Laravel 5, can't insert via request method doest not exist

Hello that's what i'm trying to do without any success:

In welcome.blade I have a foreach with some boards and subboards(random generated by user) where you can click on subboard and go something like this /subboardOne. I got this on my routes.php

Route::get('/{subboaName}', 'ThreadController@index');
Route::post('/{subboaName}', 'ThreadController@store');

then you can post a thread on this subboard via form but since i really don't know how laravel knows where he is, the form is something like this:

<form class="form col-md-12 center-block" role="form" method="POST" action="/">

this $subboardcoll->id comes from the controller, where it sends via the index function the collection:

public function index($subboard)
{
    $subboardcoll = Subboard::where('subboaName', $subboard)->first();
    $threads = Thread::where('subboaId', $subboardcoll->id)
           ->orderBy('created_at', 'desc')
           ->get();
    return view('threads.thread', compact('threads', 'subboardcoll'));
}

then i'm trying to send my form and store the thread autoinserting the subboardId but laravel doesn't recognize subboards method:

public function store(Request $request)
    {
        $this->validate($request, [
            'comentario' => 'required|max:2000',
            //'g-recaptcha-response' => 'required|recaptcha',
            //'imagen' => 'required',
        ]);

        $request->subboards()->threads()->create([
            'thrName' => $request->nombre,
            'thrComment' => $request->comentario,
            'thrImg' => $request->imagen,
            'thrSubject' => $request->tema,
        ]);

        return redirect()->back();

    }

And gives me this erorr:

BadMethodCallException in Macroable.php line 81: Method subboards does not exist.

Can you guys helpme to know why? also is there better form to do what i'm trying? im newbie on laravel, thanks

via Chebli Mohamed