NASCAR had not one but two drivers on their roofs yesterday at Talladega. Ryan Newman’s wreck was by far the most spectacular, but I had to wince at Mark Martin looking a little shook up and disgusted during the post-infield-care-center interview. I don’t blame him – or Newman – for being angry. Yes, it’s really impressive to see three-wide around the turns ten rows back, but is that really great racing? Is it good enough racing to subject the drivers (and the fans) to the types of accidents that we keep seeing at Talladega and Daytona? Is it good racing when you can cruise around the back all afternoon and finish in the top ten with drivers that fought for the lead throughout the race?
We learned (or were reminded of) a couple important things at Talladega.
- Most people do not understand the fundamental difference between the physics of drafting and the physics of bump drafting. There is a big difference between prohibiting bump drafting in the corners and prohibiting drafting in the corners
- It appears to be much easier for the new car to become airborne than the old car
- When cars are traveling in a pack, any accident is highly likely to involve more than one car
Drafting vs. Bump Drafting
Air rushing around the back of the race car creates a high pressure region at the front of the car and a low-pressure wake at the rear of the car. The high pressure in the front of the car (the car ‘punching a hole in the air’) creates a force opposite the direction the car is moving. The pressure behind the car is lower, which also acts in the direction opposite the car’s motion. So the car is fighting against a force pushing it backward in the front and a force pulling it backward in the back. If there are two cars running some distance from each other, each is experiencing two forces slowing it down: one in the front and one in the back (as indicated by the large red arrows in the top figure). Between the two cars, there are four big red arrows.
When a second car gets very close behind the first car, the air rushes over the two cars as if they were one, which removes the force at the rear of the first car and at the front of the second car. In the lower picture, there are only two big red arrows, so there is less total force working against the cars and – voila – they go 3- 5 mph faster. This is the important part: the two cars don’t need to touch to make this happen. This is plain ole drafting. You can get some more information on drafting and bump drafting in the Science of Speed segment “Drag and Drafting“.
Bump drafting is totally different physics. The leading car is running at full throttle. The trailing car is being pulled along, which means that at full throttle, it can actually go faster than the leading car. The trailing car bumps into the leading car, transferring some momentum from the trailing car to the leading car. The leading car goes faster and pulls the trailing car along with it. ‘Bump’ is probably a misnomer. Brian Vickers says in the Drag and Drafting video that he’s come away from plate tracks with headaches because he got bumped so hard – but adds that he was happy to be hit that hard because that’s the way you go fast at a plate track.
In drafting, you’re essentially removing a force by driving within inches of each other. In bump drafting, you are applying a concentrated force from one car to another. Bump drafting takes significantly more skill. The black dot in the diagram below indicates the center of gravity of the leading car. Newton’s laws: If you apply a force, the car goes in the direction of the force. The two cars bump squarely in the top diagram. The force from the trailing car pushes the first car straight ahead.
The middle picture shows two cars in a turn. There’s no way to bump squarely because one car is rotated relative to the other. If you hit the car ahead of you, you create a torque, which is a force that makes things turn. Think of the leading car as a spinner, pinned by the dot in its center. If you hit it in a direction so that the force goes directly through the center, it won’t spin. If you hit it at an angle, the car will spin. This is why you don’t bump draft in the corners. It is very easy to hit someone on the side of the bumper, sending them into a spin and wiping out half the field.
You can cause a car to spin by bump drafting in a straightaway if the two cars are not fully aligned. The bottom diagram shows that hitting a car off-center – even when both cars are going straight – is pretty much equivalent to hitting a car in the corner.
The two techniques have one thing in common: the person in the trailing car is in control and the leading driver is really just along for the ride. The trailing driver decides when to push, where to push and how hard to push. A number of drivers’ have expressed discomfort with ‘being pushed’ (drafting) too hard in the corners because an overaggressive – or an inexperienced – driver can make your car unstable fairly easily. But there is a pretty significant difference between bumping and pushing. Requiring drivers to leave space going into the turns just caused an accordion effect with cars having to back up down the line.
