Acid Dipping to Shave Weight

The stock car science blog has been a little quiet lately, mostly because I’ve been working on a really exciting project I hope to be able to tell you all about in the very near future, but also because I’ve been traveling all over the country giving talks and because there hasn’t been a whole lot of science-related news in NASCAR lately. I was just joking in my talk yesterrday that I was sort of hoping someone would try something clever just so that I’d have something to write about. And voila…

Fox Sports reporter Lee Spencer is reporting that the No. 83 Red Bull Toyota, which was selected for random testing after Martinsville, was found to have not met the minimum thickness requirements for the body panels. Spencer anticipates that NASCAR will be levying “record-breaking” fines.

mum thickness of the body panels is 24 gauge, which translates to 0.025 inches or 1/40th of an inch thick. Let’s model the side of a car as shown below, as three rectangles with dimensions as shown. Yes, I’m using rectangles to make my calculations easier. I’m considering only one side of the car and I’m ignoring windows.

The area of the sheet metal on the side of my model car is 4771 square inches. Multiply that by the thickness of the metal and you get a volume of about 119 cubic inches of sheet metal.

The density of 1018 steel is 0.283 lbs/in³, so the weight of this much metal is roughly 33.75 lbs.

If you want to thin a material, you have options. You can mechanically polish the metal, for example, grinding away a thin layer. This tends to be difficult to do with any uniformity unless you’re really set up for it.

In the lab, I often need really clean surfaces, so instead of rubbing and sanding them, I etch them. Etching is dipping a material in something that eats away at the material. In the case of steel and other metals, the etchant is usually an acid and this is the “acid dipping or chemical milling” to which Spencer refers. You put the metal piece into a bath of acid and how much material is removed depends on how long you let the metal sit in the acid bath. The metal comes out looking like new. In fact, if you take a wedding ring to a jeweler to be cleaned, what they usually do is to dip it in a mild etchant. You lose a miniscule amount of metal from the ring, but it comes out looking shiny and new.

It would be hard to tell whether a piece of metal had been etched by looking at it by eye; however, a metallurgist can examine the metal (with the paint stripped away, of course) and can analyze the etch patterns. Certain directions in a crystal etch faster than others (see Figure 3 in this paper for example), so determining whether a piece of metal has been etched isn’t too difficult if you have the right tools.

How much weight could you actually save by etching away some of the sheet metal? Let’s say that the side of the car we calculated above was etched from 24 ga to 26 ga (which takes it from 0.025 to 0.01875 inches). The weight just on that side would be reduced by about 8.4 lbs and, making a lot of approximations, maybe by 20 lbs across the car. If the thickness were reduced to 28 ga (0.015625″), you’d save 12.6 lbs. But even someone casually familiar with sheet goods would likely notice that much of a decrease in thickness.

Why would you reduce the weight of the body? The answer is back to our old friend The Center of Gravity or CG. The new car has a higher center of gravity, and the higher center of gravity means more load transfer when the car turns. One reason the new car has a higher CG is because it is taller. More weight up higher in the car increases the CG. More load transfer makes the car harder to turn.

You can lower the center of gravity by adding weight (ballast) in the framerails of the car; however, you don’t want to make the car any heavier than the minimum 3450 lbs. Spencer suggests that the RBR team made the panels thinner so that they could save weight (she claims up to 75 pounds) and then use ballast to make up for the decreased body weight. That would lower the CG. The NASCAR R and D Center ought to be able to determine intent because they no doubt know how much ballast would be appropriate for a regulation car. If there is another 20 lbs or so of ballast in the RBR car, that would suggest that someone knew that there was a significant weight savings somewhere else.

Is this a safety issue? Probably not so much. The strength of the car is in the tube chassis, not the body. You can dent the sheet metal in the body pretty easily. Ask Carl Edwards and Kevin Harvick.

There is a potential complication in what I’ve presented you with here, which is that stock cars have curves. I don’t have an accurate estimate of the surface area of a stock car, so it’s difficult for me to calculate exactly how much weight could be saved in this manner mand whether the 75 lbs Spencer suggests is realistic. My intuition is that 75 lbs would be really difficult to shave off the car without it being somewhat obvious to knowing eyes. I am in Columbus Ohio today at Ohio State University, but I will update this post when I get back to Dallas Thursday and hopefully wheedle some more accurate numbers about the surface area.

UPDATE: Well, that didn’t take long. From what I’ve been told if you figure losing about 5 mil from the thickness, that leaves you with about 6.75 lbs for the side I showed above, so maybe 13 lbs or so is about the weight saving you might expect to see. I can’t see any way you could get 75 lbs.