Stealth's S4 mountain bike hubs provide instant engagement and silent coasting. For 2016 they've been updated with a better dust seal, and now use a ball bearing under the freehub instead of a needle bearing. The S4 is available with a SRAM XD drive and will be coming in the new 12x148mm Boost size soon. Below is a display showing how the internals work using several rollers contained within gently tapered slots.
Credit
Brandon Turman
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