Description of the problem:
When pedaling backward and coasting the chain loses tension and drops on the drive side chainstay. When you resume pedaling, the chain regains tension and springs back into proper shape.
In most cases, the pedals will be moving even when you’re walking the bike forward as if you’re on a fixed-gear bike without a freewheel.
Causes of The Problem
Sticky Freewheel or Freehub
The most likely route of the problem is a contaminated freewheel or freehub which does not freely rotate because the pawls and the inner bearings are sticky and non-responsive.
As a result, when you backpedal, the rear cogs barely move and prevent the chain from rotating normally.
During backpedaling, the chainring pulls the lower part of the chain. When the chain is prevented from moving freely, it drags the cage of the derailleur towards the front of the bike.
The spring in the derailleur cannot counteract the pull and the chain sags. When you resume pedaling, the cog moves forward, and the chain is mobile and free again.
The solution is to degrease and re-lubricate the freewheel or the freehub.
First, it’s necessary to clarify the difference between a freewheel and a freehub.
The freewheel is an older system that combines the rear cogs and a ratcheting mechanism in one package.
The freehub, on the other hand, represents a hub to which the spokes attach and comes with its own ratcheting mechanism. The freehub is combined with a cassette which does not have moving parts.
If you have an older, low-end bicycle, chances are that you have a freewheel. If your bike is new and mid-range or above, you probably have a freehub + cassette.
To know with certainty, however, you will have to remove the rear wheel and examine the hub and the cogs.
A freewheel will have different cutouts in the middle compared to a cassette.
The lockring has two “dots”.
Determining whether you have a freewheel or a freehub is very important because each unit requires different tools for removal.
If you have a freewheel, you will need a freewheel removal tool that corresponds to the freewheel type.
Most freewheels use a multi-spline tool. However, some older ones (e.g., Suntour’s models) require a 2 or 4 notch removal tools that are hard to find because those freewheels are old and rare.
If you have a cassette, you will need a cassette removal tool and a chain whip which are readily available at every bike shop.
Once you get off the freewheel, it’s time to degrease it. The simplest method is to spray degreaser such as WD-40 through the entrance at the back.
Then, wash the freewheel with soapy water, dry it, and re-lubricate it by dropping oil in the same area.
Alternatively, you could also disassemble the freewheel entirely, degrease and re-grease it. However, this method is more difficult and requires more experience.
If the freewheel is beyond repair, you will have to replace it.
If you have a freehub with free ball bearings, you will have to disassemble it, (you will need 2 cone wrenches) clean it and re-grease it.
If the hub uses sealed bearings, it’s not meant to be serviced, and you will have two options:
- Do a rough service
You can spray WD-40 or another degrease into the crack in the image below. As you do it, rotate the driver to spread the chemical into the ratcheting mechanism. Then, re-lubricate it with oil.
- Replace the Hub
If the rough service doesn’t help, the freehub is practically dead and will have to be replaced.
Unfortunately, this procedure is more difficult, expensive and less user-friendly because you will have to re-lace the entire wheel to a new hub. The new hub may require spokes of a different length.
The task isn’t extremely challenging for cyclists with experience, but it requires a good number of tools – a truing stand, a dishing tool and a spoke wrench.
Loose P-tension Spring
In most cases, the culprit is either the freewheel or the freehub. However, occasionally the derailleur may also be the source of the problem.
If the P-tension spring of the derailleur has slipped or is broken, the derailleur’s cage will remain pulled forward and thus be unable to tension the chain.
The P-tension spring of many entry-level derailleurs is not serviceable, and there’s no way to get to it without damaging the derailleur. In that case, you will have to replace the entire unit.
On some models, however, it’s possible to disassemble the derailleur and re-attach the P-tension spring.
An Excessively Long Chain
In some cases, an excessively long chain may be the source of the problem. If the chain is far longer than needed, the derailleur will fail to fully stretch it.
FAQ: How Does A Freehub Work?
Both freewheels and freehubs are based on a ratcheting mechanism.
The graph below shows a freehub’s method of operation.
A freehub contains a ratchet ring (in this case the blue circle with teeth) and a set of pawls (image below or red lines above). The pawls are spring-loaded. When the rider is pedaling forward, the pawls spring out of their beds and bite against the ratchet ring.
Once the ratchet ring and the pawls become one, the cassette is connected to the hub, and the rider can transfer power to the rear wheel.
As a result, the wheel starts spinning around its axle, and the bike moves forward.
During backpedaling, the ratchet ring slides against the pawls (hence the sound) but does not engage them.
If the pawls are sticky and so is the bearing mechanism inside the hub, the ratcheting ring and consequently, the cassette cannot “break free”.
As result, the pedals will continue to spin even when walking the bike.
During coasting, the cassette and the pawls are not spinning, but the ratcheting ring continues to rotate around the axle while brushing the pawls.
Summary: What You Need To Know
If your chain gets slack when backpedaling and coasting, the culprit is probably the freewheel or the freehub.
The fixing process is as follows:
1. Get the back wheel off the bike.
2. Determine whether you have a freewheel or a cassette.
3. Get the necessary tools for removing your freewheel or cassette.
Those would be:
- Removal tool
- A Socket that can wrap around the removal tool or a wrench
- Chain whip (if you have a cassette)
4. Remove the cassette or freewheel.
5. If you have a freewheel, spray degreaser into the back entrance and wash the wheel. Then, put lubricating oil into the same back entrance until the oil starts dripping from the front.
If the freewheel still can’t rotate freely, replace it.
If you have a cassette, remove it and spray the pawl area (image at the beginning of the article) with a degreaser while rotating the driver.
If this doesn’t fix the problem, you will have to replace the entire hub (costly procedure) or service it fully. (If it’s a cheap hub, it’s probably not serviceable to that degree).
6. Mount the cassette/freewheel and then re-install the back wheel.
In some cases, the problem may be:
- Broken or slipped P-tension spring on the derailleur
- Excessively long chain