diff --git a/README.rdoc b/README.rdoc
index dd22de3e..56fa5def 100644
--- a/README.rdoc
+++ b/README.rdoc
@@ -370,6 +370,24 @@ SimpleForm has several configuration values. You can read and change them in the
 
   rails generate simple_form:install
 
+== Custom form builder
+
+You can create a custom form builder that uses SimpleForm.
+
+Create a helper method that calls simple_form_for with a custom builder:
+
+  def custom_form_for(object, *args, &block)
+    simple_form_for(object, *(args << { :builder => CustomFormBuilder }), &block)
+  end
+
+Create a form builder class that inherits from SimpleForm::FormBuilder.
+
+  class CustomFormBuilder < SimpleForm::FormBuilder
+    def input(attribute_name, *args, &block)
+      super(attribute_name, *(args << { :input_html => { :class => 'custom' } }), &block)
+    end
+  end
+
 == Maintainers
 
 * José Valim (http://github.com/josevalim)
diff --git a/Rakefile b/Rakefile
index 309ea077..97b4b62d 100644
--- a/Rakefile
+++ b/Rakefile
@@ -20,6 +20,6 @@ Rake::RDocTask.new(:rdoc) do |rdoc|
   rdoc.rdoc_dir = 'rdoc'
   rdoc.title    = 'SimpleForm'
   rdoc.options << '--line-numbers' << '--inline-source'
-  rdoc.rdoc_files.include('README')
+  rdoc.rdoc_files.include('README.rdoc')
   rdoc.rdoc_files.include('lib/**/*.rb')
 end
\ No newline at end of file
diff --git a/lib/simple_form/action_view_extensions/form_helper.rb b/lib/simple_form/action_view_extensions/form_helper.rb
index b0509730..0fc2002f 100644
--- a/lib/simple_form/action_view_extensions/form_helper.rb
+++ b/lib/simple_form/action_view_extensions/form_helper.rb
@@ -33,7 +33,7 @@ module SimpleForm
         class_eval <<-METHOD, __FILE__, __LINE__
           def simple_#{helper}(record_or_name_or_array, *args, &block)
             options = args.extract_options!
-            options[:builder] = SimpleForm::FormBuilder
+            options[:builder] ||= SimpleForm::FormBuilder
             css_class = case record_or_name_or_array
               when String, Symbol then record_or_name_or_array.to_s
               when Array then dom_class(record_or_name_or_array.last)
diff --git a/lib/simple_form/map_type.rb b/lib/simple_form/map_type.rb
index b91ca9e1..e0944f83 100644
--- a/lib/simple_form/map_type.rb
+++ b/lib/simple_form/map_type.rb
@@ -1,7 +1,10 @@
+require 'active_support/core_ext/class/attribute'
+
 module SimpleForm
   module MapType
-    def mappings
-      @mappings ||= {}
+    def self.extended(base)
+      base.class_attribute :mappings
+      base.mappings = {}
     end
 
     def map_type(*types)
diff --git a/test/form_builder_test.rb b/test/form_builder_test.rb
index 2f150d36..af1689c3 100644
--- a/test/form_builder_test.rb
+++ b/test/form_builder_test.rb
@@ -9,6 +9,12 @@ class FormBuilderTest < ActionView::TestCase
     end
   end
 
+  def with_custom_form_for(object, *args, &block)
+    with_concat_custom_form_for(object) do |f|
+      f.input(*args, &block)
+    end
+  end
+
   def with_button_for(object, *args)
     with_concat_form_for(object) do |f|
       f.button(*args)
@@ -583,4 +589,10 @@ class FormBuilderTest < ActionView::TestCase
     assert_select 'form ul', :count => 1
     assert_select 'form ul li', :count => 3
   end
+
+  # CUSTOM FORM BUILDER
+  test 'custom builder should inherit mappings' do
+    with_custom_form_for @user, :email
+    assert_select 'form input[type=email]#user_email.custom'
+  end
 end
diff --git a/test/support/misc_helpers.rb b/test/support/misc_helpers.rb
index c87c1527..c5a742e1 100644
--- a/test/support/misc_helpers.rb
+++ b/test/support/misc_helpers.rb
@@ -28,4 +28,18 @@ module MiscHelpers
   def with_concat_form_for(object, &block)
     concat simple_form_for(object, &block)
   end
+
+  def with_concat_custom_form_for(object, &block)
+    concat custom_form_for(object, &block)
+  end
+
+  def custom_form_for(object, *args, &block)
+    simple_form_for(object, *(args << { :builder => CustomFormBuilder }), &block)
+  end
+end
+
+class CustomFormBuilder < SimpleForm::FormBuilder
+  def input(attribute_name, *args, &block)
+    super(attribute_name, *(args << { :input_html => { :class => 'custom' } }), &block)
+  end
 end