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The idea of a “digital drivetrain” is simple. You have a power generator at your crankset, which converts your pedal power into electricity. You’ve then got a motor at your drive wheel. Two wires transmit the power you’re generating straight to the motor, and a few electrical components sort out the details.
As sci-fi as this looks and sounds, it is not at all a new concept. The first patent for a digital drive bike was issued as far back as 1975 and the first fully functioning digital drivetrain was shown in 1995.
The first production bike with a digital drive was released in 2012 (Mando Footloose), and currently, we’re seeing a handful of companies working on their own digital drive systems (Schaeffler, Pendix, Podbike, Bike2 & more).
Note: digital drive is sometimes known as “electrical series hybrid drive”, “serial hybrid drive” or just “electrical drive”.
Current bicycles often use derailleur gears and chains. These drivetrains have improved incrementally over more than a century, and have well and truly proven themselves to be lightweight, high-performing, efficient, and cheap to produce.
More recently, gearbox drivetrains have come a long way. While they will never be as light or efficient as a derailleur drivetrain, they are incredibly reliable and very low maintenance. Plus, when you pair them with a belt drive – the component longevity is increased and the maintenance is reduced even further.
But are these drivetrains the pinnacle of what’s possible? Or will bicycle drivetrains of the future be digital?
In this article, we’re going to explore the world of chainless, digital drive bicycles.
Advantages of Digital Drive
There are many advantages to digital drive.
Firstly, you lose the chain. This means no wear and tear on the drivetrain, no greasy fingers, lower running costs, and less bike maintenance. A bike with digital drive would essentially just need its brake pads and tyres replaced. This would be ideal for those using a bike in all weather conditions and would be great for fleets and hire bikes too.
Secondly, you end up with a fully automatic bike with an infinite gear range. This is particularly advantageous for inexperienced cyclists, as the digital drive would always have you in the perfect gear (a manual override is possible).
Top-tier gearbox drivetrains with automatic gear shifting are currently both complex and very expensive. A digital drive equivalent has much less complexity and could feasibly be significantly cheaper with the same features.
You could also add a battery (or supercapacitor) that you could charge while you ride. By choosing to send some of your pedal power to a battery, you could use it later, for example when taking off from the traffic lights or to get over small hills. And if you were using a three-wheel cargo bike, you could even charge this battery while you’re waiting at traffic lights.
In addition, you could capture energy at the rear motor while descending or slowing to a stop – this is known as regenerative braking. The motor could integrate an electrical anti-lock braking system, and when you’ve parked your bike, the motor could be mechanically locked.
A digital drivetrain could perfectly optimise your pedalling technique. The system would always have you riding at your optimal power output and cadence, which would result in the highest possible efficiency of your body.
You can also do interesting things with the pedalling characteristics, such as electronically eliminating dead spots in the crank revolution. What I mean by this is the generator resistance could be increased when you have the most crank leverage and reduced when you have the least leverage.
Another example where a change in pedalling characteristics could be useful is if you’re rehabilitating from an injury or have a disability. Let’s say it’s your left leg. You could program a digital drive to have less resistance on your left leg pedal phase and more on the right.
Digital drive also opens up new bike design options. For example, you could have interesting folding bike designs, different cargo bike layouts, and multiple people pedalling the same bike at their own perfect efficiency.
Alright, the advantages are sounding pretty sweet, but there are of course disadvantages too.
Disadvantages of Digital Drive
The big one is drive efficiency. Converting between mechanical energy and electrical energy and then back to mechanical energy is very inefficient.
When you put power into your pedals, a single speed chain or belt drive will put more than 95% into propelling you forward. In comparison, a digital drivetrain will likely lose around 20% of the power at the generator and another 20% at the rear motor. With some step-up and step-down gearing to get the power ranges right, you might lose another 10%.
In total, we’re looking at close to half the efficiency of a chain drivetrain.
To put this in perspective, a chain drivetrain would have you* riding at 27kph/17mph on the flat at 150 watts while a digital drivetrain** would have you riding a full 17% slower (22.5kph/14mph). And on a slope, it gets even worse. A 5% gradient with a chain would have you cycling at 9.4kph, and you’ll be cycling 34% slower with a digital drive (6.2kph).
*80kg rider, 15kg bike
But keep in mind, these numbers don’t take into account any energy captured by the drivetrain through regenerative braking.
Digital drive would also add at least 25% more weight to a bike.
The lightest rear hub motor is more than one kilogram heavier than a regular hub, plus you can add at least another kilogram for the front generator and wiring. And given there is no mechanical coupling between the pedals and rear wheel, a digital drive motor needs to be especially powerful to make up for it. An appropriately powerful motor could be 2.5 to 3kg, depending on the terrain.
All rear hub motors cannot take advantage of your bike’s gears to operate in their optimal RPM range for both steep uphills and flat riding. Unless you use very large and heavy motors, digital drive can only really be geared for one or the other. Combining this with the high rear motor weight, it’s less ideal for a mountain bike application too.
Digital drive also relies very heavily on software and the controller. If the motor cannot be controlled within a couple of milliseconds, it will not have the same feel as a chain drivetrain.
Ok, so the advantages are big, but the disadvantages have really stacked up too, so let’s break things down further.
Series Hybrid Electric Bikes
I’ve just discussed the pros and cons when comparing digital drive fitted to regular bikes. But things get way more fascinating when we talk about the possibilities of digital drive e-Bikes.
