Electric_Elvis

I have a DA 7800 set-up, standard crank and 12-27 cassette. When I am in the big/big combo and back pedal the chain works its way down the cassette a few cogs, binds, then falls off the crank inward. This only happens in the largest front and rear gear.

I think the obvious answer to this problem is the chain, but the chain seems ok and does not seem to be stretched at all. The chain is the correct length, if I make it any longer it will rub the derailer cage in the small/small combo.

The rear derailer seems to be alighned correctly, I have checked the hanger alighnment and it is fine. Before I spend the money on a new chain, I wanted to check with you guys to see if you had any other suggestions on what the problem could be.
Maybe the rear derailer pulleys need to be replaced, or the spring in the derailer is worn out?

CACycling

First, you should never be in the big big combo. That is cross chaining and puts the chain at a very sharp angle. Second, back pedaling serves no purpose so why do it? The extreme cross chaining coupled with back pedaling is most likely the cause of the problem, not the symptom.

Homebrew01

Yup, Don't backpedal when cross-chained.

LarDasse74

+1. Don't use the big-big combo. It causes all sorts of problems, including chain falling off and increased drivetrain wear. There are other gears that are closeto that ratio but are not cross-chaiing.

I plugged 53/39 chainrings and 12-27 cassette into Sheldon Brown's Online Gear Calculator and found that 53/27 gives you a ratio very close to using 39/19 or 39/21.

And if you feel you simply must use the big/big combo, don't backpedal. Backpedalling will get you nowhere.

BCRider

+10 on this being a bad combo. But in any event the chain falling off from backpedalling in this combo is perfectly natural. Just consider the huge cross chain angle and the fact that when you backpedal that there is no derailleur to guide the chain onto the rear sprockets. In fact it would be perfectly normal to have the chain drop outwards for pretty much any of the bigger 3 or 4 rear gears.

FBinNY 

While I disagree with the others on the policy of never using either fully crossed combination (big/big or small/small) I agree that it's inefficient and suggest you avoid it except for short situations such as for a bit of help topping a short hill.

You don't have a technical problem but need to understand that the derailleurs can only guide a forward moving chain. When riding crossed, the chain is feeding the sprockets from an angle and unguided, it is likely to climb over the side and derail. It's more likely today than it used to be, because of narrower chains and shifting gates on sprockets.

So avoid crossed combinations, and if/when in them avoid your problem by not backpedalling.

himespau 

the small small combo you speak of is equally bad.

ClarkinHawaii

I hate to show my ignorance, but why would you want to backpedal?--I mean, what purpose does it serve? It's just never occurred to me to do it . . .

o0adam0o

I think the OP back pedals to get his pedal in the right position while he is standing still and about to take off. Has happened to me before..

FastJake

Don't ride in the big/big or small/small combo. It should never be necessary to do so.

DannoXYZ 

That's possible, not sure on how much rotation is needed before the chain is pulled off. You can always lift the seat and spin forward for half a turn as well. When coming to a stop, I always shift off the big-ring and into a gear I want to use when taking off anyway.

himespau 

Sometimes I backpedal when riding with my wife. In that case it's because we're going so slow I feel like I'm going to fall over if I don't have the momentum of pedaling to balance me. Probably doesn't help, but gives me something to do when coasting at <5mph. I suppose I should just downshift to my lowest granny gear and spin, but sometimes I this is just easier.

fietsbob

"When I am in the big/big combo and back pedal the chain works its way down the cassette a few cogs, binds, then falls off the crank inward. This only happens in the largest front and rear gear."

Its a dreadful chainline , you should be putting the chain on the small chainring and a cog in the middle of the cassette,

to get the ratio that is offered by that very displaced chainring on the far right,
and the cog on the far left side of the cassette .. combination.
Running that S bend in the chain also wears it out faster too.

Electric_Elvis

Yes, exactly.

Where I live now it is 99% flat, and I can do most of my riding in the big ring up front and the 12-27 cassette. Most of the cross chaining comes when I stop at a light, and I am either on a slight incline or extremely tired. It seems like a waste to shift down in the front to only have to shift right back as soon as we get going again, so I end up cross chained.
Being aware of this problem I have tried to avoid the extremes, but sometimes its unavoidable.
But like o0adam0o said, I will forget or not realize I am completly cross chained, back pedal to get my feet in position when stopped, and the chain will get all squirlley. Dont think it has ever completly fallen off on the road in this situation, but on the stand trying to figure out the problem it has.

