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From the workshop: how to replace the belt, rear cog and chainring on a bicycle with the Gates Carbon Drive system

Updated: 7 days ago

This article describes how to replace a worn out Gates belt, rear cog and chainring on your bicycle. We assume that the bicycle is already fitted with a Gates Carbon Drive system, so we will not get into details about frame requirements such as frame stiffness, dropout configuration and so forth.


Summary

We went a bit overboard as to the length of this article. The short version is quite simply to get a Zumba bottom bracket from Thun, match it with one of the mid range cranksets from Gates and use the Gates app to find the correct belt length. If you have time to read more and want more in depth information, read on!


We would really appreciate hearing from you if you have comments or questions to this article, and we would be more than happy to update it with your insights and experience with the Gates Carbon Drive system! Write a comment below or drop us an email.


Please note that we are happy amateurs, so follow the instructions in this walk through at your own risk. A malfunctioning drivetrain can lead to dangerous situations. We are not paid or sponsored to write this article.


The full version

The Gates Carbon Drive system for bicycles is widely known for its reliability and durability. Configured correctly, the Gates Carbon Drive system lasts for years and years and thousands and thousands of kilometers. But even these components need to be replaced from time to time. Frequent belt slips is a tell tale that the components are beginning to wear out. Page 74 in the Gates technical manual gives some nice guidance on visual clues that can confirm that the components of your drivetrain are indeed due for replacement.


With any luck, you would simply return to the bicycle shop who sold you the bicycle and get replacement parts with the exact same specifications as the ones currently fitted to your bike. In that case, replacing the components of the drivetrain is as easy as replacing a worn chain and cassette. Except that you of course need to split open the rear triangle, but that is usually as simple as removing a screw or two.


The problem arises if the seller is not able to provide you with spare parts. That was unfortunately the case for us, as Oslo Sportslager has decided to discontinue our trusty commuter bicycle, the Everest Urban with Gates Carbon Drive system. Although bicycles with belt drivetrains are becoming ever more widespread, they are still far from mainstream. As such, most local bicycle shops do not provide service or parts for belt drivetrains. The interweb and international carriers come to the rescue, but even then getting hold of the right parts can be tricky, particularly in the consumer market. On top of that you need to figure out exactly what components to purchase to ensure that they will actually match up with your intended use, fit your hub and provide you with the proper belt tension and beltline.


What follows is a step-by-step guide to service a Gates Carbon Drive system, from figuring out the specifications and buying the components, to actually replacing the components. The procedure draws on the Gates technical manual, insights from other riders out there, sprinkled with our own experiences. Our drivetrain is build around the Shimano Alfine 11, se we use that as an example, but the main steps of the approach should probably the same for other internal gear hubs.


The pictures below show the case in question, an Everest Urban 2020 from Oslo Sportslager after over 15.000 kilometers of all season commuting. Shark teeth and deep rust are clear signs that the time for replacement of components has finally come. We have previously upgraded the bike from Shimano Alfine 8 to Shimano Alfine 11. Please bear this in mind if you also have the Everest Urban 2020 and are looking to service the drivetrain.


Table of contents


Preparations

The aim here is to have as little downtime as possible on our commuter bike and of course, to avoid the hassle of buying and then returning incorrect components. The best way to achieve this is to do a bit of research before starting to unscrew bolts and and placing orders. In short, read up on Gates offerings, figure out your desired gear ratio, take key measurements of the frame, and of course, check that you have the tools needed at hand.


Get to know Gates product lines

Gates offers four main product lines for bicycles. In order to get the best experience at the lowest possible cost, it is important to understand the strengths and limitations of the different product lines. Gates have a lot of information about this on their website:



In recent years the CDN, CDC and CDX product lines all seem to be based on the CenterTrack™ design. This means that these belts have a groove in the middle and that the cogs have a corresponding ridge in the middle. As far as we can understand, all of these belts have a tooth division of 11 mm and belt width of 12 mm. This should mean that the components of these systems, that is to say the rear and front cogs and belts are interoperable, as the cogs also seem to have have a tooth division of 11 mm and a teeth width of 11 mm. Note that the CDX:EXP and fixie cogs seem to have a bit wider teeth width (12 mm and 12.5 mm).


