When players swap from a Japanese joystick to a Korean arcade lever (like the Knee Lever Neo), the single most noticeable adjustment is the tension.
While a Japanese Sanwa JLF relies on a lightweight metallic spring, Korean levers utilize a solid Tension Rubber Grommet (usually made of silicone). This grommet acts as the lever’s resistance mechanism—as you push the shaft, you are physically compressing the rubber, which then snaps back to neutral with high speed the moment you let go.
However, many players make the mistake of assuming there is only “one” style of Korean tension. Depending on the durometer (hardness) rating of your grommet, your stick can go from loose and wrist-friendly to heavily resistant and tournament-bouncing.
If you are upgrade-shopping, here is our deep-dive guide to the ST Tension Rubber Grommet with Bobbin and how to choose the right rating for your playstyle.
Understanding the “ST” Sizing & Shore Scale
Korean tension rubber grommets are measured using the Shore Hardness scale. The numbers (such as 20, 25, 30, 35, and 45) correspond to the silicone’s density.
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Lower numbers (ST-20, ST-25): Mean the silicone is softer and more pliable. The lever will require less wrist force to move, though the snap-back to neutral will be slightly slower.
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Higher numbers (ST-35, ST-45): Mean the silicone is denser and stiffer. Bending the rubber requires significant effort, but it snap-returns to neutral with aggressive, hair-trigger speed.
Choosing Your Tension Rubber Grommet Rating
Which grommet should you select for your next tournament?
1. The Japanese transition: ST-20 & ST-25
Transitioning from standard spring-based Japanese joysticks is a big leap. If you go straight to a heavy Korean rubber, your hand will likely tire out within a few matches.
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ST-20: Extremely light, designed to help ease the transition from spring-based hardware.
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ST-25: Highly recommended as the gateway K-lever rating. It retains just enough resistance to keep return times fast while being incredibly easy on the wrist for long sessions.
2. The Tournament standard: ST-30 & ST-35
These represent the default and upgraded medium tiers found inside most out-of-the-box Samducksa and Knee levers.
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ST-30: The universal baseline. If your current Crown 309 Helpme or Knee lever is feeling a little too worn and “mushy,” dropping in a fresh ST-30 returns it to standard performance.
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ST-35: Perfect for 3D players who rely on highly aggressive movement inputs (like the Korean Backdash) and require quick neutral recoveries.
3. The Heavyweights: ST-45
The heavy-tension tier is built for veterans who demand massive resistance.
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ST-45: Offers exceptionally high bounce-back speed. It makes movement input very rapid, but requires a strong grip and is not recommended for anyone prone to hand fatigue.
Why Buy a Grommet Pre-Bundled “With Bobbin”?
A very common frustration for arcade stick modders is trying to separate their existing inner plastic bobbin from an old, tired rubber to reuse it Silicone is notoriously sticky, and you risk tearing the insert or losing the bobbin.
A pre-bundled ST Tension Rubber Grommet with Bobbin solves this problem. Since the inner plastic collar is pre-installed, you simply lift the old assembly out of the lever housing and drop the new one right in.
How to Install Your New Tension Grommet
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Safety First: Unscrew the e-clip from the bottom of your lever’s shaft and lift the shaft/battop up through the top panel.
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Open Case: Remove the four main screws holding your lever housing together.
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Insert the upgrade: Pull out the old rubber and drop your brand-new ST grommet with Bobbin in. Ensure the guide ridges on the rubber’s top face upwards.
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Reseal: Screw the casing back together, replace the shaft, actuator, and snap the e-clip back on!
(Note: While upgrading, if you need to optimize your button reactions too, swapping to genuine Sanwa OBSF-30 30mm Push Buttons is the perfect pairing to go with your fresh lever tuning!)
Get Authentic Tuning Parts in India
Don’t worry about paying high overseas shipping fees and unpredictable import duties on small rubber items. At Game Store India, we carry 100% original Samducksa/Crown components[cite: 1.1.1, 1.2.5] with COD and rapid domestic courier delivery.
👉 Upgrade to the ST Tension Rubber Grommet with Bobbin here!
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