Wrench Tip #29: Pulling a dent with a suction cup.
by Alex Vendler
It's happened to all of us. You park your car and head to your destination only to return and find that another motorist violated Newton's law about two objects inhabiting the same space at the same time. Crunch. Now what? Insurance kills you if you make a claim, and modern base/clear paint jobs can only be repaired by painting the whole panel. That's 4 figures minimum if you use a decent body shop.
Let's use the old bean for a sec. and really give a dent a bit of thought. Here are a couple of truths about sheet metal that are not self evident (get it? evi-DENT). OK, never mind, and look at a car with me. First off we have to accept the reality that no panel on a car is flat. Even “flat” panels on a car are actually domed. If a panel were truly flat it would look concave. In order to cancel out this optical illusion the Greeks subtly curved the horizontal elements in some of their architecture. Ah, those were the days.
Obviously the illustration here is a bit exaggerated but one gets the point and now that we are in the know about domed panels we can think about what happens when one is pushed on really hard. Especially by the bumper of some less than qualified driver's car. In order to illustrate this, I recommend you go get a beer out of the fridge. No really, do it. If you are a low brow chump like this guy your beer comes in a can and cans are a good car body simulator. Here's how you can drink and learn at the same time. Chug that road soda and when you are done take the empty can and give it a gentle squeeze while carefully watching what happens.
Note that as your finger moves the metal inward there is a fold forming along a line parallel to the circumference of the can. This is the beginning or the formation of a permanent dent. If you don't let up, the dent will remain even after you remove your thumb.
Now that we have a dent to look at let's do just that. It may come as a surprise to see that there are two high spots on either side of the depression. This looks pretty typical but it's important because those little bumps are where the metal you moved with your thumb went to get out of the way. In other words, what goes down must go up. The overall perimeter of the can can't change. It's defined by the linear measure of the metal it's made of. That means when you push on one part, another part has to move out of the way. Check out the crude drawing below:
No matter what kind of a hit a sheet metal part takes, it's going to still have the same surface area. So what, one might ask? The riser, next to a dent is the real key point to returning the panel to it's original shape. Now for our last exercise. Carefully and simultaneously press on the two high spots that flank your new dent. Pop! Out comes the dent and you never even touched the depressed part. Hmmmm this technique could come in handy.
