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Old 09-10-10, 08:40 AM   #1
jmaurer
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Drivetrain question: chain wandering off of middle chainring on a triple

All,

I'm building a rando/touring setup. All new components with the exception of the front der which is NOS.

Cassette: Shimano 9 speed 11-32
Rear Der: Shimano Deore XT
Front Der: Campy Record triple 10 sp
Shifters: Campy Veloce 10 sp brifters
Crankset: Sugino XD600 triple
BB: SKF 113mm
Chain: Shimano 10 sp Ultegra

Things have gone together well. Chainline looks good to my eye (middle chainring lines up very nicely with 5th (middle) cassette sprocket.) I'm using a Jtek Shiftmate #2 to mate the brifters to the Shimano 9 sp rear der.

Just started adjusting rear der, and it goes through the gears reasonably well. An occasional hiccup as you'd expect pre-adjustment for indexed shifting. Chain tension seems fine at both extremes of the range.

However, in 10th and 11th gear (middle chainring and largest or second largest rear cog) the chain wants to run off of the middle chainring and drop to the granny. This happens even if the front der is not involved in any way. I'm tempted to try a 9 sp chain but it's hard to believe that 0.4 mm (approx) will make a dramatic difference.

...and, yes, I've chosen a mixed breed approach and to some degree getting what I deserve. But there's evidence that it should be workable, and reasoning behind each choice.

Appreciate your guidance.

thx,
-j
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Old 09-10-10, 08:52 AM   #2
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Check the chainline. Measure from the side of the seat tube to the tip of a tooth on the middle ring, then add half the ST diameter. The total should be in the 45-47.5mm range. Campy requires the 47.5 with 32 or 35mm seat tubes. If the chainline is too narrow, the FD might hit the ST before shifting the chain onto the little ring.
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Old 09-10-10, 08:57 AM   #3
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... my first thought would be the chain, too. 10 speed chains are actually a bit narrower between the inner plates than 7/8/9, and 10 speed chainrings are a bit thinner to match. the mismatch could be enough to make the chain climb up and off a 9 speed ring.
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Old 09-10-10, 08:57 AM   #4
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I have exactly the same problem with a Shimano 10sp chain, cassette, shifters, derailleurs, and a FSA triple crank.

Someoen suggested the teeth on the chainring need some changes to the profile (more chamfer on the outboard side) to allow the chain to slip on more easily.
I have not yet tried this and my chain still drops onto the granny ring when in my two biggest cogs.
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Old 09-10-10, 09:55 AM   #5
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If the chain runs well on inline combinations but runs over the side when feeding from the more extreme angles you have a few choices.

1- check chainline, and/or and consider moving it towards the problem side to improve the line from that side. In your case inboard a few millimeters. Note that might worsen chain retention when feeding from outside angles.
2- live with it and avoid using the problem combinations
3- replace the chain with one having larger plates or more bellmouth inside the inner plates to improve it's ability to track when feeding from an angle.
4- replace the chainring with an ungated one, since the shift gate is usually the point where the derailment starts.
5- trim the FD so the the inner plate pushes the chain over marginally to help keep it on. This is hard on the FD, and only vialble if for very short term use situations.
6- modify the chainring tooth profile, which is something old school mechanic have done since the early days of derailleur bikes when chains falling off was the rule not the exception. This is the best solution because it solves the problem without causing other complications.

Here's a link to a prior response I posted for someone with a similar problem. It's post #4, start from the second paragraph.



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Last edited by FBinNY; 09-10-10 at 10:01 AM.
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Old 09-10-10, 11:27 PM   #6
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I have a similar setup. I have a Centaur triple group and I use an 11-32 shimano 9spd. The campy 10spd will work perfectly with the shimano 9spd (nearly same spacing) without any addition of a jtek gizmo. With the 32 it requires the max adjustment of the B screw. I went to an 11-34 cassette and had to change out to shimano 9 spd barcons and the Forte equivalent of the XT skeleton RD. Sounds like the Jtek isn't doing its job or the adjustmtne is out. One other possibility is that your RD hanger is ever so slightly bent causing the auto shifting by the alignment of the lower wheel to the crankset. A good LBS has a tool for checking and straightening out the hanger. I would check this out before I did anything else. Doesn't take much of a bend. Good luck.

In addition, make sure the RD arm is not bent. Doesn't take much mis alignment to cause the problem you describe.

Last edited by Deanster04; 09-10-10 at 11:49 PM. Reason: addition
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Old 09-10-10, 11:59 PM   #7
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Check that the middle chainring isn't bent. Just had one of these come through the shop on Labor Day. It was on a Trek Portland where the guy put on a cheap middle ring after replacing the stock one. It was bent for about a 3 tooth wobble. It was enough where it would toss the chain to the little ring without touching the FD when under load.
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Old 09-22-10, 09:40 AM   #8
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Problem solve-d

I tried an SRAM 9-speed chain instead of the Shimano 10 sp - cured the chainring issue nicely (and adds to the Franken-drivetrain appeal too!). On the back side, re-looked at my JTek setup. Needed to flip the pulley inside the JTek (silly setup issue) and it works famously.

Wish that I could get somewhat better chain alignment. Center chainring (triple) aligns approx with 6th cog (from largest). But not enough room for an adequate cassette shim, nor much room inboard of the crankset. So I think this is more a characteristic of the Bob Jackson World Tour frame.

Seems like mostly an aesthetic thing though, since it runs through all gear combinations very smoothly.

-j
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Old 09-22-10, 10:43 AM   #9
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Quote:
Originally Posted by jmaurer View Post
Wish that I could get somewhat better chain alignment. Center chainring (triple) aligns approx with 6th cog (from largest). But not enough room for an adequate cassette shim, nor much room inboard of the crankset. So I think this is more a characteristic of the Bob Jackson World Tour frame.

I have a couple comments:

First, the make or model of the frame is not the defining parameter for chainline - the rear hub width is. All modern frames have the centre of the bottom bracket spindle directly in front of the centreline of the rear wheel. If chainline is off it is the fault of the bottom bracket/rear hub/crankset combination.

Secondly, unless you have a cassette body that is considerably wider than the cassette, it is impossible and usually not reccomended to adjust chainline by spacing the cassette

Also, you say you have not much room inboard of the crankset - how much room is 'not much?' Because if you have a 113mm bottom bracket and change to a 109 you will move the right side in by 2mm - about half way to getting perfect chain line. Do you have 3 or 4mm of room between chainrings and chainstay? Because if you do you can swap out the BB.

Lastly, you referred to using a 9sp chain as being mopre frankestein-y, but in fact you are supposed to select a chain based ont he number of cogs in the back... since you have a 9-speed cassette, a 9 speed chain is the correct one.
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