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Discussion Starter #1
I have an '05 Giant TCR Limited. I have about 100 miles on it and started having a problem when downshifting from the big ring to the small ring. If I'm on one of the smaller cogs, the chain falls between the chain rings and stays there. It does not jam, it just rides smoothly between the two rings. This happens every time I drop from the big ring down to the small ring while in one of the smallest 4 cogs. I went through and adjusted both derailuers several times and can not get the problem to go away. Does anyone have any ideas?

Thanks!
Doc
 

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Discussion Starter #2
I just went and double checked it. It's not really falling between the rings. When the chain falls off the big ring the inside plates on the chain land right on top of the small ring teeth and it sits and rides there sliding on top of the small ring teeth and on the inside of the big ring.
 

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Some things to try

The first thing to check is to make sure that your derailleur cage is vertically as close as possible to the teeth on the large chain ring. IOW, it should just clear the big ring when it swings up or down. Second thing to check is whether the outer cage is parallel to the chain rings. A little tweaking from that may be appropriate - the range is from the inner cage parallel to the rings to the outer cage parallel to the rings. Other possibilities are to clean and lube your chain, check the chain for wear, possibly turn the lower limit screw out a bit, and double check that when you shift the front derailleur, you are getting the Shimano "big drop" shift and not just a "trim click," which is more like a half shift.
 

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It might help to be more specific about what kind of chainrings you have.
One possible cause is that the chainline is not correct. A bottom bracket spacer would move the entire crankset outward, reducing the angle between the inside chainring and the smaller cogs. I fixed a similar problem on my wife's bike that way. It's easily done.

Al
 

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Discussion Starter #6
Thanks for all the quick responses.

The bull - The chain does not spin. The crank/rings just smoothly rotate while the chain stays stationary riding over the small ring's teeth.

Kerry - I set up the derailleur per the shimano instructions. I have tried both 1mm and 3mm clearance between the cage and big ring. I set up the cage parallel to the big ring, and then later, I tweaked the back of the cage in just to see if it made any difference. It made no difference. Everything is clean and lubed. I'm definately getting the "big drop" shift and not the "trim" shift.

Al1943 - It's a truvativ rouleur compact crankset with 36/50 rings. It's what came stock on the bike. That's a good point about the spacer. When I picked the bike up from the shop I was amazed at how poor a job they did assembling it. I wonder if they left out a spacer on the crankset or bb.
 

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Doc_D said:
Al1943 - It's a truvativ rouleur compact crankset with 36/50 rings. It's what came stock on the bike. That's a good point about the spacer. When I picked the bike up from the shop I was amazed at how poor a job they did assembling it. I wonder if they left out a spacer on the crankset or bb.
I'm not familiar with those particular chainrings. Are the teeth beveled on both sides (like Shimano's) or just the outside? If the backside (nearest the frame) are flat, not beveled, you could try filing a slight bevel on the teeth to help the chain drop over the teeth.

With the chain on the inside ring and second or third smallest cogs does the chain ever drag against the side of the big chainring? This is also a symptom of a poor chainline and a bottom bracket spacer is the remedy for that problem as well. A poor chainline is usually due to something in the component or frame design rather than the bike's assembly. Bottom bracket spacers are available in 1, 1.5, and 2mm thicknesses. They go between the bottom bracket and the frame. They are inexpensive, easy to install, and are hardly noticeable.

Al
 

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Get a wider chain. It looks to be badly spec'd components from the factory. Twreck.

The problem is that this is a 9/10s crankset, meaning it might do 9 and might do 10. Maybe you can find a large outer chainring that is machined closer to the inside. Did you recently switch to a ultra-narrow or upgraded chain? Probably not as it is all new.

Take it back.
 

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about chainline...

Since the problem occurs in 4 cogs, which covers about a 16mm distance, adding a 1-2mm spacer to change the chainline is not likely to fix the problem, IMO. Spacers are intended to fix a chainrub problem at the most extreme angle (little ring and next to smallest cog) and then only if the chainline is truly incorrect. To verify correct chainline, measure from the side of the seat tube to the tip of a tooth on the big ring, then add 1/2 the seat tube diameter. The total should be 47-48mm.

