SpnKix Technical Teardown

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I backed this company on kick-starter by 'buying' a pair of electric roller skates called Spnkix.

The design looked cool and fun, but I am disappointed in the result. I thought I might as well take a few minutes to publish this as a review here so that anybody else who is considering buying them knows what they are getting.

In this post, I'm also posting some results of me engaging in my other hobby: taking things apart. I actually enjoyed this part, at least. Since this is the first time I have been the first to take apart a new category of device, I'll be through in posting photos for others who enjoy this stuff. My e-vehicle engineering experience is relevant enough that I hope you find my comments interesting too.

Teardown and Comments:

One skate shoe weighs 4.35 kg / 9.6 lb - it feels every bit of it.

Your shoes are secured in with two large buckles, each is in the style of a snowboard binding.

Steel support between the wheels underneath the foot.

The handheld remote control has a good range. It worked 30 feet away, which implies it should be reliable when used from just a few feet away as intended.

The wireless modules are prebuilt Nordic Semi RF modules. These are a digital radio and so should be better than the old electric skateboard remote controls that were prone to interference. It seems a bit wasteful to pick a two-way protocol for one way communications, though.

The nylon frame feel very solid and has a nicely done honeycomb reenforcement pattern in the thicker areas.

It seems a bit odd that there is motor wiring into each wheel, but only a motor on one side. The only plausible explanation is that there was so little space in the main body, that the only place to put the motor driver electronics was in the other wheel. This is an unusual approach, as a 75W motor driver can be almost small enough to sit in the space below the battery, but putting it in the other wheel requires significantly more parts and wiring in a wheel which otherwise could have been simple.

It has a substantial battery pack. At 125 Wh per pack, this is bigger than any laptop batteries and probably not legal to take onto an airplane.

Does not have enough connections to have a full charge balancer. The 7S2P LiO cell configuration only has 4 charge balancer channels. This will somewhat shorten battery life because the cells have different capacities and over time the stronger cell in each pair will start to get more and more overcharged and the weakest more and more undercharged. But 4 is better than none.

The motor has three thicker phase wires, and a red and a black wire in one connector and three wires in another connector. This means it is a brushless motor with a hall sensor, which is expected, since this is really the only way to get this much torque and power into a hub without using a gearbox. Kudos on not going cheap on the motor technology.

Hub motor - not disassembled

I didn't disassemble the hub motor. This is kind of tricky and dangerous to do and my curiosity was not that strong. The strong magnets in hub motors will snap the pieces around viscously. I am fairly confident it would have been like a smaller version of the normal bike hub motors. (Search youtube for “hub motor disassembly” if you're curious).

Motor connectors

I did notice that the manual suggests you replace the tires by taking off the hubs of the wheels. Lies!This is effectively impossible to do. Even after struggling to undo the screw that hold on the hub plates (the Phillips socket strips very easily), it is still impossible to simply pull off the side plate like the manual says. I can't even prise it off when the motor is in a vise. I assume this is either because it is too tight, or because the magnets of the hub motor are holding it on. You often need a special press to take hub motors apart. If the tires wear out or get damaged, you're in for a lot of trouble.

There isn't any sealing or waterproofing.

A quick high school physics calculation on the specified maximum weight, slope and speed gives a required power of 161W required per foot, which is considerably more than the 85W motor rating.

( 0.5*(81.6 kg+2*4.35 kg)*9.81*sin(5 deg)*4.17m/s )

So this is misleading, but on the other hand, it would require a fantastic amount of skill to remain balanced at top speed and top power, so this is not likely to be an issue in reality.

Given the commendable lack of second rate materials, and the un-commendable poor DFM (design for manufacturing), I seriously doubt that a first production run was profitable at $375 per pair, even ignoring the tooling costs. The tooling costs would have been substantial, even considering how slowly it was done.

Overall, this is a good effort for the first version of a product. But given the flawed concept, I doubt there will be a second.