We have tested a combination with a CDN belt and CDX cogs and found these to work well together. The ST product line has the SideTrack™ design, meaning that the components have no groove or ridge. As such, these parts would not be interoperable with the three other product lines. Its also worth noting that the ST product line only seems to be available to OEMs, while the three other product lines are also available to end consumers. Also, there have been some changes in product lines in recent year it seems, so in previous year we understand that the CDX product line were not interoperable with CDN and CDC. Mostly for historic purposes, but it is an interesting read about this over at bikeforums.net


Aside from this, as far as we can understand, the main difference between these four product lines are the materials used, and correspondingly how durable the components are. It seems the ST product line is designed for the least demanding of applications, such as the occasional recreational bike ride, while the CDX product line is on the other side of the spectrum, designed for the most demanding scenarios, such as racing, trekking and so forth. Between those are the CDN and CDC product lines, designed for city cyclists and casual commuters.


Gates have created a neat and very useful comparison of these systems in their product lines document. Among other things they pinpoint differences in materials, ability to shed debris and eBike compatibility. Its also useful to have a look at the Gates technical manual, where operating temperatures for the belts of the different systems are mentioned, see page 53 and forward.


Decide on acceptable gear ratio

Most internal gear hubs, such as the Shimano Alfine 11, provide a generous gear range. This gives quite a bit of flexibility when choosing the size of external cogs to get the gear ratio that best fits your riding style. This flexibility is particularly import for belt drivetrains. The belts come in fixed lengths and sometimes the length of the chainstay mandates a belt length that either does not exist or is hard to get hold of. In those cases you can increase or decrease the cog sizes to adjust the belt length required.


You should however avoid combinations of external cogs that will bring the gear ratio too far down. A low external gear ratio can potentially produce elevated input torque to the internal gear hub, particularly combined with a heavy rider or a loaded touring bike. If too much torque is produced, you might risk damaging the internal parts of the internal gear hub. In addition, based on the Gates product lines document, it also seems like the different product lines themselves have some limitation on maximum torque. For instance it is stated that CDC is allowed for 75 nm or less, while CDX is allowed for 100 nm or less, see page 7. These limitations seem to apply to the use of mid-drive e-motors and it is a bit unclear to us whether they are related to the belt or the cogs, or maybe both. In either case we reason this to be a matter of torque being delivered to the drivetrain, and that it is best to abide by these thresholds, regardless of whether or not an e-motor is used.


To be on the safe side, we tend to avoid combinations of cogs that provide gear ratios lower than 1.8. On the upper end we try to avoid gear ratios above 2.0 as this will result in gears feeling too heavy for most situations. Dirk Feeken has made available an interactive calculator that is very useful to figure out gear ratios and cog combinations. It even gives you a warning if the combination of cogs you have selected is likely to produce too much torque.

Screenshot of Bicycle Gear Calculator © Dirk Feeken
Screenshot of Bicycle Gear Calculator © Dirk Feeken

Check out our comparative article on Alfine and Rohloff if you would like to read more about torque. It has a lot of links to what other people have to say about this topic, which is quite a bit. The important thing right now is that you have an approximate idea about your desired gear ratio, so you can use that as a guide when selecting cogs later.


Take note of bottom bracket shell and chainstay length

In order to decide on the correct bottom bracket and belt length you need to know the specifications of your bottom bracket shell and the chainstay length.


To determine which standard your bottom bracket shell is based on, you need to know the width and diameter of the shell and whether it is threaded or not. This article is helpful in getting the right measurements. If you find that the shell is threaded and the width of the shell is 68 mm or 73 mm, you most likely have a shell based on the BSA-standard. This is the most common standard on low to mid-end commuter bikes. There are however other threaded standards as well, such as the T47, and of course a number of standards for unthreaded shells. Road.cc and Parktool have some articles that are very useful in determining exactly what standard your shell is based on. On the Everest Urban the shell is 68 mm wide, with BSA-threads.


Although you can use different cog combinations to tweak the required belt length, the chainstay length is the most important factor to calculate the correct belt length. Find your chainstay length by measuring the distance from the center of the bottom bracket to the center of the rear wheel axle. See this article for a bit more guidance on how to do the measurement. After you have taken the measurement, it can be a good idea to head over to Geometry Geeks to compare measurements. They have an enormous database with geometry specifications on all kinds of bicycles.


