot to offer: V4 engine with unique cylinder arrangement, Big-Bang ignition sequence, Unicam cylinder heads, six-speed gearbox with slipper clutch, drive-by-wire throttle grip, aluminum chassis, single-sided swing arm, six-piston brake calipers, Combined ABS, layered fairing, the most up-to-date design, high-quality paint, impressive pannier system and and and. And don't forget the optionally available dual clutch transmission.

The drive shaft of the VFR1200F shines as a highlight in its own right as well. An easy-to-maintain rear-wheel drive, contemporary modern design and extremely low-reaction, allowing not only the outstanding experience of the benefits of automatic shifting with interruption of the power flow but which can also instill enthusiasm when changing gears manually. Just to call the drive shaft of a VFR a drive shaft does not do justice to its superb operation. So instead take a respectful look at it at the next opportunity. Or even better – ride it.

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