We need to get our pedal power from the cranks
to the rear wheel somehow. Chains are clearly the most common way to propel us forward,
and if you're a subscriber to my channel, you'll have seen that I think belts are
perfect for long-distance bike travellers. There are far quirkier ways to propel
a bike too, including shaft drive, hydraulic drive and even string drive!
In this video, you'll find out all of the reasons why shaft drivetrains have
not succeeded in the bicycle world. Make sure to stick around until the end because
we will be using our shaft drive knowledge to assess the interesting new Driven drivetrain.
But first, what is a shaft drive bicycle? Driveshafts are commonly used in automotive and
industrial applications, but what you may not know is that they've been used on bicycles for
over 125 years.
Instead of using a chain and sprocket set up to rotate the rear wheel, two sets
of bevel gears transfer energy to the rear wheel via a drive shaft. The entire drive system
is usually housed in an aluminium case that doubles as the right-hand side chainstay of
your frame. Shaft drive bicycle manufacturers, who still exist today, promise no exposed moving
parts, no greasy or broken chains, and no skipping gears – which all sounds pretty good to me.
So, why have these drivetrains never taken off? Reason #1. Derailleur drivetrains are exceptional.
Derailleurs are undoubtedly the best drivetrain for the majority of cycling applications.
They are cheap to manufacture, lightweight, highly efficient, and with over a century of
innovation, they work really well too. Derailleurs can also be fitted to almost all bikes, you can
easily source replacement parts, and you can find someone who can adjust them in most towns.
As a shaft cannot be paired with derailleurs, this is a major reason why shaft drive bikes are
not more widespread. But if you prize the ability to go huge distances on your bike with almost no
maintenance, or the ability to ride in horrific weather conditions without gear adjustment, gear
skipping, or the need to clear debris from your drivetrain – this is where gearbox systems
shine, and shaft drive can be paired with an internal gear hub.
So, for the rest of this
comparison, we will be assuming someone already wants a low maintenance gearbox bike and is
deciding between using chains, belts and shafts. Reason #2. The reduced drive efficiency. A shaft
drivetrain has a lower efficiency than a chain or belt. The biggest efficiency losses are simply
due to the change in the rotation direction, once at the crankset, and once again, at the
rear hub. We don't have a lot of efficiency data available here, but in 1983 Josef Keller
compared an unspecified shaft drive with a single-speed chain and found a 7% difference in
drive efficiency in the shaft. This was between 50 and 200 watts pedaling output. Friction facts
have tested a single speed chain to be about 99% efficient at 150 watts and a belt drive train
to be 98.6% efficient.
Using this current data, it would mean that a shaft drive is around 92%
efficient. That said, chains and lubricants in the early 1980s were not as good as they are today,
so shafts are more likely to be less than 90% efficient. I know these numbers don't sound
like a lot, but let's say a 70kg rider with a 15kg shaft drive bike was riding up a hill
with a 5% gradient. After 10km of riding, or approximately an hour, the shaft drive bike
would be four minutes behind the chain drive bike. But then again, chains and belts lose efficiency
in wet and muddy riding conditions. Friction Facts has found a chain to be 94.4% efficient
in muddy and wet conditions, and 92.8% efficient in muddy and dry conditions. So, in ridiculously
muddy conditions, a shaft could technically work out to be more efficient than a chain or belt
(assuming you don't use a chaincase). Or another way to look at it – a shaft in good conditions
is as efficient as a chain in super muddy ones. Reason #3. The Extra Weight. A shaft
system also requires heavy-duty components, resulting in a weight penalty of one to two
kilograms compared to a chain or belt drivetrain. These heavy components are absolutely necessary
as shaft drivetrains undergo very high torque when a rider starts from a standstill.
a particularly big hurdle on shaft drivetrains as they use small radius bevel gears. This
results in much higher forces and distortions when compared to a chain or belt drive.