The Everest Urban has horizontal sliding dropouts. As pointed out in the Gates technical manual, this is not ideal for belt drives. The main reason is that these kind of dropouts need some sort of tensioning mechanism to prevent the axle from moving under load. In order to get the belt tension right this mechanism must usually be recalibrated each time the wheel is refitted to the frame, for example after fixing a flat. This can be a bit fiddly, particularly during roadside repairs in rainy weather, which is when Murphy's law usually mandates a flat tire..

Horizontal dropout tensioning mechanism on the Everest Urban
Horizontal dropout tensioning mechanism on the Everest Urban

When measuring the chainstay length on a bike with horizontal sliding dropouts, its a good idea to adjust the tensioning mechanism so there is leeway in the dropout both forward and backward. This makes it possible to install and uninstall the belt, as well as fine tuning its tension. Taking this leeway into account, we found that on the Everest Urban the chainstay length is 450 mm.


Calculate belt length

Knowing your acceptable gear ratio and chainstay length, you are all set to calculate the belt length. We find that the best way to do this is is to use the calculator in the app provided by The Gates Corporation. Experiment with the sliders for belt tooth count and cog sizes front and rear until you find a combination that matches your gear ratio and chainstay length. As you move the sliders up and down the app will show you the corresponding gear ratio and chainstay length. Gates also provides a desktop version of the calculator, but we find that a bit less intuitive to use.


Bring out the tools

The selection of tools needed to complete this job may vary according to your bike setup. Assuming you have a Shimano Alfine 11 hub and a square taper bottom bracket, the following bouquet will go a long way:


  • Spanner wrench with good leverage for the pedals

  • 8 mm allen key for the bolts on the crank arms

  • Crank arm puller to remove the crank arms from the spindle

  • Tool for square taper bottom bracket

  • Small flathead screwdriver to pry off the snap ring and cog on the Shimano Alfine 11

  • Gates SureFit installation tool to fit the rear cog on the Shimano Alfine 11

  • Gates Carbon Drive tension meter. Optionally, the before mentioned app from Gates

  • 15 mm socket for bolts on the Shimano Alfine 11


Selection of bicycle tools
Tools of the trade

Purchase the components

Components for the Gates Carbon Drive system are a bit pricey and often hard to come by, so its best to get it right the first time. Armed with the insights from the previous steps, odds be on our side for a well placed order and no need for returns. We shall require the following parts:


  • Bottom bracket

  • Rear cog

  • Front cog (chainring)

  • Belt


Often both belt and cogs are specified with width of teeth and tooth division. As far as we understand, at least in recent years, this is always 11 mm across components from the both the CDN, CDC and CDX product lines. Its always good to check, but most likely this is only something to pay special attention to if you are trying to match components that are many years old with newer components.


Hollandbikeshop.com, Bike-Components, ROSE Bikes and Universal Transmissions GmbH are good places to look for the parts needed. In the following considerations for each part is described in a bit more detail.


Rear cog

When choosing the rear cog you need to keep in mind which Gates product line best fits your intended use, impact on belt length, compatibility with your hub and your required gear ratio. In our case the bike is used for extensive all year commuting and is equipped with a Shimano Alfine 11 hub. Our preferred gear ratio is somewhere between 1.8 and 2.0. Taking this into account the correct rear cog for our use would be the Gates CDX 3-Cam Surefit Shimano / SRAM 24t.

Brand new Gates CDX 3-Cam Surefit Shimano / SRAM 24t rear cog, fits the Shimano Alfine 11
Brand new Gates CDX 3-Cam Surefit Shimano / SRAM 24t rear cog, fits the Shimano Alfine 11

Note that the combination of hub and rear cog implies a given beltline, in our case 43.7 mm. In contrast to a traditional chain-derailleur setup, where the chain is designed to flex sideways, the beltline must be straight as an arrow. We understand from the Gates technical manual that the beltline can be off with +/- 0.5 mm. Anything more than this will cause premature wear on the belt as well as the belt skipping and climbing on the cogs. You need to match the beltline given by the hub and rear cog when selecting bottom bracket, crankset and chainring. More on that in the next section.


Bottom bracket, crankset and front cog (chainring)

Although the bottom bracket, crankset and front cog are three separate components, we discuss them together below, as they are so closely interdependent. The crucial point here is to ensure that the selection of components for bottom bracket, crankset and chainring matches the beltline given by the hub and rear cog.


