Bicycle salespeople hear the sentence far too often: "My bike is pedalling so hard, I'd like a new one please." Offering a pedelec with the extra power to overcome the heavy gear ratios is a logical step, but perhaps not always the most customer-friendly and certainly not the cheapest. This "hard pedalling" can be due to bad bearings, the wrong tyres or the very high weight of the bike. In any case, a smaller gear ratio in the crank drive can help. Gears, gear ratios and gearing are basic concepts that you should know in order to understand your bike. A brief explanation can be found below below.
>> Part 1 of our DIY articles on the subject of drives can be found here: Chain wear: How do you maintain and repair a bicycle chain? <<
Changes due to sprocket replacement
Changing the gearing of a bike significantly changes its character. A bike that you could hardly get up a hill on before, even in the lowest gear, can be pedalled up with much less power after replacing the chainring and/or sprocket. You will be travelling a little slower, but the cadence can be kept above the rideable limit. Disadvantage: As the change affects all gears, especially on hub gear bikes, the gears for high speeds are also downgraded. Downhill, this can mean that you hardly have any resistance on the pedals even in the highest gear. With derailleur gears, individual sprockets can usually be removed and replaced.
Translation and development
In contrast to the gear ratio, the gear range does not indicate how often the rear wheel turns, but how many metres are covered; the wheel and tyre size play the decisive role here. Using the example of a Rohloff hub, the diagram above shows the not entirely linear progression of the 14 "gear steps". If you replace the sprocket on the rear wheel with one with more teeth, all 14 gear steps are slightly lower. The valuable gear hub does not have to be touched. For most riders, the smallest gear is decisive: if you lower the gear range, you can ride slower and easier uphill without the cadence becoming unbearable. A difference of one or two teeth is usually enough to make the bike significantly more "mountain-ready".
Formulas
- Translation = "Front chainring teeth" divided by "sprocket teeth"
- Unfolding = "gear ratio" times "tyre rolling circumference"
- Gear hub transmission = ("chainring teeth") divided by ("sprocket teeth" times "internal gear ratio" times "rolling circumference")
Reasons for replacing the chainring or sprocket
Removing and replacing chainrings and sprockets may not only be necessary if the gear ratios are unsuitable. It is far more common to replace them one-to-one due to wear or even corrosion. As shown in the Article on chain wear a poorly maintained drivetrain on the bike can eat up a good fifth of the pedalling power, rusty and sand-roughened parts even more.
The cost of a spare part such as a single sprocket is so low at an average of 12 euros that you should not wait too long to replace the sprocket. As with the wide chains on bikes with only one rear sprocket, such rough-running surfaces are more likely to be the reason for replacement than the actual wear of the teeth. You can tell whether sprockets are worn by looking at the individual teeth or you can feel it if the gears can no longer be adjusted properly or if there is so-called chain slippage when the chain lifts over the teeth when pedalling hard despite sufficient tension and you think you are pedalling into empty space for a short time.
Users of belt drives do not have such problems, or only after a long time. The two counterparts to the sprockets, known as "drive pulleys", wear much more slowly because they are significantly harder than the surface of the belt and are generally made of stainless material.
Visible wear and tear: time to replace
You can recognise that it makes sense to replace the sprocket by the fact that the shape of the teeth resembles a shark fin. The front and rear profiles of such a sprocket are symmetrical ex works. This means that the rear shape in the direction of travel, against which the chain pulls, is identical to the front. Due to the immense tensile forces as well as friction and dirt, the axles of the chain - to exaggerate - mill into the tooth. The shape on this side changes from convex to concave. If this is clearly visible to the naked eye, the sprocket or entire cassette should be replaced.
Interesting fun fact: Although the chainrings at the front of the crank statistically make more revolutions over the same distance than the sprockets of an 11-speed derailleur, for example, they wear more slowly. This is due both to the distribution of the pulling force over more teeth and therefore less friction per tooth and to the straighter running of the chain. In comparison, the chain pulls at a much greater angle, especially on the outer sprockets, and thus causes more wear.