Or to be more technical, the moment arm on a shaft drivetrain is approximately four to
eight times shorter than a chain or belt cog, so it needs to be engineered to handle four to
eight times more torque. These high forces also put a lot of stress on the bearings and bevel
gears which can wear out very quickly if not engineered to the right specifications. Ok, but
what if you don't care about efficiency or weight? Reason #4. The Gear Alignment. Another hurdle for
shaft drive systems is gear alignment. In order to reduce wear and increase drive efficiency,
there is an optimal distance for the bevel gears to mesh. All bicycle frames flex under a load, but
if a frame is not stiff enough for a shaft system, it can result in imprecise gear meshing. A
proposed solution has been to use CV joints at both ends of the shaft.
This would allow the
frame to flex but would also induce more friction, weight and complexity. A shaft system also
needs to be constructed to very high tolerances to achieve the optimal gear
meshing, and additionally, the rear bevel gears need to be easily
aligned by the user when installing the rear wheel. Gear alignment is a very solvable
problem but requires the appropriate frame, manufacturing tolerances, and way to achieve
the optimal distance between the bevel gears. Reason #5. The Proprietary Parts. Ok, so
you now have a super stiff frame and a shaft drivetrain that's really well designed
and manufactured. Shaft drive systems and the frames they're built around are proprietary. This
means that if you have a problem with your shaft, or can no longer get replacement parts, you cannot
switch your drivetrain to a belt or chain instead. Your bike will have a shaft
drive until the end of days. Right, before I summarize everything, let's talk
about the prototype shaft drivetrain by Driven Technologies.
This is a slightly different
take on shaft drive as it transfers power via a series of cartridge bearings that intermesh
with two circular pinion arrays. An awesome thing about the Driven drivetrain is that it doesn't
need to be paired with an internal gear hub, which allows it to theoretically provide a very
high drive efficiency in every single gear. Driven are claiming 99% or higher. In comparison, the
best internal gear hub that we've collected data on is 92 to 97% efficient depending on the
gear selected. While the Driven drivetrain has been taken up to 45 kilometres per hour on
a velodrome, there are significant challenges around making this drivetrain viable in the real
world. Managing the low RPM torque is going to be a huge challenge, and will require very advanced
materials to achieve the appropriate strength and longevity of the pinion arrays.
also need to ensure riders do not exceed the static load rating of the small cartridge bearings
too. Driven's CEO recently acknowledged these two hurdles in an email to CyclingTips, stating that
what we've seen so far is far from the final product, and that he's confident these engineering
challenges can be solved. Driven has just received a million dollars in external investment
so it'll be interesting to see if they can finally get this drivetrain off the ground. And
one final note on Driven – creating a rear frame triangle that is stiff enough to achieve precise
gear meshing is another big hurdle. This will go against the current trend of reducing frame
stiffness to improve the ride 'quality' or 'feel' of your bike.
You can learn more about the
nuances of frame stiffness in my video up here. Shaft drivetrains have a great reputation in
the motorcycle world. But in that world, the extra weight and lower drive efficiencies can be
overcome by using more powerful engines. Bicycles, on the other hand, are always best when
the effort you're putting into the pedals is rewarded by propelling you along efficiently.
I hope to see more shaft drive innovations in the future because I just think they're super cool.
But they have a lot of hurdles to overcome. Other than the additional weight and reduced
drive efficiency, they require super stiff frames, components built to very high tolerances and
easy user alignment of the rear bevel gears. Chains are still the best option for most
people as they can be paired with cheap, light and efficient derailleur gears. But if you
like the idea of a gearbox like the Rohloff or Pinion, I can highly recommend pairing those with
low maintenance, long-lasting belts.
If you like my bike nerd content, please consider supporting
this channel over on Patreon. Or alternatively, grab one of my books! The Touring and Bikepacking
Bike Buyer's Guides are updated yearly for free, and will teach you everything you need to know
about the bikes, before allowing you to compare over 220 different bikes at the back of the book.
To learn about the best bicycle gearboxes, click over here. Or if you're interested in learning
about the world of belts, click this video here..