Jeremy wrote:Doubling the power of your Smart Roadster would increase the top speed by about 28mph, or about a 26% increase in top speed.
If that power increase was obtained by increasing the engine max rpm, without changing the rpm/torque characteristic, then the acceleration times would remain similar to those with the 80hp engine, as acceleration is only dependent on the usable torque available at the wheels.
In practice, this means that performance at lower than maximum speed wouldn't improve much, unless the gearing was also changed to increase the torque available.
You know what I mean! It's highly unlikely that a 160bhp Smart Roadster (if it did exist) would rev to 13000rpm (aside from those bike engined ones). Besides, if it did, I could do double the speed in each gear, so I'd keep in a lower gear, resulting in fewer gear changes and more wheel torque in each gear.
Jeremy wrote:The big advantages that electric power provides for your application is massive low speed torque, which will reduce your times by enhancing acceleration, and the ability to accurately control the torque available at the wheel, to avoid wheelspin and get maximum traction. The simple way to do this is to mimic the Prius controller (not it's traction control system). The Prius controller monitors the motor phase current in order to control the motor torque, which is an ideal way for you to get maximum traction without being accused of using a traction control system.
That is a good solution. Luckily, I don't have to worry about the legality traction control systems as they're not banned. Having said that, the complexity of making one would stop me. I remember from my days racing radio controlled cars as a kid that I used to have a clutch that slipped at a certain torque, minimising wheelspin. It became redundant when current-limiting electronic speed controllers became available. It worked very well, although you had to set the current limits each time you went on track to match the conditions, which obviously had an effect on battery life.
Jeremy wrote:Lithium batteries have a high energy density, but lead acid batteries are often better in terms of power density. Even cheap lead acid batteries are capable of delivery extremely high power levels for short periods, just the characteristic that you are looking for. I haven't looked closely at the current state of the art with lithium, but last time we debated this here (in the context of a hill climb competition car) we concluded that lead acid still had the edge over lithium.
I'll need to re-assess, I think. I've compiled a spreadsheet of about 30-40 batteries, but it's based on the continuous rating of the cells, which I suspect is biasing the lithium cells. Perhaps I should re-do the spredsheet using the 60s values.
Jeremy wrote:A bit of work to estimate the sort of maximum torque your chosen wheel/tyre combination will take before breaking loose, together with the sort of speed profiles that you're likely to be running, should allow the optimisation of the design to be as quick as is physically possible, apart from the vagaries of conditions and the drivers skill! In many ways, an electric drive system is ideal for this type of competition, because it allows so much control, relatively simply.
The reason I've gone down this route is because it seems so ideal for the events. I have done some planning on the power/torque required but it varies so much depending on the weight of the car. I'm hoping that the rolling chassis can get down to below 500kg. The Haynes Manual says the standard car isn't much over that but I think that weight is best described as optimistic! Still, I've removed 30kg with the fuel tank, I'd assume more than 100kg with the engine/gearbox and the exhaust weighed a few kilos too. Sadly, I didn't have my scales when I removed the exhaust and the engine/'box were too heavy to weigh. I guess I'll be in a better position to make calculations when the rolling chassis is ready and I can find some scales to put it on.