This is one of the most common shared experiences in all of automobilia: You’re outside a car, trying to get in. The person inside the car is attempting to open the door lock at the same time you’re pulling the handle to open the door. A comedy of errors ensues, with each of your actions cancelling out the other’s, leaving you both pulling handles or locking buttons over and over, in a seemingly never-ending mechanical stalemate. Eventually, someone yells for someone to let go, already, and the nightmare ends. But why must it be this way? What’s going on?
Why can’t you pull the door handle while the door is being unlocked? Why would such a fundamental mechanism of a car have such a frustrating flaw? Well, the answer is simple mechanics. (Except, as we found out, there’s nothing simple about door latches.)
The most basic explanation possible is that pulling on the handle engages a little mechanical piece inside the door, a piece that bumps into another mechanical piece triggered by someone else trying to unlock the door. Alone, either one would do their job unimpeded. Do both at the same time and they block each other.
This sounds like it’s going to be really easy to understand, right? You have no idea how wrong you are.
Door locks and latches are absurdly, mind-hurtingly complicated. Just stick with us here, and you’ll see.
To understand what’s going on here, we need to look at how door latches and locks actually work in a car. To do that, our own David Tracy and I have pulled the inside door panels from two different cars, representing a range of door-mechanism tech, from old and simple to more modern and complex.
On the simple side, I’ve pulled off the door card of my 1973 Volkswagen Beetle, which is fundamentally a 1938 design, with a door latch system that hasn’t changed much since then. I know the last major update for this door system was in 1968, and the setup on my car is pretty much identical to that.
There’s really not much to it at all. Look:
Basically, it works like this: the door latches with that roughly C-shaped latch on the door, which engages with the post on the B-pillar. When latched, the C rotates down, like an inverted U, to trap the post.
Pulling the inner door handle or the trigger on the outside handle releases the latch back to its C-orientation, allowing it to be pulled free from the post. Easy!
When the lock button is down, though, a mechanism slides into place to prevent the latch from moving. You can see the flat bits of metal that make up the mechanism here:
That locking/unlocking mechanism makes direct contact with the door latch, which is directly connected to the inner and outer door handles. That means the reason why you can’t be pulling on the locking button when someone is simultaneously pulling the door handle trigger is that somehow, something is binding the process there.
I tried to draw what I could see/figure out where the camera wasn’t and with the help of some parts pictures online, but I quickly realised this was getting wildly complicated:
Now, to figure out exactly what is going on here, I’m going to need two things: (1) the whole door latch assembly out of the car and (2) to be a hell of a lot smarter.
Since I don’t really want to pull apart my working Beetle door any more than I have, I got David to agree to go to the junkyard and yank everything out of a Beetle there, which kills two birds with one socket wrench, since David is also much smarter than I am.
Here’s what David found—the first pics of the yellow door are from my car, the others from the junkyard Beetle. Have at it, David:
How An Old Latch Works
On the Volkswagen Beetle, there are rods connecting both the door handle and unlock button to the latch.
The Beetle has a striker (which Jason called a “door latch post”) on the B-Pillar and the latch that grabs it looks like a fork. Here’s the striker:
Here’s the latch in the open position (i.e. the door has been opened):
And here’s how the latch would look if the door were closed:
To understand how this thing works, have a look at the link below that I’ve called the “latch link.” It’s located on the back side of the latch shown above and rotates with the fork that actually grabs the striker (the fork and the latch link are essentially one piece). As you can see, the latch link is spring loaded, and held in position by a “latch release link.” If that latch release link moves up and out of the way, the spring will want to rotate the latch link, and thus the fork-shaped latch, releasing the striker so you can open the door.
Here’s a look at what it looks like when the latch release lets the latch link rotate:
Here’s what it looks like from the other side:
There are two ways to lift that latch release link and open the door—from the outside and also from the inside.
On the outside, pushing the door button rotates a spring-loaded actuator that works much like a clutch fork in that it uses a pivot to turn rotational motion into translation. Here’s a look:
That translation is simply the bracket shown below getting pushed to the right.
That bracket then pushes the top of the latch release link (pointed out by the red arrow), rotating the latch release link about its pivot, and lifting it off the latch link, which then rotates because of the spring, opening the door latch and thus the door.
The second way to open the door is from the inside. Pulling the door handle puts the rod shown above on the right in tension, rotating the “handle link” shown above counter-clockwise, which then pushes the right side of the latch release link down. That raises the left side of the latch release link, releasing the latch link, which then rotates to open the door.
Why You Can’t Unlock It And Open It Simultaneously
On the Beetle, you cannot pull the handle from the outside and unlock the car at the same time because of an interaction between the finger on the bracket and the nipple on the latch link.
Before we get into that, let’s look at how the lock/unlock mechanism works.
Unlocking the door on this Beetle involves moving the bracket down, either directly via the key cylinder on the outside, or via the interior lock post, which pulls on the rod shown in the photo above.
That rod rotates the lock link, sending the left side of the lock link down, thus pushing the whole bracket down. The key to unlocking the car is that, with the bracket down, the latch release link is now to the right of the bracket, so when you pull the handle from the outside and push the bracket to the right, the latch release link rotates and ultimately lets go of the latch.
Locking the car back up moves the bracket up to the point where it will no longer contact the latch release link when the handle is pulled from the outside. So the car remains locked.