Aerodynamics
One of the big reasons for the aerodynamic changes in the new car was to decrease the amount of wake behind the car. The large wake with the old car made it very difficult for one car to get up close behind another one because the swirling air from the wake didn’t provide enough downforce on the front wheels and the car got tight. The big difference between the old car and the new car in the rear is the spoiler on the old car vs. the inverted wing on the new. All of the air must go up and over a spoiler, creating a huge amount of turbulence behind the car. The wing allows air to flow on top of and below the wing, so there is less turbulence behind the car.
Let’s briefly review the aerodynamics of wings. As shown in the diagram below, air moving over the top of the wing moves faster than the air moving over the bottom. Faster-moving air exerts less pressure than slower-moving air, so a wing experiences more pressure and more force below than it does above. That’s what gives an airplane lift. The wing on the rear of the car is upside down, so there’s more force on the top than the bottom and the inverted wing on a NASCAR car provides downforce.
The key for the computational fluid dynamics simulations of the old vs. new cars (shown below, from a GM publication) is that red are areas of high pressure, meaning lots of downforce. Orange, yellow, green and blue show decreasing levels of pressure, so those colors mean less downforce. Note in the old car (on the left), there was a significant amount of downforce generated by the rear decklid, while in the new car (right), the vast majority of the downforce comes from the wing.
NASCAR cars have an aerodynamic instability problem when they get going too fast. They are fine as long as they are pointed forward, but when the car spins, the aerodynamics change significantly. The air moves very quickly along the rear window and the roof, and remember that fast-moving air doesn’t generate much pressure. The ‘shark fin’ on the side of the rear window and the roof flaps were designed to slow down the airflow because slower-moving air creates more pressure. The roof flaps and the sharksfin were designed (the story is in my book, The Physics of NASCAR) for the old car and I don’t know how much work was done to compare the effectiveness of the roof flaps on the new car versus on the old car.
A couple things I noted this morning watching the video of Newman’s crash in slow motion.
- Both roof flaps deployed at the same time. In theory, one is supposed to deploy first and then the other if it is needed. But things were happening very quickly.
- The roof flaps deployed when the car was at about 135-150 degrees from heading in the correct direction. (180 degrees would be facing backward.)
- The right rear wheel was already off the ground as the roof flaps were opening
- One the car got fully backward, it flipped over without any twisting. You would expect a car that was spinning and then got airborne to continue the rotational motion, but the 39 did a really clean black flip.
- Once the car made about a 60 degree angle with the track, the roof flaps went down again. I wouldn’t think that gravity would pull them down, so that suggests that the pressure had increased to a point where the roof flaps didn’t think they needed to be deployed.
A backward spoiler is still pretty much a spoiler. If you think about the inverted wing running backward, I wouldn’t be surprised if it were generating a lot of lift. Put yourself in the place of the people designing the car. Would you have thought to simulate the car going straight backward to see what happens? We may not even know enough about the aerodynamics of the cars to do an appropriate simulation. But between this incident and Carl Edwards’ takeoff, NASCAR needs put some serious resources into re-evaluating the aerodynamic behavior of the car at different speeds. Decreasing the restrictor plate holes, especially by fifteen thousandths of an inch (about four times the diameter of a human hair) is not going to affect safety much. The reduction took ten to fifteen mph off the cars, but it didn’t address the primary problem of plate racing, which is that the drivers are on the throttle wide open most of the time, which makes them run in a pack.
A number of drivers are suggesting that NASCAR needs to sit down with them and talk about how to make racing safer. It’s not the drivers NASCAR needs around the table. With all due respect to Ryan Newman’s engineering degree, it’s the top aerodynamics people at the race teams (some of whom actually have Ph.D.s), and people like Gary Nelson and Gary Eaker, who were largely responsible for designing the original roof flaps. NASCAR has many more times the number of people working marketing and licensing than they do on safety. The teams have some incredibly smart people in their aerodynamics departments who have spent the last three to four years trying to understand everything they can about the aerodynamics of this car. NASCAR needs to make use of those folks’ skill and talent because they simply don’t have the in-house resources necessary to do the job quickly and effectively.
