topeak-mtx-tortec-expeditionrack-adapter: sort out length

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 e62c4de7b8ae4d66fce9590d479a4fc9861352d3..5ed42a4d9487a4b92c5fce1d0950f6590c7db642 100644 (file)
--- a/topeak-mtx-tortec-expeditionrack-adapter.scad
+++ b/topeak-mtx-tortec-expeditionrack-adapter.scad
@@ -20,7 +20,6 @@ rear_elevation_nominal = 10.04;
 rear_to_front_distance = 230; // rack cross rail (fam) to very front end
 rear_to_cross_rail = 43.05; // bolt centre to rail centre, rail to rear
 rear_bolt_to_front_bolt = 155.4;
-front_to_cross_rail = 43.05; // XXX // bolt centre to rail centre, rail to front
 front_elevation_nominal = 3;
 
 cross_rail_distance = 232.09;
@@ -98,6 +97,11 @@ strap_barrel_x = rack_width_inner/2 + strap_barrel_dia/2;
 
 rack_shear_ratio = -rear_elevation_nominal / rear_to_front_distance;
 
+front_to_cross_rail =
+  cross_rail_distance * sqrt(1 - rack_shear_ratio * rack_shear_ratio)
+  - rear_bolt_to_front_bolt
+  - rear_to_cross_rail;
+
 brk_bolt_nut_top_y = -brk_nearbolt_recess_depth
   - brk_bolt_eff_len + brk_bolt_nut_th + brk_bolt_len_slop;