#1403: Another Chance to Fail Physics

Ah yes, it all makes sense now. (Flickr/tobo)

Original Air Date: 01.18.2014

   Best Moment 01:13

This week on Car Talk, if one car drafts behind another to improve its gas mileage, does the car in front suffer? Tom and Ray's answer, or lack thereof, may bring the late Physics Professor Tisza back from the grave, just to administer a dope slap. Meanwhile, dope slaps might also be in order for Jessica's mechanics, who will only discuss her car problems with her boyfriend, while ignoring her. Also, the solution to Nancy's Miata's hood problem may lie in her local tanning salon; Melanie is about to learn a very expensive lesson about what her oil pressure light means; and, one listener offers the real definition of "primitive" four-wheel drive. All this and more, this week on Car Talk.

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Tom and ray share the true meaning of primitive four-wheel drive.

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Car Drafting

by mcm350

Every high speed NASCAR race, cars traveling inline always move faster than a single car. Once a car gets out of line, it can't keep up with the line of cars.

Convertible Top that Won't Close

by d'Skippy

I can't be the only one to think that if the top needed to be warmed to be closed, why not close the top as much as possible, close all windows, run car and turn on interior heat full blast until top is warm enough to completely close?


by Rock_On

Both cars in a drafting relationship benefit. Check with NASCAR for an explanation.

Failing Physics

by WCorvi

Conservation of Energy requires the leading car to be hindered, at least a little. But, the answer to the question about drafting effects on each car is in fluid mechanics. What causes friction on a car (mostly) is having to push air out of the way in front of it, and the vacuum formed behind as a result. The trailing car benefits because (two ways to look at the same thing) there is less air to push out of the way; the vacuum pulls the trailing car forward. BUT, that same vacuum pulls back on the leading car, increasing the friction. Ferret's answer is interesting, and I haven't thought about it completely. I suppose the drag on BOTH cars could be reduced slightly if the cars are close enough together that there isn't any fluid effect between the two cars.

Getting Respect from Mechanics

by DoggyMom

Seems to me that everyone missed the point on this. Why does Jessica always take her boyfriend to the repair shop? She said that her boyfriend travels on business a lot so he wants to be with her all the time when he's home. If he travels so much then why doesn't Jessica take the car for repairs when he isn't around? Why does she wait until he's in town and then go? If she always shows up with the boyfriend then I can see why the mechanic assumes that he should be talking to the boyfriend.

Yes, it is about laminar flow

by El Maton

There is a possibility of negatively affecting the lead car. As a student of aerodynamics I learned about spoilers. The ones used on cars to improve performance. What they spoil is the laminar flow of the air as it passes over the vehicle. Depending on the shape of the vehicle, speed, and other objects in front or behind it, the laminar flow will detach from the surface of the vehicle creating turbulent flow. By spoiling the flow at the right location on the body of the car laminar flow can be maintained, thereby reducing drag. If allowed to detach on its own the chaotic processes will actually induce more separation, thereby creating more drag. Since that flow is affected by anything in front or behind it, it can be good or bad for the lead car. Depending on the speed and distance, it is possible for the follower to adversely affect the laminar flow of the lead car, thereby costing the car performance by making it fight more drag. Under the right conditions, the inverse will happen and the lead car will actually maintain laminar flow longer. Without getting into quantum physics, I will say there is a very weak potential that particles, not atoms, can affect the lead car. Especially if the rear vehicle has it's lights on.

Dignity and not get scammed by the mechanic

by Sim55

I sympathize with Jessica. As a female driver I have run into this. The parts guy would be amazed that I knew my part names. Recently I went into my local tire shop for the "tire idiot" light. They put some air in it saying it was the "cold" weather(38F) They refused to look for the leak. $250 later(new tire) for my 2013 Ford C-Max hybrid; I got a tire gauge, sensor-friendly fix-a-flat, and a air pump. For the "cold" to cause my tire to loses 20lbs pressure it would have to be -130F at which point NO ONE cares about their tires. Houston does not get that cold. Good luck Jes.

Favorite Moment: Boyfriend too shy to tell the mechanic it's her car.

Dopeslap from fluid dynamics

by Ferret

When one car is drafting another, the car in front will generally benefit from the presence of the car behind. There may be cases for certain vehicle shapes at certain distances where there is no or possibly negative benefit. Once the two vehicles get very close together there will always be a benefit. Aerodynamic drag on a vehicle arises from several sources one of which is the need for the air to close in and fill the "hole" or lack of car behind the car. Generally a second car, if close enough to affect this flow behind the lead car, will reduce the energy required to pump the air into the hole behind the lead car. This reduction in energy spent pumping air around is seen as a reduction in aerodynamic drag on the lead car, so the lead car can go faster or use less fuel to maintain speed. The cars are not magically connected. What happens is that instead of two cars each having to push air around, there are two cars acting more like one long car sharing the air they push. In the extreme, the second car comes up close enough to the lead car that they effectively become one lumpy vehicle that has a little more drag than one car. The lead car still has to do all the work of pushing air aside and it has the full amount of drag from the radiator, mirrors, wheelwells, etc so the lead car still has most of its drag and sees only the reduction in drag behind. I have extensive experience in auto racing on ovals, and have seen many cases where two cars together are able to run faster than a single car behind. You can see this during every race in NASCAR at Daytona, Michigan, Talledega, California Speedway, Texas, and some other fast tracks.

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