Body - Door Catches

Since my last post I have continued to keep on the look out for parts and to try and find a few bargains. I haven't been buying too much though as the focus is still on funding the chassis work while it is down at Juniors in Wellington.

One idea I have had squirreled away for a while now though, was to have soft close doors. I knew such a thing existed as I was looking at a BMW 7 series (they're the big luxury limousine like ones) about 5 years ago and when I closed its boot it clicked closed onto the first click of the catch, then an electric motor pulled it the boot lid the rest of the way closed. On late model BMW's they have them on the doors too now. Here's what they do.

The benefit of them is that there is no need to slam the door. You pull the door too, and the motorised catch does the rest meaning you aren't having the hassle of slamming the door and you also aren't causing damage with that sudden shock. This is of particular concern on a fibreglass car as you can get small hairline cracks appearing at stress points.

Great idea but finding these catches was easier said than done. Either not many BMW's have them or people selling them as parts cars don't understand what I am asking for. I figured there must be more cars with them on though and asked a few questions on Trademe. One guy suggested Mercedes. I found one but they wanted $450 for each latch and another $250 for the control box and I'd still have to work out how it all went together and switched. Bugger that!

At this point I started looking around in car sales yards and was told by one salesman that the Toyota Estima might have them. I tried that but they are only on the sliding side door and trying to adapt that to a hinged door would be too hard. Next suggestion was a Nissan Elgrand people mover. The salesman reckoned they had them on the rear hatch. BINGO!

I called at Dodson's (Japanese car wreckers) and as luck would have it the bosses van was a Nissan Elgrand so he knew exactly what I was talking about and whats more they had just removed the mechanism out of a wrecked one a few weeks earlier. He went and got it for me to have a look at.

It certainly is a lot more complex and a lot bigger than I expected, but I still thought I could make this work. I bought it and took it home to assess a bit more.

Here's how they work...

The door closes manually onto its first click. The first picture is with the catch open and the second picture is on the first click.

Now when it hits the first click the latch has a cam that comes up against a small metal lever that activates a micro switch (a small switch that is activated when another part presses on it. A bit like your finger pressing on a light switch to turn it on), You can see here the pencil is pointing at the cam hitting the small metal lever.  The grey plastic box is the micro switch itself.

Now when that happens power goes to the motor and it starts turning over and cranking these teeth.

The plate that has the teeth on its edge then turns and as it does so, the latch is pulled in to its second click. The pic below is a wider view of the one above. You can see the pencil at the top right is pointing to the lever that is operated as the toothed plate turns.

Viewed from the other side, the pencil is now pointing to the pin that the lever pushes against. You can see the lever coming through from the other side and making contact with the pin.

Here is a shot of the catch in the fully closed position.

Once the motor has pulled the latch onto its second click another micro switch is activated. You can see that one here.

What this one does is turn the polarity of the current around, so what was a positive feed to the motor is now negative and what was negative becomes positive.....what that does, is it makes the motor run backwards, so the arm with the teeth on it can go back to where it started (If it didn't it would permanently hold the latch on its second click and you wouldn't be able to get out of the car).


Once the motor has wound the toothed plate back to the start position another micro switch is activated to turn everything off until its required again. Here is that third micro switch.

All that switching is controlled by the little black box at the bottom right of the picture below....the brain of the mechanism if you like.

This video shows the whole thing working (I am activating it with a bit of rod so you can see what is happening, normally though, this is where the door striker would go). This video shows the latch itself.

And secondly here's a video of the motor doing all the work. You can see it pressing the lever at the top right of the picture.

Okay so the door is closed now but what about opening the door then? That's pretty easy its just a case of pulling on the lever/rod shown here.

In this build I'll be using a shaved door handle kit which means the car won't have any door handles at all. Instead what will happen is you'll push a button on the remote and it will tell this little black control box to send power to a solenoid (a mechanism that will pull the door catch release lever/rod I showed you above).

This is the actual solenoid.

A solenoid is quite simple it is a winding of copper wire around a cylinder. That is exactly what you'd find inside the solenoid pictured above. It doesn't do anything when the power is off, but when you add a current (by pushing the button on the remote) the copper wire winding becomes an electromagnet and it attracts the metal rod that is running through its core (the arm sticking out the left hand end) and pulls it in.

This video shows you what they do.

So what will happen when its installed in the car is the solenoid will be attached to the door mechanism. When I push the open button on the remote the electromagnet will come to life and the lever will be pulled. Voila the latch opens to allow the door to open up.


Look Ma no hands!











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