Frontal Area AKA How to Go Fast…

When looking at a serious gravity racer build the one thing that’s often on a designer’s mind is is how to make it fast. Assuming a steep hill and a smooth road (taking cornering and suspension out of the equation for a second) how can we make a gravity racer that goes as fast as possible?

Once you’ve done a ton of reading about physics and successful car design, you’ll find that the following points are key:

  • Rolling resistance
  • Rotational inertia
  • Aerodynamic efficiency

Rolling Resistance

Any wheel has a resistance as it rolls. This is initially from the bearings (which are never quite perfect) and from the tyre on the road. As the tyre deforms as it rolls there is a slight resistance that tries to slow it down. The “flat spot” that appears at the contact patch is essentially “pushed” around the wheel as it turns, and this movement of the rubber takes a little bit of energy to overcome.

Choosing high quality bearings and making sure that they are properly lubricated is all that you can do to minimise and losses in that area. You can spend a fortune on ceramic bearings, but I’ve read that properly lubricating a set of regular bearings has more benefit.

As for tyre losses, people typically overcome rolling resistance by using very high tyre pressures. As the pressure in the tyre is increased, the wheel becomes more “round” and the contact patch and thus the resistance associated with it, is reduced. With tyre pressure there tends to be a sweet spot – increasing the pressure makes the car faster and faster, but past a certain point a super-hard tyre will slow you down as even tiny indentations in the road will cause massive vibration as the wheels essentially have to go up and around each one. Guy Martin was in the 75psi region IIRC.

Rotational Inertia

Much like a flywheel on a regular road going car, a heavy wheel takes some energy to get up to speed, whereas a light wheel takes less energy. This means a car with heavy wheels will be slow to accelerate, as the force of gravity acting on the car takes a while to get the wheels turning and the car moving forwards. If the track you’re racing on is a short sprint, in extreme cases this may be a problem, but in practice this issue is only really a theoretical point. A point to note is that the weight of your wheels in NO WAY affects the top speed of the car. Because physics.

Aerodynamic Efficiency

Last but not least, the sticky subject of aerodynamics. Much of what I’ve read on the internet (at least initially) is just not applicable to gravity racing. A lot of what’s out there is all about generating downforce, which is all well and good as long as you have an engine to push your car through the air. Gravity racers simply need to be as slippery as possible, and while a bit of downforce at speed would no doubt help (you certainly don’t want a car that generates lift) you don’t want to be trading off too much drag for it.

So What Makes You Fast?

So here’s the important bit. Assuming speeds of 40mph plus (which are easily achievable given a steep slope and enough distance) which of the above elements are the most important? Well, in short, it’s the aerodynamics by a mile, and one specific area of the aerodynamics in particular. You can make any shape car aerodynamically efficient, but the one key factor that will have the biggest bearing (from what I’ve seen and researched) is frontal area.

Frontal Area

Frontal AreaThe frontal area of your car is essentially its “silhouette” when viewed from the front or rear. If you could do a scale drawing of the silhouette you can work out the total area of this shape, and its the total area that is very important – this is essentially the size of the hole that your car has to punch through the air. The smaller the hole, the better, and the faster your car will go.

Street Luge World record 102mph

It’s a bold claim isn’t it? But here’s how i came to this conclusion. The world record for street luge (essentially a bloke laid on a long skateboard) is just shy of 102mph. While he is wearing a slippery suit, and the luge does have some aero devices behind the drivers head to reduce drag, it is still essentially a very unaerodynamic shape. It does however have a very small frontal area – in fact, about as small a hole as you can physically fit a human through.

Fast Donnie Schoettler GF1

So then onto the gravity car world record holder, “Fast” Donnie Schoettler. His car, the GF1 has also reaches speeds of around 102mph. My initial thoughts about this car were how can it go so fast when it uses kart wheels? kart wheels have a lot of grip and must surely have a lot of rolling resistance? Well, they may do, but like the luge, Donnie’s car has a very small frontal area, and while it is bigger than the luge it makes up for this with incredibly slippery aerodynamics. The small kart wheels help enormously here, because they can be fully faired and as they are small, they do not add to the frontal area of the vehicle. In both cases though, the frontal area is the key element that makes them go fast. If you’re designing a car with the aim of going as fast as possible, keeping the frontal area to an absolute minimum is key.

Advertisements

6 thoughts on “Frontal Area AKA How to Go Fast…

  1. A quick comment on the Kart wheels and contact patch : The traditional Kart tires do not get “stickey” without heat. Gravity vehicles generate very little so this is not a factor. The biggest misconception is that the wider the tire, the better the grip. A contact patch only has as much grip as the weight that is being applied to it so it is better to have the smallest patch that still grip with the weight it is carrying. The specs on the tires I run are 10-3/8″ diameter with a 2-1/2″ narrow slick tread, on a 3-1/2″ wide wheel. They generally have more grip than I have courage for.

    Liked by 1 person

  2. Just a bit of info on this Section which just happens to be my forte – Rotational Inertia
    Much like a flywheel on a regular road going car, a heavy wheel takes some energy to get up to speed, whereas a light wheel takes less energy. This means a car with heavy wheels will be slow to accelerate, as the force of gravity acting on the car takes a while to get the wheels turning and the car moving forwards. A point to note is that the weight of your wheels in NO WAY affects the top speed of the car. Because physics. (?)
    Actually in real world testing and racing it does. My 9 time State Title winning Trike Billycart runs (after 2 years of non stop testing) a steel rim on the right rear with Alloy front and and left rear. I alter the way it accelerates and the top speed achieved by adding or taking away those rubber spoke end protectors that wrap around the rim. More and I can achieve quicker mid-range and top end speed.That’s only for one wheel for me. Less and can get off the Line faster (good for short courses) at the sacrifice of a bit of top end. My Mate (8 time State Champ) changes all four wheels weights for similar results but he runs a different class (and wheel size). Both of us can alter our top speed upto 2.4kmh or the way our machines accelerate simply by by altering the wheel weight by a few Ounces. Yes our racing is usually that close us.

    Liked by 1 person

    1. Cheers for the comment Scotty!

      I think the whole “wheel weight doesn’t affect top speed” is a theoretical statement, but in practice, as you say, it does have an effect. Given that any course will have a particular incline and length, I guess it’s a case of choosing the optimal acceleration vs speed that gives the best overall time for the run. Would you agree?

      Like

    2. The statement ” the weight of your wheels in NO WAY affects the top speed of the car” is slightly misleading. What is more correct is to say “the weight of your wheels in NO WAY affects the *terminal velocity* of the car”. Terminal velocity is a theoretical top speed reached on a given incline, where the drag forces exactly balance the force of gravity and acceleration is zero. Because acceleration is zero, there are no losses due to rotational inertia. However, terminal velocity is not strictly relevant in gravity racing because;

      1) You cannot reach it anyway. Terminal velocity is the asymptote of the velocity line – you can get infinitesimally close but never actually reach it.

      2) The distance you need to travel to get close to it is in the order of kilometers, and well beyond the length of pretty much any real world course.

      Like

  3. Another thing we discovered when building gravity karts is the aerodynamic advantages of closed/faired wheels. The turbulence caused by the top of the tire moving forward at twice the speed of the vehicle caused far more drag than we’d first suspected. Adding a set of simple fenders over the leading/upper surface of the tires saw sizable gains over an open-wheeled kart.

    Liked by 1 person

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s