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Discussion Starter · #1 ·
I have a Stronglight Pulsion 50/34 crankset with a 108mm Token ISIS BB. When I am in the outer 3 cogs in the rear and the 34 ring up front, the chain rubs against and grabs the inside of the 50 ring. I was thinking I could either move the entire BB to the right by adding a 2.0mm spacer between the BB and the BB shell on the right side which would keep the Q the same but have a slight offset on the right and left side, or I could just go with a 113mm BB which would increase the Q slightly, but keep the frame to pedal distance the same. Are there any significant negatives to either approach? Would a 2.0mm offset on the right or left side be enough to cause knee pain? I wouldn't think so, but I am not sure.
 

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not a cure...

The correct thing to do is first check the chainline. The tip of a tooth on the big ring should be in the 47-48mm range from the center of the seat tube. If it's too far to the left, a BB spacer may be in order. Changing to a 113mm will increase the tread width, but not as much as using a triple does (5mm vs 13mm).

Whether either change will provide any relief is questionable, since cog spacing is around 4mm c-c. At best it might relieve the problem in the 3rd from smallest cog. Chain rub on the 2nd cog is quite common and another of the disadvantges of a compact.
 

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lemmy999 said:
I have a Stronglight Pulsion 50/34 crankset with a 108mm Token ISIS BB. When I am in the outer 3 cogs in the rear and the 34 ring up front, the chain rubs against and grabs the inside of the 50 ring. I was thinking I could either move the entire BB to the right by adding a 2.0mm spacer between the BB and the BB shell on the right side which would keep the Q the same but have a slight offset on the right and left side, or I could just go with a 113mm BB which would increase the Q slightly, but keep the frame to pedal distance the same. Are there any significant negatives to either approach? Would a 2.0mm offset on the right or left side be enough to cause knee pain? I wouldn't think so, but I am not sure.
What size BB does Stronglight recommend?

This is not a disadvantage of compacts, but of a bad setup, or perhaps the design of a particular set. I've got full use of all 10 in both rings with my FSA. Of course I don't use the big crosses, but there's no mechanical restriction.

The 2.5 mm that the bigger BB gives should be enough. You need about 12mm in the back to get them all, and the length of the chain between the take-off of the 34 and the cog is approx 40 cm, and the distance from the take-off to the big ring (along the chain) is about 6cm. 12*6/40 = 1.8. You might not get the 12, but you don't really want it anyway, because of the chainline.
 

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Discussion Starter · #6 ·
The chainline with my old Ultegra 53/39 with no spacers on the octalink BB was 45.4 mm (47.9mm to the middle of the big ring). With Pulsion Compact 50/34 w/ 108mm ISIS BB and a 1mm spacer the chainline is 44.4mm (46.9mm to the middle of the big ring). With the 1mm spacer it is not touching in the 3rd cog at all and just slightly in the 2nd. With a 1.5 or 2mm spacer it may not rub at all in the 2nd. I noticed that with the Ultegra 53/39 there was some rubbing in the outer cog so there isn't that much difference. I think the difference is totally with chainline and not as much the compact crank.
 

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lemmy999 said:
Stronglight recommends a 108 for a 43.5 chainline. Is your FSA small ring a 34 or 36?
It's a 34. Can't quote the chainline.

Just for playing, could you loosen the crank and slide it out on the spline a bit? Wouldn't be a perfect test, but it might give some idea if a chainline fix would solve it.

Mechanically, the spacer ought to do the job, but I think I'd prefer a longer BB over creating the offset. I'm personally not all that sensitive to tread width, but I have quickly noticed (and disliked) when I've had my cleats set unevenly side-to-side.
 

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Discussion Starter · #8 ·
I did't notice 1mm on a short ride (30 mi) yesterday, but I may notice 2mm. Using a 5mm longer spindle BB would only have my chainline 0.5mm more than it was before. So this may be the best solution.

I think you are correct when you said this doesn't have much if anything to do with it being compact. The distance from where the chain leaves the small ring to the place where it clears the big ring is almost the same with 50/34 as it is for 53/39.
 

