Going Modular | Shop Talk

Tech Matters

In 1948, British motorcycle engineer Bert Hopwood proposed building a new line of motorcycles based on a “modular” engine design that used shared components to create several different power plants. He reasoned that by using common architecture and interchangeable components, development time and manufacturing costs could be reduced, which in turn would enhance profits—which were in rather short supply at the time.

Hopwood’s brief described a 125cc single, a 250cc twin and a 500cc triple, all built around a common bore and stroke. When he presented his ideas to his employer, Norton Motorcycles, they dismissed them, and shortly thereafter canned him. Twenty-five years later, he suggested a similar idea to BSA, and they likewise rejected it. Given the eventual outcome of both companies, they might have been better served listening to him.

The question is: Would it have worked? It certainly does for some manufacturers, especially the present Triumph concern, which in a previous iteration was owned by Hopwood’s former employer, BSA.

Overall, the concept makes a lot of sense. Once an engine’s bore and stroke is established, it’s fairly easy to build horsepower by adding cylinders and changing a few key components. This gives you a lot of latitude in designing your engines and keeps costs more manageable than they would be if you started with a clean sheet.

Let’s pretend we’re going to build a start-up motorcycle company, and we’re going to offer a variety of models, including an entry-level bike, a cruiser, a sport bike and a luxo-tourer. The key question is how to power them.

We could design a basic, one size-fits-all engine, and tune it, or vary its displacement slightly to suit the various applications. Unfortunately, it doesn’t always work; for example, a 750cc V-twin sleeved down to create a 350cc dirt bike motor is simply a bad plan.

Rather than create a single engine to power a number of different chassis, let’s give each bike its own power plant. 500cc is a nice round displacement, so that’s what we’ll make the single cylinder engine in our entry-level bike. If we double that, we get a 1000cc twin, which is a nice size for a cruiser, and we can arrange the cylinders in either the traditional V, or as a parallel—or even flat design. When we triple it, we arrive at 1500cc, which would make an interesting across-the-frame triple. Quadrupled, that gives us a 2-liter four, which could be arranged as a V-four, a flat, pancake-style motor, or in an across-the-frame UJM style.

What’s interesting is that each engine, different as it might appear externally, would share a majority of internal components: the rods, main bearings, pistons and rings would be common to all the motors. Furthermore, there’s no reason why you couldn’t incorporate water cooling on the larger engines, and leave the smaller ones air-cooled, or use fuel injection on certain models and carburetors on others.

As a point of interest, Detroit Diesel is a proponent of the modular concept. Their 71 series of engines were two-stroke diesels that began with the 10 horsepower 1-71 (a single cylinder engine that displaced 71ci), and culminated in the 24V-71 (24, 71ci cylinders arranged in a V). The 71 series were available as two and four-valve motors, with their cylinders inline, arranged as a V, or in some instances, as a flat pancake-style motor, and get this: nearly 70 percent of the parts were interchangeable. So, the modular concept is viable.

At this point I have to assume your question is, “Okay, if the modular concept is the best thing since sliced bread, then why isn’t it used more often?” In the first place, there are some motorcycles that require dedicated engines, with dedicated components. For instance, using a rod from a CRF450 Honda MXer or a Suzuki GSX-R1000 in something that didn’t require that level of sophistication would be expensive, and in reality serve no purpose. Of course, this doesn’t mean components can’t be shared. The bottom line is that some motorcycles demand their own engines and that’s all there is to it.

On a similar note, sometimes going modular just doesn’t pay off. When BMW introduced the /5 series in the early 70s, they offered the same motorcycle in three displacements. Since the bike was designed around a 750cc engine, what they ended up sending to showrooms were a slug of a 500, an underpowered 600 and a really nice 750, all priced within a few hundred bucks of each other. Guess which sold the best?

Lastly, I’d suggest that engineers, and the companies they work for, like to flex their technological muscles, and how better than by building clean sheet designs?

Obviously, old Bertie had some sound ideas; the modular concept works nicely for Triumph, Ducati and KTM, as well as for BMW and Harley-Davidson, and to a lesser degree the Japanese manufacturers. Too bad Norton and BSA didn’t give it a shot.