Thanks Turbotandem. The distance between the centerline of the frame and the center of the small chainring would be great to know. If you can't determine the centerline of the frame so easily (it would be no problem if there are bottle cage bosses nearby), then another way to do it is to measure from the center of the small ring to the near edge of the seat-tube, then measure the width of the seat-tube, divide the width by 2 and add it to the first measurement. On road double cranks the inner ring is often around 40mm from the center of the frame, and the inner ring on a road triple can be just 38mm. On MTB triple cranks, this is often around 43-45mm, so MTB FDs are designed for this wider position and cannot be made to shift road cranks. I'd love to know if the XTR Di2 front derailleur can get far enough in to work on a road double or triple crank. Thanks

For sure the xtr fd works with at least my road cranks with a triple. Only just barely. And fortunately for me lightening has changeable spiders because my sworks spider was to far inboard so swapped to lighting spider. And also fortunate that the way Lightning cranks work for a crossover drive at stoker involves thin shims behind the bottom brackets to get the bearing compression right. I put those shims, about 0.5 to 1mm in my case on the drive side not the timing side which moved the cranks over just enough.

Center chainring is 49 to 50mm center of seat tube to center of chain ring. The inner chainring is 42 to 43mm. I have a spider and BB spacers that could move that inboard 2 to 3mm, but then the FD would not reach that far inboard.

Weight penalty aside, it looks like the chain line flexibility of square taper cranks like daVinci could come in handy to make the FD to work on some frames.

Rather than concentrate on tooth count difference to measure the range of rings the FD can handle I think that the ring radius difference may be more important. That is the measurement that would allow the FD cage height to be adjusted close to both the outer and middle ring. A 22 tooth difference between small mountain rings should be a larger radius range than a 22 tooth difference between larger road rings.

Good point on square taper which would allow one to size a longer axle as required. Stiffness drawback as well unless you can find an axle that keeps the timing crank/chain line close to the frame at the same time. The closer the timing chain line the less bending induced in the bottom tube.

It may be the fd would work for larger rings. Spec is for 40t which is about the radius of the fd cage arc. I'm at 44t. As the ring and radius get larger the cage will need to be raised a touch higher above the rings for its smaller arc to clear the shallow at arc of the ring. Just a few mm.

FYI, Chain line on another 3x tandem.
My road tandem which also has Lightning cranks, but on a road 5-bolt spider, (configured for climbing and back-up tandem on RAAM to our TT V2R Paketa) with 3x10, crossover drive 135 rear hub (or 130 when I put in the rim brake wheel) with a Campy Mirage T FD, over 26-39-52 rings and 10spd Campy Record RD, is +-48mm to the center ring chain line and +-40mm to the center of the inner chain ring. That puts it 2-3mm closer to the frame than the XTR FD on our gravel tandem would permit.
http://bikeforums.net/attachment.php...hmentid=489221http://bikeforums.net/attachment.php...hmentid=489222

Hi Andy,

We had similar derailleur adjustment issues with our road Di2 FD. Our current FSA same-side-drive crank setup puts our granny and big ring at the same ring points as your inner and middle rings on your road tandem (40mm and 48mm or so, +/- .5mm). I just did a quick measurement with a digital caliper, so not 100% super precise... in the ballpark anyway. Our frame requires a 34.9mm FD clamp, but I would guess that Shimano adjusted the FD in/out mount point to be the same across whichever clamp size was used. Regardless, both the Ultegra 10spd and 11spd FD handle our double chainring shifting without any hesitation.

Long story, maybe somewhat useful or not...

In our case, while the Ultegra 10 and 11 spd FD can handle the innermost adjustment range without problem, our issue was with the slight overshift Di2 feature to the big ring that had the potential in our rightside drive setup, to create a really nasty timing belt snag that had the potential to destroy components and perhaps even a frame if you really cranked on it when tangled. For anyone reading this post that is not familiar with Di2 FD behavior, when the FD shift up, it deliberately overshifts then settles back a few mm. You have to be extremely careful to set the outward limit screw to put the FD outer cage plate not more than a hair off the chain when in big-ring and smallest (outer) rear cog. But even at that point, the Di2 upshift still wants to do its over-and-back thing. Then, if you have a right-side drive with timing belt or chain on the outside, it can turn into a nightmare really quickly.