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well...

You might not think this is a disadvantage of a compact, but there sure have been a lot of reports on this forum of chainring rub in the little ring and 2nd cog with 50/34 compacts.

What you may not have considered is the variation in the chainstay length between different brands of frames. Those with shorter chainstays are more likely to have this problem.
 

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Discussion Starter · #10 ·
C-40 said:
You might not think this is a disadvantage of a compact, but there sure have been a lot of reports on this forum of chainring rub in the little ring and 2nd cog with 50/34 compacts.

What you may not have considered is the variation in the chainstay length between different brands of frames. Those with shorter chainstays are more likely to have this problem.
That is a good point. I kept wondering what characteristic of the frame could be causing some to have the problems and others not.

I am not saying being compact doesn't have anything to do with the rubbing, but in my case I was dropping 2mm off of the chainline going to this new crankset. If the chainline had stayed the same and it had been a 50/36, I doubt there would be any difference as far as rubbing. With the 1mm spacer, there is no rubbing at all in the third cog and very slight contact in the second cog. While not pedaling there is no contact, but the slight vibration of the chain cause a little contact on the second cog. I really do not care how badly it rubs on the outside cog though since I am never in that gear. Actually I am usually not in the second cog either
 

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C-40 said:
You might not think this is a disadvantage of a compact, but there sure have been a lot of reports on this forum of chainring rub in the little ring and 2nd cog with 50/34 compacts.

What you may not have considered is the variation in the chainstay length between different brands of frames. Those with shorter chainstays are more likely to have this problem.
FWIW, my chainstays are 40cm, about as short as they come in standard-layout bikes. Ya gotta start tightening the STA or doing a Cervelo bendy-thing to get them meaningfully shorter.

To get rid of a single-ring rub (say, to make that 2nd cog work) with 40 cm chainstays, you need to adjust the chainline by a maximum of [4mm(distance between cogs)*6/40(ratio of takeoff-to-rub to total top chainrun)], or 0.6mm. To get rid of a rub on 38 cm chainstays - a number I'm not even sure is available - you need 4mm*6/38, or 0.631mm. A thousandth of an inch difference in chainline, more or less. Calling that a "disadvantage to compact cranks" is illogical. One particular crank/BB/bike combination, maybe, but simple bolt torques upon installation can make that much difference. As evidence against an entire product segment? Fuggedaboudit. Reread all those reports - most of them are folks mixing brands. That's fine, but anytime you do that with nearly any bike part, you have to be prepared to deal with issues that might arise.

It's no more a disadvantage of compacts as it is of doubles or triples. If I decide to convert my compact rig to a standard double or triple and need a new BB or spacers to get the chainline right, will you be pontificating that this is a disadvantage of doubles and triples?

Compacts are more sensitive to proper chainline, but where's the disadvantage in that? Gee, they force me to attend to something I ought to anyway, for efficient operation. What a shame.
 

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another extreme..

Boy, you get offended easily. Lets say that compacts are the greatest thing ever invented. There are no disadvantages. Everyone should have one. Feel better now?

I'm just pointing out the fact that a lot of posts have been made regarding the same problem. If a 1-2mm shim cures all those problems, then it's a fine cure, but using more shim or a longer spindle can create other problems. The other extreme angle, the big ring and next to largest cog becomes worse as the crank is moved to the right. If a 4mm correction is made, then another of the largest cogs should be avoided with the big ring, unless you're also a believer that there is no such thing as a too-extreme chainline, like the big ring and largest cog. Some people sincerely believe this and even go so far as to recommend "breaking in a chain" by running this combo as often as possible.

Another example of ignoring extreme chainlines occurs with a triple crank. The middle ring is only about 1mm further to the left than the big ring on a double. I treat the triple's middle ring just like the big ring on a double and avoid using the middle ring with the largest cog, since the chainline is nearly identical. The upside is that the middle ring and smallest cog are quite useable - no chainrub.

