24 June 2014

Tech Talk: Belt vs. Chain Drive

Belt drive on a modified Surly Pugsley
Chain drive on a stock Vassago Verhauen

Belt drives are one of the most exciting innovations in bicycle technology in recent years, as they have helped expose more cyclists to the advantages of internally geared drivetrains. A belt drive paired with a Rohloff SPEEDHUB 500/14 or Schlumpf Drive creates one of the most reliable and low maintenance drivetrains available.


The entire belt drive system consisting of a belt and front and rear
 sprockets weighs nearly half of a standard chain


The main advantages of belt drive systems are their long wear life and zero maintenance. Belts are manufactured as one continuous loop - an inner carbon fiber tensile cord layered with polyurethane teeth and a nylon outer coating. They do not have any moving components like the links and pins of a chain. 

Chains "stretch" as the pins, pin holes, and rollers in the links wear over time. This causes the inner and outer links to wear unevenly and the chain to lengthen overall. Eventually, a chain will wear enough that it no longer meshes with the sprocket teeth. Continued use will cause uneven wear to the sprockets and eventual loss of efficiency.


(Source: UCI)
(Source: Sheldon Brown)


On the other hand, a belt wears slowly and evenly, and the only wear that occurs is between the belt teeth and sprocket teeth. As a result, belt drive systems last at least twice as long as the longest-lasting chain systems and often five to ten times as long. Few of our customers have needed to replace their belt systems in the past 5 years, and some have ridden over 20,000 miles on a single belt drive setup.

Anatomy of Gates Carbon Drive Belt.
(Source: Gates)

Another advantage of belts is that they will not rust and do not require any lubrication. This eliminates maintenance associated with chain up-keep as well as the greasy mess that chains are known for, putting an end to grease marks on pant legs. Additionally, a belt is much lighter. A complete belt drive system of two sprockets and a belt weighs about half as much as a typical chain alone.

Although there are many advantages to belt drives, their main limitation is that they are only compatible with single speed or internally geared bikes. Unlike chains, belts can not run properly when flexed sideways and therefore cannot be shifted onto different tracks for different gear ratios. This makes them incompatible with derailleur systems but perfect for internal gearing systems.


Co-Motion Klatch Road bike with separation point at dropout/seatstay interface

Belts cannot be opened like a chain, so the frame itself must open to install the belt. This separation point must be located somewhere within the drive side rear triangle. More and more production frames are coming with split points designed into the seatstay or dropout, but these are still relatively uncommon. 

As a result of increasing interest and demand, custom builders are seeing an increase in orders for belt-compatible frames. Many of these builders are on the leading edge of belt drive innovations by developing new options for splitting frames. Paragon Machine Works also offers an easy to use production tube splitter, which was used on this 29er from Independent Fabrication.


Custom Rohloff-equipped 29er+ from Independent Fabrication with splitter integrated in seatstay

Some production steel or titanium frames can be also be modified by welding in a tube splitter, as shown on this Surly Moonlander.

Surly Moonlander frame modified for belt drive use with tube splitter welded above dropout




















Frames used with belt drives must also have a belt tension feature such as horizontal dropouts, adjustable dropouts, or an eccentric bottom bracket, because belts are not compatible with spring loaded tensioners, such as those commonly used on chain-equipped bikes.

Rocker dropout on a custom Twenty 2 Cycles
Belt-compatible sliding dropout on Vassago Verhauen.
Eccentric Bottom Bracket on a Co-Motion Klatch
Horizontal dropout on a Surly Krampus

Since belts and belt sprockets are wider than chains, maximum tire and/or sprocket size may be limited unless the frame is designed to clear the extra thickness of the sprockets. 
Gates Carbon Drive with Center Track

When initially setting up the system, the front and rear sprockets must be well aligned so that the system runs smoothly and the belt does not hop off either of the sprockets. Gates Carbon Drive's Center Track design features a spine through the middle of the sprocket teeth and a groove through the middle of the belt. This setup keeps the belt on track and helps eliminate any potential misalignment issues.

Despite the advantages of belts, chains continue to dominate the market because they are compatible with more bikes. Chains fit any bike frame, unlike belts which need frames with special features. Additionally, their length can be easily modified by adding or removing links to accommodate various sprocket sizes. Belt drives are offered in set sizes, limiting a specific belt size to a few sprocket combinations and making it more difficult for bike shops to carry replacements. For some riders, chains still offer greater piece of mind - such as long distance cycle tourists who ride through third world countries and may choose a chain because they are easier to find and replace in these areas of the world.


