If you’re building a gravity car, regardless of how seriously you’re taking it, if you want to take part in an event and have a chance of success (let’s say success is finishing in one piece) you need to build the car with the type of event you have in mind, and to a minimum standard.
If I had a million quid and McLaren ready and waiting to make the parts, I would still have to carefully consider what I was expecting the car to do before I built it. Every kind of soapbox/gravity event is different, and the quickest and most successful cars for any event will always be those that take this into account.
There are many areas of engineering and physics that go into making a good car, and what you make of each of those areas very much depends on what your car needs to be able to do well, and what range of circumstances it may find itself in. A drag car doesn’t need to be able to go round corners. An F1 car only needs to be able to do one standing start per race. Karts don’t need to consider aero or downforce as they don’t go fast enough (well, most of the time). Each race discipline has different requirements. Just because all gravity cars are called gravity cars doesn’t mean they’re all the same.
Let’s take the Redbull Soapbox race into consideration…
As you can see from the video, the “kicker” ramps cause a lot of cars to basically fall to pieces. If I was designing a car to enter this particular race, it would either have to be super-strong to withstand the forces, or have suspension, to absorb them. Cars that are not super strong, and don’t have suspension, come a terrible cropper when they hit the deck. It seems obvious to me, yet many teams don’t think of this. That may be because other people have a much better sense of fun that I do, but hey, I just want to win. Do you?
The track that you’re racing on is everything. You’ll need to take the following into account. All of these considerations are interconnected too, just to be annoying…
Incline – The steepness of the hill you’re racing down will ultimately dictate how fast you can go. If it’s not steep, and you’re only going slow, the points that follow this one will be less critical. As you go quicker, all considerations become more important as the forces involved on the tyres, wheels, axles, chassis and driver will be greater. Fun family events tend to be on shallow inclines to keep speed down, so if that’s your goal, knock yourself out – as long as it’s got four (or three) wheels and a seat you should be okay. As things get quicker, it gets a whole lot more interesting…
Surface – If the race is on super smooth tarmac with no ramps or kickers, you may not need suspension. If the road surface is rutted or bumpy, or there are ramps to content with, you’ll need to contend with that. As mentioned previously, that means building a car that is really strong, or adding suspension to help absorb the shocks.
Corners – If the track is practically straight your car will not need to deal with cornering forces so can be slim and have very low resistance (grip) tyres. If the track has sharp corners where it’s important to be able to go quick, you’ll need a car with a lot of grip, which means more rubber on the road, and a level of construction that can cope with the cornering stresses you’ll generate.
Here’s the world record holder Donnie Schoettler doing over 100mph. High speed, smooth surface, no corners.
As long as you’re prepared to do quite a bit of reading around the subject before you start your design and build (useful links to other sites here), you’ll soon work out how all of these elements can be balanced to give a car that will cope with the job in hand. If in doubt, look at the past successful cars in the race that you want to enter and consider their main features, and if that’s not enough, consider the words of my mate Des Taylor (sub 6 sec drag racer and car builder):
Build it simple and build it strong.