Pocono: Slightly Shifty

The big news for Pocono is that drivers can shift…again.  Which brings up the obvious dual questions of: Why would you want to? and Why didn’t you before?

Compare how fast the wheels have to rotate with how fast the engine rotates.  Both are measured in revolutions (or rotations) per minute – rpms.  Assuming a tire circumference of 88.6 in, tires have to rotate from 417 rpm (at 35 mph), to 1490 rpm (125 mph) to 2146 rpm at 180 mph.  The graphic tachometer on television tells us that the engine runs between 7000 rpm and 9500 rpm most of the time.

You can’t connect the engine directly to the wheels because of the difference in rotation rates.  This is where the gears come in.  A car has two sets of gears:  The first I’ll talk about is the rear end gear, which I seem to remember is somewhere around 3.8 or 3.9 for Pocono.  The rear gear reduces the rotation rate coming from the driveshaft and sends that rotation to the wheels (as shown in the diagram).  A 4.0 gear would produce a rotation rate coming out of the gear that is 1/4th the rotation rate coming into the rear gear.  If the driveshaft is rotating at 5000 revolutions per minute (rpm), the wheels would be rotating at 1250 rpm. (A 4.0 gear would mean that for every four rotations coming in, one rotation goes out.)

With a 4.0 rear gear, your engine would have to change speed from 1600 rpm to about 8500 rpm going from 35 mph to 180 mph.  The problem is that an engine produces its maximum power over a narrow range of rpms.  (It also produces its maximum torque over a small range of rpms, although not the exact same range as the maximum power band.)  You’d like to have the engine operating in the target range all the time.

This is why you need a second set of gears, which are found in the transmission.  This series of gears (usually 4, 5, or 6 different gears) gives you different sizes so you can keep the engine running near its sweet spot — regardless of how fast you’re going.  Fourth gear on most transmissions is 1:1, meaning that there is no speed change through the transmission.  On a passenger car, like the one from the gearing figure, the higher gears (overdrive) reverse the ratio.  0.50:1 means that the rotational rate coming out is higher than the rotational rate going in.  NASCAR prohibits overdrive.

In trying to go faster and faster, teams were moving their engine’s target range to higher and higher rpms – which means higher and higher costs.  In 2005, NASCAR instituted a gear rule to keep engine speeds (and thus cost) down.  NASCAR gives you a limited choice of rear-end gears and dictates the transmission gears as well.  Those choices keep the maximum engine rotation rate below about 10,000 or 10,500 rpm without having to implement a difficult-to-enforce engine rule.

NASCAR changed the gear rule for Pocono this year.  First gear can be anything you want.  Second gear can be 1.70:1 or greater, and – this is the big change – the third gear limit changed from 1.28:1 to 1.14:1 or greater.  Fourth gear stays at 1.00:1.   (“or greater” means that the first number may be larger, but not smaller.)  NASCAR still doesn’t allow overdrive.  Normally, the rule book prohibits gears between 1.00:1 and 1.28:1 except for road course events.

Why Pocono?  Most oval tracks have four turns, with the frontstretch and backstretch close to the same length.  Pocono has three turns and three straightaways:  a frontstretch of 3740 feet, a backstretch (Long Pond) of 3,055 ft and a short straight of only 1,780 feet. You can imagine that the rpm the car reaches is very different coming down the two long straights (i.e. coming into turns 1 and 3) compared to coming down the shorter straight (i.e. into 2).  What you’d like is for the engine to be turning at about the same rpm into each turn.

It seems like NASCAR’s change is too small to be meangingful – from 1.28:1 to 1.14:1 is only 0.14, right?  Actually, it’s a factor of two.  What makes a difference is how much above 1.00 the gear is.  The important thing about moving from 1.14:1 to 1.28:1 is moving from 14 to 28.

For the sake of argument, let’s say the engine is ideally in 7200 rpm in fourth gear.  When you shift to third, a 1.28:1 gear (which used to be the lowest for third), requires the engine to run at 9216 rpm (=1.28*7200) to maintain the same speed.  That takes you far away from the best rpm range for your engine.  Changing from 1.28:1 to 1:14:1 means that third gear only requires your engine to run at 8208 rpm.  That may seem like it is still a big shift; however, given the way the power and torque curve vary with rpm, it’s small enough to mean that you’re close enough to your power band for it to work. It’s a shift of about 1000 rpm instead of 2000 rpm with the 1.28:1 gear.  That gives the engine shop – and the driver – some interesting options.

This type of a rules change is, in my opinion, exactly the direction NASCAR ought to be moving to open up areas for people to be innovative.  It’s a relatively minor change in terms of enforcement.  It keeps the teams from pushing into the higher rpm ranges (and thus steeply pushing up engine costs), but it allows the engineers and the drivers to pursue different strategies.  For example, most drivers will be shifting in turns 1 and 3, but others (like Denny Hamlin) plan to shift only in turn 1.  Another aspect is how shifting affects fuel mileage.  Overdrive gears are there because the more rotations an engine makes, the more friction it has to overcome.  And, as Carl Edwards points out, every time you shift, you run the chance of screwing up and damaging the transmission.  Most NASCAR drivers aren’t used to shifting this much during a race.  Do you try for what might be a small advantage and shift at the cost of possibly screwing up the transmission?  Do drivers like Marcos Ambrose, who have a lot more experience shifting, have an advantage?  Does the engine shop adapt different strategies for drivers who are comfortable shifting compared to those who are not?

Unfortunately, this rule really makes a difference only at Pocono due to it’s unique configuration.