Don't attempt to show a wheel size if it's blank

app/models/bike_wheel_size.rb

@@ -2,7 +2,12 @@ class BikeWheelSize < ActiveRecord::Base
   belongs_to :bike
 
   def display_string
-    ["#{twmm}-#{rdmm}", "#{rdin}x#{twin}", "#{rdfr}x#{twfr}", description].join(" | ")
+    result = []
+    result << "#{twmm}-#{rdmm}" unless twmm.blank? and rdmm.blank?
+    result << "#{rdin}x#{twin}" unless rdin.blank? and twin.blank?
+    result << "#{rdfr}x#{twfr}" unless rdfr.blank? and twfr.blank?
+    result << description unless description.blank?
+    result.join(" | ")
   end
 
   def to_s