Where Did the Weight Go?
WHERE DID THE WEIGHT GO?
IGNITION
TDC
FOUR-STROKE SPORTBIKES HAVE LOST A LOT OF WEIGHT. HERE'S HOW.
KEVIN CAMERON
Back in 1980, the AMA decided to put 1,025cc "Formula 1" four-strokes into what had been an exclusively 750 two-stroke field. But both the production-based “Superbikes with fairings” and Honda’s purpose-built FWS V-4 tore up tires because of their new combination of great weight and power.
It was common for engines alone to weigh 230 pounds!
Since those distant days, four-stroke motorcycle engines have lost a lot of weight, first through the year-by-year refinement of production sportbikes and later intensively within MotoGP. Where was that weight, and where did it go?
Let’s look at crankshafts first. Both Kawasaki’s Zi and Suzuki’s GS-series four-stroke inline-fours had traditional roller cranks that were press-fit together to allow use of robust non-split roller rods and main bearings. The pieces must not only be strong enough to handle applied combustion and inertia loads, but they need added material around each press-fit to keep it tight. When I weighed such a crank from a Kawasaki, it was 32 pounds, and others at 36 were not unheard of. Forged cranks weigh just a bit more than half as much. As pistons ceased to resemble buckets and instead morphed into ashtrays, their lighter weight (thinner domes were made possible by oil-jet cooling) could be handled by more slender rods, and all that back-and-forthing metal-in-motion could be nicely balanced by lighter crank counterweights.
Consider crankshaft main bearings, which were generally hefty ball bearings of large diameter on the press-assembled cranks. The minimum stack of five of those contributed a lot to that 32to 36-pound total, but forged one-piece cranks turn on insert shell plain bearings that weigh only a few grams for each pair. Camshafts that had been solid steel became lighter and stiffer when made tubular. (Stiffness of tubes rises steeply with diameter, while the metal in the middle is largely wasted because it has too little leverage over applied loads to add much stiffness. The same is true of axles.)
Now think of cylinder heads. I once compared the weights of a Suzuki four-valve GS750 air-cooled head and a Yamaha five-valve FZR750 water-cooled head; the air-cooled part weighed about twice as much! Why? First, an air-cooled (aluminum) head may operate as hot as 500 degrees Fahrenheit briefly, while the water-cooled equivalent is held in the range of 150 to 200 degrees. To keep its pressed-in hard valve seats in place, the air-cooled head needs a substantial mass of aluminum around them to rapidly conduct heat away. In the case of four or more valves, there is the alwaysdifficult problem of cooling the “exhaust bridge”—the narrow bit of head material between the two exhaust valve seats. Surrounded by very hot action (exhaust gas at sonic speed transfers heat to metal like gangbusters—ideal conditions for heat flow), this bit of metal can become so overheated that its heat expansion permanently compresses the cooler metal around it. Then when the engine is shut down and the head cools, the bridge is effectively too small, throwing it into tension. It doesn’t take too many of such heat cycles before the bridge cracks, possibly allowing the exhaust seats to become loose. To slow down this process, air-cooled four-valve heads are usually designed with slightly undersized exhaust valves so they can be located farther apart in the head, allowing a thicker, more heat-conductive exhaust bridge to be provided between them. More metal equals more weight.
Something all engines need is a degree of emergency heat storage. In the case of an air-cooled engine, this is provided as a lump of extra metal atop each combustion chamber. Its purpose is to store and distribute the heat of full-throttle operation during the time it takes to establish greater heat flow outward along the cooling fins. In a water-cooled engine, the great heat capacity of water (it weighs only a third as much as an equal volume of aluminum, but it has four or five times the heat capacity per gram at the temperature of an aircooled head) takes care of this at much lower weight.
BY THE NUMBERS
4 Number of dead typewriters in my life
Number of unresponsive hard drives in my permanent collection
7 Number of heavy, old-tech CRT monitors I finally discarded last year
Okay, the radiator, water, water pump, and hoses do weigh something—but not much compared to lumps of aluminum.
Now think about how aluminum casting has changed.
In 1980, the crankcases of those Formula 1 i,025cc four-strokes were aluminum die-castings, notorious for being difficult or impossible to weld or to heattreat (making hot gases in the metal to expand, pushing apart layers of metal). Gas bubbles and other inclusions limited strength, requiring greater thicknesses to be provided. Weighty matters!
Then, around 1990, Yamaha, followed later by other manufacturers (Ducati most recently), adopted new die-cast technology that greatly increases part strength and fatigue resistance. Because large and complex parts could now be cast with nearforged properties, entire chassis currently cast in as few as three to four elements, then welded together. These methods saved to 35 pounds.
Adding weight to a vehicle requires correspondingly more weight elsewhere—heavier fittings, more fuel, heftier parts. Suzuki and Ducati have been leaders in the reverse process— reducing weight of major parts and thereby allowing similar reductions in most other areas.
Smaller parts whose weights have been cut in this way include chains, fasteners, wheels, brake discs, and calipers. All because lighter engines no longer required the more robust accessories that their stouter i98os-era forebears did.
Steel chassis had the fatigue resistance to survive the pounding of paint-shaker parallel twins. But once vibes were calmed by more cylinders and/or balance shafts, much lighter chassis of crack-sensitive aluminum became possible. We romanticize the mighty sit-up Superbikes of the late ’70s and early ’80s, but we wouldn’t enjoy carrying one of those chassis very far, complete with all the added tubes, plates, and gussets necessary for racetrack stability.
In design offices worldwide, weight reduction is neverending.
I ONCE COMPARED THE WEIGHTS OF A SUZUKI FOUR-VALVE GS750 AIR-COOLED HEAD AND A YAMAHA FIVE-VALVE FZR750 WATERCOOLED HEAD; THE AIR-COOLED PART WEIGHED ABOUT TWICE AS MUCH!
