Thoughts on Suspension Techniques for Drag Racing

If you’re going to drag race and win, you’ll need lots of horsepower. Well, that’s pretty obvious. Equally obvious to most of us is that in order to win you will also have to put all that power down to ground and make it stick. But how easy is it to do that? Especially on a front wheel drive car?

At INTENSE Racing we’ve made our reputation making horsepower. We like to take mild-mannered family size cars and turn them into predators capable of running down just about anything we’ll come up against. Over the last year, we’ve expanded our view and have begun exploring other aspects of good strip performance. We’ve turned some of our attention to suspension and launching techniques. We present these ideas here.

Most will agree that, the most important part of the ¼ mile run is the initial launch. We measure the quality of the launch by the 60’ time printed on the time slip. A great launch depends on good off-the-line torque, but all that is useless without a good suspension that can put and hold that power down for maximum traction. Before we can start talking about what works and what doesn’t, it is necessary that we all understand the dynamics of weight transfer and how that can affect the launch. It’s no secret that as a car accelerates forward, part of the car’s weight transfers rearward to the rear wheels. On a rear wheel drive this is a big bonus, and one that racers have been taking advantage of for years. On a front wheel drive car however, this weight transfer is a detriment to traction as it unloads the front wheels just when we need traction the most.

In order to understand not only why, but also the magnitude of weight transfer we will analyze the car in Figure 1. Figure 1 illustrates the forces acting on a vehicle. For simplicity, we will first ignore the suspension. The suspension (coil springs) also plays a role in weight transfer, but it turns out to be of a secondary nature. We’ll talk a little bit about that too.

Center Of Gravity

HCG DF DR

FA DT WF WT WR

Figure 1 - A car accelerating

Before we jump into the math, a few definitions: WT – The total downward force exerted by the weight of the car WF – The portion of the car’s weight that is carried by the front wheels WR – The portion of the car’s weight that is carried by the rear wheels HCG – The distance of the center of gravity from the ground DT – The wheelbase of the car DF – The distance of the center of gravity from the front wheel DR – The distance of the center of gravity from the rear wheel FA – The accelerating force

Now we’re almost ready to begin. Before we do anything else we’ll need to calculate WF and WR at rest, or in other words, what portion of the car’s weight is supported by the front wheels and what portion is supported by the rear wheels. In order to do that we need to assign a few values, and to keep the calculations simple we’ll use nice round numbers. For this exercise, we’ll assume that the car’s total weight is 3500 lbs. We will also assume that the weight is biased 60/40 towards the front (actually one of the cars we weighed was 61/39 so this is very close). In other words, if we assume that the wheelbase DT is 100”, then DF = 40” and DR = 60”. So at rest we have:

WF = WT X (DR / DT) = 3500 X 0.6 = 2100 lbs. (Equation 1)

WR = WT X (DF / DT) = 3500 X 0.4 = 1400 lbs.

We’ll call these two values, static loads. So in our example of a slightly front weight-biased car at rest, the front wheels will carry a 2100 lb load, and the rear wheels will carry a 1400 lb load. Now if we could keep 2100 lbs of force on the front wheels while we accelerate, we’d probably be able to launch our cars like jets off the deck of an aircraft carrier. Unfortunately, upon acceleration, weigh transfer will diminish that downward force on the driving wheels. Besides the variables that we’ve already discussed in the static load calculations, the amount of weight that will be transferred towards the rear of the vehicle also depends on the accelerating force FA that acts on the car’s center of gravity, and the distance of that center of gravity from the ground, HCG. For the sake of this next example, let’s say we want to determine the amount of weight that will be transferred on a moderately accelerating car. Let’s pick one that accelerates at a rate of 1 G. We can calculate that

FA = WT X 1 G

We’ll also assume that the car’s center of gravity is 20” from the ground. During acceleration, the downward force on the wheels can be calculated as

WF = Front static load – Weight unloaded from the front wheels (Equation 2) WF = Front static load - FA X (HCG / DT) WF = 2100 – 3500 X 1 X (HCG / DT) = 2100 – 3500 X .2 = 2100 – 700 = 1400 lbs.

