people_atease

Hi everyone,

sometime when i reverse pedal the chain falls off (at the front) ... Would it be a problem for the gear shifting setting ?

Noraml gear shifting is fine...

dabac

When pedalling in reverse, there's nothing that acts as a proper guide to the chain any more. The more cross-chained(big-big/small-small) you are, the bigger the risk of the chain derailing.

If normal shifting is fine, then don't expect gear adjustment to sort it out.

But why back pedal? Anything more than a half turn at most (a quarter turn would usually be sufficient) kinda indicates poor pedalling technique.

hueyhoolihan

could be caused by many things. easiest to check for gunk on chain and/or derailleur. just make sure everything back there is clean and lube. then check for alignment issues. (bent stuff, loose or misaligned wheel). you should and must be able to back pedal without the chain falling off.

wmodavis

What dabac said.
Bikes don't work in reverse. Only made for forward pedaling.
Derailers are designed to prevent this activity but based on the number of posts about it - it isn't working.

rydabent

However is this happens at the RD it pretty much indicates an adjustment is needed.

+1 on not back pedaling tho.

FBinNY 

IMO- the "don't back pedal and your chain won't fall off is a cop out of lazy or inattentive mechanics.

While it's hard to justify backpedaling, sometimes there's a legitimate need to, such as to position a pedal for starting from a stop, or to quickly lift a pedal on a mtn bike to clear a sudden obstacle.

If all is OK the chain shouldn't fall off when back pedaling. Obviously there's no derailleur at the bottom of the chainring to prevent it, but by the same token the chain should stay on the chainring when pedaling (forward) without touching the FD cage, or without the need for an FD at all, such as in a 1x9 setup.

Usually when the chain falls off backpedaling it's because the RD isn't vertical, so the lower loop has more angle (cross-chain) than the upper, and is a sign that the RD hanger isn't square. You can do a quick eyeball check by sighting from the top with the chains directly above each other. The lower loop should be parallel to the upper and able to hide under it nicely.

With a bit of attention any decent mechanic should be able to keep the chain from coming off when backpedaling except, maybe, in the big/big crossed combination.

neurocop

If the FD and RD throws are properly adjusted you still have to deal with the offset in the chainline, and this is greater for the big/big (and small/small) combos, and unless your hub is limited in size this problem will be hard to correct. The most you can do is to correctly set your chainline, chain length, and derraileur throw set screws, but with larger (wider) hub sprockets, you will still get some lateral pressure on the chain that will tend to force the chain of the CR's when you pedal backwards. This is true for forward pedalling as well, but drivetrains are designed to pedal forward and work better that way...to see this, just try to shift while pedalling backwards... This is especially true for the modern CR designs that have asymmetric teeth...

JiveTurkey

What adjustment would be needed? If the cable tension is set correctly to have proper shifting while pedaling forward, what adjustment could be made to allow back-pedaling without messing up normal operation?

FBinNY 

You cannot make any adjustments to prevent the chain from moving over a sprocket on the rear. It will tend to do to find a straighter chain line when the chain is coming from too much angle.

But there's a fundamental difference between the cuts of front and rear sprockets. Cassette design assumes that the chain will be fed straight onto the sprocket by the derailleur pulley, so the teeth are cut fairly square. OTOH, chainrings are designed to accept chains coming from a range of angles and the teeth are beveled to points to help them pick up chains coming from the side. In normal use a chainring should be able to pick up a chain coming from all but the most crossed over combinations without need the FD to guide it. In fact the chain should never be touching the FD cage at all except during a shift.

While it may not be possible to totally avoid the chain coming off when backpedaling, this should be a rare event, except when badly crossed. As I said in the earlier post, if the chain chronically derails when backpedaling it's an indicator the the lower loop chain angle is different than the upper loop's, which usually means a mis-aligned hanger.

I still hold that 99% of the backpedal chain fall-off issues are solvable, and this is a good demonstration of the difference between mechanics and those who hang components on a frame and hope for the best.

To the OP, if you tell us the city you're in, I might be able to refer you to someone who can solve the problem. If you're in the metro NY area, I'll do it for you for free.

JiveTurkey

Every time I've had a bent hanger, I couldn't get the shifting to work right (in every gear), which was far more noticeable than the inability to back-pedal. My point is that if everything else works well, I don't see what could really be done to fix this problem. Of course, we're getting off topic because the OP said the chain derails at the front.

people_atease: How many chainrings in front do you have and which one are you in when it derails? Does it generally do this when in the largest (outside) chainring and largest (inside) cogs in back?

BarracksSi

There's friction somewhere, and it could be just a dirty, gunky chain.

Two things I can think of that would happen with a nasty chain -- it could actually make the RD "open up" (this could also happen with dirty jockey wheels) which almost erases tension on the chain and lets it fall off, or the chain could fail to unstick from the front chainring and get caught under the FD cage.

I can backpedal as quickly as I can front-pedal on my bikes and they're fine.

