FWD, RWD & AWD

With all the hype surrounding drivetrains lately, it's time to explore what they really do and how they apply to powered boards. Given that boards haven't actually been powered for a long amount of time, and its a small niche at that, there isn't a whole lot of research and development into what performs best for specific scenarios. Our best resource is automotive sports, which has been under constant development for a century.

ON THE ROAD

FWD is easily the most uncommon choice for transmitting power to the ground in electric skateboards, and there's some obvious reasons why. The stand out one is grip, and more specifically the management of grip thresholds of the tyres as you can see in the figure below:

Here, the front tyres are under considerable amounts of load, handling torque and cornering forces simultaneously. In cars, it's undesirable because of lateral weight-shift (towards the rear) under acceleration, putting less weight over the driven wheels; this means the grip threshold is lower and easily overcome by torque. However, on a board where one is at liberty to move around, there is more dynamic weight-shift under the control of the rider. So opposing lateral forces is possible to an extent, but limited and unreliable. During turning, cornering forces are applied to the tyres in addition to torque, bringing the tyres to their grip threshold even faster. To add to this, torque-steer tries to force the front wheels to remain straight due to inertia. The result is understeer, poor cornering ability and slower overall momentum. The only real benefit for ESK8 here is with braking being on the front axle, as weight shifts to the front under deceleration and provides greater braking ability.

RWD appears the most often in performance vehicles of all types, as it corrects the main negatives associated with front wheel drivetrains. Since electric skateboards don't have to deal with issues of power transmission from the front of the vehicle to the rear, there aren't many negatives with this option. One of the main points that stand out is superior cornering grip; if you look at Formula 1 you may notice that the cars have some of the fastest cornering speeds forcing drivers to handle several G's of force, all while on a rear wheel drive platform. This is because the turning wheels maximise turning grip, without suffering from applied torque (among other factors such as downforce):

As you can see, RWD maintains a closer driven-wheel grip threshold to the needs of the vehicle, due to the lateral weigh-shift discussed previously and because the rear wheels aren't subjected to the same cornering forces. The driven and cornering wheels are also separate entities in this circumstance. But what happens when you want to apply more power? And what about the Gap between threshold and cornering grip needs? More can be done.

AWD offers a way to distribute power between all of the contact points with the ground, and all other factors remaining the same, this is one way to add power and maintain grip. However, it isn't as simple as adding power to the front, or dividing the same power by 4 wheels instead of 2. To address issue 1, if you simply added power to the front wheels, all of the negatives associated with FWD are then applied to the vehicle. With issue 2, dividing the power by 4 wheels doesn't result in any power gain. Also, it may offer a reduced traction ability compared to RWD on a high friction surface due to cornering and torque loads overwhelming the front wheels. You might ask, what is the point then? Well, its all about maximising grip opportunities. To explain further, this is how it looks when you divide power by 4 wheels:

This is how it looks when you add power to the front:

And this is how it looks when grip is independently maximised:

As illustrated here, it is important where power is applied. Simply adding power to the front is not ideal. The most crucial aspect is that the front and rear work independently, with different torque output to mitigate the differences in cornering loads while being able to put more power down than RWD vehicles. A common ratio is 30:70 to the front and rear respectively. With the predominant powered axle at the back, and a small amount of power added to the front to take advantage of the grip threshold gap at the front wheels. This is a very aggressive component of some of the fastest vehicles, and partly behind the fastest lap records in the world of cars.

You could expect that similar results transfer to electric skateboards, and it may even be more crucial considering a skateboards lacking ability to manage understeer or oversteer conditions. Another thing to note is that AWD ESK8's offer front braking, which is beneficial for the same reasons and should be taken advantage of in a similar fashion like our RX-1.

OFF-ROAD

Low friction surfaces are an entirely different animal. One of the main things to consider is that there is almost always a degree of slip on these surfaces, there is no real way of applying large amounts of power while staying under the grip threshold, since it is so low. Therefore, the most common pursuit is to apply the same power at all 4 points and have controlled slip. This allows the vehicle to be predictable and fast under almost constant conditions of wheel slip. Think rally cars. This is often done by dividing power through 4 wheels, instead of adding to the front. This is probably the most applicable to off-road electric skateboards, since sliding skateboards is already well concreted in traditions and this power distribution offers similar characteristics on surfaces like dirt. 

Nowadays, there are complicated systems that manage power variably at each wheel, and performance vehicles utilise these systems to gain traction where they can. This is most apparent in hybrid and electric vehicles, such as the blistering fast Volkswagen ID:R. Electric skateboards are capable of similar achievements to scale, and the future is bright.

KEY TAKE AWAYS

  • Power is only going to be relevant where it can be best utilised.
  • AWD only offers more grip if it's used efficiently.
  • FWD sucks.
  • Off-road is an untamable beast and is best embraced.

Disclaimer:

This is subjective, simplified information and is open to error.