531phile 

You should still be able to backpedal with the big/big combo and not have the chain fall off. Could be a combination of things. I'd check to make sure the front derailleur is adjusted properly. Once that's done and it still does it, check to see if the "L" or Low limit screw on the rear derailleur is set so the derailleur goes all the way inwards without causing the chain to fall inwards (towards the spokes). After that, if it still does it, you might want to increase the cable tension on the rear derailleur by turning the barrel adjuster counter clockwise a half turn at a time. Just make sure that you are able to downshift to the harder gears, if you can't you have too much tension. You need to find a balance. If it still drops while backpedaling. Maybe your bottom bracket isn't the optimal length. Also, your frame's chainstay might not be optimally designed for a good chainline some frames just have poor chainlines. In this case you can't do anything. Just make sure you don't cross the chain. I've worked on a 2010 Specialized Hard Rock and that frame has one of the worse chainlines I've ever seen. The bottom bracket length might not be optimal on that frame or the chainstay angle could be out of wack.

Scrockern8r

Do you find it necessary to run 39/27? If not, it may be easier to run the rear cog in the same gear and drop the front ring for a stop.
Sometimes I tend to shift the front ring while in stop and go traffic and leave the rear cog in my "cruising" gear. Which is the 4th cog down.

I find it makes a good high/low and saves travel over the cog for each stop.

Maybe try that?

Pwnt

Which side is the chain falling to? It may be a little bit of a limit screw issue as well. But +1 on everything else that has been said.

FBinNY 

Take a glance at your bike and it'll be obvious that neither derailleur has any effect when back pedaling, so spare yourself the effort of making non-relevant adjustments.

If backpedaling while using crossed combinations is important to you, you might solve the problem by using a chain with more bellmouth or chamfer on the inside of the inner plates. This will improve pickup when coming from an angle. You could also fool around with chainline, or the profile of the chainrings. But the problem is so minor in the scheme of things, and easily avoidable, and so isn't worth fixing.

Tom Pedale

I got a new set of wheels for my Tarmac. While I don't ride the "big-big" combo, I noticed the chain would come off the next to largest cog in back while pedaling backwards on the big chain ring. I checked the chainline and it appeared that it was off. To fix, I put the biggest spacer I could fit between the cassette and hub while still being able to securely fasten the cassette lock ring. Problem fixed.

It's also possible that since I've been running the same cassette for several years that the fit between a new chain and this cassette along with the slightly off chainline caused this problem.

o0adam0o

Well i like to lift my whole bike with one hand and spin forward with the other hand just in case any ladies are watching. It also makes my 20lb bike look like its 15lbs

silver_ghost

"A guy walks into the doctor and says 'Doc, it hurts when I do THIS..."

zacster

Every now and then I find myself in the big/big or 2nd largest cog. It is fine while riding, but if I get off the bike with it still in that combination the chain invariably falls off from by getting pulled from the tension of the derailleur. It's kind of comical as I can do a 50 mile ride with no problem and the chain falls off when I get home.

clasher

Why do people have such an aversion to using their front dérailleurs?

Homebrew01

What the heck does any of this have to do with the problem ?

OP, you've already got your answer.

Electric_Elvis

I wonder if the fact that I am not using the 1mm spacer that goes on the hub prior to the cassette has anything to do with this.

The short story is that I was in a race this spring, somebody bumped me and bent the rear derailer hanger. Got a new hanger and different derailer, but the bike never shifted correctly. Got the derailer hanger alighnment tool and adjusted it, but shifting was still slightly off. Although, I dont think it was shifting that great prior to the bent hanger.
Tried various things, got a new cassette, nothing worked and just dealt with the poor shifting. Recently I got a new rear wheel, and when installing the cassette noticed that it fit tight without the spacer. So I tried it without it, and with some adjustments the shifting was the best its ever been, until I noticed this new issue I have been having. Its a PT wheel, but I just checked there website and it doesnt look like they make 10spd only hubs.

I can see how not having that spacer would cause the chainline to increase, and especially in the big/big combo cause problems. I'll take everybodys advice and just avoid the extreme combos!