Evolution of the Sportbike Motorcycle | Cycle World

Evolution That Made Today's Sportbikes Possible

How did we get here?

sportbike evolution illustration

Evolution of the Sportbike

Illustration by Jim Hatch

Today’s 1,000cc sportbikes have more than double the power yet weigh 20 percent less than the well-loved classic sit-up literbikes of the 1970s and early ’80s. With much lighter wheels and tires yet more capable brakes, today’s bikes stop and turn with agility impossible to their forebears. Chassis, swingarms, and forks of improved design provide stiffness essen­tial to stability and hard braking yet are laterally flexible enough to stay hooked up in corners on unsmooth pavements. Most recently, the coming of aerospace-derived electronic rider aids supplement human reactions to better control wheelies and wheelspin regardless of the bike’s angle of lean.

The big change in motorcycle structure has been a switch in the last 10 years to casting methods that provide “near-forged” properties. This has made engine cases, swingarms, and chassis lighter while allowing control of local material thickness. Classic casting technology entrained films of aluminum oxide (which forms instantly on liquid aluminum exposed to air), which later acted as planes of weakness in the mate­rial. The resulting limited strength was compensated for by using thicker, heav­ier sections. Today’s casting molds are often filled from the bottom with low turbulence, allowing aluminum oxide films to rise harmlessly atop the liquid metal—not be mixed into its volume.

Cast chassis and swingarm parts can have complex interior bracing (as on current-model R1) and are designed to reduce expensive assembly welding to a minimum. Racing chassis assembled from machined and press-formed sheet parts looked trick in the 1990s, but their construction was too slow and expensive for production use.

The big air-cooled engines of the Kawa­saki Z1, Suzuki GS1000s, and Honda CB900F had the classic fins-everywhere look older motorcyclists love, but they were heavy, with engine weights well over 200 pounds. Air-cooling limits the highest safe compression ratio, reducing torque. Sure, you can buy 11.0:1 pistons for your Z, but 9.0:1 was about as high as any prudent manufacturer could warranty. And the crack-prone cast aluminum of that era responded poorly to revs. As Rob Muzzy said of his championship-winning 152-hp at 10,250-rpm Superbike engines of 1982, “Things start to go bad in a hurry near eleven [thousand].”

Today’s engines, in World Superbike form, rev to more than 15,000 rpm reliably, allowing them to make almost 50 percent more power than 1982 Super­bikes. With liquid-cooling, pistons survive at compression ratios a third higher than the 9.0:1 or 9.5:1 of the days of yesteryear—numbers that translate directly to increased torque. The cooler pistons run, the longer it takes for fatigue cracks to form. To operate at today’s higher revs, stronger internal parts are essential—forged rather than cast pistons, shot-peened alloy steel or titanium (the fracture-split rods in the YZF-R1) connecting rods rather than the lower-spec steel rods of before. Titanium valves—40 percent lighter than steel—are a key to today’s higher peak rpm, assisted by valve springs made from ultra-clean, super-fatigue-resistant Japanese wire. On his way back from victory circle at Daytona one year in the 1980s, Fujio Yoshimura stopped to say to me, “This is the first time this race has been won using Japanese valve springs.”

Compare modern and classic parts

The rear unsprung weight of a CB900F was 68 pounds, a huge 14 percent of vehicle weight. The Z1 front wheel with its two hefty (7mm thick!) brake discs was 50 pounds. Thin tubes are basic to lightweight structure because all their material is far from their centerlines, giving maximum leverage against twisting or bending. Classic 1970s to ’80s bikes had 36 and 37mm fork tubes but upper tubes today are 50mm and up: They have to be able to handle increased braking loads from today’s sticky tires. Thirty-five years ago frames were made of steel tubes of 1-1/8-inch diameter; today they are aluminum, many times that diameter, and flattened to combine the bending resistance to handle braking with the lateral flexibility that keeps bikes hooked up in corners. The same is true of swingarms: Welded-together tubing is replaced by light, highly twist-resis­tant box structures, their beams flattened for the same reason main frame beams are.

A suspension revolution was in progress in 1980, as fork dampers with fixed-size damping orifices, as crude as door-closers, gave way to today’s washer-stack-controlled damping, which keeps damping force proportional to damper-rod velocity. Today’s rear units separate damper oil from the gas pressure that keeps it from cavitating (low pressure pulling the oil apart), but in 1978 at Imola I watched Bud Aksland pour the fluid out of Kenny Roberts’ TZ750’s rear suspension unit; it looked like a foamy chocolate shake. Instead of damping, it bounced.

Getting the most press are today’s electronic rider aids

Some old-timers reject electronics as unmanly, but their origin is in the development of digital flight controls by NASA during Apollo. They increase safety and performance by handling problems for which human reactions are inadequate. From NASA they went to military and then commercial aircraft, to Formula 1, production autos, and then from MotoGP to production motorcycles. In general, electronic aids make motorcycle behavior smoother and more predictable and at their best are almost imperceptible in their operation.

Least talked about because least under­stood are tires. Hugely expanded capabilities extend the options available to riders. In the old days, people spoke admiringly of “cornering as if on rails,” but today’s tires allow us to change line while in corners, giving riders more control than ever. How long before lean angles increase to drag shoulders?

As each piece of the system evolved, other elements had to catch up. We enjoy the fruits of this and ponder where the high-performance motorcycle will go next.