Is There a Best Displacement for Motorcycles?

[Hugh Janus]

Kevin Cameron (Robert Martin/)

This is clearly a stupid question. Because people use motorcycles for so many different purposes, this is like asking which  fighter aircraft of World War II was the best. Best defensive interceptor? Best bomber escort? Best carrier fighter? Ground attack? Night? The requirements for each category are so different that they make the question pointless.

Yet in 1972, there was the late professor Gordon Blair, announcing that the ideal displacement for a motorcycle—at least for lap time and at that moment—was somewhere around 375cc.

Why? Because, with Honda winning the Daytona 200 in 1970 with a derivative of its new CB750 four, and a BSA 750 triple taking the 1971 race, it was a shock to see the great race won in 1972 and ’73 by little Yamaha 350 twins with less than half their displacement.

That time the clear cause was tires. The then-new 750 two-strokes from Suzuki and Kawasaki tore up their tires while the 350 twins cruised. Tires continue to define limits today; MotoGP races are won by the best tire management.

It happened again in 1982-83 when Freddie Spencer rode Honda’s lighter, lower-powered three-cylinder NS500 against the might of the four-cylinder rotary-valve Grand Prix establishment. Again, a surprise; he became world champion in 1983 with the least power in the paddock.

As new product was released in the late 1980s, Honda’s CBR1000F boldly pushed on toward 600 pounds. But the emerging sportbike market was seized by Suzuki’s revolutionary GSX-R750, which was, in round figures, 100 pounds lighter than its apparent competitors. This would not be the last time the market was seized by making a motorcycle lighter, less bulky, and easier to ride; think of the 1998 intro of the 1,000cc Yamaha R1, which was hailed as “feeling like a 250.”

How much power can we use? Through the sportbike era the cry had been “Too much is just enough!” But today, we have to ask: What’s wrong with the KTM 890 parallel twin’s 120 hp in a (claimed) 400-pound package? Let’s do the numbers: The CBR1000F’s 602-pound wet weight plus a 175-pound rider is 777 pounds, which must be thrust forward by 113 rear-wheel horsepower (RWHP) for a power-to-weight of 6.88. Compare to the 890′s maybe 111 RWHP: 405 + 175 = 580/111 = 5.22 lb./hp.

Okay, I’m comparing 1987 with 2020, which is unfair. But the point I’m trying to make is that today’s smaller, lighter bikes are achieving greater overall performance than the admired giants of the past, and seasoning the result with lightness and agility at the same time.

Now let’s factor in Honda’s World Superbike homologation special, the CBR1000RR-R. Weight is maybe 443, wet, and CW’s dyno found 165 RWHP. So 440 plus the 175-pound rider is 615, then divide by 165 to get 3.74 pounds per horsepower. Pretty impressive.

But let’s consider lightness and agility. We know that Dorna’s new electric bike class has run at selected MotoGP events. The spec Energica electric racebikes generate extreme torque and accelerate hard. But perhaps because of their 589-pound weight and its effects on tire durability (to say nothing of agility), their lap times remain slower than those of Moto3, which is powered by (drum roll) single-cylinder 250cc engines making roughly 55 hp.

Do I hear a chorus of angry “Yabbuts”? Yes, certainly, more powerful bikes usually lap more quickly in races when ridden by experienced pro riders. And they are quicker away from the stoplight for anyone with the knack.

What happens when Mr. Checkbook shows up at the trackday with his 214 hp missile? Do others, impressed by his financial achievement, meekly load up and leave? No, the frequent result is the local ace on a banged-up nothing of a bike (he has grind marks on his fork caps) repeatedly passing the Checkbook Special on the brakes, on the outside, over and over until its rider becomes frustrated enough to run off, crash, and do 10 or 15 grand of damage. Multiple riding modes and anti-spin electronics are great, but even in these modern times there is still no Win Mode. That can come only from the rider, whose skill may very well be a replacement for displacement.

How much performance can we, as average riders, actually use? As our experience and skill increase, we may be able to use more. But not always. I’ve seen a fair number of riders who were pretty good on a 125, a bit slower on a 250, and a bunch slower on a 750 or 1000.

There’s another point to consider. As motorcycle manufacturers have tackled tightening emissions limits, they’ve had to shorten cam timing to prevent loss of fresh charge out the exhaust during the overlap period (so-called “hot” cams begin to open the intake valves well before TDC, and the exhausts don’t close until some time after TDC, providing a “window” through which charge loss can take place). As it happens, the shorter the cam timing (giving less valve overlap and lower emissions of unburned hydrocarbons), the broader and easier to ride the powerband becomes. Especially if the manufacturer is compensating for the shorter timing by opening the valves further (higher valve lift). This is more appreciated now that the competitive madness of the sportbike era is behind us. Bikes can now have good performance without need of spiky torque curves.

Here I have to repeat a story I’ve told before. Back when Honda was considering building a light, powerful 900cc four-cylinder sportbike, there was concern that average street riders might find such a bike to be just too much. Accordingly, a numerical “rideability index” was devised and a prototype was built to showcase the qualities the research had identified. As you’d expect, focus groups of average riders loved the bike. What was not expected was that professional riders also went faster on the highly rideable bike than they did on their highly tuned hot rods.

Thus, there is a difference between a bike that’s just frightening and hard to ride and a bike that actually does what you ask it to. Enjoyment of life is the goal, so it’s sensible to seek bikes that make us happy.

Why do we ride motorcycles? Is it just good fun that we can afford? Do we seek a lick of too-muchness, scaring ourselves back to basic sanity? Is it the healing isolation of no phone, no talk, no distractions, and eyes on the horizon? Is there an element of the peacock’s feather display? Do we hope that “When something comes up to me, he don’t even try” (thank you, Brian Wilson)? Does it allow us to feel we have, through the multiplying effect of the motorcycle, “take[n] up arms against a sea of troubles” (thank you, Bill Shakespeare)?

Despite the reams of psychoanalytic tripe that have been written about “the motorcycle,” it’s different for everyone. I’ve seen really large men on extended-and-lowered wheelie-bar-equipped ‘Busas making not-too-graceful U-turns on Route 92 in front of the Daytona Speedway. I saw, at least 25 years ago, four young women laughing and shouting as they struggled with the weight of the Sportsters they were riding through a two-stoplight town in New Hampshire. Blip the throttle as you look over at the banged-up twin next to you. Are there grind marks on its fork caps?

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