NASCAR has a history of being a reactive organization – as Carl Edwards noted last spring, when he said “I guess we’ll do this until someone gets killed, and then we’ll change it”. For a few scary moments Sunday, I was afraid we’d reached that situation. Take the initiative and solve this problem before someone gets killed. Don’t tells us that your rules weren’t the cause of the problems. Slowing the cars down is not enough: It is time for a major change, whether that be repaving Talladega to decrease the banking (as the late David Poole suggested), a major re-design of the aerodynamic safety equipment on the car, and/or introducing a significantly less powerful engine that could be run without plates.
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Thanks for the science behind the talk (and there is a whole lot of it out there after the race this weekend). Nascar does a great job of fostering the personal connection we fans have feel to the drivers. It’s one of the things that makes this sport so different from others. But that doesn’t work in a positive way when drivers are put in seriously dangerous situations. Seeing Mark Martin and Ryan Newman airborne and spinning out of control left a sick feeling in my stomach. While these high speed wrecks look cool, that pause between a big wreck and waiting to see if the driver is okay is just not worth it.
Nice analysis, Diandra. I saw the race and Newman’s disturbing post-crash comments. There isn’t going to be an easy fix to this, but constant wide-open throttle and large packs is not the most entertaining racing.
Great post – as usual. Thanks DLP!
Primarily, there were 2 major changes with the COT over the old car –
1) safety – and as a result of all the cage improvements, the CG of the car is considerably higher. There’s little chance the impact that overturned the 5 would have flipped the old car
2) the wing – less drag, cleaner air, closer racing – what it appears to need is a “spoiler flap” that would pop up under the wing and seal the back deck lid and valance to the underside of the spoiler once the car get’s pointed backwards. Same fundemantals as the roof flap and easy to install – no gap and no resulting airflow would mean no lift due to the wing.
Tim – love the idea of a ‘spoiler flap’ – NASCAR should be talking to you!
I agree with what you said Diandra, but why dont the same cars that race at the 1.5 milers when they wreck flip like at Daytona and Talledaga?
I mean they are running faster at these tracks, even if they dont bump draft as much I still see them have wrecks and they dont seem to lift?
I would love to see them take cars from a era that didnt have as much problem with this, say the 1970s or eighies cars, put them in the windtunnel and get the numbers and use that as a baseline!
You are right on about the cars high cg look at Mcdowells crash at Texas,however I would do away with the stupid wing altogether and the new cot nationwide cars don’t have them.
Tim has the right idea. It has been discussed before and was revisited again on Nascar radio following the wreck that was Talledega. Why Nascar won’t institute a smaller engine for Daytona and ‘Dega is beyond me. That solution is certainly cheaper than repaving or re-designing those tracks. It seems that the science almost dictates a smaller engine so 1) the drivers don’t run wide open through the turns 2) and would allow the pack the spread out by virtue of driving styles (how fast, slow, deep or wide a driver goes into a turn, exits a turn, etc.) and 3) create more interesting racing.
(Diandra, I attended your lecture at UVa and love your book.)
Great explanation, but I’m afraid of too much focus on the wing. A backwards wing “should” not generate much lift, though the angle of incidence of it in relationship to the deck lid may generate some lift. I noticed the bottom rear of the car had a large pocket as the car was upside down. Could it be possible that as the car does a 180, the air gets disturbed and is forced under the car into this pocket (does the flat slab bumper contribute?), creating a positive pressure area great than the pressure above the car? If this is true, engineers would only have to devise a way to equalize this pressure. I’m not one (yet) but this got me thinking Thanks again for another great article.
I’m no engineer, but it seems to me that limiting horsepower creates the bunching. The drivers need to have throttle control instead of driving with trail-braking. I say free up the HP (no restictor plates) and have NASCAR set the the gear ratio for 4th gear (1:1) and then set the rear axle ratio at something that would not allow full-throttle down the front or back stretch. This would allow more throttle control from the drivers and not drop so far back if they do have to get off the throttle, because they would have the HP to catch up. The drawback would be the amount of blown engines!
