Why You Won’t Find Carbon Fiber In Most Land-Speed Record Cars

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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.

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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!

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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.

Aerodynamics

https://www.instagram.com/p/CwfwFWIsKaS/

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.

The Model

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

Where:

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):

Model Inputs

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:

Model Outputs 1

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:

Model Outputs 3

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:

Model Outputs 4

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:

Model Inputs 2

Now, the tractive force will be:

Model Outputs 5

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:

Model Outputs 6

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:

Model Inputs 3

Now, our acceleration results become:

Model Outputs 7

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.

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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:

Model Inputs 4

Here are the drag and tire force results at 200 mph:

Model Outputs 8

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:

Model Outputs 9

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:

Model Inputs 5

Here are the drag and traction results:

Model Outputs 10

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?

Model Outputs 11

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.

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87 thoughts on “Why You Won’t Find Carbon Fiber In Most Land-Speed Record Cars

  1. Thanks for another great article Huibert – the engineers involved with LSR stuff are absolutely amazing.

    I still remember Richard Noble coming to my school after setting the land speed record in Thrust 2 (I think he was fundraising to keep the car in the UK, it ended up on display at the Coventry Transport Museum). Meeting the man and seeing the car were like encountering an alien and their spaceship; 633mph seemed like such an unimaginable number.

  2. First of all, thanks to Huibert.

    A very interesting design was the Carbinite LSR.
    The wing has a neutral profile for high speed and flaps are used to provide downforce. Not sure, but I believe that flaps have to be actuated by hand since the rulebook does not allow actuators controlled by computer.

  3. The Bloodhound Land Speed Record project https://www.bloodhoundlsr.com/ which has been trying for over 15 years to break 1000 mph (rocket engine, solid metal wheels and extensively carbon fiber, but an awful lot of titanium and aluminum too.) They’ve managed to hit “only” 628 mph in testing thus far. Turns out it takes a lot of money to got that fast, one of the biggest engineering issues being the behavior of the car at subsonic vs. supersonic speed. The project has gone bankrupt more than once.

  4. Top speed is a matter of aerodynamics, power and final drive ratio. Speed record attempts are already expesnive enough (don’t their tires cost a fortune ?), no need to pay for useless stuff.

     The “golden age” of road top speed frenzy was the 80’s – that was a time when, at least in Europe, sports cars magasines were making a big deal out of the top speed of a vehicle and would take points away for “not enough”.

    The 80’s were incidentally the time when Cx became a thing too, and started hovering around 0.30 and lower for production cars. The Audi 200 (was it the 5000 in the US) could do 230km/h (143-ish mph) with 182hp. The Opel Calibra (never sold in the US) was good for 150mph with 204 hp (and a very low Cx of 0.26). The regular non-turbo Calibra (115hp) was good for 125mph. I once was driving behind one in France, it took the guy literally minutes to get there, but he did.

    Both were heavy cars for their time.

    On the flip side, my 1990 Euro Spec CRX (non-vtec ZC engine) was a little rocket, but I was never feeling too comfortable above 190km/h, it was simply a bit light to my taste. We were discussing cars a lot with a friend back then and agreed that autobahns or not, a sub-1000kg car like the CRX is not born for 200km/h+ speeds, no matter it could go there, and had that unofficial “rule” that you’d have to be above 1200ish kilograms at least to safely cruise at 200km/h+ speeds in all conditions.

    The Euro spec CRX vtec (the equivalent of the JDM SiR) could do a real 220km/h, and I wonder what it must have been at these speeds. It is true it was a good 80kg heavier than the ZC (going symbolically above 1000kg), but that was all in the front, so the rear was probably behaving worse.

    Incidentally, all 1.6 dohc CRXes (vtec or not) in Europe came with a non-powered, old-Cadillac steering ratio of 4 point someting steering turns. The one thing you could not do in these was quickly and comfortably compensate an oversteering – you had to turn that wheel a million times.

    The lowly, much less expensive 1.4 Civics of the same generation came with a nice, fast, 3.5 turn to turn, powered, steering rack.

    The same rack came in the JDM SiR CRX (factory-limited to 180km/h like all Japanese domestic market cars at the time).

    My little theory on this is that the Euro specced 1988-1992 Civics were simply too fast and too short in their DOHC versions to have a fast and direct steering slapped on them. This would throw you off the road at high speeds.

    Add to that that the CRX’es wheelbase was another 10 inches shorter than the already short Civic’s wheelbase (which is why in local Rallye races people would always prefer to run Civics vs CRX’es, although it was pretty much the same car).

