Hyosung RX 125 D SM [2007-2010]: A Supermoto for the Urban Adventurer

Introduction

The Hyosung RX 125 D SM is a motorcycle that defies easy categorization. Part streetfighter, part scrambler, and fully committed to the supermoto ethos, this 125cc machine carved out a niche for riders seeking affordability, versatility, and a dash of rebellious style. Produced between 2007 and 2010, it’s a bike that feels at home threading through city traffic, hopping curbs, or exploring gravel backroads. But does it hold up as a practical daily rider? Let’s dive into the details.

---

Design & Ergonomics: Function Meets Aggression

The RX 125 D SM’s design is unapologetically bold. With its high-mounted fender, slim bodywork, and upright riding position, it screams "supermoto" from every angle. The 33.5-inch (850mm) seat height might intimidate shorter riders, but it pays off with commanding visibility in traffic. Taller riders will appreciate the knee-friendly ergonomics, though the narrow seat can feel unforgiving on longer rides.

Hyosung opted for a steel frame that balances durability with a manageable 280-lb (127kg) dry weight. The 17-inch cast wheels (110/70 front, 130/70 rear) wrapped in street-oriented tires hint at its asphalt bias, while the long-travel suspension (250mm front, 200mm rear) suggests off-road capability. The real standout is the inverted front fork—a rarity in this class—giving the bike a premium look and improved damping over bumps.

---

Engine & Performance: Small Displacement, Big Personality

At its heart lies a 124cc, air/oil-cooled single-cylinder engine with a DOHC 4-valve setup. The carbureted mill produces 12–13 hp (8.9–9.5 kW) at 8,500–10,000 rpm, with torque peaking at 9.9 Nm (7.3 lb-ft). These numbers won’t set records, but the engine’s character is surprisingly lively.

On the Road:
- Below 6,000 rpm, the RX 125 D SM feels docile, even sluggish. But keep the throttle pinned, and the engine wakes up around 7,500 rpm, pulling eagerly to its 10,000 rpm redline.
- The 5-speed transmission shifts with a satisfying click, though finding neutral at stops can be finicky.
- Top speed hovers around 65–68 mph (105–110 km/h), making it ideal for urban commutes but strained on highways.

Fuel Efficiency:
Hyosung claims a frugal 58–65 mpg (4.0–3.6 L/100km), which translates to 120–140 miles (193–225 km) from its 2.1-gallon (8L) tank. Perfect for city runs, less so for touring.

---

Handling: Where This Bike Shines

The RX 125 D SM’s lightweight chassis and supermoto geometry make it a riot in tight spaces.

City Performance:
- Flickable steering lets you dodge potholes and split lanes effortlessly.
- The upright riding position provides excellent visibility, though windblast becomes noticeable above 50 mph (80 km/h).

Off-Road Manners:
Don’t expect Dakar-level prowess, but the long suspension travel soaks up curbs and gravel roads with ease. The 10.8-inch (275mm) ground clearance avoids scrapes on uneven terrain, though the street tires limit traction on loose surfaces.

Braking:
The single 240mm front disc (with a 2-piston caliper) offers strong initial bite, while the rear drum brake feels adequate but lacks modulation. Upgrading to sintered pads is a wise move for aggressive riders.

---

Competition: How It Stacks Up

The 125cc supermoto segment has always been competitive. Here’s how the Hyosung compares:

| Model | Hyosung RX 125 D SM | Yamaha WR125X | Aprilia SX 125 |
|--------------------|------------------------------|-----------------------------|-----------------------------|
| Engine | 124cc air/oil-cooled SOHC | 124cc liquid-cooled SOHC | 125cc liquid-cooled 2-stroke|
| Power | 13 hp | 15 hp | 22 hp |
| Weight | 280 lbs (127kg) | 265 lbs (120kg) | 242 lbs (110kg) |
| Key Strength | Low maintenance | Refined performance | Lightweight & aggressive |

Yamaha WR125X: More polished and powerful, but pricier. The WR125X’s liquid-cooled engine thrives at higher RPMs, but its parts cost more.
Aprilia SX 125: A 2-stroke beast with nearly double the power, but demanding maintenance and fuel mixing.

The Hyosung’s edge? Simplicity. Its carbureted engine and steel frame are easier (and cheaper) to repair than fuel-injected rivals. For new riders or budget-conscious buyers, it’s a compelling choice.

---

Maintenance: Keeping Your RX 125 D SM Alive

1. Carburetor Care
The Sung Lim PD carb is reliable but sensitive to ethanol-blended fuels. Clean it annually and consider a rebuild kit after 10,000 miles (16,000km). Pro tip: Install an inline fuel filter to prevent clogs.

2. Oil Changes
Hyosung recommends fresh 10W-40 oil every 1,800 miles (3,000km). Don’t skip this—the oil-cooled engine relies on frequent changes.

3. Chain Maintenance
The O-ring chain needs lubrication every 300 miles (500km). Swap it for an X-ring chain (like DID’s 520VX2) for longer life.

4. Brake Upgrades
- Front: EBC HH sintered pads improve stopping power.
- Rear: Converting to a disc brake (using parts from the RX125D-E) costs ~$200 and adds control.

5. Suspension Tuning
The stock suspension is soft for aggressive riding. Stiffer fork oil (15W) and a preload-adjustable rear shock transform handling.

MOTOPARTS.store Recommendations:
- High-flow air filters for better throttle response.
- LED headlight kits for night visibility.
- Aluminum skid plates for off-road protection.

---

Conclusion: A Underdog Worth Considering

The Hyosung RX 125 D SM isn’t perfect. The seat could be softer, the mirrors vibrate annoyingly, and the exhaust note is more lawnmower than symphony. But it’s a bike that grows on you. For urban riders who value low costs, easy repairs, and the occasional dirt-road detour, it’s a scrappy underdog that punches above its weight.

While newer bikes offer flashier tech, the RX 125 D SM remains a testament to no-frills motorcycling—a canvas waiting for your upgrades. Whether you’re bolting on crash bars or tuning the carb for a bit more pep, this Hyosung invites you to make it your own.

Ready to personalize your ride? Explore our catalog of aftermarket parts at MOTOPARTS.store—where every adventure begins with the right upgrade.

---

Specifications sheet

---