Sitrep: the car’s still not running yet, but at least it’s heavier now! Adding weight to a race car feels fundamentally wrong, but what can you do. As a famous philosopher once said: “Rules is rules and if we didn’t have rules then where would we be?”

As discussed in the previous post, the Miata was too light for the class given the bigger motor. After a cup of tea, and coopting a few ideas from a Spec Miata ballast system, I came up with a way to add 100 lbs to the car in a place that would improve the balance of the car (offsetting the driver’s weight), and keeping it low down and close to the center of the car.

The idea was simple: procure a stock passenger side seat base, then fab up some steel and lead to fit between those rails. Use a 1/4″ pieces of steel on top of the rails, hang ~1 1/4″ of lead under that, and sandwich all of that together with another 1/4″ piece of steel from underneath.

Here are the stock rails in place, sans seat. Click here for my highly detailed technical drawing showing dimensions and clearances.


The previous picture shows the rails completely intact (minus the cable between the adjusting lever and its partner on the opposite side. The problem is, the ballast wouldn’t fit with the adjusting lever and locking mechanisms in place, so all of that had to go. Since the mechanism that holds the rails steady was lopped off, I got the rails lined up where I thought they’d work for me, and tacked them in place.


After mocking things up, a buddy with, let’s say, access, to some serious machine shop equipment helped me source and cut out the steel and lead (band saws bigger than my dining room table are cool). A few holes (that I even drilled almost straight! #miatathings) later, and some stainless hardware, and we can get an idea of exactly what we’re working with.


I needed 94 lbs, I wanted at least 100 lbs… came out damn near perfect. Online material density calculators are your friend here. I mean, you COULD do some old fashioned math, but where’s the fun in that? Essentially: 12″ x 12″ x 1 1/4″ of lead add up to around 75 lbs, 12 x 12 x 1/4″ (bottom plate) and 15 x 10 x 1/4″ (base plate to go on the rails), come out to be about 20 lbs. Cut a few holes, then add the rails and hardware, and we get 102 lbs and some change. After tacking everything up, doing an initial mock up, fully welding the base plate to the rails, it was time for a final mock-up before completing the welds on the rail sliders, just in case I found myself needing to adjust things.


You can see the bottom sandwich plate hung underneath the base by a pair of bolts to make sure the clearance is as expected. And sure enough, there’s enough. Not exactly a ton, but enough. The downside for “not a ton” is that the whole 100 lb lump needed to be assembled out of the car, and then muscled into place.

After finishing up the welding on the sliders (to make sure they don’t slide no more), it was time to slap a couple coats of paint on everything, because lead poisoning is no fun for anyone. Eventually you’ll end up walking into rooms and then forgetting….what were we talking about again?

My welds look great….after a coat of paint 😉


The whole thing assembled on the bench, It’s up on the wood so that I’d be able to get a wrench underneath to snug up the nuts.


And finally, resting in its home.


What a ridiculous amount of time and effort put in to making a race car slower. Dumb.
Aww who am I kidding… the only way my car could be any slower right now is if it were sitting on cinder blocks.


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