Tim’s idea of the wing flap is probably the only way to keep the wing and have a car that won’t turn over when backwards.
The other thing all cars have going against them is the weight concentrated up front (engine) that makes Newman’s flip easier and weight on the lower left (lead/moly to make tech weight) that makes Carl Edwards twisting flip easier.
Oh, David Reynolds is right about the air underneath the car. NASCAR mandates the cockpit be sealed so air can’t pass through the car and out the rear of the cockpit. (That creates downforce – remember Carl Edwards famous missing oil tank cover).
So that is a difficult area to increase air pressure.
Diandra,
I love the way you explain things. I don’t have a scientific bone in my body or a scientific brain cell in my head but, I always understand what you write. Thank you. You make learning fun.
Good post, Diandra. How about NASCAR issuing each team a chip that would limit the rpms the motor turns, taking off the restrictor plate and mandating a gear rule that would not let the cars exceed maybe 175? Reducing the engine rpms by, say, 1000 rpms and limiting the gear ratios and removimg restrictor plate would slow the cars down, but they k would still have good acceleration, like on the non-restrictor plate tracks. I think the wing flap might be a good idea also. Jack
If you want to address power – they can keep the same engine, they just need to mandate a smaller carb, not a restrictor plate. A plate just means the drivers hold the throttle wide open to make sure they are getting max power available and “trail brake” with their left foot if they get too fast or close to the car ahead. Trail braking with a WOT (wide open throttle) slows the car with the added bonus of increasing the gas-to-air ratio – which means? Less power on acceleration (or when the driver lifts his left foot) as the air:gas ratio shifts towards more gas! To address this, engine tuners at a plate race generally run larger air jets in the carbs to lean out the engine (run more air) – which translates to more power. The problem here is that a leaner engine is a much hotter engine – was anyone listening to Jimmie Johnson talking to Knaus about how long he can run close without overheating? That’s all the result of running lean. At least with a lower CFM-rated carb, but fully operable, unrestricted, and set with proper jets for the perfect +/-14:1 air:gas ratio, then the drivers would have power available through the whole power curve – instead of just a steep peak at max HP. Combined with a properly mandated rear gear, they could hold speeds to whatever they liked and the driver would find it desireable to modulate speed with his right-foot, instead of his left. And no BS about costs – all the engine builders have a separate plate engine program already.
The plate decreases overall HP by restricting airflow to the outside of the venturies in the carb. But it also slows acceleration by interupting the smooth transition from the bottom of the carb through the top of the intake manifold – all the fancy grinding and smoothing a good carb guru does is out the window – air/gas cannot flow smoothly, it gets really turbulent inside the manifold. With all the turbulence, the engine is not as responsive. The turbulence evens out somewhat once the car is under acceleration and the extra vacuum pulls the air/gas more directly, but that takes a split-second to respond and is never as smooth as an unresticted engine. Slow response = low acceleration = follow-the-leader = big crashes
Lower hp won’t fix the issue of the packs as they will still run wide open for the lap because the cars will still be within their handling “window” at the lower speeds. If anything they need to take down force away at these tracks and run unrestricted so the drivers are forced to lift at these tracks. I would propose a smaller wing that will flip forward when reversed, harder tires, less depth to the splitter and a redesign to the roof flaps to make them deploy faster. Also I would not allow yellow flag pit stops at these tracks, this will also work to break up the packs. This problem does not appear to be as bad at Daytona because the inherent differences of that track place a premium on handling so the drivers cannot just run wide open and trail brake to stay in the pack.
These comments about restricting the engine in other ways seems irrelevant. What’s the point in having more throttle response if you never need to lift anyway at these speeds?
It looks like there are issues with the car. But I think without question the track configuration is just plain insanity. It is today and it was in 1969 when it opened (196 mph pole speed, even with driver boycott)
They have to knock down the banking dramatically.
It seems to me that Nascar has precipitated this by mandating certain components. They control rear shocks, gear ratios, springs and suspension designs that limit originality.
