chichi

Does anybody have any advice on converting an FAS SLK 9 speed triple to a compact double?
I am hoping to convert to SRAM wireless 2x11 and I had no idea of the difficulty of finding 130mm and 74 BCD chainrings that might make this possible.
The crankset set we have has 172.5 stoker arms which I would like to keep and they also seem difficult to find on new cranksets.

Andrew R Stewart 

I would not think a 130mm crank bolt pattern as being anything near "compact". A 38T minimum is pretty large for that. The images I pulled up show a two piece crankest. The chainline is set by the attached axle and won't change, shift inwards, to maintain the rings and cassette remaining centered WRT each other. I suggest seeking another solution. Andy

Kontact

A quick search shows that Stronglight sells 74 BCD 32t 5 bolt chainrings. Since you ought to stay within 16t difference between rings, you would pair that with a 48t 130 BCD 5 bolt middle ring and not use the outer ring position.

This has several problems.
1. You might not have the chainstay clearance for a 32t inner.
2. You might not have the front derailleur low range to get the derailleur to shift down to small ring.
3. You might not be able to mount the front derailleur low enough to engage a 48t high ring on your frame. Some bikes built before the compact era have problems.

You may be able to figure out 1 and 3 with a piece of 32t size cardboard.

You may be able to fix 1 and 2 with a spacer under your right cup, depending on BB system (but likely not for Mega Exo). You might also be able to simpl7 swap your current front derailleur clamp, designed for tandem 60mm chainline, for a standard road claim designed for up to 45mm chainline. Some adapters can vary the derailleur clamping distance via angle, which can buy a few more mms.

Some tandems have adjustable chainline built into the frame.

So I would say it is possible for nothing more than the cost of two new rings and some shorter chainring bolts. But it will be hard to know for sure without trying a derailleur. Could you borrow one to try?

chichi

I have been trying to find a 52 or 50/38 that would both attach to the 130 bolt circle.

I think I might have better luck finding those rings than my original idea of mounting a 32 or 34 on the 74 bolt circle and a 48 on the 130.

Andrew R Stewart 

The problem with a 48T ring mounted on the inside of the 130 step is that most all pinned/ramped 48T rings will be intended for the outside position. Flipping them around to the inside means their shifting aids are all wrong, not flipping them usually means the ring bolts will stick out of the 48T ring (its recessed seats being on the wrong side) and snag on the chain. Running a symmetrically cut ring would work but even with "home made" W cut mods the shifting won't be that of a modern ramped and pinned ring's. Andy

Kontact

Or just use non-ramped rings. They shift.

Andrew R Stewart 

"symmetrically cut ring" is what a non ramped/shift aided one is. For those of us who have learned how to shift a bikie well traditionally cut rings can be all well and good. But so many current riders have grown up being able to shift front rings when under some power (too often too much power, hence chain suck, lack of shift completion and chain derailment issues being so common here). So my advice is more for them then us. Andy

chichi

I have completed my conversion of 3x9..12/36 to 2x12 11..50. I am very happy with the results.

This is a list of the changes
SRAM AXS Red road front and rear derailleurs
Cycles Chinook AXS Extended derailleur cage
Wolf Tooth Road link
10mm longer B screw with head ground down to clear derailleur body
FSA tandem FD mount with " point " filed down about 1/8"
Mount for inner ring milled down .125 mils
House of Tandems 34 tooth inner ring
FSA 50 tooth mounted at middle ring position
1.2 mm spacers on outside of chainring bolts, chainring bolts mounted with allen drive facing in
SRAM PG-1230, 11- to 50-tooth Eagle cassette.
KMC DLC 12 chain

A side benefit which I was not expecting was improved braking.I am using Paul Klampers, front caliper has a short pull arm, the rear has a campagolo arm, they are spaced for Hope ventilated discs.The braking is much improved with the SRAM levers vs the Campy levers.

PromptCritical 

What rear hub and free hub body are you using?