Thursday, 20 November 2014

The No Compromise gravity racer

Building a gravity racer is battle of compromises that limit its performance. But what if you could have your cake and eat it? Here is a proposal that might allow you to do just that

An obvious compromise on a gravity racer is the wheel size. Bigger wheels have less rolling resistance but they create more aerodynamic drag. Also, most gravity racers use some form of bicycle wheel that are not designed for cars. Because bicycles bank into turns they never experience a significant side load. On a gravity racer, though, they take a big side load with resulting distortion and, the bigger the wheel, the greater the distortion. Likewise the tyres are also designed for cycles that bank into turns so they are optimised to give the least rolling resistance when vertical and generate the most grip when banked.
Another typical gravity racer compromise is the ratio of the track to wheelbase. Tyres do not like load variation and their load to grip curve has a sweet spot where they work best, but either side of this point their grip falls away. But when you corner in a four-wheel vehicle, weight transfer takes load off the inside wheels and adds it to the outside ones. The result is none of the tyres are working at their optimum. The solution is to have the lowest centre of gravity and widest track possible to minimise weight transfer. While this is not a cure, it does reduce the problem. But, under most gravity racer rules, the maximum allowable track and dimensions wheelbase create a very square ratio that is dauntingly nervous to drive.
With these challenges in mind, how can you start to reconcile any of the compromises? How about we throw away the gravity racer textbook and start again. Now, about those big wheels stuck out on the end of the axles creating loads of turbulence and aerodynamic drag. Why not bring them inside the bodywork by narrowing the track? I’m talking very narrow, maybe 200-300mm. The body already has to be a certain size to enclose the bulk of a driver so why not take advantage of the aerodynamic price you’ve already paid and use it to cowl the wheels. Yes I know this creates other problems but stay with me on this; all will be explained. Think instead about the wheels you could fit in there; you could certainly up-size from those weedy 20 inchers everyone seems to use. Plus, the drag in a straight line is going to be super low without all that axle stuff cluttering up the airflow.
This is fine in a straight line but most gravity courses have corners and a car with a 200mm track will fall over at the first sniff of a bend. How can we tackle that? What if we can fool the car into thinking it’s a bicycle?
Instead of rigidly mounting the axles to the chassis, what if we pivot them in the middle and, instead of just one for each wheel pair, we have two, parallel to each other. If they are able to pivot freely then, as the car turns a corner, the axles offer no roll resistance. That’s right, the body will roll outwards onto its bump stops and then the car will fall over. Hardly an improvement its true, however, a bicycle would do the same if you didn’t lean it into a bend and you can drive this car like a bicycle.
Whether they know it or not, cyclists reverse steer as they approach a bend. That is, they steer the wrong way first, which starts the bike falling over into the bend, before they then catch it with the steering. The bend is then negotiated with the bike leaning in perfect equilibrium between gravity and the cornering force. This has great advantages like moving the centre of gravity toward the inside of the turn and cancelling all side forces on the road wheels. Suddenly we have no nasty forces making the wheel distort and the tyre is kept within its sweet spot.
If we had the proposed pivoting axles set-up on our gravity car, then we would have all the same conditions as a bicycle, except on four wheels. Each pair of wheels would lean with the car and, with no roll resistance, there would be no weight transfer between them. Suddenly, instead of working against the best interests of the tyres, we are working for them.
There’s one more refinement. While the car would have no problem staying upright on the move, just as a bicycle does, how would you stop it flopping over when standing still? You could have fussy arrangements with stabilizer wheels you could pull up when on the move or some can of lock on the pivot mechanism. The latter would be hilarious if you forgot to disengage it before the first bend. Alternatively, if you mounted the chassis onto the swing axles lower than the upright pivots, then body roll would tend to raise the car. However, gravity would be trying to keep the body as low as possible generating a force that keeps the body upright. This force would be quite small and easily overwhelmed in motion but should be about enough to stop it flopping around embarrassingly in the paddock. It is worth noting, however, that this would result in an unequal load across each pair of wheels when leaning.
So, there it is, the no compromise gravity racer. It is rather unconventional but new ideas by definition are. The question is would it give a real world advantage over the conventional designs? Certainly it would make better use of the tyres and ditch a bundle of aerodynamic drag. In addition, being much narrower than normal cars would be the equivalent of having a wider road and straighter turns.
There is still a bundle of subtleties in how you design the steering and suspension while achieving these objectives. None of them are insurmountable and I may expand on this in a future post if there’s the interest. For now, why not let the idea marinate in your creative juices and see if ‘how could you’ turns into ‘why wouldn’t you’? It has to be said that some gravity racer rules unhelpfully specify a minimum track of 500 or even 800mm so this would limit where you could run the car. But, if you are going for maximum performance regardless, then it has clear advantages.