Simple_form adding class to form

It's intended behaviour.

So if you want to change it you should monkey-patch simple_form_css_class method in this file.

it works for me like this

= simple_form_for @model, html: { class: "form-horizontal"} do |f|
  ...

it compiles to

<form accept-charset="UTF-8" action="/numbers" class="simple_form form-horizontal" id="new_number" method="post">

this works for me config.form_class = 'form-horizontal' and keep new_model class