WR = Rear static load + Additional Weight Added to the rear wheels WR = Rear static load + FA X (HCG / DT) WR = 1400 + 3500 X 1 X (HCG / DT) = 1400 + 3500 X .2 = 1400 + 700 = 2100 lbs.

It may seem surprising, but in this example our driving wheels lost 700 lbs of downforce! Now lets see what happens if we throw a bunch of horsepower on the car and try and launch a lot harder. Let’s see what happens if we try and accelerate at a rate of 1.5G. Incidentally, an average of 1.5 G of acceleration corresponds to a 60’ time of roughly 1.58 seconds.

WF = Front static load - FA X (HCG / DT) WF = 2100 – 3500 X 1.5 X (HCG / DT) = 2100 – 5250 X .2 = 2100 – 1050 = 1050 lbs.

WR = Rear static load + FA X (HCG / DT) WR = 1400 + 3500 X 1.5 X (HCG / DT) = 1400 + 5250 X .2 = 1400 + 1050 = 2450 lbs.

We added more power so that we could try and launch harder, but the weight transfer increased at the same time further hurting our traction. This illustrates the fact that as we add more and more power to our cars and then try and launch harder and harder, we are increasingly trying to walk an ever-narrowing line. In this second example the load on the front wheels went from 2100 lbs down to 1050 lbs. This is pretty bad as we lost half of the available downforce! When we want this car to accelerate, we lose a huge amount of the downforce necessary to give us the traction that we need. That would be bad enough, but unfortunately the story gets a little worse. In the examples above, we completely ignored the car’s suspension. When you add a suspension to our test vehicle, the forces already explained will continue to act on the car. However in addition, the acceleration acting on the car’s center of gravity will tend to torque or rotate the car about the rear wheels. As this occurs, the car’s center of gravity moves rearward, biasing the static loads more towards the rear and further increasing the weight transfer towards the back. We recognize this at the track when we see a car’s rear end sag and the front end lift a little as the driver launches the car. We will talk about what you can do to your suspension to minimize weight transfer a little later.

Intuitively we can see that as a car’s suspension is stiffened this secondary effect can by minimized, and in so doing, the car’s actual weight transfer can be closer to that predicted by the above calculations. Some people have even considered locking their suspension in order to minimize this effect. While the math supports this in straight-line acceleration, we at INTENSE think this is not a good idea. A very stiff (or worse yet, a locked) rear suspension can induce oversteer in a panic maneuver. Imagine for a moment that you’re in the back half of the track running in excess of 100 MPH and something happens that causes you to have to swerve. A locked rear suspension can cause the car’s rear end to lose traction and come around putting the car in a spin. For this reason alone, we recommend that people not do this. We think that stiffening the rear end with things like spring blockers or similar devices is fine, as long as the rear suspension still has enough travel to absorb bumps or control the car’s shifting weight during maneuvers. A locked rear suspension simply cannot.

INTENSE recently began experimenting with wheelie bars, and so far the results have exceeded our expectations. With wheelie bars, one of our cars has been able to obtain back-to-back 1.58x 60’ launches. What has impressed us even more is that this was done with a heavy car, relatively speaking. Intuitively one can see that it requires more force and traction to accelerate a heavy car, and we will present some information to support that a little later.

Wheelie bars can allow considerably more weight on the front driving wheels than our weight transfer formulas above indicate. They do this by one simple phenomenon; they extend the car’s effective wheelbase. If the bars are stiff as ours are, and also adjusted down on the ground, again as ours are, the new effective wheelbase will be the old wheelbase plus the length of the bars. This can be seen in Figure 2.

Center Of Gravity

HCG DF DR

FA DT WF WT WR

Figure 2 - A car accelerating with wheelie bars

The bars that we’ve been using on our cars are roughly 59” in length. They attach to the rear sway bar mid- bracket hardware, located a few inches behind the center of the rear wheels, and under the rear bumper support. Again, for simplicity, we will say that the car’s new wheelbase DT has increased from 100 inches to 160 inches. When we now calculate the weight on the front wheels of our 1.5 G launching car we have

WF = Front static load - FA X (HCG / DT) WF = 2100 – 1.5 X 3500 X (HCG / DT) = 2100 – 5250 X .125 = 2100 – 656.25 ≈ 1444 lbs.