If you have a BSA bottom bracket shell, the best and most trouble free option in our opinion, is to choose a Zumba bottom bracket from Thun. These bottom brackets are made specifically to match up with CDN cranksets from Gates and will provide the correct beltline. Page 32 in the Gates technical manual contains a super useful table that shows exactly which Zumba bottom bracket you need, depending on your hub and selection of rear cog. Since our bottom bracket shell is 68 mm and the beltline is 43.7 mm, we shall require the Thun Efficient Zumba Kranklager GBL440 BSA 68 mm. This short blogpost from 128.bike is a useful read about the Zumba bottom brackets from Thun.


With the bottom bracket all sorted, it is time to pick a crankset and chainring. The most straight forward option here, in our opinion, is to choose one of the mid range cranksets from Gates. An overview of cranksets from Gates is available on their website and in their technical manual, starting from page 19. Note that quite a few of these cranksets are only available to OEMs. By choosing one of the mid range cranksets, i.e. S250, S150 og S100, you will be certain that the crank arms fits the spindle of the Thun Zumba bottom bracket and that the correct beltline is achieved.


The only drawback buying these CDN cranksets is that they come with a CDN chainring already attached. In our opinion, the CDN crankset itself seems very durable and can most likely be used for extensive commuting, and probable touring also. The CDN chainring however is another cup of tea. It is made of glass-filled nylon composite and would probably wear out way too fast with extensive all year use. Luckily Gates in recent years has made CDN and CDX components interoperable, assuming you dont have an old version of CDX. This makes it possible to purchase a CDX chainring separately and replace the CDN chainring with that. Both chainrings are 5-bolt 130 mm BCD and matches the spider of the CDN crankset.


As with the rear cog, take into account which Gates product line best fits your intended use, impact on belt length and your required gear ratio when choosing the front chainring. We ended up using a CDN S150 crankset with a CDX chainring of 48t in order to ensure durability and get the belt length right.


If for some reason it is not feasible to use one of the Zumba bottom brackets from Thun, you can of course figure out the parts needed for the bottom bracket, crankset and chainring one by one. This would however require you to measure and match up the beltline yourself, which seems like a daunting task. Obviously, an eccentric bottom bracket which can be adjusted horizontally will make that task less risky. In any case, this video from Gates gives useful guidance in how to measure the beltline. It varies how well distributors specifies the components from Gates, so it can be useful to head over to shop.carbondrive.net to check the complete specifications. Simply search in the upper right corner for product description, e.g. CDNKGA50-AM for the S150 crankset.


Belt

The two most important considerations when choosing belt is obviously the belt length, and as with the cogs, which Gates product line best fits your intended use. Belt lengths are usually specified in both mm and number of teeth. Since the belt length calculators from Gates only provide information about belt length as number of teeth, the tooth count is the safest measurement to use when choosing belt. It can be a good idea to doublecheck that the tooth division of the belt matches the tooth division of the cogs. At least for current versions of the CDN, CDC and CDX product lines tooth division seems to always be 11mm, but on older product lines this could be different. If the tooth division across components does not match, the cogs and belt will not work together.

New CDN belt in the rear and a used belt (15 000 km) from an older product line in front. Note wear and different tooth division
New CDN belt in the rear and a used belt (15 000 km) from an older product line in front. Note wear and different tooth division

As mentioned earlier in this blogpost, CDN and CDX components are interoperable, assuming you dont have an old version of CDX. So in an effort to save some money, we first tried to get away with a CDN belt with 118 teeth. As expected, this belt worked flawlessly with the CDX rear cog and CDX chainring described above. However, after about 700 km the we started having problems with the belt. It started with some occasional belt slips under heavy load. As the wear progressed, the belt started to climb on the rear cog. After 860 km the belt was in practice broken and would not engage properly.


In all fairness we should say that this is not a problem with the belt, but with our use. Gates explicitly states in their product description that the CDN product line is designed for casual urban cyclists and city cyclists who occasional commutes, and that the CDN offers an entry-level option for more recreational riding, also not for all year use. This obviously holds true.