Solution to this problem came with the 11sp FD electronic adjustment feature, something that is not present on the 10spd FD version. This electronic adjustment can be made with either the shifters or with the E-Tube software. Combined with the mechanical limit screws, the electronic adjustment allowed me to tune the FD shift range to keep the over-and-back upshift from interfering with our outer right-side belt setup. It is an extremely picky setup adjustment that probably does not lend itself to going mainstream. When done right, it works great, if minutely off it can be a catastrophe.

--- end if story ---

Just found some interesting Di2 XTR / road-cross setup info here... Tinkering: Shimano XTR Di2 & Road Di2 Shifters ? Perfect for CX & Monster CX |

They discuss some 2x11 setup issues that are similar to Andy's. They also mention wishing that "a custom, ovalized spacer set could be fabricated". Hmmm, my cammed shim idea has a customer ;) Either that, or maybe an aftermarket "narrow clamp" that brings the FD in closer to the seatpost. Ah, next career..., I'm just waiting for Carbon3D printers to become available and at 10% the current cost.

Super post twocicle! One thing readers could consider is a cross bike with 74mm bottom bracket should resolve any tight spacing issues. And to be clear the xtr di2 3x11 road can be made to work as our bike does, but depends on a very refined adjustment of the chain line that not all cranks can accommodate.

With your "outboard" right side drive and triple chainrings, does that not widen the Q factor? How do you fit all that between the frame and the cranks arm?

On a side note, how do you find the stiffness of the "outboard" right side belt? The spider and bottom bracket should be stiffer than a crossover setup. With the rear belt spoket effectively bolted directly to the chainrings. And you get to pick stuff single bike cranks. But the distance of the belt line to center of the frame would greatly increase bending in the bottom tube, as a square of the distance from the frame center. Not sure if the gain in more direct drive at the right spider factors better than the flex in the tube. Probably hard to compare all things equal but configured with a left side belt tight to the frame. If it's a close call and the crossover drive allowed a di2 3x or even 2x setup I'd probably accept the crossover drive to get that.

The "monster cross di2 tinkering" article posted by twocycle suggest the FD can shift up to 48t chain ring.

Q. I do not believe there is a real difference between 10 and 11 speed chain rings. This from some of the top mechanics in my home town. And I am using 10sp chain rings on my 11 speed set up. No issues. The "tinkering" article suggests such a difference limits the chain ring size on an 11 speed set up. I don't believe so.

I know "twocycle" make sense for a tandem alias, but I had created that after we froze on a ride... ie: two icicles ;)

Yes, your and our Di2 FD setup issues are mostly due to running narrow cranksets + triple cranks. The FD adjustment would not be an issue for anyone running a standard tandem width triple with chainlines more in the order of 50-52mm.

Our outboard" right side drive and triple chainrings is implemented using a standard FSA road triple crankset, so no wider than their OOTB product... 157mm. We actually lessened that to 156 by removing one of the pedal insert washers (not needed with Speedplay pedals). As I had offset the cranks to the driveside (improve chainline by putting the wave washer on that side), doing the pedal spacer thing helped to reverse that effect on the pedal stance.

I definitely feel the right side belt is locked into the drive. To put it another way, to me it feels more like a single bike's drive than a crossover tandem drive which seems to be a fraction more vague. Interesting experiment too, but at some point I'll probably return our tandem drive setup to a crossover + triple setup and with a Di2 triple config. I'd love to get a set of Lightning cranks to play with, but haven't antied up the moola yet.

See more facts about my di2 setup with operational synchro shifting here: http://www.bikeforums.net/tandem-cyc...l#post18624475