I'm looking for a little direction re tubing sizing for a custom frame I'm planning.

For the record:

a) No actual designs for the frame currently exist...it's still a concept. Hoping to make a start on designs over the next 2 - 3 weeks with the frame builder.
b) After much frame testing I'm happy with all my chosen sizes / geo / spec. It'll be made from 853 tubing.
c) I'm perfectly happy that the experienced frame builder will be able to identify the 'correct' tubing for my needs, however...
d) I just want to understand the pro's & con's of tubing sizing & their effect on compliance / comfort.

So...a few questions for further thought & consideration:

1. Does the size of the seat tube affect compliance i.e. a 27.2mm internal seat tube will be more compliant than a 31.6mm?
2. Does the shape / bends of a seat stay affect compliance i.e. straight v 'S' bends?
3. Is compliance / comfort affected by the size of the TT i.e. too thick walled / big a TT and you get a harsher ride...too thin walled / narrow a TT and the bike becomes a bit noodly.
4. Is compliance / comfort affected by the shape of the TT i.e. oval v round?
5. Can anyone direct me to a site / forum / guru who I'll be able to discuss this in more detail?

Thanks in anticipation.

If you haven't already, I'd suggest speaking to Shand.

C) is the way to go

but for reference after having a fair number of frames made, tt and dt diameters do affect twist stiffness levels and a frame that handles well loaded or under a strong rider will be on the rigid side of average. A very common combo is a 31.8mm tt and 34.9 dt, or a size down at 28.6 tt and 31.8mm dt. Wall thickness varies by material but has much less effect on stiffness. I'd generally expect to find the latter combo too flexible for loaded mtb use but a lighter rider on a smaller frame may not. Those same tubes could make a nice light road bike though. I don't believe that stay shape has much if any real effect on comfort unless you go from one extreme to another. Seat tube also, larger can certainly feel stiffer to pedal but std size has always been fine for me and the post makes most difference to comfort. What's difficult in all this is quantifying it all so I'd say its best to work with a good frame designer builder and have a reference point to go from.

1. Does the size of the seat tube affect compliance i.e. a 27.2mm internal seat tube will be more compliant than a 31.6mm?

More to do with seatpost extension, material and diameter (in that order)

2. Does the shape / bends of a seat stay affect compliance i.e. straight v 'S' bends?

No, in fact bending many steels will work harden them and make them stiffer

3. Is compliance / comfort affected by the size of the TT i.e. too thick walled / big a TT and you get a harsher ride...too thin walled / narrow a TT and the bike becomes a bit noodly.

Not that you would notice unless running solid steel tyres and wheels...and even then,,,,

4. Is compliance / comfort affected by the shape of the TT i.e. oval v round?

See above, in a lab with really good kit maybe but never on the trail

5. Can anyone direct me to a site / forum / guru who I'll be able to discuss this in more detail?

MTBR.com frame building forum. But TBH I would start with some basic engineering texts and theory. Or just leave it to the builder.

SSP, I'd agree that in the oft-misused vertical compliance sense tube changes make so little difference to vertical deflection, but if we agreed that a lot of perceived harshness is really a lack of twist or lateral flex causing a very rigid feel then those changes in tube dimensions can make a notable difference. A difference that many describe as 'more comfy'. Comfort is certainly about fit more than flex though, imo.