Stock Car Science

Yaw makes the new car easier to drive. So why did NASCAR make a rule limiting the amount of yaw allowed in the car?

Initially, NASCAR was steadfast that they weren’t going to make a rule about yaw.

They [complain] when we’ve got too many rules and then they want us to create more." –John Darby

Last week, NASCAR made two rules that allow a limited amount of yaw. One degree of toe is allowed in the rear wheels, and the displacement between the front of the rear tire and the back of the rear tire must be no greater than 5/8". There are two rules because there is more than one way of angling the rear tires. Toe is the slant of the tires when you’re looking at them from above, as shown below.

Toe usually refers to the front tires, but in this case, teams are angling the rear tires so that the rear end of the car is displaced relative to the front end of the car, thus putting the car in yaw. (And yaw helps the car turn.) The second way you can get the tires at an angle is to offset the axles. The 5/8″ rule accounts for both toe and offsetting the axes

One concern expressed by a couple crew chiefs (notably Mike Ford, crew chief for the No. 11 car) is how NASCAR is going to measure the one degree accurately, because the toe of the rear housing should be measured relative to the rear ring gear and you can’t do that without disassembling the rear end. I understand that NASCAR will measure the 5/8″ rule and then if there is a question that the car isn’t in compliance, they’ll do the more careful measurement after the race. They’ll also probably randomly measure some cars as well. The desire to take a chance with getting away with something is likely going to be tempered by the precedent set last week when NASCAR assesed the Haas CNC cars a 150-point penalty and suspended not only the crew chiefs, but also the car chiefs.

I did a calculation to see how much offset one degree of toe in the tire would give you. It comes out to about a half inch. Two degrees of toe would give you about an inch difference between the front and the back of the tire.

That’s the what of the rule. What about the why? Why did NASCAR feel compelled to tighten even further the box the teams have in which to maneuver? Eliminate the black helicopter arguments: (NASCAR wants Jeff Gordon/Hendrick Motorsports/Dale Earnhardt, Jr. to win; NASCAR just wants to take all the fun out of racing) and what do you have left?

One argument advanced is that the cars look "look silly" dog tracking down the straightaway, and that the NASCAR rule change is due to pressure from journalists and fans. If commentators hadn’t pointed it out, I think the yaw escalation would have gotten much further along before the average fan started noticing. Few fans see the cars struggling to make it up the scales.

Marc from Full Throttle pointed out that the Nationwide cars are pretty yawed out, but NASCAR hasn’t made any rules restricting yaw there. There’s a slight difference in the origin of the yaw: Nationwide cars are twisted because of changes to the body, while Cup cars are yawed due to changes to the rear housing.

Why does that make a difference? When you start changing the rear housing geometry, you’re changing how things mechanically interact with each other. When you change the body shape, you’re changing how air flows over the car. Changing the mechanical working of the car has potentially more serious consequences. For example, the rear housing has the rear-end gear and all the gears in the differential. Most of these gears mesh at 90-degree angles to each other. Try offsetting pinion gears by a couple degrees and see how well they work. I talked to people from two different teams (neither of which was having great success with the yawed out setup) and both said they saw major problems with gears binding, gears getting chewed up, or excessive heating when they ran extreme yaw setups on chassis dynos. One engineer mentioned that he suspected the Penske cars had to machine significant modifications into the rear differential to allow the gears to work properly at those high offset angles, and to offset the increase drag you would generate due to the additional friction between the mismatched gears. I believe it was at Martinsville that the 00 had its rear end housing catch on fire (there was a spectacular photo in NASCAR Scene) and I wonder whether that might have been related to rear-end housing experiments.

The situation is, in some ways, similar to the coil-binding issue. In the old car, teams limited the car’s travel (how far the car moves up and down) by setting up the front springs so that the coils of the spring actually touched. When the coils ‘bound’ (as shown below), the car couldn’t sink any lower into the track.

The problem with coil binding is that cars are designed to run on springs. When you coil bind, the car depends entirely on the tire to absorb any bumps and shocks, and tires (in which 1 psi corresponds roughly to 60 lbs/inch of spring rate) aren’t designed to do this. The transition from having a spring to not having a spring is quick and some drivers had a very difficult time coping with the unexpected things the car might do.

There are some similar issues with the yawed out cars. Some teams caught onto it much faster than others, and some drivers adapted to driving a yawed out car faster than others. Just as the coil binding set up caused the tires to take on a task they weren’t engineered to handle, moving the rear axle at an angle other than the one it was designed for can create stresses the axles weren’t designed to deal with.

The Young’s Modulus characterizes the stiffness of a material. Your bones are much stiffer longitudinally (along their length) than they are laterally (perpendicular to their length.). Your bones are usually supporting the weight of your body when you are standing (i.e. the bones are being stressed along their length). Most breaks are due to stress perpendicular to the bone. All structures are designed for a particular use: When you use them in ways other than they were designed to be used, you are likely to find that they don’t perform quite right.

NASCAR also noted that cost was a concern. John Darby said that,even though the curved rear-end housing used by Penske was legal (actually, the words he used were “not illegal"), the idea that everyone was going to get rid of their existing rear end housings and replace them was a bad idea. The crew chief of the 77 car, Chris Carrier (from a very nice NASCAR Scene article by Jeff Gluck that unfortunately doesn’t appear to be available on the web), swears that the parts they used were off the shelf.

“It’s not something that was made in Moscow in the basement of some retired scientist who was under the influence. We bought them.”

Chris, by the way, is quickly becoming my candidate for most quotable crew chief. When you click the link, search for ‘rocket’.

Jeff Gordon publicly commented about the yaw issue, as he did about the coil binding. There tend to be two groups of people in NASCAR: Those who think the spirit of the law should be enforced and those who think the letter of the law should be enforced. Chris Carrier’s comments are an example of the former. Denny Hamlin gives an example of the latter when he said (As quoted from the same NASCAR Scene article by Jeff Gluck),

“It is good that NASCAR is not letting it get too out of hand because these cars were made to drive straight…They (NASCAR) wanted to set a level playing field and all the cars to look the same around the race track, and I think that’s the way of getting them back in their hands.”

I used to let my physics students use a crib sheet for tests: one page, anything they wanted written on it. Creating the crib sheet forces students to winnow out the main ideas of the unit because they can’t write down everything. Or so I thought.

This worked really well for quite a while, until some students used a reducing photocopier to get much more information on the page than they could if they had to write it out themselves. This didn’t bother me much, and the students who were doing this weren’t scoring any better (or worse) on average than the students who didn’t.

The reduction process escalated until students were bringing in essentially every word of the class notes. They had so much in front of them that they couldn’t find anything during the test. There was an inverse relationship between the amount of material on the crib sheet and the grade: The more material a student had on their crib sheet, the lower their grade. More significantly (to me), students were skipping the most important step: Identifying the main ideas of the unit . I finally had to make a rule that the crib sheets had to be written in pencil or pen - no mechanical copying allowed. And some students complained about it just as much as some crew chiefs are complaining about this rule.

That’s where I think NASCAR is with yaw. They came up with the new car and new rules to address issues they felt had gotten out of hand. There are always some innovators who find ways to make things better or easier. Those are the folks who thought of offsetting the rear axle to increase yaw. There are also always the people who take things as far as they can get with them, which forces the teacher (or the sanctioning body) to make additional rules. NASCAR clearly recognizes that a little yaw is a good thing, otherwise they would have outlawed yaw entirely.