There should never be an occasion when you need to shift from the big ring and smallest cog to the little ring. Usually, the transition shift from the big ring to the small ring is made from one of the largest 2-3 cogs, when it's obvious that the conditions warrant a lower gearing range.

Much more likely is a chainring ramp/pin problem or simply the need to loosen the left limit screw a bit more. If you loosen the screw enough, you should get enough cage movement to throw the chain off the little ring to the left side. Then you need to tighten it 1/8-1/4 turn.
 

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Discussion Starter #10
C-40... I adjusted the cable tension for the FD according to the shimano instructions. That means that when I shift down to the small ring that the low limit screw (even though originally adjusted with no cable tension) is not the limiting factor. The cable tension is. So now that makes me think that I have too much cable tension.

If I remember correctly the shimano instructions have you set the cable tension by going into the big ring and big cog and then increasing the cable tension until there is 1/2 mm clearance between the chain and the cage. But adjusting it this way it means than when I shift to the small ring the derailleur's position is not determined by the low limit screw, but rather by the cable tension. This probably means that the derailleur is not moving the chain towards the inside far enough on a down shift. So instead of using Shimano's instructions for setting the cable tension, it's probably just better to set the cable tension so that the derailleur just barely sits on the low limit screw (i.e. just take all the slack out of the cable when shifted into the small ring).

I'll give that a try when I get home. I bet that's what the problem is.
 

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Discussion Starter #11
OK, no luck. I took all the tension out of the cable. I even set the low limit screw so far out that the chain would off the ring to the inside when on a large rear cog. Still when on a small cog, dropping from the big ring to the small ring the chain falls in such a way that the chain plates ride right on top of the small ring teeth.

I also tweaked the FD cage in and out. While it affects other aspects of the shifting, it does not affect the fact that when the chain drops from the big to the small it won't drop all the way down onto the small ring.

Lastly I took the chain rings off the crank and ensured the indexing pin on the big ring and the indexing mark on the small ring were aligned correctly. I also made sure the rings weren't installed the wrong way. I retorqued the chain ring bolts. The problem is still there.
 

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Start over.
Disconnect the front der cable.
Inspect cable and housing for damage and lube with a light lube.
Your chain should be in the small ring.
Make sure the der height and angle are right (2mm over the big ring teeth and parallel)
Shift into your biggest cog in back.
Set the inner der limit so the der doesn't rub the chain.
Attach the cable and tighten. There should be no slack in the cable.
Shift into the small cog in back.
Shift (slowly) onto your big ring. Adjust the outer der limit so the chain doesn't rub.
Try all shifting combos, making sure it won't kick off to the inside or outside. Tweak limits slightly if necessary. Also ajust cable tension slightly if necessary.

If this doesn't work take it to a shop.
Good luck.
 

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Discussion Starter #14
I tried all kinds of variations on adjusting the front derailleur so I knew that wasn't the issue.

I called Truvativ this morning when they opened and they are aware of the problem between the Rouleur crank and the ultegra 10 speed chain. They said to have my bike shop call them and they will send out new rings.

Thanks for the input everyone.
 

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Larry Lackapants
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Hello,

Actually I have this problem when riding my shimano 600 cranks with it's original chainwheels (52 42)
I have a HG 70 9 speed chain and cassette. Obviously the chain is quite narrow and it matches the distance between the centerline of the smaller chainwheel and the outside of the large one. The problem disappears when I change the chainwheels to a 39 -53 105 pair.
I think the cause the problem disappears is
1. Larger difference between tooth count
2. The 39 chainwheel's centerline is closer to the bigh chainwheel.

You can try and turn around the samll chainwheel and see if the chain doesn't drop more easy on it.
Or you can slightly file the teeth of the small chainwheel to bring their top "closer" to the big chainwheel. (It's not very hard and not destructive either. If you want to try this method tell me and i'll try to make a drawing to describe it)

Good luck
br

Edit: I use the 39 -53 with the shimano 600 crankarms
 

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fsa compact too

I had the same problem with my fsa compact pro and a campy 10 drivetrain. I believe the problem is that the rings were designed for both 9 and 10 systems but the 10 speed chain is really too narrow for the clearance between the two rings. I was told that new 10 speed only rings would fix the problem.
 
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