For a while, I was the guy who said, "Mobilon bands are fine." The popular opinion in the retro-gaming scene was that they're low quality and wear out too easily, but I knew that wasn't an accurate complaint.
The primary problem is, unsurprisingly, the user. Mobilon 90mm elastic bands have been used as belt replacements in the Famicom Disk System (FDS) for decades, and that longevity by itself makes it seem like a reliable option.
But Mobilon bands do not a proper drive belt make, and it's not because of the usual complaints. During installation, it's incredibly common to stretch these bands too far. The material's elastic deformation profile isn't the same as true rubber, and it catches most enthusiastic installers by surprise. And as it turns out, over-stretching isn't the only issue.
However, the deeper issue is that Mobilon bands have a number of structural characteristics working against them. To start, the most commonly available replacements are sold with the wrong fold length. Fold length is the vital metric used to determine the native size of the loop and how firmly it will tension onto the motor and spindle pulleys.
Most importantly, the rigidity of these bands is completely wrong. In fact, I would extend this criticism to all of the aftermarket options available today. They are simultaneously too small and too rigid.
In fact, you can fact check me! let's look together:
The motor pulley, spindle pulley, and distance from centers of rotation are FIXED. As in, they do not change.
Therefor, the following established formula can be used to calculate the belt circumference
Plugging in our fixed hardware dimensions yields a total operational circumference of 244.90 mm. Because a belt is measured flat, the required installed fold length is exactly half of that: 122.45 mm.
Wait! Rubber stretches, and we want it to. Otherwise, our belt will just flail loosely around the spindle pulley. Rubber drive belts are widely understood components, and in this application, the physics are directly comparable to a cassette tape player.
With this information, we can make an informed judgment that a brand-new belt should be between 5% and 10% smaller than the physical path circumference to maintain proper tension.
Let's apply that data:
This is why Shore Hardness and elasticity are critical considerations for rubber components.
Rubber comes in many different durometers. You can't just buy a literal brown office rubber band, slap it into a Famicom Disk Drive, and call it a job well done. Even with true rubber, the hardness and the material's Young's Modulus (its resistance to elastic deformation) are crucial factors to ensure the band stretches safely, wears correctly, and applies the exact intended load onto the delicate motor bushings.
We've all heard the collective moans about how notoriously unreliable the FDS is, and maybe you've been guilty of complaining about it too. But I wouldn't call that entirely fair when we consider that the majority of our historical maintenance and repair documentation is based on "close enough."
If you're serious about preserving your Famicom Disk Drive and you don't mind paying a little extra for a part built to the exact physical specs of the hardware, you're going to see a massive jump in reliability—and you won't have to open up your drive to replace a melted or snapped band nearly as often.
The primary problem is, unsurprisingly, the user. Mobilon 90mm elastic bands have been used as belt replacements in the Famicom Disk System (FDS) for decades, and that longevity by itself makes it seem like a reliable option.
But Mobilon bands do not a proper drive belt make, and it's not because of the usual complaints. During installation, it's incredibly common to stretch these bands too far. The material's elastic deformation profile isn't the same as true rubber, and it catches most enthusiastic installers by surprise. And as it turns out, over-stretching isn't the only issue.
Better Luck Next Time
Many amateur installers have nasty, dirty little fingers. What I mean to say is that they have grease on their hands from handling the internal teardown of the Famicom Disk Drive mechanism. This finger grease is all-too-easily transferred to the spindle pulley or to the band itself, causing immediate slipping.However, the deeper issue is that Mobilon bands have a number of structural characteristics working against them. To start, the most commonly available replacements are sold with the wrong fold length. Fold length is the vital metric used to determine the native size of the loop and how firmly it will tension onto the motor and spindle pulleys.
Most importantly, the rigidity of these bands is completely wrong. In fact, I would extend this criticism to all of the aftermarket options available today. They are simultaneously too small and too rigid.
Danger, Science!
You might be asking how I can be so confident. The answer? Science. Okay, not just science... I also happened to source original, intact OEM Nintendo bands along the way, which gave me some invaluable clues. But still... science! Let me explain.In fact, you can fact check me! let's look together:
The motor pulley, spindle pulley, and distance from centers of rotation are FIXED. As in, they do not change.
- Motor Pulley Diameter (d): 7.85 mm ⌀
- Spindle Pulley Diameter (D): 45.00 mm ⌀
- Center-to-Center Distance (C): 78.75 mm ⌀
Therefor, the following established formula can be used to calculate the belt circumference
L = 2C + pi/2(D + d) + (D - d)^2 / 4C
Plugging in our fixed hardware dimensions yields a total operational circumference of 244.90 mm. Because a belt is measured flat, the required installed fold length is exactly half of that: 122.45 mm.
Wait! Rubber stretches, and we want it to. Otherwise, our belt will just flail loosely around the spindle pulley. Rubber drive belts are widely understood components, and in this application, the physics are directly comparable to a cassette tape player.
With this information, we can make an informed judgment that a brand-new belt should be between 5% and 10% smaller than the physical path circumference to maintain proper tension.
Let's apply that data:
- Reducing the 244.90 mm circumference by a conservative 5% gives us 232.66 mm.
- Dividing that in half gives us an optimal manufactured native fold length of 116.33 mm.
More Science!?
There's more to this than just pure belt geometry; there's material science as well. The biggest issue with Mobilon belts, if we ignore the fact that the commonly recommended size is physically incorrect, is that the risk of over-stretching is too high in inexperienced hands.This is why Shore Hardness and elasticity are critical considerations for rubber components.
Rubber comes in many different durometers. You can't just buy a literal brown office rubber band, slap it into a Famicom Disk Drive, and call it a job well done. Even with true rubber, the hardness and the material's Young's Modulus (its resistance to elastic deformation) are crucial factors to ensure the band stretches safely, wears correctly, and applies the exact intended load onto the delicate motor bushings.
Wrapping Up
Where did the Mobilon bands and other generic rubber belt replacements go wrong? Simple: an absence of science. The accepted belt length in the hobbyist community has been a hand-wavy "close enough" for decades. Does that sound like a familiar trend with the Famicom Disk System?We've all heard the collective moans about how notoriously unreliable the FDS is, and maybe you've been guilty of complaining about it too. But I wouldn't call that entirely fair when we consider that the majority of our historical maintenance and repair documentation is based on "close enough."
If you're serious about preserving your Famicom Disk Drive and you don't mind paying a little extra for a part built to the exact physical specs of the hardware, you're going to see a massive jump in reliability—and you won't have to open up your drive to replace a melted or snapped band nearly as often.
