New Bike Cable Stretch Question

I bought my Cross Check on July 16th and rode it about 150 miles by the time it had it's 1 month free tune up at the LBS where I bought it. So I've had it almost 2 months and it's got at least another 100-150 miles on it in the past month (250-300 total). It has a compact double chainring set up. I was in the small chain ring in the front and middle cogs in the back (maybe toward the bigger cogs a little), when I came to a stop on a 1-2% slow, short hill. I took off from my start, was under slight load when I shifted from small to large chainring and the chain shot past the large chainring and got caught between the chainring and my lovely chainring guard. I could not pedal, so coasted to the side of the road (I had taken the lane and there was a car behind me, but we were going slowly). I moved my bike onto the sidewalk where I could not pull the chain out, so I flipped my bike upside down on the lawn, pulled out the part of the chain that was stuck, pedaled and voila - chain and chain rings were playing nicely. Took all of about 30 seconds. On the way home I did some shifting in the front while on the flats with no problems whatsoever.
I think this was most likely due to cable stretch as the bike is still "new". Wondering what you think? I realize you shouldn't shift under load, but it's not like I was grinding up some epic hill. I probably should have shifted in the rear cogs if I had to shift at that moment since the distances aren't so significant and the chain is going "downhill" so to speak.
Anyway I'm having fenders mounted tomorrow and will have my LBS (not the one I bought it at) check/adjust the derailleurs, etc.
I appreciate any insight. Please....educate me...I'm a sponge just slurping up all the juicy knowledge here. ;)

Sounds like a limiter screw adjustment issue?

(there are two on the derailleur - one keeps the chain from overshooting the small ring and falling off to the inside, one keeps the chain from overshooting the big ring and falling off to the outside)

Thanks Knotted. Yes that's what I was thinking, but since it only happened once I wondered if it were just the circumstances under which I was shifting or was it more cable stretch, or a combination of both? I have adjusted this very limit screw on day 2 of owning the bike when I dropped the chain 3/4 times shifting from small to large chainring! Mind you this was a brand new bike at that time, so you can imagine I was p*ssed! They should have more thoroughly checked things out before letting me walk out with it.
Anyway, thanks for your quick reply. ;)

Knot is right - limiter screw. Chain stretch would cause the derailleur to not shift to the big ring. Put the chain on the smallest cog in back and the big cog in front. Then look at how close the chain is to the outside plate of the front derailleur. If the adjustment is correct, the chain should have only about 1 mm of clearance.

Thank you both. I know which screw is the high limit screw but when one says to "tighten" it as did all the online articles it assumes I know which way to "tighten", so initially I turned the screw counter clockwise, which caused the chain to fall off again, then remembered "righty-tighty, lefty-loosey". I turned it clockwise and it's better now. I will still have the LBS mechanics, who from this shop I trust, check and make adjustments.
Thanks to you both.

Why would this happen just after two months? Do limit screws come loose?

Oh, the reason I had to look it up online for additional help is because when I turned the high limit screw I didn't really see the derailleur cage move inboard or outboard.

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Originally Posted by Velocivixen

Why would this happen just after two months? Do limit screws come loose?

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If I look at your earlier post this happened on day 2 originally so I don't think it's "coming loose" it sounds like it was not set right from the get go...good idea to have a pro check it out, especially one you trust...he/she will give you guidance about that.

It's not unheard of for limit screws to suddenly be out of adjustment for no obvious reason, but it's also not very common. If your bike is that new and the chain is coming off, then it most likely wasn't adjusted correctly from the start, as others have already said.

