In the history of motorcycling, few engineering ideas have shaped the industry as profoundly as the inline four-cylinder engine. Today it’s a familiar layout — smooth, powerful, reliable. But between the 1930s and 1970s, the concept evolved from niche experimentation to full-scale revolution. This is the story of how the four’s rise redefined performance, changed global manufacturing, and set the stage for modern motorcycles.
Early Experiments: The 1930s Foundation
While V-twins dominated the early decades of motorcycling, a few bold manufacturers began experimenting with inline-four designs. These early efforts were complex, expensive, and decades ahead of their time.
One of the earliest pioneers was FN from Belgium, whose inline-four models like the FN M.70 (1930s) introduced the world to the concept of smooth, vibration-free power. In the U.S., Henderson and ACE built luxurious four-cylinder cruisers that offered unmatched refinement — but the manufacturing cost was enormous, limiting popularity.
European brands such as NSU and BMW began testing high-performance configurations, laying the groundwork for future development. Still, inline-fours remained a rarity, reserved for wealthy buyers or racing departments.
The Post-War Shift: Technology Meets Ambition
After World War II, motorcycle engineering accelerated dramatically. New metallurgy, improved fuel, and better machining allowed designers to push for higher rpm and greater power output — areas where inline-fours excelled. Yet the complexity and price of building four-cylinder engines kept them out of reach for most riders.
Brands like Gilera and MV Agusta built exquisite Grand Prix racing fours that dominated the 1950s and early 1960s. These machines featured dual overhead cams (DOHC), multi-carburetor setups, and speeds previously unimaginable on two wheels.
But these bikes were exotic, crafted in small numbers by engineers obsessed with perfection. No mainstream manufacturer had yet figured out how to bring the benefits of four cylinders to everyday riders.
Engineering the Inline-Four: What Makes It Special?
The inline-four layout offered several key advantages that made it ideal for performance motorcycling:
- Balanced primary and secondary forces: smoother operation and reduced vibration.
- Higher rpm capability: smaller pistons and shorter strokes allowed for higher rev limits.
- Compact width: narrower than a flat-four or V4, ideal for cornering stability.
- Linear power delivery: predictable acceleration and refined throttle response.
These traits would ultimately redefine what riders expected from a motorcycle — but the world had to wait for the right moment.
1969: The Honda CB750 Changes Everything
When Honda unveiled the CB750 Four in 1969, the motorcycle industry entered a new era. Many bikes are called “revolutionary,” but the CB750 truly earned the title.
Key innovations included:
- SOHC inline-four engine producing around 67 hp — an astonishing figure for the price.
- Electric starter — unheard of on a high-performance motorcycle.
- Front disc brake — a first for a mass-produced bike.
- Reliable carburation using a bank of four Keihin carburetors.
- High-volume production with strict quality control.
Honda didn’t just build a powerful motorcycle — they built a machine that ordinary riders could depend on every day. The CB750 was smooth, robust, fast, and affordable. It triggered a global engineering arms race that reshaped motorcycling for decades to come.
The 1970s: The Inline-Four Becomes the Standard
By the mid-1970s, every major manufacturer had joined the four-cylinder revolution. Kawasaki launched the fearsome Z1, Yamaha brought out the XS Eleven, and Suzuki released the GS-series. These machines pushed performance far beyond what twins and singles could offer.
Inline-fours became the backbone of sportbikes, touring machines, and even everyday commuters. Their combination of power, refinement, and reliability proved unbeatable.
How the Four-Cylinder Changed Motorcycling Forever
The rise of the inline-four reshaped almost every corner of the motorcycle world:
- Racing: high-rpm power became standard in Grand Prix and later in superbike competition.
- Engineering: DOHC, multi-valve heads, and advanced cooling systems grew from racing into road bikes.
- Culture: the “UJM” (Universal Japanese Motorcycle) era was born, changing riding demographics and expectations.
- Global manufacturing: Japanese production methods set a new quality benchmark.
From smooth touring to explosive top-end power, the inline-four expanded what motorcycles could be — and who could ride them.
Why This Era Still Matters
The evolution of the four-cylinder engine isn’t just a technical milestone. It represents a shift in philosophy: the idea that performance and reliability should be accessible to every rider, not just a privileged few.
For those who appreciate classic engineering and the heritage of motorcycle innovation, the story of the inline-four is a reminder of a time when bold ideas and fearless experimentation reshaped the world on two wheels.
And even today, whether on a vintage CB750 or a modern superbike, the spirit of that revolution still lives inside every smooth-revving inline-four.