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
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.
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.
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.
The 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.
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.
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.