Suspension.Pdf

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.

Load more