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Old 04-25-11, 06:40 AM   #1
RichP
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Chain rattles against front derailleur

Hi, newbie here,

This problem has come and gone and now it's back. It appears in the higher gears on my Giant Cypress the chain rattles a bit against the front derailleur. The bike has gripshift-type gears, and if I rotate the shifter with my left hand beyond it's final click, I can see the derailleur moves a bit away from the chain and alleviates the noise. I checked out the "Sheldon on creaks" article but it didn't seem to address this. The noise is not constant, i.e. it seems worse on the right pedal downstroke, and doesn't happen at all when the gears are not engaged (like going downhill).

Any suggestions? Thank you!
Rich
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Old 04-25-11, 07:34 AM   #2
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You need to adjust the trim of the front derailleur. it's done by adjusting the length (tension) of the cable. There's an internally threaded rotating sleeve where your cable leaves the lever, turn it to tighten the cable and solve the problem. But don't get carried away because it also changes the trim on the inner sprocket, and how the FD shifts. Do it in small increments so you have good shifting and the FD cage doesn't rub.
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Old 04-25-11, 10:16 AM   #3
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The issue can get worse if you're riding with what is called "cross chaining". This is where you leave the front set to the largest chainring and then shift to one of the last couple of biggest sprockets at the rear. This cobination causes the chain to have to connect the front and rear with a large angular difference. The angle coming off the front chain ring can then result in the chain dragging on the cage of the FD.

The normal advice is to avoid the extreme combinations like this. If you need the gearing that this gives you're best off to shift down to the middle front ring and that will let you use the middle sprockets in the rear and your chain line will be straighter and not scuff the FD's cage.
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Old 04-25-11, 11:10 AM   #4
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FBinNY: thanks! I will try this later today.

BCRider: very interesting, this is the first time I've heard of cross chaining. After a google search I see similar info to what you say. I've actually been riding with the two biggest gears about 80% of the time...I just like to max out the resistance and speed I guess, I had no idea this is harmful. What do experienced riders do when they want max resistance?
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Old 04-25-11, 11:18 AM   #5
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The two 'big' gears refers to the sprocket diameter. The highest resistence is actually the big front and smallest rear combo which is a nearly straight chainline, and is perfectly acceptable.
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Old 04-25-11, 11:53 AM   #6
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Sorry I'm probably confusing the gear size and the number on the shifter. I usually ride with the left/front gear at "3" and the right/back gear at "7" and that's the highest resistance, but it sounds like this is ok? I'll pay attention to the actual line the chain makes in this gear today.
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Old 04-25-11, 12:11 PM   #7
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Engineering has unfortunately made derailleur bikes more consumer friendly
by eliminating the riders ability to do the things that would simplify your situation.

friction shifting, 'thumbshifters' and such as the left bar end lever,
on current Shimano indexed bar end shifters,
let you move the lever just a little bit to solve the chain drag ..
typically done after each gear change..
but that is deemed to narrow the mass consumer potential to old guys like me..

I have to admit , the rapid push button like click between 1 shift and another
is an engineering marvel .. when new they snap right into place ,
right out of the box , from just the work done in China,
by all those people putting the parts on new bikes,
and wrapping and packing them for shipping to your local bike shop.

surprisingly pretty adequately adjusted , the Big Box Stores leave things be,
and just put the bars on and the seat and the pedals and put it on the sales floor.

Last edited by fietsbob; 04-25-11 at 12:19 PM.
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Old 04-25-11, 12:17 PM   #8
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chain rub

Quote:
Originally Posted by RichP View Post
Sorry I'm probably confusing the gear size and the number on the shifter. I usually ride with the left/front gear at "3" and the right/back gear at "7" and that's the highest resistance, but it sounds like this is ok? I'll pay attention to the actual line the chain makes in this gear today.
What you're describing is the chain rubbing the outer edge of the front derailleur cage when you're in the highest (most difficult to pedal) gear. As an earlier poster explained, you have insufficient cable tension. That's why you have to rotate the grip shifter to its most extreme to correct the problem temporarily.

As that poster explained, you can reset the cable tension one of two ways:

- back out a barrell adjuster (counterclockwise to loosen) somewhere along a cable stop for your front derailleur cable. Maybe on the downtube? If you don't have one of these, then

- set the rear shifter so that it's in any middle cog, then shift the front gripshift to tmove the chain onto the smallest (inner) front chainring. The cable should be slack now. Loosen the binding nut (top of front derailleur), grab the cable with a pair of needlenose pliers and pull gently until the cable is taught along its length, then retighten the nut while holding the cable tight.

You should now have adequate shifting to the outermost front chainring without rubbing. If not, then you'll have to make one final adjustment, the "high" screw on the front derailleur.

Depending on the model, there will be two screws visible, one to limit the travel of the cage in the inward direction, and one to limit it in the outward direction. The limiting is due to a small cam that rotates and contacts the bottom of one or the other screw as the cage is moved with the cable tension (outward) or the return spring (inward). While turning the cranks (have a friend hold the rear wheel up, or use a stand, or flip it upside down), shift back and forth on the front chainrings paying particular attention to that small cam as it moves against the screw ends altenately. The screw that limits the outward movement is the "high" screw. That's the one you want to adjust.

- back that screw (counterclockwise again) out about a quarter turn and then repeat the shifting. You'll notice that the cage goes out farther than before. If this solves your rubbing problem, you're done. If not, try another 1/4 turn. That should do it.

Cheers!
Phil G.
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