CDN and CDX belts
The furry fellow in the back is a CDN belt after 860 km (no longer usable), brand new CDX belt in front

If you use your bike for regular all year commuting, or more demanding applications like touring, the CDX product line is definitely the right choice. After we wore out our CDN belt, we changed to a CDX belt. That CDX belt has now 2 288 km on it, including winter conditions, and still runs super smooth. It shows no signs of wear, and will most likely work perfectly for many thousands of km to come.


Install the components of the drivetrain

With all the tools and components at hand, everything is set to finally make the actual replacement of parts.


Start by removing the pedals from the old crankset. Then use the crank arm puller to remove the crank arms from the spindle. With the crank arms out of the way, unscrew and remove the bottom bracket. Give the inside of the bottom bracket shell a good clean. Loosely screw the new bottom bracket into the shell, drive side first. Once the new bottom bracket is in place and both cups are finger tight, fasten the drive side and then the non-drive side with the square taper bottom bracket tool. According to the instructions from Thun, the cups should not be greased. This is probably because the cups are of plastic and may be damaged over time by grease. Of old habit we went with grease anyway, but its probably a good idea to stick to the instructions from Thun.


With the bottom bracket in place, attach the crank arms to the spindle. These should be fastened quite hard. Here you should definitely not use grease as this will reduce the friction between the spindle and the crank arms, and you want them to be as tightly coupled as possible. Once the crank arms are in place, unscrew the CDN chainring and fit the CDX chainring. Use 12-14 nm of torque to fasten the chainring bolts.


With the bottom bracket, crankset and chainring all set, move on to the rear hub. Obviously the rear wheel should be off the frame at this point. Start by removing the old rear cog. You need to remove the cassette joint, dust cover and snap ring in order to get to the rear cog. With the rear cog off, simply replace it with the new rear cog. Use the Gates SureFit installation tool to ensure that the rear cog is securely seated on the Shimano Alfine 11 hub. For a bit more guidance on how to partly disassemble and assemble the Shimano Alfine 11 hub, see this article.


With the rear wheel still off the bike, gently split open the frame and install the new belt. To our knowledge the direction of the belt is of no significance. Once the belt is in place, refit the rear wheel. Take great care when seating the belt onto the cogs. These belts are extremely durable once correctly installed, but they are very susceptible to damage if they are flexed sideways or forced onto sharp edges. It is imperative that the belt is simply put onto the cogs. Under any circumstances, do not not roll or pry on the belt, as you may be accustomed to with chains.


The last step of this odyssey is to adjust the tension of the belt. This is done by adjusting the horizontal dropout tension mechanism of your bike. A belt with too low tension might slip and wear prematurely. A belt that that is too tight will create excessive drag in the drivetrain and prematurely wear out the bearings in the hub and bottom bracket. As shown in the picture below, we prefer to use the analogue tension meter from Gates in order to get the correct tension.

Gates tension meter
Tension just where it is supposed to be

The previously mentioned app from Gates for calculating belt length, also contains a function for checking belt tension which can be used instead of the tension meter. You need to pluck the belt as you would with a guitar string. The app records the frequency at which the belt vibrates and advices on what frequency a correctly tensioned belt should emit. As a simple rule of thumb, if the belt skips, the tension is too low and if the drag seems excessive, the tension is too high.


The pictures below show our drivetrain before and after the replacement. Notes that snubbers are supposed to keep the belt in place when the frame flexes under load, but they should only be used when specifically mentioned in the instructions and they should not make contact with the belt when the bike is still. Using a snubber when not recommended may cause adverse effects to the drivetrain.


Maintenance

There really isn't much maintenance needed to be done on a correctly configured Gates Carbon Drive system, but it can be a good idea to regularly spray the belt and cogs with water. This would particularly be the case during winter months to get rid of salt rather sooner than later. Its also a good idea to check the tension every now and then, even though the belt itself will not stretch over time like a chain. This article from Tern bicycles gives some nice advice on basic maintenance.


At this time, there is not much more to do other than to roll out and enjoy thousands and thousands of pretty much maintenance free kilometers! We haven't seen Gates give an exact number, but some say a Gates CDX belt can have a lifespan of up to 30.000 km. The one we replaced during the course of this article had 15.000 on it and was still working fine.

Gates drivetrain with snow
Behold, a maintenance free drivetrain, impervious to the most hideous of conditions

Further reading

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