Throwing a chain off the big ring.!?

[Road Hazard]

Well, a certain incident in today's stage of le Tour inspired me to ask this question:

Does anyone else have a hard time tuning their front derailleur so that there is no rub when on the big ring and little cog, AND no significant chance of throwing the chain off the big ring when shifting?

I have tried tuning both my Shimano 105 triple and SRAM Force compact for hours on end. For both gruppos, there seems to be a choice: either set it so there is no rub in the big ring/little cog combo, or, risk overshifting the front derailleur so that if you shift too excitedly (happens) you thrown the chain on the outside.

My tuning technique if you want to call it that, is to:

a. release all tension in the shifter by downshifting into the little ring;
b. unhook the cable;
c. use a wrench to pull the cable as taught as possible without really forcing it;
d. tighten the bolt on the cable/front derailleur mech;
e. run the gears on the bike stand to get into the big ring (usually needs a couple twists of the barrel adjuster)
f. adjust the limit screw so that when in the 11th cog, there is no rub on the front derailleur.

Seems to work on the stand, but in the field, I tend to throw the chain off the big ring unless I'm really careful about it.

Any advise, or is this just one of those things where the difference of a few micrometers in adjustment makes all the difference?

 

[ustabe]

c. use a wrench to pull the cable as taught as possible without really forcing it;

That's your problem. You need only enough cable tension to hit the next chainring consistently with the chain running in any rear cog, and to allow the chain to run (barely) without rub on the front cage when in the big-little (highest gear) combination. This actually requires a bit of slack in the front derailleur cable when the chain is on the smallest ring.

 

[Road Hazard]

This actually requires a bit of slack in the front derailleur cable when the chain is on the smallest ring.

Ah I should have known. Another game of opposites, like golf. Want to loft the ball, drive the club downwards.

Ok off to slack out the front cable and see what's what.

Thanks for the reply.

 

[Notso Swift]

I actually do it your way, ustabe, because that it what I have been taught, slack on the inner (or more accurate, set the inner stop then have no tension on the cable) but I can still throw the big ring.

Only when on the big and shifting to the smallest rear (never happens other wise) and when at a moderate cadence and about to make a big effort... like about to sprint. THere is no way you can replicate it on a stand as there is a good 75 kg of forcce being applied. This is despite a very slight rub for a fraction of the revolution when on the biggest rear cog
If I am already on the smallest rear and shifting to the large front I never have an issue, though I guess I am not putting as much force in on a front shift

 

[ustabe]

I actually do it your way, ustabe, because that it what I have been taught, slack on the inner (or more accurate, set the inner stop then have no tension on the cable) but I can still throw the big ring.

At my age I'm finding it more productive to work on my pedaling finesse over brute torque. Also, because I learned on friction on the downtube I will probably always soft-pedal for the instant before I feel the chain snick onto the new cog.

Last spring I found that I was very rarely throwing my chain off both rings of my DA 7800 setup, so I set the limit screws extremely close. For what it's worth, my front derailleur cage is set as low as possible (barely 1mm from the big ring) and barely angled outward at the rear. I haven't had a single drop in months. Knock on wood.

 

[dsut4392]

I actually do it your way, ustabe, because that it what I have been taught, slack on the inner (or more accurate, set the inner stop then have no tension on the cable) but I can still throw the big ring.

Ditto. You don't want slack in the cable, but you don't need significant tension either. I'm not sure about 105 or Force, but on UltegraSl at least has two trim positions in the big ring. If you set too much tension on the cable (and set the limit screw so you can't over-shift), you can only shift into the lower trim position. So you lose the ability to trim the derailleur position according to whether you are in the big or small cogs in the cluster.

To remove the chain rub, make sure the derailleur is aligned properly.
1.Set the derailleur as low as you can while still being able to shift smoothly. Unless you're running Rotor rings or similar, you should be able to set it close enough that throwing the chain off the outside is virtually impossible. You should have a gap of only a few mm at most.
2. Line up the outer plate of the derailleur with the front chainring (looking from above) - it should be parallel to the chain when you are in the big ring/small cog.
3. Then set the cable tension as above.
4. Then set your upper limit screw to stop yourself overshifting.

Also agree with insinuations that people who have learned on friction shifters and flexy old chainrings have a lot less of these drivetrain problems with modern systems. If you're throwing the chain right past the big ring, you could probably fix it by setting the limit screw, but alternatively there's no reason you couldn't just ease of the gas a little, and not throw the shift lever quite so far...

 

[Notso Swift]

dsut
That is pretty much identical to what I do, and yes have checked front alignment and space to top (I actually have it really tight, way less than a mm)
I was thinking that the front ring could be buckled?
THe only time it happens is when I have just shifted and then am jumping, so we may be travelling at 45-50 then I am kicking for my sprint 65 on our usual crit course (300mt gentle down slope), so fast and BIG effort

ustabe, I grew up on Superbe Pro, but they we were fitting 7 speeds in the space of 10 (or 11) I think the outer alignment (and inner) was closer to straight

 

[frankenbike]

Old post, but... an easy fix I found was to rotate the FD forward a little, nose in tail out (just very slightly). The inside of the tail is responsible for initiating an outboard shift and the nose is resposinsible for stopping one (and also potentially for jamming up if it , the nose not the chain, goes too far out). So, for the same outward shift impetus you get better blocking against over-shift. Or said otherwise, for enough blocking, you now get enough shift impetus. My particular derailleur is not precisely aligned the way you usually read to do it, but it now works great. This stuff isn't rocket science. You just have to really look CLOSELY at what's going on and watch it many times.

 

[frankenbike]

Re:

I forgot to add the important key words for my setup that might make this information useful (or not) to someone. There's probably a good reason my derailleur needs this fix. It's a 6 speed derailleur on an otherwise 10 speed system! This rotation is effectively reducing the profile width of the derailleur. Yeah, that sounds pretty dicey and certainly I was prepared to replace it, but for the moment it seems to be working great. Of course that means my observations might not apply to every normal use case, but I think the mechanism I described should be a pretty universal fix or at least hack that could help this kind of shifting problem.