Вам необходимо изменить следующие строки в /config/initializers/simple_form.rb
:
config.wrappers :default, :class => :input,
:hint_class => :field_with_hint, :error_class => :field_with_errors do |b|
...
b.use :hint, :wrap_with => { :tag => :span, :class => :hint }
b.use :error, :wrap_with => { :tag => :span, :class => :error }
...
config.error_notification_class = 'alert alert-error'
для
config.wrappers :default, :class => :input,
:hint_class => :field_with_hint, :error_class => :errors do |b|
...
b.use :error, :wrap_with => { :tag => :small }
...
# delete hint
config.error_notification_class = 'alert-box alert'
Здесь вы можете увидеть diff .