I'm trying to override a function in Mage_CatalogRule_Model_Rule but I can't get it to work.

config.xml

<config>
    <global>
        <models>
            <rule>
                <rewrite>
                    <rule>Apptha_Timer_Model_CatalogRule_Model_Rule</rule>
                </rewrite>
            </rule>
        </models>
    </global>
</config>

/app/code/local/Apptha/Timer/Model/CatalogRule/Model/Rule.php

class Apptha_Timer_Model_CatalogRule_Model_Rule extends Mage_CatalogRule_Model_Rule {
    protected function _convertFlatToRecursive(array $data) {
        //not called
    }
}

What am I missing? I've been able to override controllers and blocks just fine.