Onyx's Sprag Clutch Hub
Conventional thinking says that the quicker you want your hub's clutch mechanism to engage, the more drag you'll have to put up with when you're coasting. This is because engagement time is most often associated with the number of points on the drive ring and the number of pawls - the more biting edges on the pawls and the more points on the drive ring, the more drag there will be as they all rub against each other when you're coasting. Having a quick engagement time is nice because then there's very little 'dead spin' in your crank arms, and having no lag in pick-up means you're putting the power down sooner. There are already hubs on the market that offer next to no lag, of course, but they're also plagued by a fair bit of clutch drag, which means that the bike isn't rolling as freely as it could be. That might be small beans for a lot of riders who don't even care if their brakes are dragging, but having as little drag as possible is important to a lot of riders, and especially to cross-country and road racers.
| The Onyx hubs employ a sprag clutch that allows for essentially drag-free performance when coasting, but also instantaneous engagement when you begin to put the power down. They system is also completely silent when the rider is coasting.|
Onyx's rear hubs employ a sprag clutch that runs essentially drag-free when not engaged, but it also manages to offer what feels like instant engagement when it's time to put the power down. That near drag-free performance has made them the choice of some pro racers who, while being sponsored by another hub company, are looking for every possible advantage. Sprag clutches are not new - automotive companies, including Ferrari, have been using them in transmissions - and Onyx has been manufacturing sprag-equipped hubs at their Minnesota factory for four years now, but the system certainly seems to make a ton of sense for use on a bicycle.
This clear plastic model shows the two sprag clutches that are found inside of the Onyx hub shell.
The sprag clutches sit at the center of the hub shell.
So, how does the Onyx hub run nearly drag free and offer instant engagement? Think of its sprag clutch as a one-way bearing that employs a number of asymmetrically shaped sprags (the small metal pieces that looks like roller bearings in the photo above) instead of bearings. The shape of the spring-loaded sprags means that they allow for the barrel that's attached solidly to the freehub body to rotate freely in one direction, but they then wedge tightly against the barrel when force is applied in the opposite direction. Onyx says that the system can withstand any sort of torque that a rider's legs could ever apply, and that the mechanism is actually greased for life right from the factory with ISOFLEX LDS 18 Special A grease. In other words, don't mess with it.
The Onyx hub has a lot going for it, but there's no denying that its weight might turn some riders off - it's going to be heavier with the extra steel inside of it that makes up the sprag bearings, and Onyx refuses to use aluminum freehub bodies due to gouging and reliability concerns. A 12 x 142mm Onyx hub with an XD driver and 6-bolt rotor mounting weighs in at 364 grams, while the Center Lock version comes in at 463 grams. Those numbers are a bit higher than other options out there, but Onyx says that the lack of drag is of far more importance than the 50 - 100 gram weight penalty.
All those separate metal pieces are referred to as sprags, and they allow the freehub to rotate in one direction but wedge tightly against its barrel when torque is applied in the opposite direction.
The barrel that extends out from the freehub body is what the sprags grab ahold of.
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