Goodbye, Black Sabbath – The story behind the development of iruka C (Part 2)
The new iruka C, after facing shipping delays due to parts delivery issues and defects in received components, has finally arrived at dealers across Japan.
Here is the press release for the launch.
Following the previous article on how the iruka C was conceived, this time I’ll cover the process behind deciding its specifications.
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When I shared my idea of creating a more affordable 5-speed comfort version of the iruka, our Taiwanese coordinator Magum and President Chen of our manufacturing partner, immediately understood. After all, I’ve been working with Magum for seven years and President Chen for five.
Taiwan is home to an enormous number of bicycle parts manufacturers. We began searching for components that would meet my requirements right away.
First up was the saddle. The original 8-speed model uses a slim design from VELO (left), while the new 5-speed model features a thicker, cushioned type from CIONLLI (right).
Next up are the tires. While the original 8-speed model uses Schwalbe’s 1.25-inch treadless tire “Kojak” (left), the new 5-speed model features a 1.50-inch tire from GMD with a tread pattern (right). The difference in width is obvious when viewed from the front. The thicker the tire, the greater the air volume, which increases deformation and rolling resistance, reducing agility a bit but enhancing shock absorption and stability. Since there were no off-the-shelf 18-inch tires with a tread pattern I liked, we ended up creating a custom tread design for the iruka from the mold up.
The changes to the saddle and tires give the new 5-speed model a slightly lower propulsion efficiency compared to the original 8-speed model. However, it offers improved cushioning against road vibrations and shocks, resulting in a gentler and more comfortable riding experience.
Next up is the handlebar. The original 8-speed model features a compact 490 mm flat bar (left), while the new 5-speed model uses a 520 mm riser bar with a 20 mm upward curve (right). It’s not immediately apparent in the photo below, but the riser bar is also offset 6 degrees to the rear, providing additional comfort. This brings the handle grips closer together, making it easier to maintain a more upright and relaxed riding posture compared to the original 8-speed model.
Lastly, let's talk about the drivetrain. We started by calculating the gear development (GD) —the distance traveled per pedal rotation—based on the gear ratios, and then considered the optimal combination of chainring and sprocket. Here are the gear ratios for the Alfine 8 used in the original 8-speed model and the Nexus Inter 5 used in the new 5-speed model:
Alfine 8:
1st gear: 0.527
2nd gear: 0.644
3rd gear: 0.748
4th gear: 0.851
5th gear: 1.000
6th gear: 1.223
7th gear: 1.419
8th gear: 1.615
Nexus Inter 5:
1st gear: 1.000
2nd gear: 1.277
3rd gear: 1.622
4th gear: 2.070
5th gear: 2.630
Anyone familiar with drivetrains—not just in bicycles—might find the Nexus Inter 5’s ratios a bit surprising. Even the lowest gear has a 1:1 ratio, meaning this hub does not offer any reduction gearing. That’s because it is designed with the assumption that electric assist will provide additional torque at low speeds. Since the iruka is not an electric bike, we needed to select a chainring and sprocket combination with a smaller difference in tooth count. After evaluating several options, we decided on a 40T chainring paired with a 22T sprocket. Here are the resulting developments:
Original 8-speed model:
1st gear: 2.2m
2nd gear: 2.7m
3rd gear: 3.1m
4th gear: 3.5m
5th gear: 4.1m
6th gear: 5.1m
7th gear: 5.9m
8th gear: 6.7m
New 5-speed model:
1st gear: 2.4m
2nd gear: 3.1m
3rd gear: 4.0m
4th gear: 5.0m
5th gear: 6.4m
As a result, the chainring on the new 5-speed model is approximately 30% smaller than that of the original 8-speed model—40T (right) compared to 56T (left)—giving the side profile of the bike a distinctly different look.
Even before coming up with the idea for the new 5-speed model, I had been planning to add some vibrant new colors to the next batch of iruka bikes.
The iruka frame isn’t painted. Instead, like Apple’s MacBook and iMac, it uses an anodizing process. The aluminum frame is immersed in an electrolyte solution and an electric current is applied, creating a porous oxide layer on the surface. Dye is then absorbed into these microscopic pores. The result is the familiar anodized finish, which is both durable and colorful. For reference, the anodizing process creates billions of tiny pores per square centimeter on the iruka frame’s surface—a truly microscopic world.
To choose the new colors, we exchanged anodized aluminum pipe samples with our partners in Taiwan. Although flat color swatches are also available, using curved pipes for samples gives a much more accurate sense of how the colors will look on the frame.
As a result, we decided on the following five colors:
Silver, the signature color since the launch, will of course remain.
Storm Gray will continue as well, but with a darker tone than before.
Black is also staying. Initially, I wasn’t very enthusiastic about it, but it was strongly recommended by our international distributors. When we finally produced it, it sold out immediately—proving my lack of taste. This time, we’ll increase production.
The new Blue color. This is essentially our brand color, as it’s also used in the iruka logo.
The other new color is Red. Previously a custom color for Indonesia, it’s now being promoted to a standard color.
Now, about the name. At first, there was no official name, and between us, we simply called it “New version” or “Alternative.” However, as development progressed, I found my mind naturally settling on a single name:
iruka C. The “C” stands for Comfort.
At the same time, I decided to rename the original 8-speed model to iruka S, where the “S” stands for Sport. Each model will feature a model identification decal on the front of the seat tube, as shown below.
This change also helps us escape what I call the “Listen to the song ‘Black Sabbath’ from the album ‘Black Sabbath’ of Black Sabbath’” situation, where our company name, brand name, and model name were all the same: “iruka’s iruka called iruka.” Now, with the models named iruka S and iruka C, it feels more like “You just listened to ‘Wild Anthem’ by Anthem from their album ‘Anthem’.”
Goodbye, Black Sabbath.
Masaki Mark Kobayashi
Founder and President, iruka Inc.