    The Peugept 309 Gti 16 (160hp for under 1000kg) was a legend, did a real 220km/h, and reviews said (and praised) the fact that it was “not a beginner’s car” at speeds above 200km/h (to be read as “bless the 90’s, no manufacturer would dare selling this thing nowadays by any means“).

    The short story – light weight is often the ennemy of high speeds.

  5. Weight is king! I talked to a guy with an Opel GT powered by a blown big block Chevy (you read that right). He had that tiny car ballasted to over 4000 lbs.

    My friend who was racing a street bike filled the rear swing arm with lead shot, the number plates were made from half-inch steel and he had another one-inch steel “skid plate” under the chassis.

  6. Great Article! Thank you.
    I highly recommend attending Speed Week at least once in your life. It is a motorhead’s dream. You can walk right up to the cars and chat with the (very brave) drivers. Everyone is generally pretty chill and happy to tell you about their custom creations.

    Well… almost everyone, I ran into Boyd Coddington there one time. He was definitely the the Boyd Coddington of the group. Funny that he couldn’t find anyone willing to help him un-stick his $500k motorhome axle-deep in the brine.

    1. It’s really an awesome event to check out at least once. While we were in staging I saw Rod Millen getting ready to run. He had pretty well stripped down to his undies (somewhat hiding behind the door of the car) to change into his fire suit.

      Can’t say I expected to see that, but it was definitely what I’d classify as a more open experience than most race venues.

  7. 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 “

    That’s exactly what it is. Reducing weight matters if you want to speed up, slow down and take corners fast.

    With a land speed car, it’s almost all about the aero… and having enough power to go fast of course.

  8. Also, a lot of land speed cars are hand built in garages, hot rod shops, etc. Places where a good craftsman can form aluminum but would rarely dabble in casting carbon molds or doing hand layups. Work with what ya got….

    Apologies if it is mentioned already (meeting in 5 minutes, had to skim, will come back and nerd out later)…..

  9. That reminded me of an excel spreadsheet I did during my last internship where I studied the impact of several parameters on the performance of a turbojet.

    Huibert, you could have a VBA code run several simulations with your excel and show them as an array of curves, displaying the influence of one or more parameters in a single graph. It’s a great way to show sensitivity to a parameter and highlight non-linearities.

  10. Huibert,
    I was under the impression that the really fast cars were running on solid aluminum wheels with either a little bit of polyurethane in the form of buttons on the wheel or simply some texture machined into the rim of the wheels and no tire at all. I would think that a solid aluminum wheel would have much less friction than a pneumatic rubber tire. Or even those fancy “tyres”.

    Anyway, going from relatively grippy rubber tires to bare aluminum at the higher speeds must make for an interesting in-between zone of performance.

    I hear that some folks are trying all wheel drive. Also didn’t someone try tracks like on a snowmobile once? All I can find is this
    https://m.youtube.com/watch?v=93c0cWCou-k
    Note: sitting on top of a 211 mph snowmobile on the salt is beyond bonkers.

    1. Hugh, solid aluminum wheels are pretty much required for rocket powered cars but not for wheel driven ones. The friction coefficient would be much too low. Little rubber or polyurethane nubs are being tried by some teams and may prove fruitful but I didn’t see anything like that this year.

  11. A very excellent article. The lack of carbon fiber in LSR cars is not something I’ve ever thought about, but it does make a good bit of sense.

  12. Thanks for stirring up the brain cells again.

    That air density is cool for aero, but just horrible for conventionally inducted internal combustion engines. I remember wondering why a guy with a custom bike with a similar engine build to mine ran so poorly in the results a few years back and then the light bulb in my head illuminated and I went oh yeah less oxygen.

    1. It’s amazing what the lack of oxygen does for these motors. I was surprised how many seemed to be running rich, presumably because the owners didn’t compensate the carb jettings for the higher altitude.

      1. More likely they were running rich so that after 5 miles of wide open throttle they don’t overheat. And because as long as the engine doesn’t overheat or stall in the pits probably nobody cares. Come to think of it, running really really rich at the end of a run would cool down the engine nicely and normal heat management slows the car down at speed.

  13. Great article. I think a good part of it, too, though is that these aren’t teams with lots of sponsorship money and CF is terrible ROI—CF is expensive, requiring more expertise and tooling. Metals are also much better known quantities, can be repaired and modified more readily and in the field, inspected for stress or other damage, and they tend to fail less suddenly.

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