### Sebring & Sun: The Races

#### By dcb on Mar 21, 2005

My son, my dad, and I went to the 12-hours of Sebring
race yesterday! It was a great time... perfect weather and lots to see
and do and experience. Oh, and there was a car race as well :-) The
Audi R8 has owned this race for years! The above isn't the actual car
that won, but the same model. It beat out Corvette CR-6s, the new Aston
Martin DBR9s, Porsche 911s, Maserati MC12s, Lola EX257s, Saleen S7Rs,
Dodge Vipers, Ferrari 550 Maranellos, and others, After 6 years,
Audi will debut the R10 next year, which should secure the gold for
another generation.

If you haven't been to a car race before (I must be one of the few
that hadn't), Sebring is the first and longest race in the annual American
Le Mans Series. It's a "street race" (unlike Daytona's NASCAR style) on
a 3.7 mile loop
on an old WWII airfield with flat hairpin corners that require
strategic down shifting and braking. The longest straight segment allows
cars to approach 200mph! Average lap speed can exceed 120mph. This year 37 cars started (and 18 finished)
the 300+ lap race. Each car has a pit crew and several drivers. There
are four classes of cars (each capable of different top and cornering
speeds) racing together, making it a very different race than, say, the
Indy 500.

Like a golf tournament, there are 4 days of racing
(Wed-Sat) leading up to Saturday's main event. Many thousands of people
lined up on Wednesday to drive their RVs and U-Haul trucks and School
Buses and other interesting forms of transport onto the grounds for an
extended campout/party. Some built scaffolding along the course on
which they placed sofas! It's March after all - Spring Break time.
People watching was as entertaining as the race itself! A friend of
mine has gone every year since 1959.

There were also many
interesting "sponsor" displays, including race cars in various stages
of (de)construction, allowing fascinating views of the internals of the
cars.

As we watched the cars fly by our viewing area (we set up
our canopy and chairs on a grassy knoll between turns 6 and 7), my dad
and I discussed the stresses involved in engines that propel
these cars ~1300 miles at an average speed of well over 100 mph, much
of it accelerating out of a total of over 4000 turns over 12 solid
hours. Each piston cycles about 5 million times! We wondered, at red
line speed, how fast those pistons move (12 hours is a
long time to talk :-). Are those piston heads traveling faster
(inside the cylinder) than the car itself? Reflecting on riding my bike
at 20mph, I knew my feet (acting like a piston) don't move that
fast... so I guessed that car pistons at red line would move slower than a
car's top speed.

Well, I had to figure it out. The math is easy.
Obviously piston speed is tied to RPM and Stroke Length, and just indirectly related
to the car's forward motion (you can spin the engine in neutral at a
standstill). At one of the displays I found out that the typical stroke length for these types of engines is
2.5-3.5 inches, and the typical red line is 8,000-10,000 RPM. Note that
RPM measures the crankshaft rotation rate, not the camshaft, which is
rotating at half that speed, as shown in this animated GIF:

Since speed is distance/time, and since the piston head travels
2\*StrokeLength (up and down) for each cycle, the \*average\* piston speed
is:

Therefore,
a car traveling at nearly 200 mph, at the engine's red line, will have pistons
traveling at an average of only around 51mph! I was right.

But wait... that piston starts and stops 18,000 times every minute, accelerating to
the next stop just 3 inches away (at a huge "G" force). Average speed does not
really answer the question. I had to figure out the peak piston
speed. Since it literally explodes from a dead stop (it's a combustion
engine after all), possibly the peak speed exceeds the speed of the car?

It
turns out that the peak speed equation is complex, with sin/cos kinds
of rotational acceleration factors. I took the easy way and looked up
stats from several types of race car engines using Google. The ratio of
peak to average piston speed is consistently very close to 1.6:1.
So at Sebring, the pistons never traveled faster than about 82 mph.

Even
a Formula 1 (NASCAR) car, with an engine that can red line at 19,000
RPM,
that has a stroke length of about 1.65 inches and a top speed ~240 mph,
will have pistons that average: 2\*19000\*1.65 = 60mph, and peak
at: 95mph. See footnote below.

So, why did I include "Sun" in the Subject Title? Check out the Rearview Mirrors. We're a sponsor!

http://lordcaffeine.com/wordpress/index.php?cat=2*At
19,000 rpm, 316.7 revolutions and 1,583.3 ignitions take place each
second in the BMW F1 engine. 9,500 engine speed measurements are made,
the pistons cover a distance of 25 metres, and 550 litres of air are
drawn in. In the P84, maximum piston acceleration was 10,000g. Peak
piston speed was 40 metres per second.*

Posted by

Mike Mon March 23, 2005 at 09:07 AM EST #Posted by

Dave Brillharton March 23, 2005 at 09:23 AM EST #Posted by

c.cloveron June 19, 2005 at 06:52 PM EDT #Posted by

Jean Winchesteron July 01, 2005 at 05:00 AM EDT #