If the chain has not been replaced at the same time as the cogwheel, the drive may run a little rough for the first few kilometres as the parts have to get used to each other. Here, an extra portion of lubricant helps to significantly reduce the almost painful running noise, at least for technology fans.
Pinion replacement instructions
Sprocket replacement for hub gears, geared bikes or single-speed drives: If only one sprocket is installed at the rear, it is screwed onto the hub with a thread like a union nut. To prevent it from accidentally unscrewing from the hub when pedalling backwards, there is usually a lock ring in front of it, which must first be loosened. Caution: Its thread is the exact opposite of that of the sprocket! Mnemonic: The cogwheel tightens itself by pedalling, so it is loosened against the direction of rotation of the wheels, while the counter ring is loosened with the direction of rotation. This can be done very safely with such a "puller for counter rings", experiments with hammers and screwdrivers usually leave notches on the ring and fingers. Costs: around 15 to 20 euros.
Without the counter ring, you can unscrew the sprocket against the direction of rotation of the wheels. However, as this can be very tight due to the pull of the chain over thousands of steps, you definitely need good tools here. Pliers like these "CP-1.2" from Parktool grip the teeth firmly without damaging them like pipe wrenches and the like (do not use them!). At the same time, they have sufficient leverage to loosen even stubborn threads. So-called chain whips are cheaper but more fiddly. When fitting the new sprocket, proceed in exactly the opposite way, aligning the web or the widening around the thread to the hub side. Tip: Lightly grease the thread. The sprocket does not need to be particularly tightened, this is already done by the chain when riding. The counter ring can be retightened after 20 kilometres.
Today, derailleur gears rely on so-called "cassettes": The sprockets are not pushed onto the hub individually, but as a block or as a block with several individual sprockets. For example, you can remove the smallest sprocket and add a large one on the hub side to make it more suitable for mountain biking. The cassette or its end ring is loosened with a socket spanner, as a nut for a ratchet they are much cheaper.
The end ring is opened again against the direction of rotation of the rear wheel. However, as derailleur gears have a freewheel, the entire cassette would rotate instead of the ring, which is why you have to use a tool such as pliers or a chain whip to hold it in place. Counter-holding is not necessary for fastening. Caution: If the bike has a Campagnolo groupset, a differently shaped tool may be required for the lockring.
Some force may be required to remove the cassette. The profile (red) on which it sits is often made of aluminium, and the cogs eat into the material to a minimal extent. There are hubs for Campagnolo-compatible cassettes and those for all other brands, and the number of sprockets must be the same, e.g. 11-speed. Attention: The new cassettes/sprockets only fit in one position on the hub. Then tighten the lockring with just a little force and you're done.
5 tips for buying the right spare parts for sprockets and chainrings
If you want to put new teeth - i.e. new sprockets and chainrings - on your bike yourself, you need to make sure you get the right spare part, while the actual screwing is actually child's play. The following tips will help you not to "sell out".
- A cogwheel on the crank, called a chainring, can also wear out, although this takes a long time. It is only fastened with Allen screws and is replaced one-to-one. Important: The four or five "arms" of the crank result in a bolt circle that must match that of the chainring. It is usually stamped into the chainring behind the abbreviation BCD and is 130, 110 or 74 mm. The new chainring must correspond to this.
- If you want to change the cassette and fit a larger "biggest sprocket", you should know the capacity of your rear derailleur. The longer its tension arm is, the more difference between the largest and smallest it can handle; two or three teeth more are usually not a problem.
- Quality: All manufacturers offer sprockets and cassettes at very different prices. For frequent riders, the second-highest or highest quality level is worthwhile as they wear out much more slowly. Occasional riders are well advised to go for the mid-range; an 11-speed cassette, for example, costs around 50 euros. Cheap products will only make you work again unnecessarily early.
- The threads of sprockets for hub gears are standardised; however, there is a type that is significantly smaller (BMX bikes). Please pay attention to the designation "single speed" or "for hub gears". Tip: buy a stainless steel sprocket.
- Different sprocket diameters require different chain lengths. If the chain can no longer be tensioned with a new sprocket, it must be shortened or lengthened by one or two links.