Not necessarily THE solution, but something to check for.

FBinNY 

The question as to whether this occurs only in crossed combinations, or more generally is a valid one. If it's only in combinations that are rarely used the OP should live with it, and we might remind him not to ride in crossed combinations. But if it's more chronic, or chronic enough to become a nuisance it can be fixed.

In my 45 years riding bikes, including those in the dark ages when they were far less reliable than today, I've never owned, nor would have tolerated, a bike that routinely dropped chains backpedaling. I'm not saying a chain never came off backpedaling, only that is was a very rare event, well within my low range of tolerance for mechanical issues.

DiabloScott

Also could be a worn out chainring - if the teeth are sharkfinned the chain will act similarly to being gunked up. When backpedalling, the chain will hang on below the tangent point on the top, and then let go suddenly.

people_atease

I had 2 chainrings in the front.. and it was in the bigger one when it derails..
Last time it happened when I was panic and back pedalled as I was losing balance with my clipless shoes (silly, i am new to that ) The rear chainrings should be at no.6 or no.7 counting from the left side.

people_atease

I just got my bike few weeks ago, I think it's bad adjustment instead as I tried to mess with the screws on the FD and RD since the gear shifting was not so smooth and i couldnt get it to the Biggest chainwheel at the back.

btw it's didnt drop off frequently, but the gear shifting is smooth now. Just that I wonder why it fell off and caught between the crank and the wheelchain set.

people_atease

Btw when my bike chain is on front chainwheel biggest and rear chainwheel biggest the chain is kind of touching the side of the FD, is it normal? I am using a 9speed sora. (like the tail too far outward chain touching the side)

FBinNY 

OK, it sounds like it dropped to the outside with the chain coming from the outer side of the cassette. This is a combination where the chain shouldn't derail, as the line is normally pretty decent.

Take a few minutes to make a few simple measurements. You'll need a friend, a straight edge about 26" long or so, and a smaller ruler. Shift the bike to a gear similar to when it happened. Have your friend hold the straightedge vertical against the rear wheel, just forward of the derailleur. Measure the horizontal distance from the chain to the straightedge, on both the upper and lower loop.

I'll bet 2 beers (imported) or a bottle of Chain-L that the lower loop is farther out than the upper.

BTW- I should add that there other possibilities, chain wear, chain lube, chainring wear or tooth profile, but start with a measurement because it's free and will conclusively confirm or disprove that the RD alignment is a likely factor. If it is have the hanger checked and see if that solves it, if it isn't derailleur alignment, then you can move on to the other possibilities.

bkaapcke

On a RD, adjusting the low limit stop 1/2 turn, moving it towards the spokes will eliminate this problem. bk

FBinNY 

Definitely not, won't/can't help, and might cause harm.

The problem is described as the chain falling off to the outside (towards the crank) when on the outer chainring, and one of the outer cassette sprockets. Since the low limit is to the inside, and in any case has nothing to do with the position of the RD except on the innermost sprocket it can't do anything to help the OP.

OTOH, adjusting the low gear to allow the RD to move inboard, even by as little as a half turn could cause the chain to over-derail past the largest sprocket and into the spokes. Not a good thing at all, and possibly a very bad thing.

Limit screws are exactly what they are called. they limit the innermost and outermost position of the derailleur, no more no less. They have absolutely nothing to do with any of the intermediate positions of the derailleur, which are controlled by the cable.

jack002

That's the "large/large" chain combination, you should never use it. You get the same ratio with the small chainring and one of the medium cogs. Most modern bikes are not intended to ever be shifted into that combination. (The 10speed cassettes have even more restrictions like that as I understand)

Old Hammer Boy

May also be not enough tension on the "B" adjustment to pull up slack properly. Also, idle wheels on RD may need lube. If they are not free, they could allow the chain to slacken up too much.

people_atease

so touching is normal ? I am not going to use it but it seems to be one of the elements to show if the gear shifting mechanism is working perfectly? no?

FBinNY 

the chain touching the inside of the cage when crossed large/large is normal. The chain comes from a range of angles depending on which rear sprocket it's coming from. Normally the FD is trimmed so the outer plate just clears when the chain is coming from the outermost rear sprocket to the large chainring. But when the chain comes from the extreme inside angle it'll rub on the inner plate.

Some systems have the ability to trim the FD over slightly so you can correct it as you ride. You might wonder why they don't just make the cage wider, and the answer is that it would make shifting sloppy, so they make it wide enough to clear the combinations you're expected to ride, and no more.

cobba

https://cycle.shimano-eu.com/publish/...faqs_0012.html

RollCNY

Difficulty in getting the rd to shift into biggest gears, and issue with backpedaling issues really suggest bent RD hanger. On a new bike, that is not super uncommon. They have been shipped, and stored, and handled, and that never really gets checked in assembly.

Best means to check it is the tool that bolts into the hanger, and then measures the distance to the wheel as you swing it. You can make one yourself with any solid bar and a drill press.