Why Dirt Racers are Stacking Springs

The use of stacked springs has become commonplace among many top Dirt Late Model teams. It wasn’t always that way. As far as I know, the first test using stacked springs in a dirt late model occurred in March of 1998 at Eldora Speedway. Coincidentally, I was there, and I was surprised.

We were there, I assumed, to test a new setup arrangement I had developed for the upcoming Dream race that seemed to work out very well. My friends at the test wanted to see how stacked springs would work, so we tried them too. Although the stacked spring concept as it was arranged then did not work well at the then high banked Eldora track, it would eventually gain traction for use at lower banked tracks.

The stacked spring concept began with desert racers in the Baja Pro Trucks as far as I can ascertain. I’m sure someone will correct me if I am wrong on that. Anyway, those trucks needed a compliant spring that gained rate, but it needed to be very long due to the high amounts of suspension travel. The same is true for the right front and left rear corners on many Dirt Late Models in today’s racing.

The use of stacked springs, first in the right front of the Dirt Late Models, allowed the car to run on a very soft spring rate on entry to the corners until a heavier rate was needed through mid-turn and then off the corners. Now, in today’s racing, the stacked spring concept is being used at the left rear also and sometimes in the right rear of those cars.

If you use stacked springs in your car, or you are thinking about switching to stacked springs, here are some important tips and technical information you will need to know. This mostly comes from professionals in the racing spring business I have talked to and who work directly with race teams. I admit that I knew very about the use of stacked springs before I researched and sought out the help of these individuals.

Definitions Of Stacked Springs – Basically there are two ways to run stacked springs. One is called Stacked springs because all you are doing is stacking two springs in series, with the same or different rates, to achieve a longer spring that is a softer rate than either of the springs used.

If you stack a 10 inch 400 ppi and a 6 inch 400 ppi spring, you get a combined rate of 200 ppi. You may not be able to find a 16 inch 200 ppi spring, so this provides more spring length to use. We’ll go over how to find that rate later on.

The other way to use stacked springs is to use the Dual Stage system. This system is also a stacked spring in that you use two springs on top of each other in series. The difference is that you also use a stop mechanism that eliminates the travel of one of the springs at some point of shock travel to where the car is then riding on only the one spring.

With the Dual system, you can run a softer spring on the top and a stiffer spring on the bottom. As the shock travels, the divider between the shocks moves up the shock body. At a predetermined (by you) point, the divider hits the stop and the top spring is no longer able to compress.

Since only the bottom spring is now the only one working, its rate is what the car runs on as long as the shock is compressed at least that far. So, for example, if we want to run a 450 ppi (pounds per inch) spring through the middle and off the turns, but require a much softer spring rate to enter the corner with, we can stack a 300 ppi spring on top of a 450 ppi spring.

Using the Stacked formula to find the combined rate, we use the Top Spring rate times the Bottom Spring rate divided by the Top Spring rate plus the Bottom Spring rate. To find the example rate, we multiply 450 times 300 = 135,000. Then we add the two spring rates to get 750. Dividing the two, 135,000 divided by 750, we get a 180 ppi combined stacked spring rate.

So, on entry, our car rolls in on a spring rate at the right front of 180 ppi. Once the shock travels to a predetermined compression, the top 300 ppi spring is eliminated and the car now runs on the bottom 450 ppi spring at the right front until the shock de-compresses and the spring divider is no longer contacting the stop.

Another Dual Rate System – I will throw this in at this point. There is another Dual Stage system being used by some teams. It involves using a bump spring, or bump stop, on the RF corner. Just like the Dual system I described above, the rate changes as the shock travels.

With the bump spring, or for that matter bump stop, the car is suspended by the normal ride spring until it travels a predetermined amount. Then the bottom of the shock body, plus any stackers, contacts the bump and the ride rate is now the combined rate of the ride spring plus the bump rate.

The drawback to trying to go this route on dirt is that most bump springs, and especially the bump stops, do not have much travel before they coil bind or compress to solid. Some teams will stack two bump springs to get more travel, but now that stack equals a much softer bump spring rate.

Two 500 ppi bump springs that are stacked will equal a spring rate of only 250 ppi. To get back to the 500 ppi rate of just one spring, you would need to stack two 1,000 ppi bump springs.

Which Do I Run? – The choice of which to run, Stacked or Dual, depends on which corner you will be working with. Generally, the Stacked system works best on the LR corner to provide a longer spring to help it stay loaded when the LR corner hikes coming off the corners.

The same Stacked system will not work very well on the RF because the soft rate would cause the RF corner to travel too far. And, it would not produce the higher rate needed to gain loading for the RF and LR corners coming off the corners. So, we use the Dual system on the RF.

The combinations for each, the Stacked and Dual systems, are many. You will need to determine what you want to accomplish and then decide on the spring rates that will get that done for you. For the LR corner, make sure you end up with a combined rate that will work with the RR spring rate to balance the car. Too stiff and the car will be too tight in and through the middle. Too soft and the car will be too loose in and through the middle.

Important Considerations – Here are a few important considerations and tips on how to put together your stacked systems. You cannot just install two stacked springs in your car on any corner and just go racing. There is some shop preparation that is required.

