when pedaling backwards chain falls

Question

When pedalling backwards chain falls from front single sprockets.

It happens only when on rear cassette chain is on the smallest cog.

Any suggestion ?

Image shows a moment before the chain completely falls.

(It's a simple kids bmx style bicycle)

Answer 1

That amount of chain slack is caused by the derailer spring being too weak to pull the chain through and maintain tension on the top. This is ultimately cause by too much friction in the drive line.

Before doing anything too drastic, a clean and lube of the chain and derailleur might solve the problem. I would remove the derailleur pulleys and clean and lube the bushes (maybe even replace them) Look at the hub freewheel/free hub and ensure it is coasting freely. A hub service could be in order.

Next most likely cause is the derailleur spring is weak, which can happen over time. Replacing the derailer is usually the best option (A weak spring indicates an old deraileur, so likely it is worn out).

In the cross chained position as shown the drive line is more prone to this kind of problem as the chain angle has increased friction. Ideally a drive line in good working condition would not do this even with a crossed chain, but some remain persistently stubborn in which case the best option is learning to ride in a way that avoids it happening (i.e. don't back pedal in high gear).

Answer 2

Do not pedal backwards when chain is on small sprocket. There's no benefit to reverse pedaling. While there is a ratcheting technique one can use in a technical situation where a full forward rotation of the crank is not possible, I would argue that during such a technical situation, the highest gear the bike has to offer is not the appropriate one to be in.

The problem stems from the necessary evil of cross chaining. This is chain line that is at an extreme angle to the fore-aft line of the bike. It happens when the chain is on the extreme ends of the cassette. A proper chainline (where the chain runs in a line parallel to the centerline of the bike) generally would run at or very near the center of the cassette. For a 7s cassette, you may find the chain to be parallel to the bike centerline when the chain is on the 4th cog (from either direction, largest or smallest cog).

I'm not getting into the intricacies of multiple chainwheel bikes, but essentially, a biker minimizes cross chaining by selecting gear options that utilize the various front chainrings which are more in line with your rear cog selections. Instead of using the chain on the large chainring and larger rear cogs to achieve a lower gear, you could shift the chain to the middle or small chainwheel and select a middle rear cog. Because gearing is duplicated in multi chainwheel bikes (for example a 3x8 drivetrain does not have 24 unique gears--especially ones a person can feel a difference--rather, some gears will be duplicated. Of the 8 gears that can be selected when the chain is on the large chainwheel, some will be duplicated when the chain is run on the smaller chainwheel but at a different cog in the rear.

What this has to do with cross chaining is that to minimize the chain angle (and thus, excess wear, friction and noise of the system), one selects a different chainring and different cog that maintains a straighter chainline while the pedalling (the gearing) feels the same.

When you're cross chained and the cranks are rotated backwards, chain tension is reduced. So first, the chain has reduced purchase on the sprockets. Next, the rearward rotation and extreme chain angle cause the chain to catch a tip of a cog tooth (the same one it's riding on or the one next door), and that, in combination with reduced chain tension, causes the chain to attempt moving off the current cog and derail or skip along on the teeth of a nearby cog.

Falling off the front happens similarly when chain tension is reduced, the chain has reduced purchase on the chainwheel, the extreme angle causes the slacker chain, rotating backwards, to "catch air" instead of the next tooth of the chainwheel. The chain drops off as the rotation continues. The whole thing is exacerbated with a normal (a Non-narrow/wide) chainring. The thinner profile of the normal chainwheel allows more side to side movement of the chain. Great for shifting between multiple chainwheel or derailing off a cross chained system.

Potential fixes if this is unbearable, are many. You can retune the system beginning with checking the front chainline. 48-50mm would be desired. This is the distance from the center of the bike (center of bottom bracket shell) to the point 1/2 way between the chainwheels of a 2x; the tips of the teeth of a 1x; the tips of the teeth of the center chainwheel of a 3x. A 48mm front chainline would increase the chain angle when running on the small cog. Chainline is somewhat of a fixed parameter depending on the bottom bracket and the crank itself. Your problem would be improved by a 50mm front chainline.

Next option is to reset the H limit setting of the rear derailleur. There should be no influence of cable tension when doing this. You could turn the barrel adjuster at shifter or derailleur (if present) full clockwise. This reduces cable tension. Ideally, release the inner cable from the derailleur pinch bolt. Turn the H limit screw on the derailleur so that the tips of the teeth of the upper, jockey wheel of the derailleur line up under the outside edge of the smallest cassette cog. The jockey wheels teeth should be a hair outside the plane of the cog teeth tips. This is all observed and determined from a perspective directly behind the derailleur. Turn the cranks, listen for excess chain noise. You could turn the cranks in reverse, slowly loosen the H limit screw until the chain starts to make noise, then tighten H limit screw back about 1/4 to 1/2 turn from the point the chain noise just begins. This is all done with the chain on the largest front chainrings.

You'll then have to reconnect your inner cable to the derailleurs pinch bolt (use care that the slack is removed from the inner cable BUT not pull the cable so much that the derailleur cage begins to move. From this point cable tension is set by manipulating the barrel adjusters. A good beginning point for the barrel adjusters is about 2 full rotations out from fully tight. Gives you room to work tension either way.

There are additional fixes like replacing a chainring with a narrow-wide chainring. Using spacers--either for the drive side crank or the chainring--to manipulate the chainline. These are more expensive, complicated and can affect other aspects of the shifting.

I recommend the easiest option. Refrain from pedaling backwards in high gear.

Answer 3

In my case, I had something similar and the cause turned out to be lots of dirt on the derailleur pulley wheels. The dirt wasn't on the bearings. It was on the teeth. However, the sticky dirt on the teeth caused lots of friction when the chain travelled through the derailleur, meaning the derailleur spring wasn't strong enough to overcome the friction, and back-pedaling didn't work well.

So I would first check the teeth of the derailleur pulley wheels. If there's dirt, use a flat blade screwdriver to remove it.