Belt Drive

Advantages
  • No moving parts to wear (belt itself)
  • Lasts at least twice as long as a chain
  • Lighter than a chain
  • Does not stretch
  • Quieter than a chain
  • Does not require lubrication
  • Continuous loop cannot separate - no pins to pop out
  • Cleaner, no messy oil
Disadvantages
  • Only works on internally geared or single speed bikes
  • Only works on frames with built-in tensioning method
  • Only compatible with frames that have a belt splitter
  • Carbon tensile cords can be damaged by mishandling or trail impacts
  • Newer technology is harder to source in some areas
  • Limited belt sizes means not all bike types are covered, especially bikes with long chainlines like cargo bikes and recumbents
  • Not compatible with most suspension frame designs - belts cannot tolerate changes in effective chainstay lengths
Chain Drive

Advantages
  • Does not require frame with a belt splitter
  • Length is adjustable
  • Easier to replace and service, more commonplace
  • Does not require frame with built-in tension method
  • Metal links are resistant to trail impacts
  • Can be used with a tensioner
  • Compatible with suspension frames
  • Repairable on the road/trail
Disadvantages
  • Susceptible to rust
  • Chain wears over time
  • System wears faster than belt system
  • Pins can back out unexpectedly and links can separate
  • Must be lubricated & cleaned regularly
  • Efficiency decreases overtime due to dirt and wear

Because of the low maintenance and high reliability that belts offer, we see more riders adopting belt drives for all riding styles.

While belts and chains have their own distinct advantages, the choice often comes down to a rider’s intended use and riding style. If you are trying to decide between a chain or belt drive system, contact us to start a conversation about which is best for the way you ride.

19 comments:

  1. Can we convert a stock Surly Krampus with Knards 29 x 3 to accomodate a Rohloff and Belt drive combination ? if the answer is yes, any mods required ?

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  2. Yes! We've built a few Krampuses with that setup. Here's one example: http://cyclemonkeylab.blogspot.com/2014/03/monkey-ride-neils-surly-krampus-29.html

    The frame must have a belt drive splitter welded into the seatstay. We offer this service, or you can find a local framebuilder who may be able to.

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  3. I have a first-gen Trek District. I wanted the bike for the belt drive but it didn't hurt that it was also a gorgeous machine. Unfortunately on cool nights the normally silent drivetrain is creating a lot of noise (the nylon creaking against the "chain" ring)-- any ideas on how to return the bike to it's previous, near zen-like silence? Thanks! -Nathan

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    1. Gates recommends using silicone spray, which works well but needs to be reapplied periodically. We've been having good luck using Rohloff's biodegradable chain lube. One application seems to work well for several months of off road riding. We typically needed to re-apply silicone every week or two of regular off road use (60-75 miles).

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  4. Have you ever seen a successful marriage between a Rohloff Hub and a belt drive system on a RWD recumbent trike? I have heard there are other belt manufacturers apart from Gates who make longer drive belts. I have also seen an example where Gates used 2 belts to complete a set up on a tandem bike. Is modifying an existing trike to split the frame and accomodate a tensioning method feasible or totally off the planet?

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  6. Which drive is more suitable for electric bicycle?

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  7. Which drive is more suitable for electric bicycle?

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    1. Depends a little on your usage and bike setup, but we like belts for most applications, including e-bikes. If the bike has a front or rear hub motor, the belt/chain does not see any additional loads, so nothing changes relative to a standard bike. If the bike has a mid-drive motor, there is increased load on the drivetrain, which will lead to reduced lifespan for chains and belts. There is also an increased chance of the belt ratcheting over the rear sprocket with a mid drive system when using a super high power motor, say over 2kW. If the frame is properly stiff, this will not be very likely though.

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  9. Above you mention the need for silicone or Rohloff chain lube. Doesn't this then even the clean vs dirty playing field as they would both be oily? How often do you see belts needing some sort of lubricant?

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  10. The silicone spray dries and stays clean. The chain lube does collect some dirt, but you really only need a few drops to address the noise, so the system will stay relatively clean if you don't overdo it.

    In environments where it rains on and off throughout the year, belts typically do not need any lubricant or maintenance whatsoever. In dry climates, especially when riding off road in dusty conditions, you should expect some squeaking to appear every couple of weeks (silicone spray) to once every several months (chain lube) depending on what you are doing to mitigate the noise.....based on our dry for 75-80% of the year Northern California conditions.

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  11. why do you say belts are not compatible with spring loaded tensioners

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    1. To date, Gates has not approved the use of an external tensioner with their belts due to concerns about a roller contacting the back of the belt potentially damaging the carbon cords inside. This may be changing in the near future, but to date Gates only approves the use of their belts on frames that adjust to tensioner the belt.

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  12. How many gears has a belt drive system,only 8 ?

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    1. The belt doesn't have any gears. If you combine with a gear hub you'll get multiple gears.

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  13. Who cares if gates approve it ?
    I design RC helicopters and we use gates belts WAY outside of gates design spec. Idler pulleys are sometimes used in belt tensioning with no issue. And they last very well in even these extreme environments.

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  14. I'm thinking of purchasing a single speed belt drive bike (Bike Friday Pakit) to use with my friction front drive motor (ShareRoller). Is removing the rear wheel for tire repair or change more difficult than with a chain drive? I know the IGH models are kind of a pain for wheel removal which is why I'm tending towards the single speed.

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