Underbone Vs. Backbone Motorcycles

If you’ve been to or if you live in South East Asia, you’ve no doubt seen a special type of motorcycle known as “underbones.” But what makes them so unique? And what sets them apart from “underbone” motorcycles? In this blog post, you’ll discover:

Underbone vs. backbone motorcycles – definitions & differences
What sets an underbone motorcycle apart from a scooter

Prefer to get this information in podcast form? Listen to the 30 Minute Motorcycling Podcast episode about underbone motorcycles:

Quicklinks:

What’s an Underbone Motorcycle?
- Underbone motorcycle vs. Scooters
What’s a Backbone Motorcycle?

What’s an Underbone Motorcycle?

The frame for an underbone motorcycle consists of structural tubes and body panels often made from either plastic – a sturdy design that can fit several people. It’s no wonder why these bikes are common in Southeast Asian countries, where it’s not rare to see 3 or more people riding together on a single two-wheeler (something I call “riding Vietnam-style”). Some other names for underbones include:

“Motor bebek” (Indonesia)
“Kap Cai” (Malaysia & Singapore)
“Papi/Papaki” (Greece)

The transmission of underbones is also interesting. Often, they have semi-automatic gearboxes, where you still shift through the gears with a regular floor shifter, even though there’s no clutch lever. The engine is positioned between the rider’s feet while the fuel tank is under the seat (since underbones have no backbone frame where the engine is usually mounted). As an example, take a look at the 2004 Suzuki FX125 pictured below without covers:

Original image by Spinnanz at English Wikipedia, CC BY-SA 3.0, via Wikimedia Commons

However, despite the difference in design, the drivetrain is almost identical to a regular chain-drive motorcycle. The Honda Super Cub (AKA The Honda 50) is probably the most well-known underbone motorcycle. Some other examples of underbones include:

Honda Wave
SYM VF3i
MCX Raptor (Philipines)

Underbone motorcycles vs. Scooters

In Southeast Asia, “underbone” is synonymous with scooters. In Malaysia, there’s even a national racing league for underbones called the “Malaysian Cub Prix.” With that in mind, there are some differences between the two. For example, scooters tend to have their engines near the rear wheel, unlike underbones. Underbones also have bigger wheels than scooters, so their engines are mounted where they are. But the most significant difference is that underbone motorcycles use semi-automatic gearboxes, while scooters generally rely on a CVT transmission for that “twist and go” feel.

Read more about the CVT transmission in the blog post below:

CVT Transmission - What Is It?

Underbone Motorcycles vs. Scooters – More Info:

“Scooters vs Underbones – What’s the difference?” – Motodeal.com.ph

In a mechanical brake system, there’s no liquid whatsoever.

Instead, power is delivered through a metal cable that tensions as you squeeze the brake lever - similar to how many motorcycle clutches work.

Although not so much on modern motorcycles, these brakes are highly common on bicycles and e-bikes.

What’s a Backbone Motorcycle?

The most significant telltale sign of a backbone motorcycle is its frame. As the name implies, backbone motorcycles have frames that act as their “spines” and hold all crucial components, such as the engine. Therefore, a backbone frame is easy to build and is cheaper than an underbone frame. But there’s a catch. While backbone frames are cheaper to build, they’re not as robust. Because of this are, therefore, mainly found on cheap and small displacement motorcycles.