Ballast: Tungsten vs. Lead

NASCAR penalized a couple of teams recently for dropping tungsten ballast on the racetrack. Let’s revisit why those chunks of tungsten are there in the first place.

Ballast is weight added to a racecar to meet overall weight requirements as specified in the rules. And while you could just put the ballast anywhere convenient, why not put it where it will help the car handle better?

As of 2020, NASCAR requires a racecar to weigh at least:

3,200 lb (1,451 kg ) minimum without driver and fuel
3,400 lb (1,542 kg)  minimum with driver and fuel

The fuel cell holds about 17.75 gallons and the E15 fuel weighs about 6.2 lbs per gallon, which means a full fuel tank weighs about 110 lbs.

But even with a relatively hefty driver, the use of materials like carbon fiber and aluminum means that most cars don’t make minimum weight.

Why Build Cars Lighter that Required?

Because that allows you to put the weight where you want it. Thanks to load transfer, a car turning left shifts its weight to the right. An accelerating car shifts weight from front wheels to back wheels and vice-versa for a braking car.

Grip is proportional to how hard a wheel is being pushed into the track. Left-turning cars have a challenge keeping weight on those inner wheels, so putting ballast on the left side of the car helps with left-side grip.

Lead Ballast vs. Tungsten Ballast

People like to say that tungsten is heavier than lead, but that’s only true when you’re comparing two pieces of metal the exact same volume.  I learned this lesson when I was working on my book, The Physics of NASCAR.

Kirk Almquist, then Elliott Sadler’s car chief, brought me two blocks of metal the same size.  He held them out for me to see, but didn’t let me hold them.  They were both a rather unimpressive dull grey.

“One of these,” he said, “is tungsten.  The other is lead.   Which is heavier?”

I pulled up my mental periodic table, which is nowhere near as neat as the one I’m showing below.

Lead and Tungsten are two of the annoying elements whose symbols have nothing to do with their names because Latin.

• Tungsten has the atomic symbol is W because the element was discovered as part of a mineral called Wolframite , which has a lot of tungsten in it.
• Lead is Pb, which comes from the Latin plumbum, meaning “liquid silver”. Actually, plumbum means ‘lead’, as pointed out by a commenter. I knew I should’ve taken Latin in high school.

Tungsten is atomic number 74 and lead is atomic number 82.  With a few exceptions, the larger the atomic number, the heavier the atom. This isn’t one of those exceptions. The atomic table tells us that if we have one mole of tungsten atoms, it would weigh 183.85 grams, or about 0.4 lb. One mole of lead atoms would weigh 207.2 grams (0.45lb). A lead atom is about 1-1/8 times heavier than a tungsten atom.  So I guessed lead.

Kirk got a big smile on his face and handed me the blocks.   The tungsten block was a lot heavier than the lead block.

Why Was I Wrong?

This is extra embarrassing, because I trained as a solid-state physicist, which means I studied how atoms form solids. A tungsten block is heavier than the same sized lead block because you can fit more tungsten atoms into the same volume. Tungsten atoms are just a little more social that lead atoms and they don’t mind being squished in.

These metals form regularly ordered patterns when they form solids.  The more closely the atoms pack in, the higher the density of the solid. Tungsten is 1.7 times denser than lead and about 2.5 times denser than a typical steel.

• The density of lead is 0.410 lb/in³, which means a cube of lead one inch on all sides weighs 0.41 pounds.  
• Tungsten has a density of 0.70 lbs/in³. A cube of tungsten one inch on all sides would weigh 0.70 lbs – 1.74 times more than the same sized cube of lead.

Ballast is installed inside a car’s frame rails, so a typical cross-section for ballast is 2-5/8″ x 3-5/8″.  

• Twenty-five pounds of tungsten would be 3.75″ long
• Twenty-five pounds of lead would be 6.40″ long.

Using tungsten allows you to more precisely place the weight and thus more precisely impact your car’s handling.  

No Tungsten Ballast Allowed?

Tungsten is outlawed in many lower-level racing series because it’s expensive.

Stock Car Steel and Aluminum Company will sell you a 35-lb piece of tungsten that is 2-5/8″ x 3-5/8″ x 6″ long for $1876.88.  A comparable piece of lead would cost you somewhere around $100.  Lower-level series don’t want to price the teams out of competition.  When one team gets an advantage, all the others are going to have to do the same thing to keep up.  It becomes a cost issue.

Why is Tungsten Expensive?

Tungsten is rarer than lead and it is just plain harder to work with. A melting point of 6192 °F. Compare that with lead’s melting point of 621°F. Shaping tungsten into bars requires a heavy duty furnace and a lot of energy.

Tungsten is difficult to machine because it is hard and brittle, unless it is very very pure. Making it pure makes it more expensive.  Even without being really pure, tungsten ballast is still about a hundred times more expensive than lead.

You might remember Chase Elliott being disqualified after winning the 2013 Snowball Derby because his car was found to have tungsten ballast instead of lead. That disqualification gave the win to Erik Jones

Another Problem with Tungsten

There is a trend now of making jewelry from tungsten and tungsten alloys, which raises a big problem. If your gold ring gets stuck on your finger, you can cut it off.  You can’t do that with tungsten.  (The solution is that you have to take advantage of the brittle nature of the material and crush it without crushing the fingers.)

Note: This post was revised and expanded from an earlier post original posted on 12/9/2013