Remember a few years ago when Ray Evernham brought a car to Daytona I believe that was a radical departure from the norm. Jeff Gordon ran some laps with it but the team was quietly told to take it away and never show up with it again.
One of Dale Sr’s comments after winning the Daytona 500 was he was sorry that car had to go on display for a year because fo the shock work they had done.
Nascar is so afraid of anyone getting even the slightest edge they legislate out the creativity of the builders. You may call creativity cheating, but these guys are pretty smart and secrets don’t last long in gasoline alley. The competition will soon figure it out and duplicate it.
How about fewer rules and more innovation. That will make for better racing.
It seems to me that Nascar has precipitated this by mandating certain components. They control rear shocks, gear ratios, springs and suspension designs that limit originality.
Remember a few years ago when Ray Evernham brought a car to Daytona I believe that was a radical departure from the norm. Jeff Gordon ran some laps with it but the team was quietly told to take it away and never show up with it again.
One of Dale Sr’s comments after winning the Daytona 500 was he was sorry that car had to go on display for a year because of the shock work they had done.
Nascar is so afraid of anyone getting even the slightest edge they legislate out the creativity of the builders. You may call creativity cheating, but these guys are pretty smart and secrets don’t last long in gasoline alley. The competition will soon figure it out and duplicate it.
How about fewer rules and more innovation. That will make for better racing.
I believe Dale Jr said it the best during a post race interview. He said it wasn’t Nascar’s fault about the racing at Talladega, it was their faults for over engineering.
The tracks have to keep up with innovation, and then Nascar has to try to stay one step ahead also.
Nascar has issues being proactive, but as long as teams are trying to improve performance Nascar will have to put rules in place that combat those improvements.
Maybe just going to an IROC car would solve all of our problems. But then again, none of us want to see that ingenuity lost.
Nice analysis as usual. My limited intuition says turning the wing around won’t change the effect from downforce to upforce, but probably would reduce the magnitude (probably by a lot). It’s still further around the bottom than the top, hence the air has to go faster around the bottom, hence less pressure on the bottom, hence downforce. But I’d sure like to see some modeling, and wouldn’t be surprised to be wrong!
I think it’s the shape of the car’s underside that causes lift when it faces backwards.
I think Daytona and Talladega are just going to have to get on with the digging and reduce the banking so that the plates can be removed.
Either that or gear ratio and rpm limits (electronic or otherwise) to keep the speeds below 200 or whatever is deemed safe.
first off this with talladega really got started before practice. All the science is good but from a strategy stand point i was viewing practice. I was right the 48 car was dragging it brakes in the straight with nobody around him. This was premeditated so knaus and gordon could lobby for a sudden rule change. Believe it or not knaus has nascar by the balls. He is the media he is on tv more than some racing talk show host. Cant people see it the same thing happened with president last year obama manipulated people to vote for him. The same thing has been happening in nascar but some teams turn the wick at right time and get mike helton thinking. It is straight from the art of war book. He threatens to throw nascar under the bus on twin and that is how that happened. There is more to it than what people are seeing nascar as a sanctioning body either moves real fast or slow as molasses in winter time. Racing this year has been on the edge of what is called in pro wrestling biz a work. Unexplained cautions and last lap finishes at tracks where passing is easier on back stretch like daytona. Nascar can control strings to puppet show. I am on twitter trying to get nascar to go to nureburgring for ppv event the worlds most dangerous track 12 miles long. Nascar needs to change to beat 48 team you need a f1 paddock girl 36 24 and whole lot more and a top 5 car. You got to force an error. Finance and romance is every mans weakness. Drop an hotty into concord in jan have her find knaus let mother nature takes it toll. 1.5 million i could make it happen. nascar neeeds to go ahead and let teams run data logging systems it would give more parity. Nascar needs to find a way to absorb energy on roof of cot through rubber body bushings and add more gussets to roll cage. A driving uniform changee is needed similar to a sportsbike. World of outlaws need it as well. Pit confirmation buttons are needed as well. The cot nose was made by somebody who attended woodstock that is where the problen is.