From a starting weight of 2100 lbs we’ve only lost 656 lbs. of downforce from the driving wheels. This is a big improvement over the previous loss of 1050 lbs.

This can be improved upon even further. In this last example, the bars were set low to the ground, but they were also set to zero pre-load. That means that with the car at rest, the bars did not support any of the weight of the car. At the time of this writing, this is as far as we’ve experimented with the wheelie bars. With a zero preset, we’re able to hold the full torque of the Turbo GP and the MP112 cars without spinning at the starting line. As we add power to all of our cars however, we are certain that we will start spinning our tires again and will be looking for a way to increase the downforce on the drive wheels, therefore traction, of our cars. One potential way to achieve this will be to increase the preload of the bars. Here’s why:

From Equation 2 above, the total weight WF on the driving wheels is equal to the static load minus the weight that is unloaded from the wheels during acceleration. Increasing the static load and simultaneously minimizing the weight that is unloaded can maximize WF. We’ve seen from the last example that the presence of the bars decreases the amount of weight transferred away from the driving wheels by virtue of the fact that the wheelbase is increased. Now all we need to do is to increase the static load on the driving wheels (while the car is at rest). We see from Equation 1 that the weight on the drive wheels is proportional to the distance that the center of gravity is from “the rear support point”. So as we start to support more and more of the car’s weight by the wheelie bars, we actually move the car’s center of gravity further forward, relatively speaking. We’ll illustrate this with an extreme example. For the sake of argument, let’s say that we’ll support the rear of the car’s weight entirely on the wheelie bars.

The static weight on the front wheels then becomes

WF = WT X (DR / DT) = 3500 X (120 / 160) = 2625 lbs.

The static weight on the wheelie bars will be

WR = WT X (DF / DT) = 3500 X (40 / 160) = 875 lbs.

Now if we launch this car at the same 1.5 G:

WF = Front static load - FA X (HCG / DT) WF = 2625 – 1.5 X 3500 X (HCG / DT) = 2625 – 5250 X .125 = 2625 – 656.25 ≈ 1969 lbs.

This illustrates that a technique such as this could allow a FWD car with an obscene amount of torque to hook at the line and achieve dizzying acceleration. In all seriousness though, this was an extreme case for illustration purposes only and we’re not ready to do this just yet, at least not for the entire length of the track. But it does illustrate that some pre-loading of the bars will result in an increased downward force on the drive wheels.

If you managed to stay awake while we talked about the relative evils of a non-functioning rear suspension while drag racing a FWD car, you should be starting to wonder what makes running wheelie bars (even with zero preset) any better. The wheelie bars that we’ve designed for our cars are extremely rigid in the vertical direction, so this certainly does beg the question. At INTENSE we take racing seriously, but we’re even more serious about safety.

We spent a lot of time thinking about just how to fabricate wheelie bars that are very stiff on the one hand, but will also get out of the way if the car has to undergo panic maneuvers, on the other hand. Looking at the picture of the wheelie bars on the turbo car, it can be seen that they’re really made up of three parts. The first and largest part is the lower part that attaches to the rear sway bar hardware. This part attaches to the frame of the car on one end and holds the 4” wheels in a knuckle-like bracket at the rear end. It also contains some cross bracing for rigidity, but it’s important to note that the bracing ends a few inches before the knuckles. The other two parts are the diagonal tubes that attach to the bottom of the bumper support on one end and to the knuckle-like bracket at the other end. These tubes provide up-down adjustability of the bars, and prevent the wheelie bars from deflecting up during hard acceleration. If the car has to swerve, the bars were designed so that the short section of unsupported thin-walled chrome-moly tubing just ahead of the knuckles will bend out of the way. The diagonal tubing will not provide any side-to-side support as they are pivoted at both ends. We’d rather trash the bars and go home early than take a chance on wrecking.

