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If you’re tracking a car in HPDE or racing in a series where weight doesn’t matter, obviously make the car as light as possible. As Colin Chapman said, “Simplify, then add lightness.” But some racing series are based on power to weight ratio, and in this case, lighter is not always better.
The NASA Super Touring series, Gridlife, Pineview Challenge Cup, and many others use the weight of the car divided by horsepower as the primary balancing factor. And why not? Mathematically, that seems like a good way to create parity between different cars. Well, if they were all racing in a vacuum.
In the real world, air has resistance, and overcoming resistance requires power. Drag increases at the square of the speed, so there’s about twice as much drag at 100 mph as there is at 70 mph.
Let’s take a look at two cars with identical shapes. They both have a frontal area of 20 square feet and a coefficient of drag of .48 (typical race car with windows open). The cars differ in weight, one is 1800 lbs the other is 3000 lbs.
If I input those numbers into the RSR calculator, I find out that it takes about 81 hp for the lightweight car to reach 100 mph, and about 85 hp for the heavyweight car. So weight is not a large factor here. For both cars, 60 hp is lost to drag.
Now I’ll increase the speed to 120 mph to simulate the back straight on the closest track to me, Watkins Glen. The lightweight car needs 137 hp to reach that speed, while the heavy car needs 143 hp. For both cars, over 110 horsepower is lost to drag. If these cars were in the same class based on 12.5 lbs/hp (GLTC or NASA ST4), the light car would have 7 hp remaining at 120 mph. The heavy car would have 97! Because these cars have the same power to weight ratio, they both accelerate at the same rate. But once they reach 120 mph, one car can barely maintain that speed, while other has power to spare.
In other words, at a faster track, you want power at the expense of weight. To illustrate that, let’s see what happens in OptimumLap when we add power, but keep the power-to-weight ratio the same. In the following table, all cars are at 20 lbs/hp. I’ll run simulations on three very different tracks, Watkins Glen, New York Safety Track, and Pineview Run. (Conveniently all within 100 miles of me.)
As you can see, the heaviest car is the fastest at every track. It’s a small difference at Pineview, and a significant 1.5 seconds at Safety Track, but a whopping 4 seconds at Watkins Glen. The simulator shows the difference in top speed is 12 mph at WGI. Wow. A difference of 4 seconds and 12 mph with cars that are supposed to be equal!
But don’t lighter cars corner faster? Not really. Friction (grip) depends on weight, and the more weight, the more friction. Don’t lighter cars stop faster? Not really. A lighter car still has less grip because of less weight. The simulator factors all this in so we don’t need to.
But it’s worth noting that OptimumLap is a single-point-mass calculator. It can’t factor in elevation changes, track camber, or the fact that your car has four tires. When cornering, the outside tires are loaded more, and on a heavier car, even more. At some point you get diminishing grip from the outside tires, and so lighter cars do grip and change direction better than heavier cars. I just can’t simulate that in OL.
Aerodynamic lift affects lighter cars differently than heavy cars, as well. If your car makes downforce, then the lighter car has an advantage. If your car generates lift (most street cars have a Cl of around .3 or .4, meaning they lift at speed), then the heavier car has an advantage.
Another way lightweight cars are handicapped is by having to run skinnier tires. In many series, a lightweight car might be limited to a 205-width tire, while heavier cars could use 225, 245, or whatever. And what’s the logic behind that? I’m not sure. As you can see, the lightweight cars are already at a power disadvantage (anywhere outside of a vacuum), and limiting their tire width only makes it worse.
Here’s the current rules for GLTC, and you can see that lighter cars are penalized with narrower tires.
|Weight||Slick||100 TW||200 TW|
|2250lbs or less||205||245||265|
|2251 lbs – 2600 lbs||225||255||285|
|2601 lbs – 2950 lbs||245||275||295|
|2951 lbs – 3300 lbs||275||295||315|
|3301 lbs or more||295||315||335|
In addition, if you win a GLTC race, they add 150 lbs to your car. On a lighter car, this is a greater penalty than a heavier car. For example, on a 2200 lb car, it would go from 13 lbs/hp to 13.9, and on a 3000 lb car, from 13 to 13.65.
In the NASA system, not only are lighter cars penalized with narrower tires, but as cars get lighter, they take additional penalties to HP. So if your car weighs 2450 lbs, it takes a .4 lbs/hp penalty. If it weighs 2250 lbs, that’s a .5 penalty. And so on. This is completely the opposite of how it works in the simulation, because lighter cars need more power to be competitive.
For example, if you race in NASA TT6, you can have a Miata at 2140 lbs and 110 hp, with 205 tires. Or you could have the same car at 2450 lbs, 128 hp, and 225 tires. Do the math and the heavier car has both a better power to weight ratio, and wider tires, and would win in a simulation and the real world.
Ideally, there should be a reverse corrective factor that balances the lbs/hp ratio for cars of any weight, so that lighter cars get a bit more power, and heavier cars less. Let’s take a look at what that factor could be. Ideally, you’d like to see the 1800-lb car, the 2400-lb car, and the 3000-lb car in the same second at all tracks, not 4 seconds apart.
If I do a corrective factor to figure lbs/hp like this:
Then I get simulated lap times like this:
The heaviest car still wins at Watkins Glen, but it’s only a 1 second difference, not 4 seconds. At Pineview, the lightest car wins, but only by 3/10ths. And at New York Safety Track, it’s mostly a draw until the cars get heavier than 2400 lbs, but then the greatest difference is only 2/10ths instead of 1.5 seconds.
Naturally the formula could be adjusted slightly, by using a different median weight (2600 lbs instead of 2400 lbs, for example), or changing the hp factor from .016 to a lower or higher number. In any case, for a series that used lbs/hp series for classing, they can make it more fair by using such an adjustment. Of course they won’t, but data supports that they should. I mean, ideally, they’d have a different adjustment for every track….
I wrote the rules for the Pineview Challenge Cup series, and those rules also use lbs/hp as the basis. Pineview is a slower track (72 mph top speed in this simulation), so there’s much less in the deltas. One of the benefits of this series is the thin rule book, so I’m not tempted to complicate things. But if we ran our series at any other track, I might make an adjustment like this.
Full disclosure, I’m a weight weeny. My street Miata with a full interior, rollbar, splitter, and wing weighs in at 2070 lbs dry. It’s just about as light as I can get it while still keeping it streetable. So I’m not the guy who says that heavy cars are better, I’m the guy who drills out his keys to save weight!
However, the reality of it is, if you race in a series that uses power-to-weight ratio as a balancing factor, you’re probably better off with a heavier car and more power. This often gets you into a wider tire size, as well. Adding lightness is adding slowness.