When a digital drive bike is used in conjunction with a battery, it’s sometimes known as a Series Hybrid Pedelec. Digital drive bikes already have all of the components of an eBike, so fitting up a battery to not only provide some power assist but also offset the extra weight and transmission losses just makes sense.
The only production bike you can currently buy with a digital drivetrain is the Mando Footloose. This bike is at least a decade ahead of any other bike, so the drivetrain is not a particularly refined product. From all reports, it feels more like you’re walking up a stair climber rather than pedaling a bike, and the lag between the pedals and wheel torque results in a lot of surging.
Part of the issue here is the software. If the controller uses basic algorithms and cannot control the motor quickly enough, or provide appropriate feedback to the rider through the generator – the digital drive will not feel natural.
One reason why the Mando Footloose is the only digital drive bike available is that the German approval body for eBikes has until very recently classified digital drive bikes in the same category as motorbikes and mopeds, rather than as a bicycle. German bicycle shops attempting to sell the Mando Footloose even risked a €5,000 fine.
A more advanced digital drive system that never made it into production was found on the EE-SpeedBike. Using advanced software simulation of a chain drive in combination with special control algorithms to manipulate the components, it has been described as riding just like a regular bike.
The EE-SpeedBike weighs 29 kilograms (64lbs), can travel at 45kph, and get 80km of range with medium assistance. It’ll be interesting to see where this technology heads next – it looks like Schaeffler might be using some of the patented control electronics from this bike in their new ‘Free Drive’ system.
Digital Drive Efficiency Analysis
Ok, we need to talk about efficiency again because it’s often viewed as a real sticking point for digital drivetrains – but it need not be.
Firstly, drive efficiency is a lower priority for some cycling applications. Commuter bikes, bike rental fleets, and especially cargo bikes are examples where the possible extra weight and reduced range are far less important. And in any case, you can easily install a bigger battery to achieve the required range.
But the difference in drive efficiency to a chain drive bike might not be as large as we previously discussed, given the right circumstances, and provided you have smart ways to save and use power.
When the motor is doing the majority of the work, digital drive e-bikes are closest in drive efficiency to chain drive e-bikes.
For example, if we compare a mid-drive e-bike to a digital drive e-bike with 200 watts coming in from the battery, when we cycle at 50-watts power output, we can theoretically achieve similar drive efficiencies. This is because there is only a small amount of energy lost to the generator.
I found a great thesis by Edgar Tournon, a student at Claude Bernard University, who explained that with his digital drive system, effective drive efficiency of close to 75% is possible on a 5km flat course, and it’s 62% if you add a 50 metre hill in the middle.
The reason the efficiency is so high is that energy can be recovered by the motor when slowing for traffic lights or cycling downhill. This is something that I didn’t factor in my earlier efficiency estimate – in stop/start situations, digital drive bikes can be quite efficient.
Edgar also determined that his drive system could even keep up with a regular chain drive bike on a stop/start city commute, provided his digital drivetrain had an efficiency of around 70% and he could use regenerative braking.
But perhaps it’s not the bike that needs the efficiency optimisation, but rather, the human.
Improving The Efficiency of Human Bodies
Another promising aspect of digital drive is that the system can make you – the rider – more efficient.
Our bodies work most efficiently at a nearly constant load, rather than a highly variable one. To maintain a constant load, we need to always select the right gear ratio and cadence for different riding situations. We don’t always get this right, but a digital drive could – offering the most efficient extraction of power with the lowest levels of body fatigue.
Even More Possibilities for Digital Drive
Ok, I’ve got a few more interesting takes on digital drive and its possibilities.
Electric motors have very fine control over how much torque output they offer, which gives us the opportunity to intentionally limit torque and mimic what it feels like to have gears!
There is absolutely no reason to have ‘fake’ digital gears, but hey, it could be fun, and I like fun.
You could use a digital drive bike as a workout machine by programming it to offer outdoor interval training without a change in cycling speed. In fact, there are already very expensive cycling products that intentionally slow you down so you can train effectively (Airhub Pro).
Alternatively, you could put your digital drive bike onto a stand, and train indoors while charging up a battery. And if the digital drive identifies that you have strength imbalances between your legs, it could even apply more resistance to the leg that needs additional training.
Another interesting idea is that digital drive bikes can easily be coupled with solar power. With a handful of panels, perhaps you won’t even have to plug in your bike in the future.
It was not long ago that professional photographers were certain that they would never switch to digital cameras. But through incremental changes in technology over time, digital cameras are now the choice of professional photographers around the world.
So, will digital drive follow a similar path?
When compared to a chain on a bike without a battery, I think it’s unlikely. The low drive efficiency, extra weight, and inability to efficiently work on flat and hilly terrain make it an uphill battle (pardon the pun).
But when it comes to bikes with a battery, I’m certain that digital drive will be incredibly useful in the coming years. With the fully automatic mode that perfectly optimises your pedal efficiency at all times, this is going to be advantageous for anyone who swings a leg over a bike.
Digital drive will be ideal in poor weather conditions, given the lack of wear, reduced bike maintenance, and lower running costs. The system will also be cheaper to manufacture.
The best application currently is for cargo bikes, personal transport, and hire bikes – expect to see digital drive on these bikes in the next year or so. I’m also really looking forward to the new bike designs that will come about as a result.
So, what do you think? Will digital drive bikes be big in the future?