As for why you can’t open the door from the inside when it’s locked: When you lock the car from the inside or outside, the bracket goes down and rotates the lock link into a certain position. Trying to pull the interior handle leads to an interference condition between the lock and handle links.
OK, so now that you know how the lock works, imagine the car is locked as shown above, and you decide to pull the handle. The bracket is going to move to the right, and the finger will be just above the nipple. Unlocking the car (which moves the bracket down) will not be possible because the nipple and the finger will interfere.
Thus, you’ll have to let go of the door button and then try unlocking the again.
Pulling the inside door handle (represented by the bottom right blue arrow) will also restrict the door from unlocking. Looking above, you can see that as you pull the interior door handle, the handle link crashes into and puts pressure on the lock link. So when you pull the unlock post (represented by the red arrow), it won’t be able to rotate the lock link out of the way, and the door will remain locked.
So there you go: On the Beetle, opening the car door at the same time that someone’s trying to unlock the car simply won’t work because of that nipple/finger interaction and the lock link/handle link interaction.
I think the biggest takeaway here is, holy crap, car locks and latches are complicated. They’re like the human foot of cars. Human feet have the highest concentration of bones and are mechanically really complicated, likely the most mechanically complex system in the body, and yet they rarely get any respect outside of podiatrists and I guess foot fetishists.
Seriously, it’s easier to picture how engines work in your mind than this shit. These locks are making my cerebellum ache, and this is from a pretty crude and simple car!
What must more modern locks be like? Well, I’m going to pass to David’s big, pulsating, dripping brain to yet again show you:
How A More Modern Latch Works
A modern door latch mechanism like the one in my landlord’s 2001 Oldsmobile Alero grabs onto a metal striker on the B-pillar like this one:
The latch is bolted to the ends of the doors like this:
Here’s a closer look, showing the latch in the open position (the position it would be in when your door is open).
When you close the door, the striker hits the angled part of that latch, rotating it and causing the latch to grab onto the striker. The white plastic “latch release” shown above then moves upward and holds the latch in place so that the latch holds the door firmly in the closed position like this:
Notice how the white piece is now up, holding the latch in place so you can’t just push your door open. For the latch to open—and for thus, the door to open—your door handle sends forces through a series of linkages that ultimately pull that white plastic piece down into the position shown in the photo before the previous one, allowing the latch to rotate into the position shown in that photo, and thus release the striker.
Now let’s look at how your door handle does this. The handle is attached to the upper rod shown in the photo above, and the lock/unlock button pushes and pulls the lower rod.
First, we’ll look at the door handle, which puts the upper rod into tension, rotating “Link A” shown below counterclockwise (the rods are removed in the photo below).
This rotation of Link A, pushes up on the left side of “Link B” causing it to rotate about its pivot point (the silver dot in the centre of the picture), ultimately pushing the right side of Link B down. (Note: If you’re opening the door using the outside handle and not the inside one, you’ll be pushing down directly on the right side of Link B at that large hole just above the blue arrow).
From there, Link B pushes on another link (Linc C shown above), which then pushes on a black link that I’ve labelled “Link D.” That black plastic link, Link D, goes into tension, and pulls the white plastic latch release down, allowing the door to open.
Why You Can’t Unlock It And Open It Simultaneously
Just as there are two points for the door to be opened (the top left yellow doughnut is for the interior door handle, the big hole just below and in front of the right yellow doughnut is for the outside door handle), you can also unlock and lock the car from inside or outside, as shown above.
In either case, locking the door acts upon link D, pushing it out and away from Link C (Link D moves towards you in the picture below).
Here’s how that looks in animated form. Watch the plastic “Link D” move out from under Link C:
Since opening the door requires putting Link D in tension so that it can pull the white latch release, moving link D out from under Link C means pulling the door handles will simply move Link C downward. This won’t pull on D, so the door will remain closed.
The reason why you can’t open and unlock the door simultaneously, then, is quite simple: As soon as you’ve pulled the door handle on a locked car (again, locked means Link D has been moved from under Link C), you’ve put Link C where Link D needs to be in order to unlock the car.
In other words, unlocking the car while pulling the handle will simply result in Link D—the black plastic piece that’s meant to pull on the white latch release to open the door—crashing into link C, and not moving under Link C. Once you let go of the handle and move C up out of the way, Link D will still be outboard (locked) until you push the button to unlock the car again. Then it will move in, and Link C, Link B and ultimately the handle will be able to force it down and ultimately release the latch.
So basically, on the Oldsmobile, the problem is that pulling the handle to open the door moves linkages to a spot where the latch release mechanism is trying to move to unlock the car.
It’s incredibly complicated considering most people hardly spend much time thinking about automotive latches, but we had to get to the bottom of why so many have “dorked” their cars trying to open the door while someone was locking it. It’s just a matter of linkages interfering.
As for why they were designed this way, we’re not sure. It doesn’t seem entirely necessary (like, why not remove the nipple and finger from the VW?), so it seems deliberate. Whether it’s meant to provide a major safety benefit, I’m not sure.
Got all that? Good. You just wanted to know why you’ve had that annoying experience of handle-button back-and-forth stalemate fighting and look what it got you.
Some things we use every day and absolutely take for granted are secretly deliriously complicated. I feel like that’s the real lesson here, and not just because figuring out exactly how these latch systems works is like brain CrossFit or something.
I feel humbled. This is probably healthy.