One of the big car-geek events that has been on my bucket list is Speed Week, held every August at the Bonneville Salt Flats in Utah. The same place where Gary Gabelich set a record speed of 622.407 mph on October 23, 1970, is also the site of an annual event where anyone can bring pretty much anything they want to and drive it as fast as they possibly can. There are, of course, safety-related rules must be followed, but otherwise it is a run-what-you-brung type of event. The whole thing is organized and run by the Southern California Timing Association (SCTA) and each year attracts around 500-550 cars and motorcycles of every description imaginable. There are production based sedans, pickup trucks, production motorcycles, custom bikes, streamlined bikes, bikes with sidecars, antique roadster-based cars, tank cars (sometimes called Lakesters), and the stars of the show: full custom streamliners. You see a bit of everything on the Salt Flats, but what you rarely see is carbon fiber — a material found in almost all of the fastest cars these days. Here’s why.
No Signs of Carbon Fiber
While spending the week walking around the pits and looking at all the different cars and the way they were built, I noticed something odd. Race cars, especially the ones playing at the top of their field, are usually showcases of many innovative and new technologies. Look at any recent Formula 1 or Indy car and you will see copious use of carbon fiber, Kevlar and other high tech materials in an effort to keep weight down and make the car as fast as possible. Looking at these land speed record (LSR) cars, I saw none of that. The only concession I saw to light weight were aluminum body panels, hand beaten over a streamliner’s frame. Why? Is light weight really not critical to these cars? The streamliner with the aluminum body panels still weighs over 5000 lbs; that’s quite beefy!
Do You Need It?
The more I thought about it, the more I began to wonder if light weight might actually be a detriment to high speed runs. Remember, Formula 1 cars have to accelerate and brake quickly and corner at high G levels. Land Speed Record (LSR) cars don’t have to do any of that — they drive in a straight line and have several miles to get up to speed and several more miles to come to a stop at the end. Formula 1 cars also drive on nicely paved tracks with lots of grip. LSR cars run on salt, which although it seems to stick to absolutely everything, doesn’t provide much grip for tires.
The other factor that comes into play with LSR cars is their extreme high speeds and their aero implications. At 300-400 mph, aerodynamic drag becomes the dominant factor determining how much faster you can go. It takes a lot of force to push a car through the air at those speeds, and all that force has to come from the tires pushing the car forward.
At some point, the force of the drag trying to slow the car down will equal the force the tires can provide to push the car forward. When you reach that point, you cannot go any faster, no matter how more power your engine might have. All that extra power would just cause the tires to spin but wouldn’t make the car go any faster. At that point, what you want is more tractive force from the tires, and since you cannot make the salt any stickier, the only thing you can do is push down harder on the tires to get more friction between the tires and the salt.
You could do this by adding wings, like a Formula 1 car does, but wings add drag. The best and easiest way to increase the friction between the tires and the salt is to make the car heavier. Since we have plenty of acceleration space, the extra weight probably won’t slow us down. In fact, if we have enough power in our engine, the extra traction the weight gives us will help us accelerate quicker and counteract the impact of the added weight. There are limits, of course. We don’t have infinite space to accelerate, and even though our acceleration space is measured in miles, when you’re trying to get up to 400+ mph, those miles get very short very quickly. It becomes a delicate balance between weight, engine power, and aerodynamics.
To look at this in more detail, I built an Excel model of a LSR car so I could change various parameters and see what their effect would be. The equations are pretty straightforward; the most complicated one is for calculating aerodynamic drag:
Drag = 1/2 x rho x V² x Cd x A
rho = the density of the air
V = the vehicle velocity
Cd = Coefficient of drag as measured in a wind tunnel or calculated using CFD software
A = frontal area of the vehicle, i.e. the area of the vehicle when seen from the front.
Other things we need to take into account are the friction coefficient between the tires and salt, the weight of the car, the weight distribution between the front and rear axles, the diameter of the tires as well as the tire rolling diameter (SLR), and the tire rolling resistance. If we later want to figure out how much engine power we will need then we also need to know the axle final drive ratio as well as the gear ratios in the transmission. Here are all the inputs we need for our model along with some initial values for a streamliner running at Bonneville (7,000 ft elevation):
With these values, we can start calculating things like the aerodynamic drag as well as the tire drive force. For the vehicle shown above, these numbers are:
As you can see, for a vehicle that weighs 5,000 lbs with a drag coefficient = 0.15 and frontal area = 0.72 square meters, which is quite possible for a streamliner, the drag at 400 mph would be 1588 N while the tire tractive force would be 4445 N. Clearly, the tractive force is more than enough to propel this vehicle up to 400 mph and above as long as we have enough room to get up to that speed.
