I am building a tandem with a triple on the back. The frame has a 135mm rear spacing.
Standard road triple chainline is assumed to be 45mm, while MTB is 47.5-50mm. I will be using a 10 speed cassette (SRAM 110-28 OG1070).
If I want to set it up for optimum use of outer two chinrings and use the inner ring as the occasional bailout hillclimbing ring, then I am led to believe that I want to pull the chainling in a bit.
My plan is to use an old 118.5mm BB to mock up the spacing and then estimate the BB spindle length required to set the crankset up as close to the frame as possible cointingent upon achieving 45-47mm chainline.
Is this a reasonable approach?
Second question is twofold.
1. what length BB spindle? (assuming no data from the above experiment)
2. what are the arguments to run the same BB upfront, or two use a narrower one to aid Q factor versus ideal chainline for timing chain.
Though your frame's 135 rear spacing might help, using a shorter than normal BB axle can make shifting to the small ring quite difficult. It could also cause the chain, when on the middle ring, to brush and "ting" against the outer ring in otherwise useable f/r gear combinations. You don't say what crank you have, but you will probably have best results with the BB that the crank manufacturer specifies for road triple use.
Modern chains are quite flexible side to side. A narrower BB up front will work in many cases. If your frame's chainstay configuration permits, you can experiment with moving the rear timing ring to the inside of the crank spider while leaving the front timing ring is its normal outside position.
BTW, we run a SRAM 10s 11-28 cassette on our PeriScope triple. I've found that it's an excellent choice for a road tandem.