In my last article I talked a lot about the left rear axel weight  which is the hot thing now in dirt racing.

I had a bunch of questions about the difference between that and just bolting lead on the chassis. So, I think I should talk a little bit more about that and try to clear up some of the misconceptions.

When I first became aware of it around the middle to the end of last year, my first reaction was ‘increase unsprung weight? You’ve got to be kidding me!”

Then I sat back and started to break things down and understand what was actually going on here.

Actually, it’s a genius simple solution to the bone head suspension rule changes that plagued the sport last year.

Like I said in my previous article, some really smart guy came up with a better way and the big money spenders came down hard on him.

After refinement of his original idea, it would have come down to a different birdcage and an additional shock on the left rear. Now, we are really escalating costs by stepping into the world of tungsten weight.

Let’s forget about the cost for a minute and dig into the tech behind it.

Axle weight acts different than left rear chassis weight. Since the chassis weight is sprung weight it is free to transfer throughout all four corners of the car.

But, a very heavy concentration on the left side rear of the chassis won’t transfer as much to the right rear to give side bite on the way into the corner. Forward traction is also best when both rear tires are biting at their highest potential.

So, by increasing left side weight, your car will loose traction as well as side bite.

Now, think about adding lead to only the left rear unsprung section. This will still allow the sprung weight mass to move around the same. The same amount of weight (roughly) will move on to the right rear through the spring.

But, will help hold traction in the left rear when the limiting chain snaps tight.

This is the difference between increasing left side weight and only increasing the left rear unsprung weight.

Now, let’s think about the fact that some of the weight will transfer across the rear axle as well.

Since we are running a beam axle in the rear, weight is free to move across the rear axle. Some of the weight increase on the left rear tube will also transfer to the right rear. This will help increase traction on the right rear as well.

I know some of the more experienced racers are probably saying this is crazy because for years we have been concentrating on limiting the unsprung weight. We have been spending a ton of money on lighter components just to get the unsprung weight to a minimum.

In the case of a four link car, where we need to run a limiter just to keep the left rear tire from camming over center, this seems to work to hold traction when running topped out on the limiting chain.

I still don’t recommend this for other racing classes where you don’t run a chain.

I had some questions after my last article asking how the tire looses traction when it hits the chain.

Think about a tire being flexible when it has air in it. When the suspension hit it’s limit, the centripetal force of the spin of the tire will start to expand and the stretch of the tire will be released and begin to loose traction.

I go over all of this in my racing secrets book where I explain slip ratio. The added axle weight will slow the release of the stretch in the tire and help hold traction longer.

Go to You Tube and look up a slow motion video of a drag race car leaving the line and watch the left rear tire. The initial lurch will sink the car. Then the expansion of the tire will begin to lift the car. The same ting happns on the left rear of our car.

It looks now like they will be outlawing tungsten to help keep costs in check. I think this will be a good move. We don’t need to step into the world of tungsten.

Now let’s think about rotating weight. I heard people are also putting on hubs filled with lead or solid brake rotors. This will also increase rotating weight. This also opens up an entire other facet of slowing acceleration and slowing the effectiveness of the braking system.

This is also something I go into more detail in my book when talking about weight transfer. Then same thing is happening when the axle is spinning as what happens when when weight transfers around an axis.

It will take longer for your motor to accelerate your car and it will be more difficult for your brake system to stop the car.

I’ll let you know right now, I’m not a fan of increasing the rotating mass of the left rear just to increase left rear axle weight.

Leave the heavy hubs and rotors on the shelf. Solid axles are probably the exception because they are so small in diameter, the inertia will not increase a hole lot.

So, I’ll give you a hint where I’m kind of leaning on this. Could we fill the left rear axle tube with lead while first inserting a steel liner just big enough to give clearance for the axle.

Oh well, just thinking out loud.

We’ll talk later.

Be safe,

Kevin