topeak-mtx-tortec-expeditionrack-adapter: introduce front_to_rear_elevation_change

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>

diff --git a/topeak-mtx-tortec-expeditionrack-adapter.scad b/topeak-mtx-tortec-expeditionrack-adapter.scad
index 688d468fa4a8999cfd9d4b73db77f585cce95b1e..49441beaa1add203df6e31652fe5cda2eabd4bd0 100644 (file)
--- a/topeak-mtx-tortec-expeditionrack-adapter.scad
+++ b/topeak-mtx-tortec-expeditionrack-adapter.scad
@@ -73,6 +73,9 @@ $fs=1;
 
 bolt_z = -brk_block_z/2;
 
+front_to_rear_elevation_change =
+  rear_elevation_nominal - front_elevation_nominal;
+
 main_sz_rhs_z = max(min_on_rail_sz_z, beside_strap_sz*2 + strap_w);
 main_sz_lhs_z = min_on_rail_sz_z;
 
@@ -95,8 +98,7 @@ block_y_min = adapt_main_top_y - main_sz_y;
 
 strap_barrel_x = rack_width_inner/2 + strap_barrel_dia/2;
 
-rack_shear_ratio = -(rear_elevation_nominal - front_elevation_nominal)
-  / rear_to_front_distance;
+rack_shear_ratio = - front_to_rear_elevation_change / rear_to_front_distance;
 
 front_to_cross_rail =
   cross_rail_distance * sqrt(1 - rack_shear_ratio * rack_shear_ratio)
@@ -393,9 +395,9 @@ module ForRackForDemo(){
   rack_rail_y = rack_rail_y_of_elevation(elevation);
 
   rotate([atan(
-               (rear_elevation_nominal - front_elevation_nominal) /
-               cross_rail_distance
-               ), 0,0])
+              front_to_rear_elevation_change /
+              cross_rail_distance
+              ), 0,0])
     translate([0, rack_rail_y, brk_block_z/2 + rack_rail_y*rack_shear_ratio])
     children();
 }