Next, we can calculate how long it would take for this vehicle to get up to 400 mph and how far it would need to travel to get there, assuming we have enough engine power to keep the tires at their limits of grip at all times. This becomes a little more difficult to do since aerodynamic drag increases as speed increases. Since the force available to accelerate the car is equal to the force provided by the tires minus the aerodynamic drag force, the total acceleration force slowly decreases as speed increases:
From this graph you can easily see that if we were able to continue increasing speed, at some point, the available acceleration force would drop all the way to zero. This is the point where the aerodynamic drag is equal to the tire tractive force and no further acceleration is possible. We will have reached our terminal velocity. With this data, we can now calculate how far the vehicle has to travel in order to get up to speed:
It takes more than 6 miles for our car to get to 400 mph. If we had the salt flats all to ourselves, this might be acceptable, but for Speed Week, this is way too far. The tracks the SCTA normally lays out on the salt are 9 miles long: 1 mile for acceleration followed by 4 timed miles and finally 4 miles to stop. Speed is measured as an average over each of the four timed miles and the speed that is recorded is the fastest of those four readings. So, in reality, you have 4 miles to accelerate followed by the last timed mile.
Given this track payout, if we want our car to go 400 mph at speed week, we need to get up the speed within 4 miles. Clearly, our car will not get there. From the graph, at mile 4, we are only going about 330 mph and at mile 5 miles we are going about 360 mph, so our average over the last timed mile would only be 345 mph.
So, how do we get our car to accelerate quicker? We need more traction from our tires. We could try making our car heavier to give the tires more traction. Let’s make our car 1,000 lbs. heavier:
Now, the tractive force will be:
Tractive force has increased from 4445 N to 5334 N. Aerodynamic drag is still the same so we should have more acceleration force available now. Unfortunately, we also have a heavier car to accelerate and the result is no better performance:
We still need over 6 miles to get up to 400 mph. There is a very slight improvement, but this is because the drag force is a smaller percentage of the available tractive force, not because we’ve made a significantly better vehicle.
We need a way to increase the tire tractive force without making the car heavier, and the way to do this is to shift the weight around, so we have more of the vehicle weight sitting on the drive wheels. Assuming we have a rear-wheel-drive vehicle, we need to move some weight rearward in the car. Let’s assume we can move a bunch of heavy stuff to the rear and increase our rear weight distribution from 50% to 60%. We’ll also drop our weight back down to 5000 lbs since the weight increase didn’t work:
Now, our acceleration results become:
We’re still not quite there. It’s still taking 5 miles to get up to 400 mph, but you can see how playing with these inputs can shift the results and how the overall weight and weight distribution can have a significant impact on our performance.
Now, let’s look at a more common example. Most of the cars at Speed Week are based on common production cars. These cars have a much higher coefficient of drag and much larger frontal area. They also start out lighter than our streamliner.
Of course, we aren’t going to go 400 mph in one of these, but is it possible to go 200? Let’s find out. Here are the inputs for our new car:
Here are the drag and tire force results at 200 mph:
Obviously, the drag force is far greater than the tire tractive force which tells us we can’t get there. If we look at the acceleration graph, we see the same thing:
We can’t get to 200 mph, but we can quite easily get to 160 mph before we start to lose traction. That tells us there’s hope. We just need more tractive force. What would happen if we made our car a lot heavier? Let’s add 2000 lbs:
Here are the drag and traction results:
Our tractive force is now greater than the drag so we should in theory be OK, but do we have enough space to get up to speed?
Yes, we do! We can even go a bit faster than 200 mph. In fact, by the time we get to mile 4, we will be going over 220 mph! And all we did was make our car heavier. We did nothing else. Two thousand pounds is a lot of weight to add, and we wouldn’t need all of it if all we wanted was to just break 200 mph, but you get the point. More importantly, adding that much weight means adding a lot of ballast, and we can decide where that ballast goes. If we put most of that weight towards the rear of the car, then at the same time that we are adding weight, we could also increase the rear axle weight distribution and get even better performance.
Each of these models assumes we are talking about wheel-driven cars and that we have an engine that has the horsepower needed to keep the tires at the limits of traction or else all bets are off. If we don’t have the power, then we certainly won’t get the speed. But what these models do tell us is that saving weight in LSR cars by using exotic materials is actually counterproductive. The only time it might make sense is if the only way to get the speed you want is to change the weight distribution. Then, using lightweight materials might let you add ballast in places that give you the weight distribution you want. If you don’t have that problem, go ahead and make your LSR car heavy. It could actually make you faster!
One last point. Because of the tire traction limitations we’ve been discussing here, there is a limit to how fast wheel driven cars can go. So far, the record for a wheel driven car is 503 mph set by the 4,950 pound pound Team Vesco Turbinator II on October 1, 2018 at Bonneville. If you want to go significantly faster, you have to move up to a rocket powered car in which case the tire question becomes moot. Your top speed now just depends on the power of your rocket motor.