Time, Distance, Speed and HorsepowerUpdated: Mar 2, 1999
I catch him in the corners, but he just pulls away from me on the straights... sound familiar ? The racetrack can be the source of some very interesting illusions. One of the most common is the time versus distance illusion: Time Versus Distance Suppose that in the middle of a second gear corner you get within one car length of the car you're chasing. That's about 20 feet that your nose is behind the nose of the car ahead. For this example, let the corner speed be 40 mph for the slow spot of a second gear corner. At 40 mph you're covering about 59 feet per second. So, when the nose of your car is 20 feet behind the nose of another car at 40 mph, you are 0.34 seconds behind. Now, imagine that you accelerate exactly the same as the car ahead, and that you exit with exactly the same cornering speed, keeping exactly 0.34 seconds behind the other car. As you accelerate down the straight, keeping always exactly 0.34 seconds behind, the distance between the two cars increases. For example, if you stay behind by exactly 0.34 seconds, when you reach 4500 rpm in fifth gear, about 110 mph, the cars will be travelling 161 feet per second, and your separation will become 54 feet. It looks like you need a new motor, but its just an illusion, you were a constant 0.34 sec behind. It looks like he is pulling away... but he isn't. The car ahead seems to pull away, but in reality, the time separation has never changed. Just an illusion. But here is another illusion that can be even more difficult to envision... imagine that you are right on the tail of the car in front of you, zero separation coming out of a 40 mph turn. That means that the nose of your car is about 10 feet behind the nose of the car ahead. So, at that instant, you are 0.17 sec behind the car ahead. Then, if your cars are exactly matched in horsepower and aerodynamics, the car ahead will keep its nose 0.17 sec ahead of you all the way down the next straightaway. At 110 mph that will be a distance of 27 feet. So, it looks like the car ahead pulled away from you.... gees where did he get that horsepower... but in reality, you were a constant 0.17 sec behind. Think time... not distance. In reality, the drafting effect will reduce the aero drag of the following car more than the leading car, so if you have matched cars, the following car will actually have an advantage, but that's another story...
For a new driver like me, one of the toughest issues is corner exit speed. It doesn't take much speed difference at the corner exit to make a big difference in position by the end of the next straightaway.
Corner Exit Speed This one is not an illusion. Corner exit speed is king. Based on the performance of a typical spec racer, as documented on my Dyno Test page, it is possible to calculate the time and distance separation of two identical cars when the only difference is the corner exit speed. I won't bore you with the math, but I use a program called MathCad to perform the calculations. If two identical cars exit a 2nd gear corner with a 2 mph speed difference, here's what happens as they head down the straightway:
As you can see, Car number 2 is at a distinct disadvantage due to only a 2 mph difference in corner exit speed. At the end of 1000 ft from the corner exit, car number 2, which had the slower corner exit, is 0.103 seconds behind car number 1. At their speed of 94 mph, that will be a distance of about 14 feet. On many tracks, this scenario can be repeated many times per lap, leading to lap time differences of up to a second a lap. Just by exiting every corner 2 mph faster.
Corner Exit Speed Versus Horsepower In the previous examples, the cars have been assumed to be identical. But, what would happen if car number 2 had more horsepower ? In the following calculations, Car number 1 has 100 hp and exits the corner at 50 mph. Car number 2 has 102 hp but exits the corner at only 48 mph. Here's what happens to these two cars as they head down the straightaway:
From this data, you can see that it takes more than 2 horsepower to make up for just a 2 mph difference in corner exit speed. At the end of 1000' from the corner exit the slower exiting car is still .067 seconds or about 9 feet behind, despite having 2 more horsepower. The additional 2 horsepower is just not enough to catch the car that exited the corner only 2 mph faster. There are three very important things to learn from this: corner exit speed, corner exit speed, and corner exit speed. And don't forget, an additional 2 horsepower is difficult to get out of this stock engine. If you could get an additional 5 hp, here's what would happen:
From this data, you can see that a slow cornering car is gonna need to have about a 5 horsepower advantage in order to catch up with a more skilled driver who can exit the corners 2 mph faster. In actuality, the speeds shown above are probably a few mph faster than a spec racer will really achieve, since I have neglected the windage losses in the transmission, and those losses are significant at high speed. However, even if the top speed calculations are off as much as 5 mph, the relative times and distances are still within a few percent of the numbers shown here. The moral of the story is that a small increase in corner exit speed will
have a greater affect than a couple more horsepower. Richard Shelquist |
