Hondas Better Braking System

HONDAS BETTER BRAKING SYSTEM

IS THERE LBS IN YOUR FUTURE?

KEVIN CAMERON

BRAKE! YOU GET ON THE BINDERS FOR AN EMERGENCY stop. Fortune favors the experienced rider, who instinctively and from long practice balances his braking effort correctly between front and rear brakes, according to the load each wheel is carrying, and gauged to available traction.

Separate brakes permit a full range of front/rear braking ratios, however, from the panicked novice’s 100 percent rear to the roadracer’s 100 percent front. Neither of these extremes gives maximum stopping power on the street. Rear brake only gives about 25 percent of maximum stopping power; front only may work on the racetrack, under ideal conditions, but this option, too, delivers less than 100 percent of max stopping power on a street motorcycle. Correct division of braking effort requires a sensitive, educated response located somewhere between these extremes. Could there be a way to narrow the range of possible choices, to rule out the extremes?

Anti-lock brakes (ABS) solve a different problem: how to automatically detect and prevent wheel locking. They do nothing to prevent novice users from braking with the rear wheel only. ABS does enhance safety by avoiding wheel

lockup, but only at some penalty in cost and complexity. The price of ABS is, for the moment, not for all riders.

Therefore the question remains; what can be done to ensure that all riders can brake safely and powerfully, using simple technologies with little impact on price?

Honda has developed a system that purports to answer that question. The new CBR1000F sportbike introduces what the company calls its Linked Brake System. LBS applies both brakes to some degree whether hand lever and foot pedal are used together or separately. Two discs are fitted at the front, each gripped by a three-piston caliper. A single disc is used at the rear, gripped by a single three-piston caliper.

When the rider pulls the lever, fluid pressure from the master cylinder activates the two outer pistons in each front caliper. While the right front caliper is bolted conventionally to the fork leg, the left caliper is mounted on a mechanical linkage. As brake torque develops at the left caliper, this linkage applies the reaction force to a second, or servo, master cylinder. Its fluid output is routed through a special proportioning valve, to the outer pistons on the rear caliper. Thus, applying the hand lever alone delivers twothirds of the maximum possible front-brake force, and also delivers a smaller but proportioned force to two-thirds of the rear caliper.

If the rider applies the foot pedal alone, fluid is sent directly to the center pistons of all calipers, and through the left front caliper and the servo master cylinder it is linked to, fluid is also sent indirectly to the outer pistons of the rear caliper.

In neither case is the rider able to commit the classic panic error of applying only the rear brake; going for the pedal alone splits the braking effect, with three calipers working to give a decent but not maximal rate of deceleration, while reducing the chance of an outright rear-wheel lockup.

The special proportioning valve is the heart of the system; it is a non-electronic device made up of passive valves, pistons and springs. Its job is to adjust rear-brake torque to be proportional to front-brake torque, except when braking becomes so heavy that significant forward weight transfer makes rear-wheel lockup more likely. Therefore as frontbrake torque rises, the valve at first increases rear-brake torque at a high rate. At higher front-brake torques, it shifts to a lower rate of increase. At still higher deceleration rates, a “cut valve” holds rear-brake torque constant. Finally, at rates of deceleration so rapid that the rear wheel is significantly unloaded, a decompressor valve actually reduces rear-brake torque by progressively pulling fluid from the rear caliper-line.

While a rider determined to lock wheels can do so with LBS, the system does eliminate the most hazardous extreme choices in front/rear brake proportioning. It also non-electronically and cost-effectively eases the difficult problem of obtaining maximum, correctly proportioned brake action. □