TOO TIGht!
If you go with an 18 inch radius and a 3% grade you are not going to be able to get much up it with a Bachman Shay. on my RR's Valley division I have a 3.3% grade, which my Bachman Shay can get six or seven cars up. That grade has 22 inch radius curves on it. As the radius goes down, the drag on the locomotives and cars goes way up. at my club, where my Bachman Shay has gone to live on a branch with slightly more than 18 inch radius curves and a grade appoaching 4.3% the Bachman shay can handle only a meager 3 or 4 cars. it every extra inch of radius on your helix will help you out 3 times. 1. longer run to gain height = lesser grade needed. 2. longer distances between overlaping tracks= more overhead clearance and 3 drastically reduced rolling resistance on locomotives and equipment.
The only places on my extensice layout where the Bachman Shay was happy has Curves of more than 20 inch radius, and low grades . were I to design a layout with the Bachman Shay in mind I would have a minimum of 20 inch radius, with a 22 inch radius greatly preferred, and grades no more than 2% preferably 1.5% My Bachman Shay has gone to the club, as it shares the only section of track on my home RR it is happy with with a PFM 3 truck shay which will not negotiate the sharper curves elsewhere on my RR. The PFM Shay, with new motor and NWSL gear reduction can pull more than twice the Bachman. At the club, my Bachman Shay may end up as a switcher at the big coal mine I am planning, because it is a weakling when it comes to pulling the hills.
The Bachman Shay is a fine looking and ok running machine, but it is way to weak to suit me. I have two Westside class A climaxs that can outpull a Bachman shay on 18 inch radiuss of any grade, which is silly, since the Shay is three times loner the the class a climax, and has an extra power truck!
My two cents worth
Bill Nelson