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For over a century, chains have been used in bicycle drivetrains. They are cheap, lightweight, efficient, repairable, and found in bike shops all over the planet.
But technology has come a very long way since the 1800s, especially when it comes to textiles – so, perhaps a rope, or “String Drive” bike is the ultimate solution?
In this article, I’ll introduce you to a very special chainless drivetrain that uses Dyneema ropes to propel you forward. These bikes were produced from 2011 to 2021 and used similar ropes to those found in fancy bicycle spokes, paragliding lines, sailing ropes, and even human-powered helicopters.
Here are some quick facts about String Drive to whet your palate: it’s completely grease and oil-free, it has 19 non-overlapping gears, you can change the gears while stationary, and the gears don’t ever require adjustment.
To prove that String Drive is no gimmick, Ferenc Szonyi (an ultra-endurance cyclist) used this drivetrain in the insane Race Across America, a non-stop 4,800 km cycling event (3000 mi). In just 11 days and 10 hours, the athlete on the String Bike completed the route. That’s a crazy 422 km (or 262 mi) per day!
Of course, there are downsides to a rope drivetrain too. So, let’s dive deep into the technology and determine if we will see ropes on bicycles again.
How Do String Drive Bicycles Work?
I know it looks crazy and complicated, but it’s actually rather simple in operation.
You pedal in a circular motion just like normal, however, instead of rotating a chainring, the crank arms move two levers that pull on ropes attached to either side of your rear wheel. The levers take it in turns to pull your wheel forward, with the ropes winding and unwinding neatly on the rear drums.
Incredibly, Dyneema ropes have a higher strength under tension than a bicycle chain.
A 3.5mm rope can handle over 16,700N of force, which is twice the industrial standard for bicycle chains (8000N). To put this into context, the best Tour de France sprinters don’t even hit a quarter of the tensile strength of these ropes. That’s pretty impressive considering these ropes float in water!
The gear ratios are changed by moving the front pulley up and down recesses in the lever arms. There are 19 gear options, where the highest gear is about three times larger than the smallest (304% range). This is equal to a traditional road bike drivetrain (53-39tT + 11-25T), or Shimano Alfine 8-speed internal gear hub.
You can choose between three rear drum sizes that can either give you lower gears for the hills, or higher gears for the flat.
The ropes typically last 1000 to 2000 kilometres, and as they don’t absorb water, they can be used in wet conditions and even snow. That said, grit significantly affects longevity, so while it can handle extreme conditions, it performs best when the conditions are good.
Let’s talk about advantages.
The Advantages of String Drive Bicycles
To start, this drivetrain runs smoothly and silently without any grease or oil. This virtually eliminates drivetrain maintenance, and means that you’ll never get grease on your hands or pants ever again.
Once set up, String Drive doesn’t require any gear adjustment, and the gears won’t ever skip when you change them. In fact, you can change gears any time with this drivetrain: that’s while stationary, while coasting down a hill, and even under a full pedal load.
There is also no rear derailleur to damage or bend out of whack, and while the lever arms are large and exposed, they sit high and out of harm’s way.
Interestingly, the advantages we’ve discussed so far are all shared with an internal gear system paired with a belt. Let’s now discuss the advantages exclusive to String Drive.
Firstly, the pedals automatically rest at the most favourable starting position, so you can just get on your bike and ride.
While most drivetrains wear out the front and rear sprockets, as well as the chain or belt, the only regular consumable on String Drive is the ropes, which are about $10 each. The spare ropes can be easily stored inside your seatpost, and anyone can change them in about five minutes without removing the rear wheel.
String Drive is also designed to minimise the ‘crank dead spot’. It does this by decreasing the gear ratio at the top and bottom of the pedal stroke, and increasing it when you can exert the most force. This provides torque to the rear wheel over a longer duration, which is said to be more efficient – it has the same effect as using an oval chainring on a chain drive bike.
When you remove the rear wheel, the entire drivetrain stays with the frame. This makes for easy wheel removal and it’s great for transport too – you can put the dirty wheels into bags, and the bike travels cleanly. The bike will also stand up by itself without its rear wheel fitted.
When these bikes were available, the price was quite reasonable. City bikes were just over €1000 (~US $1100), which was excellent considering that the frame and many of the high-quality components were manufactured in Europe.
The last advantage is that the gears are spaced very close together, which allows you to almost always find the perfect gear ratio. This is not dissimilar to the Enviolo hub we recently analysed. To illustrate just how close these gear ratios are, each step on an 11-21t cassette (1-tooth difference between each gear) is still 50% larger than what String Drive offers (7.5% vs 5%).