As for mixing brands being the problem, the majority of compacts are made to fit standard 109.5mm shimano double octalink BBs, standard 108mm ISIS BBs, or have their own integrated BB supplied with the crank. There's no reason for any of these designs to be mismatched, unless the manufacturer of the ISIS BB fails to follow the published standard. There probably are some square taper compacts in use, in which case, failing to purchase the correct spindle length and taper (JIS or ISO) could be a problem. That's where a simple chainline check is in order.

http://www.chrisking.com/pdfs/ISIS Drive Complete Rev D.pdf
 

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No offense taken. I've always held that they're not for everyone. You seem to post at every opportunity as if they're not for anyone, and I'm simply trying to figure out why. I'm trying to get to how you figure it being a problem inherent to compact cranks, rather than to the user's particular circumstances. I'll confess to a pro-compact bias, if you'll confess the opposite.

I'm not sure I understand your thoughts that a chainline adjustment means that you shouldn't use the big/next-to-big. I understand avoiding cross-chaining, but to the extent that it's true for a compact, it's true for a standard, too, as the fronts remain the same distance apart. If you are suggesting that the compact has to be further out than a standard because of the size difference, I understand your point, but I disagree with it. It doesn't take a 4mm outward movement to correct for the difference in chainring sizes, but only a few thousandths. Some folks have made extreme adjustments trying to get the full cogset (including the small-small) to not rub, but that's an error of understanding. A compact does push the limits, and one of the 'compromises' is that the smal-small will rub with everything in perfect running order and with a good chainline. What it takes to fix a second-cog rub is well within normal chainline parameters. We also don't know how many of our friends having difficulty have BB shells that have been faced one too many times, or that have gone down hard enough in a crash that the rear has been moved a few tenths, or how many frames were simply welded crooked. Any of those could create an adjustment that seemed extreme, but was in fact creating a perfectly normal chainline.

Each of Campy's six compact crank models (Mirage, Veloce, Centaur x2, Chorus, Record) are square-taper. Some of them require a 102mm, BB others a 111. Even with the correct BB, a taper'd can be overtightened enough to cause trouble. It's one of the shortcomings of the square-taper, as mentioned in the ISIS standards doc. And the ISIS standard allows a .5mm variance in the cl-shoulder distance that effectively sets chainline for a particular BB - which is enough to cause or cure a small ring / small-1cog rub.
 

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Discussion Starter · #14 ·
danl1 said:
And the ISIS standard allows a .5mm variance in the cl-shoulder distance that effectively sets chainline for a particular BB - which is enough to cause or cure a small ring / small-1cog rub.

Actually, this may be part of the problem with the Token BB. It doesn't have a squared up shoulder like my old FSA ISIS BB, but instead has a rounded shoulder.
 

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The gold ring...

is the shoulder that provides a stop for the crank.

My best advice is to measure the chainline as accurately as possible from the side of the seat tube to the tip of a tooth on the big ring, then add 1/2 the ST diameter. If it's 47-48mm and you still get the rub, add 1mm of shim. If danl1 is correct and it only takes a few thousandths of an inch to correct the problem, then forty thousandths (1mm) should work wonders.
 

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Context, please...

The mention of “A few thousandths” was in an entirely different context. That was a calculation of the extent to which chainstay length differences might be responsible for rub. Later, I similarly described the extent to which the ring-size difference (the thing that makes a compact a compact) contributed to a potiential chain-rub problem. This was only to show that chainring rub is not an inherent problem with compacts, but rather of some setup problem.

Regarding the issue of correcting a rub, I calculated that it should take approx. 0.6mm/cog. Reading the OP’s posts, changing cranks decreased the chainline by 2mm. That’s a compatibility/setup issue, and irrelevant to the “few thousandths” discussion. Adding 1mm spacer corrected one cog, and got close to fixing the second. Going to 2mm of spacers will restore his original chainline, and by every indication should correct the problem entirely. If it happens not to, it will be within a few thousandths, as the other variables will have then been accounted for and extreme chainlines corrected.

A 2mm chainline drop would have caused a conventional double to rub, though perhaps not quite as badly.
 
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