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So. Calif.
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Discussion Starter · #1 ·
On my Chours 11sp, one of Campy's tech documents lists chainline = 43.5 mm.

Chainline defined as distance from center of bottom bracket shell/housing, to the inboard face of the large chainring.

I am thinking of installing a Quarq Riken powermeter to replace powertap wheel. The Riken is built around a (gasp!) SRAM S900 crankset (I think).

SRAM lists their S900 crankset's chainline = 47.5 mm .

So basically, chainrings would be about 4mm further outboard.

Obviously the front Chorus derailleur will need to be reset and adjusted.

>> Can anyone else think of what unintended or unforeseen issues could arise?

It also seems the big chainring/big cog alignment will be even less optimal, but nothing I can do.
Of course, I don't routinely ride in big/big, but there are many times when big/big for a minute or so, is desireable for getting over a short hill.
 

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So. Calif.
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Discussion Starter · #2 ·
Looked at SRAM & Campy drawings more carefully, and here is another difference between 11sp Chorus and SRAM S900 chainring positioning.

I haven't actually measured my Chorus crank to verify, this is off the drawings. Center-to-center spacing (or "pitch") between small & big rings:

Campy = 7.7 mm
SRAM S900 = 6.0 mm

So the SRAM chainrings are actually 1.7mm closer together. I'm assuming the Quarq powermeter spider would exactly mimic the stock S900 spider.

Net effect of this is, in the SRAM small ring / small cog configuration, chain is more likely to snag or catch onto the big ring and do an unintended shift to big ring.

In fact, this unintended "snag" occasionally occurs on the Chorus 11, if I hit a bump or on a rough road. Lucky for me, temporarily riding in small-small combo is extremely rare, much more so than big-big.
 

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The traditional 43.5mm dimension, used on nearly all double cranks, for many years, is from the center of the BB shell to a point, centered between the two chainrings. It makes no sense, since you can't mesaure to a point in space, but that is the definition.

Triple cranks are defined in a sensible manner to the tip of a tooth on the middle ring.

I have a Campy drawing that shows the dimensions of the current 11 speed double crank. The distance from the center of the BB shell to the inner face of the big chainring teeth (not the shifting ramps) is 46.3mm. The cog spacing is 7.7mm, which is about the same as the spacing used as far back as 7 speed chainrings. You will get the correct 43.5mm chainline if you subtract half the chainring spacing and add half the tooth thickness.
 

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So. Calif.
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Discussion Starter · #4 ·
C-40,
We might be looking at different drawings for 11sp.

I'm seeing center of the BB shell to the inner face of the chainring's thick body, not the teeth = 43.5 mm.

From the inner face of ring's thick body to inner face of teeth = 2.0

So, center of the BB shell to inner face of teeth = 43.5 + 2.0 = 45.5 mm, differs slightly from your 46.3 calculation (but i could be wrong!).

I am leaning toward getting the Quarq powermeter from Competitive Cyclist, who offer a very liberal "no questions asked" return policy. That gives me an escape route, should I encounter unforeseen hiccups with the SRAM S900-based powermeter on the Chorus 11 drivetrain.
 

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My drawing shows 44.3mm to the thicker portion of the big ring. It all works out to the proper 43.5mm chainline, which is to the center point between the rings, not the thick body that you mentioned.

The Campy drawing that shows the 43.5mm dimension is not to the thick portion of the chainring. The dimension is between TWO centerlines that both have the same type of dashed line that indicates a centerline. That means the center of the BB shell and the center between the rings. If the right side extension line was referencing the thick portion of the chainring, it would be a shorter line and not dashed. You may need to look at a large screen version of the drawing to see the dashed centerlines.

www.campagnolo.com/jsp/en/downloader/docid_275.jsp
 

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So. Calif.
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Discussion Starter · #8 ·
Got around to installing the Quarq Riken powermeter with SRAM S900 cranks, onto my Chorus-11 equipped bike.

Surprise -- and not in a good way. The SRAM chainrings are indeed about 2mm further outboard, than the Chorus-11 was.

I know this because,
1) measured from chain catcher to the chain on small ring.
2) chainring position relative to the undisturbed Front derailleur.

The design of SRAM's GXP style of crankset and bottom bracket is such, that the left-right position of the crank axle is fixed by the NDS cup/bearing.

Eg, read: GXP: External Bottom Bracket | Truvativ | SRAM
" GXP captures the left-side bearing between the spindle and the left crankarm and allows the drive-side bearing to float axially on the spindle. There's no need to side-load bearings during adjustment. "

There is actually a small gap between the DS bearing and the backside of crank spider. And, there's no way to adjust & reduce this gap, to move the chainrings more inboard to match the Chorus.

I suppose, a spacer between the NDS cup & frame would axially shift the entire crank to the left ... but a spacer is specifically not recommended or deemed necessary on road frames, per SRAM.

BTW, I did reconfirm my BB shell width is within spec of 68.0 +/- 0.5 mm. I measured at 3 spots (10, 2, and 6 oclock) : 67.64, 67.63, and 67.64 mm.

Unless the crank was manufactured wrong -- unlikely -- my two choices seem to be:

1) use "as is", readjust front derailler, and hope the chainrings pushed outboard ~2mm don't cause unforeseen issues
(probably not, it's less than 1/2 chain width).

2) put a ~2mm spacer between NDS cup & frame ... contrary to SRAM install instructions.
(I don't care for 2nd guessing the engineer's designs).

Have I overlooked anything on the SRAM/Quarq installation ??

The SRAM GXP design looks decent , but I still think the Campy Ultra Torque is the simplest, best designed, and most foolproof crank system out there.
 

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So. Calif.
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Discussion Starter · #9 ·
update ...

Haven't been riding because of cracked rib :-( , but here's the mechanical update, and a duplicate posting at the "Components & Wrenching" sub-forum.

Finished installing the Quarq Riken w/ SRAM S900 crankset.

Sure enough, the FD limit screws and cable tension had to be readjusted to deal with chainrings that are about ~2mm further outboard.

As a diagnostic, I also temporarily installed crank without the NDS cup, to see how far inboard the chainrings would go.
Result: Only about 1.25mm further in, and there was some rubbing.
So, any spacer under the NDS cup would be limited to <1mm, maybe 0.7mm ... not much.

Basically, it seems the design of the SRAM crank (or Quarq spider?) inherently pushes the chainrings ~2mm further out than Campy Ultra-Torque 11speed.

Regarding shift quality, on the workstand:
The SRAM "Powerglide" 10sp chainrings definitely do not shift as fast or as quietly as the Campy Chorus 11sp.
Shifting under load, to be determined.

Possibly this because the SRAM rings are optimized for a 10sp SRAM chain, but I think it's more likely the big ring's pin & ramp design.

The SRAM big ring has only 2 pairs (4 total). The Campy big ring has 4 pairs (8 total) pins. I've also had good shifting with Stronglight rings (6 pins total).

I may switch to Stronglight or Praxis rings ... no direct experience with Praxis but have heard good feedback.
 
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