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. Torque is 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. Driven will 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.