Are Chainless Shaft Drive Bicycles a GENIUS or TERRIBLE Idea?

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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..

As found on YouTube

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