Why Your Handling Does Not Change
Have you ever wondered why fuel burn-off typically does not change the handling of a race car, at least not a great deal? I have been asked many times when helping teams setup their cars, what will the car do when the fuel burns off? My standard answer is, I have never seen much change. And that remains true, at least when we have achieved a more balanced setup.
I have setup cars that have run 200 lap races on one load of fuel and not seen any appreciable change in handling. The car burned almost twenty gallons of race fuel, or 120 pounds of loss of loading behind the rear axle. Why didn’t the handling balance change with such a large loss of rear loading and dramatic change in front to rear percent?
I thought – the same as you are probably now thinking – there must be a reason why such a dramatic change in rear weight, front to rear percent, and overall weight does not seriously affect the handling. So, I studied this at length and came up with a probable cause concluding that there are compensating and offsetting effects from fuel burn-off.
To understand what changes in the car when fuel burns off and we lose load at the point where the fuel tank is mounted, we need to understand the whole picture of what is going on with our race cars.
Suppose we have a car that has equal load on the two axles when the fuel tank is full. This would represent a 50/50 weight distribution front to rear measured at the axles. The Center of Gravity (CG) would then be midway between the two axles. When we go through the turns, the lateral force trying to take the car to the wall is centered at the CG. The force, and the resisting tire forces, would be equally distributed between the two axles.
Understanding The Moment – There is a concept called Moment Arm (MA). There is an influences on the car associated with MA related to the force acting at the CG verses its distance from the axles. Since the CG is centered between the two axles in our example, there is equal force applied to each axle. The moment is equal distant in front of and behind the CG. Whenever the CG moves closer to one of the axles, those tires will have more lateral force applied to them and be more stressed.
If the car is truly balanced with a centered CG, then there will be equal distribution of the lateral force at both ends and the car will remain neutral in handling. If the lateral force exceeded the traction we have, the car would slide, but equally front and rear.
Also, if the CG was moved forward, then the lateral force would cause the front tires to lose traction first as the force overcame the traction. That is the essence of moment arm, the closer to an axle, the more force is applied to that axle as a percent of the total lateral force. Are you staying with me so far?
Traction Verses Vertical Loading – The amount of traction a set of tires will produce is partially determined by the amount of load those tires have on them. The more load, the more grip. We can easily understand that when fuel burns off, the rear tires won’t have as much load on them and the rear tires will lose grip. The front to rear percent changes also to more front percent.
At the same time, since the fuel load is cantilevered behind the rear axle, as we lose load in the tank, we gain an even greater percent of load on the front tires. It’s a double whammy. This car should go loose with less rear loading and more front loading, but it doesn’t.
Here is where teams become confused as I was initially until I figured this out. So, here we are, losing rear weight and rear percent of sprung loading. That loss of rear weight and resulting move of the CG towards the front changes the lateral moment too.
The lateral force is now located closer to the front axle and those tires have to resist the greater lateral force. Since the load is also increasing, they are able to resist the extra lateral force.
At the rear, we have less loading due to the fuel burn-off reducing the traction of the rear tires. At the same time, those tires have less lateral force to deal with since the CG moved forward, so they do not lose grip. This is because they are less influenced by the moment affect, so the loss of grip is offset by the loss of lateral force.
Balance of Losses – The changes caused by movement of the center of force coupled with the change in rear loading and percent of load will equal out for the most part. The tradeoff might not be exactly equal, but both changes move in the same direction, the gain of grip to resist the gain in lateral force at the front and loss of grip to go along with the loss of lateral force at the rear helps to maintain a neutral handling car.
Different setups may experience different results. I believe that a balanced setup is less prone to handling changes due to the effects described above and maybe that is why some cars can lose performance the longer the race goes on while others don’t.
Movement of the fuel load, or fuel tank, forward will not change the offsetting affect. This is because if the fuel load were over the rear axle, that would remove the cantilever affect as fuel burns off. So, the front will gain less percent of sprung weight per pound of fuel burn-off.
At the same time, the movement of the CG forward will be reduced and so as the change in rear load becomes less, the change of CG movement forward becomes less. Again we are closer to equal in the trade-off.
Different Designs of Cars – All of this does not change with the different designs of cars for the most part. If we run a high rear percent like some dirt cars do, the center of lateral force, or CG, is closer to the rear tires than if the car were a 50/50 weight distribution car.
With a high rear percent car, initially the rear tires will be over loaded as more of the lateral force will be acting on the rear tires because the CG is more to the rear. Especially on dirt, they may break loose sooner than the front tires due to the overall lack of grip on dirt.
Even though they will be somewhat more heavily loaded than the front tires, the lack of grip won’t translate to more resistance to the lateral force. Maybe that is why teams with a loose car, that is in reality a tight car that goes loose, will run more rear percent and end up with a more neutral car. They are fixing the tight condition and that solves the loose condition.
The problem with that is this. As the fuel burns off, the loading moves to the front giving the front tires more grip while the CG also moves forward towards the center of the car which puts more equal lateral force on the two axles. As the front gains grip, the car will go loose later in the race.
I’ve always said that dirt cars should run less rear percent and it has always worked out better that way to keep the car more neutral throughout the race. Adding rear percent is a crutch for a car that won’t turn. Like we said, most cars that won’t turn end up tight/loose. That is usually wrongly interpreted as a loose car that seems to be fixed by adding rear percent.
Conclusion – So, the next time someone asks you how your cars handling will change as the fuel burns off, tell them it won’t. If they ask why, tell them you have worked hard to create a balanced setup and that there are compensating factors that prevent the car from changing its handling balance. If they want to know more, refer them to this article.
Sources:
Coleman Racing
www.colemanracing.com
800-221-1851
Smileys Racing Products
www.smileysracing.com
866-959-7223
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