jyl

What causes a chain to derail (come off the chainrings) when the rider is backpedaling?

My friend's daughter brought her bike over because her "chain was coing off". I didn't find any problem with it on the stand or on a test ride. I'd tuned it up last year and everything was still fine. Later she told me it happens when she is pedaling in reverse. Why she would do that I don't know, but who knows why teenage girls do anything. No comments about the nicely trued and tensioned wheels, crisp braking, etc. Sigh.

I told her to bring the bike back. Now I need to know what I should look for/do about it . . .

This is a vintage Univega touring bike with, if I recall correctly, Suntour friction shifting and double chainrings. Nice bike.

fietsbob

Dont back pedal.. IF chain is not dead on in alignment it's just going to happen..


have her watch what happens while you demonstrate ..

onbike 1939

This can be caused by a stiff chain link. Watch the chain carefully as it passes over the smallest sprocket and you may see it jump at the stiff link. You can fix this by lubing the link then, using a cloth in both hands, twisting the chain against itself on either sides of the stiff link.

FBinNY 

If the bike test rides OK, specifically if it shows no inclination to derail of the rings when pedaling forward and you don't need to have the FD cage rub to keep it on, there's a good chance that the problem is at the rear derailleur.

Because a stiff link, chain friction, or a slightly damaged chainring tooth can also be factors, start by lubing the chain and checking for stiff links, or a dinged chainring.

Then if you still don't see anything, check that he RD cage is plumb (parallel to the wheel plane) by shifting to a gear where the cage is close to vertical. Set a yardstick or broom handle against the right side of the wheel (tire on is fine) and observing from behind to se if the RD cage is parallel.

I'll bet a bottle of Chain-L against a beer, that the chain is derailing to the inside, and that when you check, you'll find the RD cage tilted in at the bottom. That's because when bikes are dropped or mishandled, the RD hanger gets bent inward. That puts the lower pulley inboard of the sprocket, which means that the chain in the lower loop is coming from a more inboard angle than it does on the upper loop.

So the cure is to straighten the hanger (which is important in it's own right), and to correctly set the inner and outer limits so the RD works correctly, and cannot put the chain into the wheel.

fietsbob

RD banged up in the Uni Bike racks ... it happens ..

Little Darwin

A rear derailleur pulley mechanism is designed mostly to keep tension on the chain, not to align the chain with the chain ring to allow for successful back pedaling. In fact, for the extremes (highest and lowest gears in back) the chain line frankly sucks, and since the front derailleur can only help keep the chain from derailing at the top of the chain ring, it makes sense that back pedaling could lead to dropped chains since it tries to take up the chain on the bottom of the chain ring.

While ideally things should work anyway, unless everything is perfect, back pedaling is the most likely way to cause a chain drop. So, since there is no inherent benefit to pedaling backwards more than a half revolution to position pedals if you misjudge a corner or something, I think that the real answer is to tell the rider not to pedal backwards.

jyl

Thanks! I will check all that - stiff link, hanger alignment, RD limit, etc. Last time this bike came to me, the hanger was bent causing all kinds of malf. I fixed it by eyeball but may not have done a good job.

FBinNY 

I agree with the basic premise, but disagree that dropping chains when backpedaling is normal or acceptable, except maybe in the most mis-aligned combinations.

The front derailleur is used to effect shifts, but doesn't, or shouldn't, touch the chains in normal operation. So that means the chain is expected to stay on without help, and it does. Therefore, if the line is good enough up top, it's good enough underneath (if it's the same line).

That said, chain retention will be better on top, because the normal riding load tension keeps it home, and the FD cage is there as sort of a safety in case it wants to derail. So there's not as a reliable prevent to chains falling off when back pedaling, and some will occasionally, but only occasionally. Falling off routinely means that something is wrong and can be corrected.

IMO- chains falling off more than once in a rare while is unacceptable on a derailleur bike.