This works for me, but i would like to solve this with ajax without reloading the page. Choosing the values from the Dropdowns and submit the data per POST Method, so i can return the value with "getParam( name )". This question is related to my own question Reuse chosen dropdown field value on frontend
{% set field = craft.fields.getFieldByHandle('jahrgang') %}
<form action="tourdefrance.html" method="post">
<select class="dropdown" name="modelljahr" size="1" onchange="this.form.submit();">
{% for opt in field.settings.options %}
<option name="fields[myFieldHandle][]" value="{{ opt.value }}">{{ opt.value }}</option>
{% endfor %}
</select>
{% set modelljahr = craft.request.getParam('modelljahr') %}
{% set entries = craft.entries.section('mountainbikes') %}
<form action="tourdefrance.html" method="post">
<select class="dropdown" name="modell" size="1" onchange="this.form.submit();">
{% for entry in entries %}
{% if entry.jahrgang == modelljahr %}
<option value="{{ entry.title }}">{{ entry.title }}</option>
{% endif %}
{% endfor %}
</select>
{{ craft.request.getParam('modell') }}
Thanks a lot!