Understanding understeer.

As the Winston Cup Series prepares for this weekend's race at Texas Motor Speedway, crew chiefs and engineers are gearing up to do battle with a force they can't even see.

With corner entry speeds of 200 miles per hour, aerodynamics will play a significant role in determining the outcome of Sunday's race. Drivers and teams will spend all weekend looking for the perfect handling package for Sunday's 500-mile event, only to see that package altered by the effects of air around their car. Understanding those effects and knowing how to deal with them could be the difference between winning and losing.

Here Terry Laise - Lead Chassis and Aerodynamics Engineer, GM Racing - talks about understeer, the effects it has and a lot more:

Q:
How do you define understeer?

Terry Laise:
Understeer is when you try to turn into a corner and the car doesn't want to turn. You'll hear a driver say his car is 'tight' or 'pushing,' which means it feels like it wants to push in the corner up towards the outside retaining wall.

Aero-understeer can be identified because, as you go faster, this becomes worse. Aero forces go up as the square of the speed. The force is the square of the speed change, so the faster you go, the worse the aero-understeer would be.

We'll probably hear quite a bit about understeer - or an aero-push - this weekend at Texas. You'll especially hear about it from guys running in traffic.

Q:
What accentuates the aero-push in traffic?

TL:
If a car is in front of you, the air acts as though those two cars are more or less the same car, from the aero point of view. That removes the front downforce from the front of the car running behind the leader. You'll typically still have all of your rear downforce, but you lose some of your front downforce and that makes for aero-understeer. It actually makes it the other way for the car in front - it makes it aero-loose. But, generally in large packs, the situation we encounter is one of aero-understeer.

Q:
Once a car develops an understeer condition, what ill effects does that lead to in the car itself?

TL:
If you get into this situation, you have a tendency to slide the front tires more. You have to put more steer-angle in to get the car to turn, so you get the tyres operating with a greater slip-angle on them, which isn't particularly good for the tyres. It kind of manifests itself into the driver having a harder time turning. Because he has a harder time turning, he gets on the throttle later because when he accelerates more, it puts more forward thrust in the car and it tends to want to go straighter. So, these drivers are forced to lift to control the car.

Q:
Can a crew chief work to compensate for an anticipate aero push before a race?

TL:
Traffic, in general, causes a loss of aerodynamic downforce at both ends of the car, because in general, you don't just have a car in front of you or just a car behind you. You have cars around you and that leads to more of a global loss of downforce. If you're dependent upon your downforce balance for handling and it goes away, that means you end up with whatever your mechanical balance is - mechanical balance being whatever setup is under the car - springs, shock package, etc. I think some of the more sophisticated crew chiefs work toward making the mechanical balance and the aero balance in line with one another so that when you lose some total downforce, the balance stays the same. Ideally, you'd want the car to be balanced the same whether it had downforce on it or not. You can't defeat this all the time because sometimes you do only have a car behind you or a car in front of you. You can't catch all of the circumstances because the aerodynamic effects are different. But on average, the best thing to try to do is to have the car balanced such that when total aero is lost the car is still well balanced. Some do that and some don't.

Q:
How do you go about addressing this in your role?

TL:
I try to address it in the development process. We certainly try to design a car that is well balanced and has maximum downforce within whatever guidelines NASCAR gives us when we do a new body. We're going through that exercise right now with the 2004 Grand Prix. But, as we develop the cars -going to track tests with the teams, doing simulations and all - we try to help them get to a point where the aero balance is such that it can be in line with the mechanical balance.

On a race weekend, it's pretty difficult. We only have a couple of practices and it's really hard to go beyond having a plan. The best thing to do is to test it ahead of time and have a plan for things to do if the car goes one way or another way.

Q:
What kind of information, if any, do you share with a driver with regard to the effects on an aero-push?

TL:
It's good for a driver to know the places to be. Where you are, with regard to traffic, has a significant effect on what happens to your aerodynamics. We've done substantial wind tunnel testing with cars in various drafting positions and we've shared that with the drivers. We've seen the effect on the downforce because of those positions, and we've gone to our drivers and said, 'Here is what causes this,' and, 'Here's a place to be, here's a place not to be. If you need to pass, here is a scenario coming up on a car - what position you should be in behind him and when you should jump out to pass him and in what direction.' We've done a lot of that work and we've shared it with the drivers and the driver response has been really amazing.

Dwight Woodbridge (GM Racing engineer) and I did the work together on that a couple years ago. I told him, I said, 'The first thing we're going to do is get [Dale] Earnhardt and let's sit down and talk about this.' It was amazing. Dale looked at what we had and said, 'Yep, that's right. Yeah, you've got that right. You know, you guys didn't look at this spot here. You ought to think about this here.' He was right on the money.

Earnhardt, of course, always said he could feel the air and could feel what it was doing. I don't know if he could really feel it, but I know that he knew what the air was doing better than any other driver. And, it may very well have been that he had just run so many races in so many positions and was so observant that he learned it all.