Pretty much every bicycle tinkerer out there should be vaguely familiar with the Nuvinci. While it may look like just another internally-geared bicycle hub from the outside, that's where the similarities end. Instead of sets of planetary gear trains, the Nuvinci hub contains a miniature CVT, or continuously-variable transmission. This particular flavor of CVT makes use of several balls rolling between two dish-shaped wheels. The axis of the balls can be adjusted from a shifter on the handlebars, changing the mechanical advantage of the system and creating an infinite number of "gears" within the available range of the device. Nuvinci prefers to call this system a "CVP", or continuously-variable planetary. Unlike many other CVT designs, the input and output shafts are co-axial, which means the mechanism can serve as a bicycle hub with almost no extraneous components.When I first heard about the Nuvinci, I have to confess that I was skeptical. How could something with such small contacting surfaces efficiently transfer the non-trivial amounts of power that a cyclist can generate, all while not slipping under high loads and still being easy to shift? It didn't help at all that Nuvinci originally hyped the new hub as part of a big cruiser bike instead of something more practical. Nothing screams "novelty" quite like associating your product with a genre of bicycle most commonly found hanging from garage rafters or abandoned in a shed.
Nonetheless, I pored over the computer-generated animations of the mechanism and read about the hub. I still couldn't quite picture how this was possibly as great as it was being pitched. Fast-forward a few years, and a shiny orange Cross-Check shows up at work, complete with a Nuvinci hub and not much else. It's a demo bike that a Nuvinci rep hauls around for shops for curious mechanics and customers to try out. Skeptical but curious, I took it for a spin around the West Bank. In short, I was shocked at just how good the hub felt. My preconceived notions of slipping or drag failed to materialize. I tried sprinting hard on the flats, climbing up hills, and bombing down the other side. Whatever was inside this hub must be pure witchcraft.
My brief test ride was just that, brief. A few days later, the bike was gone, destined for some other shop. Fast forward another few months, and a guy in a nice black polo shirt shows up at the shop with a weird-looking hub. It turns out it's a new version of the Nuvinci that was released mere weeks earlier. He says that they managed to shave about 3 pounds off of the hub, make the shell notably smaller in diameter, and increase the overall range. In a world where hubs usually tip the scales at a few hundred grams, shaving 3 pounds (about 1400 grams) off of a hub's weight is almost incomprehensible. We talked for a good while about the technical details of the new hub compared to the previous two versions, and I compiled some juicy details below.
By the way, I learned that the "witchcraft" allowing such amazing performance from such a small mechanism is the type of oil inside the hub. The shell is about half full of a special "traction fluid" made by Valvoline. It is normally an oily liquid, but when compressed between two surfaces at high pressure, it somehow turns temporarily into a semi-solid that aids in traction between the contacting surfaces. Imagine smooth steel parts that have the strength and hardness of metal, but stick against one another like they are coated in rubber. That's pretty impressive technology.
NuVinci N170
- Released in 2006 (?)
- Non-drilled flanged (compared to the N171)
- Internals are supposedly extremely similar to the N171
NuVinci N171
- Released in 2008 (?)
- 250% range
- 0.5 underdrive to 1.75 overdrive
- 3460 grams, or 7.6 pounds
- easily identified by the lightening holes drilled around the flange between spoke holes
- Shifter: grip shifter with "inchworm" indicator, 1.5 turns.
- Shift pod: mounts outside of dropout, dual-cable
NuVinci N360
- released fall 2010
- 260% range (increased from 250%)
- 0.5 underdrive to 1.8 overdrive (increased from 1.75)
- 1.8 minimum primary gear ratio allowed (decreased from 2.0)
- 135mm spacing (same)
- 2450 grams, 5.4 pounds (decreased about 30% from 3460 grams, 7.6 pounds)
- Shifter: grip shifter with "rider on the hill" indicator, 3/4 turn. (Cable replacement is WAY easier than on the older version, according to rep.)
- Shift pod: mounts inside of dropout, dual-cable (better protected than previous version)
- $399 MSRP (reduced from about $450 for the N171, I believe)
Temperature range: The hub has been tested at about -5 Farenheit and worked fine. As the temperature drops, the traction fluid tends to perform less favorably. If you got the hub cold enough, it would theoretically be able to slip under load. The rep assured me that even if the outside air temperature is well below zero, the hub should quickly warm up to an acceptable operating temperature after you start riding. He admitted that the hub has not been tested in extreme Minnesota conditions, where a bike could be left outdoors to reach ambient nighttime temperatures of well below zero. The slice of the population that would be riding a bike the next morning in that weather is small, and the number of those people riding a NuVinci is likely about zero.
Whenever someone asks the NuVinci guys about the efficiency of the hub, they seem to get a runaround answer. The most simple chain drivetrains have been measured as around 98% efficient under ideal conditions. This means that 98% of the energy put into the cranks should be transmitted intact the rear wheel. Introducing any other elements into the system can only decrease the efficiency. Many common internally-geared hubs have an efficiency around 92%, with the lost energy going into friction in the many added bearings and gear contact surfaces. Note that neither of these numbers take into account anything other than the drivetrain. Losses to wind resistance, tire rolling resistance, non-ideal lubrication, and many other things add up to a much bigger piece of the pie. NuVinci won't come right out and state an efficiency percentage for their mechanism, likely because it is not a selling point. We are left to assume that it is somewhat less efficient than a modern internally geared hub, or they would surely be touting their hub's performance as a big advantage.
I've been thinking about building up a mountain bike around a Salsa A-La-Carte frame, and have been thinking about using the new Shimano Alfine 11-speed hub when it becomes available this fall. After talking to the NuVinci rep, I had to step back and reconsider my options. I think I will still go with the Alfine for a handful of reasons, but the NuVinci is beginning to look pretty good these days.
Reasons I'll likely use the Alfine 11-speed over the NuVinci N360 include:
- better mechanical efficiency
- better hub weight
- better overall range
- Alfine can be serviced and maintained; the NuVinci is sealed for life
- the new Alfine trigger shifter looks awesome and is intuitive for MTB riding
Wheelbuilding dimensions for the N360
(note that the N360 hub shell is NOT the same as previous versions!)
- 135mm OLN
- 123mm flange circle diameter on both sides
- 26.7mm left center-to-flange distance
- 25.5mm right center-to-flange distance
- 2.9mm spoke hole diameter (I measured 2.75mm, but was using a clunky caliper)
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