Had it not been for engine-braking, I might never have been born. One day in the 1920s, my mother and her parents were sleeping in a bus driven by my sometimes overconfident teenaged uncle. He’d let his speed get away from him while descending through the Cascades and had burned his brakes. He was sweating the possibility of not making one of the corners ahead. My grandfather, awakened by the swaying, came forward. Seeing the situation, he said, “Okay, you get on the brakes, I’ll pull the hand brake, and you see if you can get this thing down a gear.”
They pushed and they pulled and my uncle was able to make the downshift, engine buzzing at peak revs. Knowing that the resulting engine-braking now had their speed under control, my grandfather returned to his seat and his sleeping.
Every time you close the throttle, you feel engine-braking slowing your bike much faster than just coasting would do. The reason is that 15–25 percent of an engine’s gross horsepower goes to overcome friction, mostly of pistons and rings. On a high-powered bike that can be 35–50 hp worth of drag, and when you combine that with hard braking that transfers a lot of weight to the front tire, it can unload the rear tire enough to slide or even begin hopping up and down. When I watched MotoGP video from 2002, there was Valentino Rossi, entering corners looking as though his rear tire was sliding out from using a Mick Doohan-style thumb brake acting only on the rear wheel. But there was no thumb brake. What I was seeing was the drag of engine-braking on Rossi’s V-4 Honda RC211V, making his rear tire slide on corner entry.
Particularly if rear-wheel hop results, this disturbance brings the danger of loss of control. In the 1990s, AMA veteran Tom Kipp dealt with engine-braking in the most fundamental way, by just pulling the clutch during corner entry, then reengaging the drive at the throttle-up point. Like working out your income tax on a pad taped to the gas tank, this called for exceptional concentration.
Back in the early 1970s, Phil Read worked very hard with MV Agusta to make its new four-cylinder 500, revving to more than 14,000 rpm, competitive with the fast-improving two-stroke Yamahas. Film shot from a helicopter at Vallelunga in Italy shows the MV and Read accelerating and top-ending better than the Yamaha but losing ground on corner approach. Braking harder with the disc brakes he’d brought to MV, Read had discovered the problems of engine-braking—wheel hop and sliding.
When Honda fielded its 19,000-rpm oval-piston four-stroke NR500, engine-braking was a problem in a big way. Engineers then developed what today is called a slipper clutch: When the engine drove the rear wheel, its torque passed through the entire clutch stack, but when the rear wheel drove the engine, as it does when you close the throttle, a rotary ramp mechanism lifted half the plates. By allowing the clutch to slip before engine-braking could drag or hop the rear tire, the worst of the problem was solved.
The same concept was applied to Honda’s 1,000cc FWS racer that Freddie Spencer and Mike Baldwin rode at Daytona in 1982 (tire trouble made this race a Yamaha benefit). Soon other makers adopted it for racing. Today, slipper/assist clutches have become widespread on stock production bikes.
When MotoGP began in 2002, the combination of potent carbon brakes and big high-compression engines was too much for slipper clutches. Now there was braking instability. Max Biaggi said at the time chassis oscillations developed so fast that you were on the ground before you could respond.
Why? With a typical racing six-speed gearbox and at a given speed, the engine turns about twice as fast in first gear as it does in sixth. That means in order to have smooth engine-braking response, you need a different slipper release setting for every gear. At Motegi, I spoke with Colin Edwards, who said clutch testing with Aprilia’s three-cylinder “Cube” MotoGP bike was never-ending. He’d do a couple of laps to check the latest setting, then pit to tell the engineers, “Yeah, it’s better in turns 3 and 6, but now it’s worse in 1, 4, and 9. Then they’d add a 0.005 shim, send me out, and it would be different again.”
Bottom line? A passive slipper wasn’t enough; an active system of some kind was required.
That active system required that the on-board ECU be able to operate the throttles of at least one or two cylinders. Then, as the rider braked hard on closed throttle and engine-braking threatened to overcome rear tire grip, the computer would throttle up the engine just enough to cancel unwanted engine-braking. During those early years, spectators were surprised to hear some bikes sounding like lawn-mower engines during corner approach. That was the sound of just one cylinder being throttled up in the new torque-canceling role.
When full throttle-by-wire control was later adopted, it simplified engine-braking control because now all the throttles were operated electrically. Production sportbikes now share this system so their riders can select the degree of engine-braking cancellation they prefer. This has become the new normal.
Some propose that such things as engine-braking control are morally wrong because they take aspects of control away from the rider. Let’s think about that. As World War II began, fighter pilots found themselves acting as engine-management systems as well as having to fly the airplane. Propeller pitch, throttle opening, engine rpm, supercharger boost, and impeller drive ratio, plus fuel-air mixture were all his responsibility as well. The American innovation of the constant-speed propeller handled two of these extra variables but, in combat, pilots still had too much to do. German engineers then created the kommandogerat, or “control apparatus,” a hydro-mechanical computer that integrated all functions into a single power lever; move it this way for more power, the other way for less. This made Fw 190 pilots more effective by allowing them to concentrate exclusively on flying the aircraft.
More recently, much technology has gone into increasing a naval aviator’s chances of picking up that third wire when landing on a pitching carrier deck at night. These technologies have made pilots better able to do what they do best—fly. Similar technologies applied to the motorcycle are doing the same for riders, making it easier to concentrate on riding.
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