A little earlier we mentioned that if running wheelie bars is a little too extreme for you, there are things that can be done to your suspension to help your existing setup. We list them here:

• Stiffer springs – Stiffer springs will allow less body movement thus resisting the center of gravity from moving rearwards. • Stiffer struts – Aftermarket struts with a higher damping rate will also control and reduce body movement during a launch • Lower the front only – Some of the INTENSE cars run Eibach lowering springs in the front and stock springs in the back. This biases more weight to the front wheels effectively moving the center of gravity forward. • Taller rear tires – Some people run 28” skinnies in the back to bias more weight forward. • Run high air pressure (within the tire manufacturer’s specs) in the back tires. Besides giving you a lower rolling resistance, it will also raise the rear slightly more. • Spring blockers in the rear.

These tips will all help control the weight transfer on a launch, but it is important to remember that weight transfer is primarily governed by the formulas described earlier.

There is one other thing that is likely to help that we will be testing shortly, airbag suspension in the rear. The reason we like this is that airbags can offer many of the advantages listed above. For example, when you get to the track you can raise the pressure in the airbags to raise the rear end a little higher. The added pressure will also help to curb some of the weight transfer. But there is one key component found in airbags that we find the most attractive, and that is that they offer a progressive compression rate. In other words, as the car’s rear suspension contracts, the amount of force exerted upwards by the airbags increases non-linearly.

Many people are aware that the formula that describes a spring’s deflection as force is applied is:

F=kd

In this simple formula, F stands for the amount of force applied to a spring, k stands for the spring constant (or the stiffness of the spring), and d stands for the amount of deflection in the spring. This formula tells us that the spring deflects in a linear fashion, that is, if twice the force is applied the spring will deflect twice as much and so forth. That is, up until the coils bind.

The formula that best describes the behavior of an airbag suspension is Boyle’s Law, or

PV=k, which can also be written as P=k/V

In this case, P stands for the pressure applied (analogous to F from above), k is a constant that depends on the gas, temp, etc. (different from the k above), and V stands for volume (analogous to d from above). The fact that P is proportional to 1/V means that this is not a linear relationship. For example, doubling the force or pressure on the airbag will reduce its volume or length by ½, tripling the force reduces its volume to 1/3 and so forth. This is probably best described by the curves in Figure 3.

Limit of car’s suspension travel Coil Spring Airbag Deflection

Force

Figure 3 - Force versus deflection

The slopes (k) of the two lines in the figure are arbitrary, but they show that for all but small forces applied, an airbag can support a greater force while compressing less than a coil spring. This is why you see them on trucks.

OK, all this theorizing is great, but there’s nothing like some hard numbers to help make the point. Table 1 shows some of the hardest launching cars at the time of this writing (12/03).

60' time Avg. Accel Weight Horiz. Force G (lbs.) (lbs.) johnt 1.583 1.49 3575 5316.67 Digital Ken 1.579 1.49 3190 4768.17 Zooomer 1.547 1.56 3060 4765.04 BadSSEi 1.698 1.29 3675 4750.15 Scott 1.667 1.34 3470 4653.54 johnt - pre bars 1.711 1.27 3545 4512.75 GTPMORAD 1.659 1.35 3255 4407.41 BadGTP 1.644 1.38 3030 4177.96

Table 1 - 60' Launch Comaprisons

First of all, INTENSE would like to congratulate Zooomer and Digital Ken on their hard launching cars. We don’t know any of the details of their custom rear suspension limiting modification that they’ve made to produce launches like this, but we think their results are phenomenal. Besides that, there are a few noteworthy observations one can make from examining this table. The first is the striking improvement that johnt’s car has had in dropping 1.2 tenth in its 60’ time. Many realize that this is worth about .25 seconds in the quarter mile. It can also been seen that before installing wheelie bars, johnt’s car could only put down about 4500 lbs. of force down to the ground before the tires went up in smoke. After the bars were installed, the same car can put down on the ground a force of 5300 lbs. and hold it! That’s an improvement of 18%. Another observation one can make is that the wheelie bars have allowed this car to hold more accelerating force onto the ground than any other car to date.

This write up is a snapshot of where our thinking is at today regarding suspension. As mentioned already, we are nowhere near done testing and learning more about what we can do to get more power down to the ground. We have yet to experiment with various pre-loading techniques of the wheelie bars and airbags to name a few. As we learn more we’ll continue to present that in our Technical Tips.

All content and graphics are (c) 2003 INTENSE Enterprises, Inc. http://www.INTENSE-Racing.com Don't make excuses. Make horsepower.