String Drive has shown some significant advantages, so what are the downsides?
The Disadvantages of String Drive Bicycles
One reason why String Drive never took off is that the drivetrain cannot be installed on any bike. Instead, a specially designed frame must be used – one with the right guides and attachments, and with notably wide dropouts to fit the two rear drums.
Not only did the frame need to be special, but almost all components and spare parts were exclusive to StringBike, making them hard to obtain.
The design of the drivetrain itself also made it a hard sell. This is because the left side drum occupied the same location as a disc brake rotor, which meant the drivetrain was only compatible with rim brakes. While rim brakes offer acceptable performance, disc brakes have become the industry standard – limiting the versatility of this drivetrain.
Modern drivetrains are ideally eBike compatible too. Unfortunately, there was no easy way to make a String Drive electric bike as the components occupied the same space as mid-drive and rear hub motors.
The Dyneema ropes also wore out quickly compared to other drive options. The best chains offer top-level performance for between 4000 and 7000km, while modern belts can last upwards of 30,000km (both with contamination). In comparison, Dyneema ropes only lasted 1000 or 2000km.
A grip shifter needed to be used to change gears on a String Bike. Some users found these shifters stiff to rotate, especially when they were wet. Grip shifters don’t integrate very neatly on drop bar bikes compared to modern brake/shift levers either.
As you need lever arms, ropes, and drums on both sides of the bike, there was a weight penalty of approximately one kilogram (2.2lbs) compared to a derailleur bicycle.
A String Drive bike was also likely less efficient than a chain drive bike. Converting from rotary motion to reciprocating motion, and then back to rotary motion is never particularly efficient, and additionally, this drivetrain has many sliding surfaces that generate friction.
That said, the drive efficiency might just be superior to a gearbox or internal gear hub. The fact that four amateur cyclists could average 33km/h for 24 hours on String Bikes (Hungaroring 24H race), or that an endurance cyclist could cover over 420km per day (in the RAAM) suggests that drivetrain losses are not unreasonable.
The String Drive gear range was also small by modern standards (304%). It worked fine for most urban environments, but it became a bit limited if you wanted to climb both steep hills and have the gears for high-speed cruising too. For context, most road bike drivetrains now offer a 450% gear range and off-road drivetrains regularly exceed 500%.
Why Didn’t String Drive Take Off?
With the pros and cons laid out, you should have an idea about why String Drive didn’t take off.
Ultimately, the proprietary frame and components made it a really difficult sell, as did the clunky grip shifter, additional weight, lower drive efficiency, and short wear life of the ropes.
Furthermore, String Drive didn’t come out at a great time. It was developed just as disc brakes were becoming standard on bicycles, right when electric bikes were turning into the largest area of growth in the bike industry, and just as belt drive and internal gear systems had finally become a suitable alternative to chains and derailleurs.
Let’s finish this article with some of String Drive’s interesting quirks.
Other Interesting String Bike Quirks
Firstly, you can pedal backward and the bike will move forward! This is because the levers will always pull the ropes, no matter the direction your pedals are moving.
You also have the opportunity to set different gear ratios for each crank arm by using different-sized rear drums. This could be useful for those with a leg injury, or for training up a leg with a muscular weakness.
The reduction in ‘crank dead spot’ is another fascinating quirk. As I previously mentioned, String Drive was designed to decrease the gear ratio at the top and bottom of the pedal stroke and increase it when you can exert the most force.
While this sounds like a genius idea, it’s actually inconclusive whether there is a biomechanical advantage to using an elliptical driving path. According to carefully controlled biomechanical studies, your ankle movement changes when you ride with non-circular chainrings, and this change in movement effectively cancels out any possible additional power generated at your knees and hips.
That said, some people prefer the ‘feel’ of non-circular chainrings, and others say that they notice less strain on their knees, so I think reducing the crank dead spot is still an idea worth pursuing.
Summary
I honestly thought String Drive was a bit silly when I began investigating it. But after analysing every aspect of its design, I’m actually really impressed. The entire drivetrain is well thought out, and I had no idea that Dyneema rope tensile strength was so high.
The ropes are clean, maintenance-free, cheap, and easy to replace. I can certainly see how the designers imagined their bicycle drivetrain revolution, but unfortunately, they did not anticipate just how important disc brakes, electric motors, gearboxes, and belts would be in the decades to come.