First off, a longer spring will tend to bow more than a shorter one, no matter how good your spring is. This may be a problem that can be solved. On most quality springs, the ends are 180 degrees opposite where the tips on each end are pointed. This tends to offset the bowing tendency.

If you stack springs, you will need to arrange the springs so that the bowing tendencies of the two springs offset one another, just like the design in individual springs. This takes time and experimentation. How do we do that? We eliminate bowing by rearranging the location of the two springs after compressing the system on a commercial coil-over spring compressor to find a position for the springs that has less bowing.

Adding bearings to the top and bottom of the assembly will also help prevent bowing of the springs. The two springs must be allowed to twist, or wind and unwind, as the shock travels. That is the way springs work all of the time.

What Else You Need To Make This Work – Most teams, if they are going to get the most out of the stacked spring systems, will hire a shop that has a coil spring force measuring rig, or they will buy their own rig, to help in the setup of the stacked spring system.

These rigs measure force at predetermined compression amounts of the shock/spring combination. This is not a hard concept to learn and understand if you follow along with the overall picture. To do that, we need to mentally separate force from spring rate, they are two entirely different things.

Force is the overall work that the spring is doing based on how far it is compressed. A 500 ppi spring that is compressed two inches has 1,000 pounds of force. It gains 500 pounds of force for every inch it is compressed. The designation, 500 ppi, means that for every inch of compression, the spring gains 500 pounds of force. That is why it is labeled 500 pounds per inch.

In order for us to maintain our ride heights we had using our old one spring system, we need to measure the shock length at ride height and record that number. This is true no matter which corner of the car we are working with. With the LR using the Stacked system we also need to know how far the shock will extended when the rear is hiked up so that we install a Stacked spring system that is long enough.

At the LR, we compress the Stacked spring and shock in the Force rig and adjust the spring height so that the force at compressed ride height is the same. If we compress the shock to the ride height amount and we end up with too much force, we back off the adjuster until the force is the same. Then our ride height will not change when we install this Stacked system.

For the RF corner, we will be running a Dual system. For this, in addition to finding ride height force, we also need to know how far the shock compresses at mid-turn when running a single spring. This motion produces its own force that we need to duplicate in the Dual system.

At the RF, we do the same as we just did with the LR for the overall Dual system ride height. For the transition from the stacked rate to the single spring ride rate, we need to know the average shock travel at mid-turn. Then we use our Force rig to find that force number in pounds.

We usually want to hit that mid-turn force number at some point after the spring separator has contacted the stop that eliminates the softer spring and when the RF corner is riding on only the higher single spring rate. The total amount you end up compressing only the single spring is something you will have to experiment with.

This timing has a lot to do with how long into the corner you want to be on the softer, Dual spring rate, before transitioning to the single higher rate. That is why most consultants suggest on-track testing to tune the transition point.

Shocks To Use With Stacked Systems – The shock package you will need for your stacked system is a little different than what you might be use to for single spring applications. We need to understand what we are trying to accomplish and then think out our shock rates.

For the RF, we need the shock to control the force levels we are working with at mid-turn and off the corners. By control I mean rebound settings in the shock. Those forces will be already known if you run through what we discussed above. They will be higher than standard because one of the benefits of the stacked Dual system is that it travels farther and generates more force.

For the LR corner, we want the shock to extend more easily and therefore the rebound rates for that shock would be lower than normal. Also, when we exit the corners, there will be some extension initially, then some compression of the shock and spring, so we might consider increasing the compression setting to take advantage of that to create more loading of the LR tire for late exit traction.

Outlawing Stacked Springs – It has been discussed within come sanctions, and enforced in others, that stacked spring setups be disallowed. The reason stated is to save the racer money. Many of these very same sanctions allow bumps stops and bump springs. Let’s examine the argument.

Cost is relative to many things. We know springs last a very long time. So, any investment in springs for setting up stacked spring systems will be a one time, or long term investment. And it can be argued that the teams may have the springs needed for the stacked spring systems already lying around the shop, unused. Now the cost is only the hardware needed to go on the shock and those parts are relatively cheap.

If bump stops are allowed, we know they don’t hold up to the rigors of dirt racing and need to be replaced on a regular basis, so the cost of those systems, while relatively cheap in the beginning, becomes more expensive in the long run.

The force rig that should be used by the teams to setup the stacked spring systems can be rented, or shopped out to a consultant, much like we do with our shocks. If the team does invest in the force rig, it is basically a spring tester designed to rate the spring/shock combination. Many teams already have those, and if not, they need them.

Getting a car to handle better makes for better racing and it is safer. The reason it is safer is because the cars no longer need to force the front end to turn like before these systems came out. With less sideways action, there is less contact between the cars and less overall damage from contact.

I really don’t see what the big deal is. Modern super late models on dirt run custom built engines that cost upwards of $40,000 in some cases and a stacked spring system cost pales in comparison. The difference between the stacked system and a single system is around $300-400 for parts. One crash will eat up more parts that that. So, I’m not convinced there is any realized savings in outlawing stacked spring systems for dirt racing.

Conclusion – The use of stacked springs in either configuration can make your car faster and easier to drive. If you run stacked spring systems, or intend to convert to stacked systems, be sure to follow the simple rules we have presented. Also, consult with your spring supplier, shock supplier and those who manufacture the force rigs so that you can get the most out of your application.