I am having difficulty deciding what to buy to set up a single side drive train. I have been searching the posts and intranet and am still confused. I would prefer the rear to be a 2 X 10 and would like the big ring to be a 53 or 52. The small ring I would like to go as small as possible without having shifting problems. Right now my much heavier tandem (probably 10+ lbs heavier) has an Ultegra triple and I use a 11-27 for all riding including some mnt climbs.

Do you buy a triple rear crank and use the big ring position for the timing chain and then use a single ring crank on the front? (would require the purchase of a 40T ring and a larger granny with the appropriate BCD)

Or would buy double cranks and add the extra timing ring?

Does anyone know if a Sram s900 or s905 will work for a single side build? Should I be able to put a 40T ring outside the 53?

You will have to measure the crank to see if there is enough room between the big chainring and crank arm. The clearance often decreases as you get in closer to the centre. We use DA7800 crank arms with a 39/56 and 53t out board timing ring. on those cranks there isn't room for much less than a 53.
Also keep in mind that if you use standard double cranks with a 145mm spaced rear wheel your chain line will be out a bit. We use 130mm.

Thanks. I am building the frame to take a 142 12mm thru axle so I can use a chris king mnt bike hub laced to velocity fusion rims. Wow 39/56. I will be getting the design spec tonight or tomorrow. I was assuming the chainline would be fine. Hopefully I can just use a spacer on the bb if necessary. What do you use for you front crank? Thanks for the help

I think the 142mm thru axle is effectively a 135mm hub (someone can correct that if I am wrong) so the chain line should be near enough. We use DA 7800 cranks front and back.

How about some pictures of your setup?

Also a pic of disc mounted to a tri spoke.

wow.... it is hard to tell but it looks like 39/56 are mounted in their normal position and the extra 53 timing ring is added on the outside with a spacer. Where do you get the spacer and longer chainring bolts?

My implementation using a Gates timing belt. These FSA cranks have plenty of clearance to do SSD. Easy to modify to use a normal timing chain if desired and would suggest using 39t or 42t timing chainrings.

Plenty of photos and detail provided in the link above. Enjoy.