How can I use sub-controller WITHOUT providing the directory name?

With routes.

PHP Code:

$route['someMethodA'] = 'subdirectory/ControllerA/someMethod';
$route['someMethodB'] = 'subdirectory/ControllerB/someMethod'; 


http://yoursite.com/someMethodA would then actually trigger /application/controllers/subdirectory/ControllerA/someMethod.

Thank you! That works - but seemingly not with default_controller...

$route['default_controller'] = $isSiteB ? 'SubDirectory/ControllerB/index' : 'ControllerA'/index;
Success: http://siteB.com/SubDirectory/ControllerB/index
404: http://siteB.com (expecting it to trigger SubDirectory/ControllerB/index)

Suggestion?