Jun 172011
 

According to @tomjensen100, the oil pans confiscated from the Joe Gibbs Racing cars weighed between 20-30 lbs.  Lee Spencer posted some nice pictures of them.  A normal oil pan weighs about 4-5 lbs.  Why in the world would you make your car heavier?  It’s not always the weight of the car, it’s how the weight is distributed in the car.

When a car accelerates (and by accelerates, we mean here the physics use of the word, which includes  braking, speeding up and/or turning), the way the weight of the car pushes on the wheels changes.  It’s called load transfer.  (P.S.  There is a difference between ‘load transfer’ and ‘weight transfer’.  Don’t get them confused around Bob Osborne.)

The body of the car is suspended (relative to the wheels) on springs and shocks.  When you brake, the body rotates a bit toward the front.  When you accelerate, it shifts toward the rear.  When you turn left, weight shifts to the right and when you turn right… well, we’re at Michigan this week, so we don’t really care what happens when we turn right, do we?

Shift happens because of torque.  Torque is the ability to rotate.  It’s like force, but slightly more complicated:  torque depends on force, but also on how far the force is from the rotation point.

Go to the nearest door and try to open it by pushing on it close to its hinges.  Doesn’t work so well, eh?  Push a little further away – should be easier.  (If it’s not, make sure you are using a door that does actually open outward.)  There’s a reason the handles of doors are as far away from the hinges as possible.

The torque you can exert on something is proportional to the perpendicular distance you are from the point about which the thing is turning.  When you use a cheater bar on a wrench, you’re moving the force (that’s you pushing or pulling) further from the nut you’re trying to turn, which makes it easier for you to turn.

Now pretend you’re trying to tip over a car.  (Insert your own snarky hockey comment here.)  You  have two choices – the car on the left and the car on the right below.  Their centers of gravity are marked by the circles.  Both cars weigh the same and you’re going to push from the left as shown by the arrow.  Which one is easier to tip over?

Imagine you're the arrow.  Which car is mostly likely to tip over?Hopefully, you pick the one on the right.  It’s center of gravity is further from the ground (the car rotates about the point where the right tire touches the ground), so it’s easier to tip.  It’s the same reason football players crouch down — it makes them harder to tackle.  The lower your center of gravity, the more stable you are.

When a car turns, it’s the same thing as applying force through the center of gravity.  A large force tips the car over, but a smaller force just rotates the body relative to the wheels.  If  car is turning left, the rotation of the car puts more force on the outside tires than on the inside tires.

Why does that matter?  The grip each tire provides is proportional to how hard it’s being pushed into the track.  The first law of racing is that you can only go as fast as your least grippy tire.  Maybe you have 1200 lbs of force on your right rear coming out of turn 4- if you only have 200 lbs pushing the left front down, you aren’t going to be turning very quickly.

You’d like to have all four tires pushed into the track with comparable amounts of force.  The less load transfer, the better.  The biggest complaint from the drivers when they switched to the new car was that the higher center of gravity made them feel like they had moved from sports cars to SUVs.

The center of gravity (CG for short) is determined by how weight is distributed in the car.  Everyone has to make the same minimum weight, but it makes a significant difference where the weight is located.  The goal is to get the CG as low as possible.  Teams have been making things like dashboards out of carbon fiber to decrease the weight and give them the opportunity to add ballast lower in the car.  A heavier oil pan would lower the CG, which would decrease load transfer and make the car faster.

Another possibility mentioned by John Darby (thanks to Bob Pockrass for noting it) is that the bottom of the oil pan could have been contoured to change the aerodynamics underneath the car.  You don’t want air getting under the car.  What air does get under there you want to flow smoothly out.  NASCAR doesn’t let the teams do much with the underside of the car – but it’s one of the few places where there is still some room to explore.

I remember asking a crew chief whether something was legal.  He squirmed a bit and finally said, “Well, it’s not illegal”.  NASCAR leaves grey areas – fewer and fewer of them, but they are there.  The crew chiefs and car chiefs scan the rule book trying to think of things they can do that are not specifically prohibited.  On the one hand, you can argue that running a piece that wasn’t previously submitted to NASCAR for approval is illegal.  But the rulebook (thanks Jeff Gluck) contains very little in the way of specifics for the oil pan.  It does, however, contain the magic phrase “must be acceptable to NASCAR officials”.

  2 Responses to “When Oil Pans Put On a Little Weight…”

  1. Great Article, but I got two questions:

    1. What is the difference between the Center of Gravity and The Center Of Mass?

    2. What is the difference between ‘Load Transfer’ and ‘Weight Transfer’?

  2. [...] When Oil Pans Put On A Little Weight [...]

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