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's 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,
and currently, we're seeing a handful of companies working on their own digital drive systems.
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 very long way. While they'll 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 video, we're going to explore the world of chainless digital drive bicycles.
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 tires replaced. This would be ideal for those using a bike
in all weather conditions, and it 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. 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 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 the traffic lights.
In addition, you could capture energy at the 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 optimize your pedaling 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 could also do interesting things
with the pedaling characteristics, such as electronically eliminating dead spots in the
crank revolution. What I mean by this is that 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 pedaling 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, and more on your 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 pedaling the same bike at their own perfect efficiency.
Alright, the advantages are sounding pretty sweet, but there are, of course, disadvantages too.
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 forwards.
In comparison, a digital drivetrain will likely lose around 20% of the power at the generator,
and another 20% at the 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 27 kilometers per hour on
the flat at 150 watts, while a digital drivetrain would have you riding a full 17% slower. And on
a slope, it gets even worse. A five percent gradient with a chain would have you cycling
at 9.4 kilometers per hour, and you'll be cycling 34% slower with a digital drive.
But keep in mind these numbers don't account for any energy captured 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 3 kilograms.
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 motor weight,
it's also 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.
Okay, so the advantages are big,
but the disadvantages have really stacked up too. So let's break things down further.
I've discussed the pros and cons when comparing digital drive fitted to a regular bike. 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 e-bike,
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 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 e-bikes 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 Euro 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,
can travel at 45 kilometers per hour, and get 80 kilometers 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.
Okay, 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 five-kilometer flat course, and it's 62% if you add a 50-meter 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 into my earlier
efficiency estimate. In stop-start situations, digital drive bikes can be quite efficient.
Edgar also determined that his drive system would even keep up with regular
chain drive bikes 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 optimization, but rather the human.
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.
Okay, 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.
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.
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 optimizes 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 applications currently are 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?