Sunday, February 10, 2008

Drag Racing Data

A new season has started for NHRA drag racing, which brings a new set of data from the Winternationals out in Pomona.

A run from Friday qualifying showed an average acceleration of over 5.0 g's over the first 60 feet, which makes the 3.0 g launch of the Space Shuttle look lame. A run on Saturday was even quicker at the start, over 5.1 g's, but two thousandths of a second slower for the full quarter mile. It looks like it will be quite a season if Sunday's elimination runs match what was done in qualifying, with six cars running over 330 mph at the top end.

Data first, then calculations and an explanation of terms like "Top Fuel".

What do the data show?

Friday: Right Lane: Antron Brown runs 4.495/330.07
Antron Brown's incremental times:
60ft-0.862 sec., 330ft-2.152, 660ft-3.059/278.40mph, 1,000ft-3.824

A time of 0.862 seconds for 60 feet (18.288 m) corresponds to a constant acceleration of 49.2 m/s2 or 5.02 g (5.02 times the acceleration of gravity).

Antron Brown has moved from winning drag races on a motorcycle to Top Fuel. IMHO, the driver matters more on a motorcycle. Top Fuel is more about setup by the crew. The driver 'just' has to keep calm and the car straight at 300 mph while fighting the forces produced by such high acceleration.

Saturday: Right Lane: Larry Dixon runs 4.497/325.14
Larry Dixon's incremental times:
60ft-0.850 sec., 330ft-2.152, 660ft-3.053/278.98mph, 1,000ft-3.822

A time of 0.850 seconds over the first 60 feet corresponds to a constant acceleration of 50.6 m/s2 or 5.16 g.

Note that these accelerations mean that there is a force of over 800 pounds pushing a 160 pound person back into the seat. Five times the force of gravity is trying to pull your arms back and your hands off the wheel. That is why the Beach Boys sing about "being pushed out of shape" so it is "hard to steer" in one of their 60s ballads about racing cars, only they were talking about 1 g acceleration, max.


Assuming constant acceleration, the formula is a = 2*x/t2. You can do this yourself without doing any conversions by taking the 60' time T and evaluating 3.73/T2 to get the acceleration in units of "g", the acceleration of gravity. The number comes from 2*60/32.17 where 32.17 is approximately g in ft/s2.


Drag racing is about acceleration, not top speed. Quick at the start means you are at a higher speed sooner, so you cover the quarter mile in less time. The fastest run did not have the highest speed (which was over 334 mph).

You might wonder how it is possible for a car to accelerate at over 5 g's. If I told you that the car is going almost 100 mph after traveling just 60 feet from a standing start (roughly zero to 60 in just over 0.5 seconds), you would think I was making this up. One key is having an engine that makes about 1000 horsepower in each of its 8 cylinders, while another is having tires with really soft rubber that has a coefficient of friction (when in contact with a carefully prepared concrete surface) in excess of 5.

It helps that "Top Fuel" refers to the use of a mixture of 90% nitromethane and 10% methanol as fuel. (I think they are back to using 90% nitro.) Like in high explosives such as nitroglycerine, the nitrogen in nitromethane acts as an oxidizer. This fuel needs very little air to burn, so you can stuff more fuel into the engine than with gasoline (less air means more fuel, so they can burn several gallons per second) and it detonates almost like dynamite. Seriously, these cars literally explode off the line. You have to feel it (the shock wave hitting your body when the throttle opens) to appreciate it.

The real trick, however, is in the clutch. They simply drop the clutch with the engine at full rpm. The engine runs at a constant rpm corresponding to 330 mph, and the clutch slips at a rate that keeps the tires stuck to the pavement. This is done with a very complicated, yet entirely mechanical (no electronic computers here!) multi-stage clutch system. They first drop a tiny disk that slips a lot, then more, then more, until it goes to full lockup at the top end.

[Correction: See comment posted today. I was given misinformation somewhere along the line ... You can get a fair description of the centrifugal clutch described in the comment by looking at the lines about "racing karts" in the Wikipedia entry linked here.]

By the way, sometimes they literally explode, but safety rules and good engineering keep that to a minimum and kevlar blankets and straps help confine any debris. Nevertheless, they do have to rebuild the engine completely, including replacing all of the head bolts, after each 5 second run even when all goes correctly. During eliminations, this engine rebuild has to be completed and the car ready to run in 90 minutes. That alone is something you have to see to believe.

Other details:

Top Fuel cars must weigh at least 2300 pounds after the race, should you want to work out the forces involved.

I am using the standard acceleration of gravity of 9.80665 m/s2.


Nitrohelper said...

The clutch has five 10" discs, the engine revs to 8200 rpm at the start.The clutch is full centrifugal & has six levers engaged prior to stepping on the throttle. The rpm's are pulled down to around 6800 at about 2.5 seconds,and then climb back up to over 8000 at the finish line. The clamping force on the pressure plate is increased during the run by a hydraulic piston that moves away from the plate at a controlled rate to allow 12 addition levers to add clamping. All the levers are engaged after three seconds. Then the amount of adjustable weight (grams) on the levers determine when the clutch locks-up.

Doctor Pion said...

Thanks! Because it is a centrifugal clutch, that means they "pedal" it with just the throttle?

Nitrohelper said...

That is correct, the cars have a clutch pedal that holds the six primary levels in a neutral position for putting the car in reverse&forward and to prevent heat build-up. When staging the car,after both cars are pre-staged, the driver takes their foot off the pedal, the engine is then pulled down about 300 rpm. the driver then stages with the brake, and hits the throttle when they see yellow. The engine goes from 2500 rpm (idle) to 8000 in two tenths of a second. At any rpm above idle it's not possible to push in the clutch pedal.

Baba Doodlius said...

It's my understanding that they designed the space shuttle to have a mild acceleration on purpose, to avoid squashing astronauts like big, white-suited bugs.

by the by, you're a physics guy - do you happen to know anything about the QTL?