Front derailleur adjustments are like black magic for many bike mechanics. There's a lot goin on up there. To get a better understanding of how your limit screws work, shift your bike so that the chain is on the small chain ring. Examine the limit screws and locate the ends of the screws where they interface with the inner part of the front derailleur. While watching this area, pull the derailleur cage (the curved silver part) toward you. Watch how the distance between the derailleur and the end of the screw varies with the position of the derailleur cage. Notice how, when you pull the cage toward you, you see the gap on one of the screws gets smaller until the inner derailleur actually touches the screw. This is how you can identify your high limit screw; the one that keeps the chain from falling off the rings to the outside. That screw that is touching the derailleur is limiting how far out it can move. Look at the screw and the derailleur and imagine what would happen if you tightened (righty-tighty) the screw that is touching. The screw would go further into the derailleur and would further limit the outward mobility of the derailleur cage, thus, keeping your chain on the big ring.

Now, mind you, a properly tuned front derailleur requires a delicate balance. Much like life. If that screw we were talking about is tightened too much, then it will prohibit the derailleur from moving out far enough to push/allow the chain to move onto the big ring.

Of course, none of this really matters if your other front derailleur adjustments aren't correct. If your derailleur is clamped too high above the crank (an often overlooked adjustment), it's going to throw off your limit screw adjustments. If your derailleur got bumped and the cage isn't parallel with the rings, it's going to throw things off. There are numerous other issues that can cause problems with front shifting, but I'm tired of typing, so we're not going into that right now.

This is why front derailleurs cause so much grief. It's like trying to fine tune a see-saw designed for six people. The trick is making all parties cooperate.

To the OP: Like I said, front derailleurs are tricky and bike mechanics aren't perfect. My advice would be to go back to the shop and tell them exactly what the deal is. Make sure you ride the bike when you pick it up, before you put it in or on your car.

If the problem occurs again, either get a new mechanic or teach yourself.

Most importantly, congrats on the new whip!

It's the cable that moves the derailleur, so you should make the adjustment and then shift it to see if the adjustment helped. It's quite possible you could adjust the high limit screw and it doesn't make an obvious difference because the cable isn't moving the derailleur far enough for it to hit the limit. You can do the shift manually (by pulling on the cable or pushing the derailleur cage) to be sure to hit the stop.

Quote:

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Originally Posted by Velocivixen

Oh, the reason I had to look it up online for additional help is because when I turned the high limit screw I didn't really see the derailleur cage move inboard or outboard.

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Thanks to all of you. I knew what screw was the high limit screw and figured out, based on what you said, that it needed to be "tightened" (turned clockwise). However Mama Coosa, you explained the visuals well. I got my fenders installed yesterday at my regular, trusted bike shop (Portland Metro has over 76 bike shops!), NOT where I bought the bike from. I have never, ever had problems with any of the bikes I bought at my trusted bike shop, but they're not a Surly dealership and nobody, none of the Surly dealers had this bike in "Robin's Egg Blue", so that's why I ended up in SE Portland to buy the bike. My trusted shop is in Beaverton, so they've got me all fixed up.
Thank you all for your step by step help & explanations. If this ever happens again, I now know which way to turn that screw!
I have watched some excellent youtube videos, particularly the ones by "intownbicycles" and Mike on there is wonderful at explaining derailleurs. I even sent him an email to their shop saying so! ;)

@DebW, also, my front derailleur has a bottom mount cable, so unlike the photos in my Parks Big Blue Book volume 2, I can't really grab the cable and pull - at least not in any obvious way. Thanks again.

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Originally Posted by Velocivixen

@DebW, also, my front derailleur has a bottom mount cable, so unlike the photos in my Parks Big Blue Book volume 2, I can't really grab the cable and pull - at least not in any obvious way.

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Unless you have continuous housing all the way, you can grab the cable along the downtube. Just pull it away from the downtube - the derailleur should move. (However, once when I did this at the co-op, the patron's brifter slipped instead.)

And if you do trying pulling the cable to make the shift don't expect it to hold the change you made because when let go, the gear will shift right back where you started from. When I initially did this on my bike I was expecting the shift to hold the change and thought I'd done something wrong.

Hey, those are some good ideas. Hopefully there will be no "next time".