Over the past couple of years I've probably confused the hell out of a lot of people on these forums, more often than not with my own personal philosophies on vehicle design. I've probably seemed a little abrasive towards some members, which certainly has not been my intention (Daniniowa and Garlic are first to mind sorry gents!). I just think sometimes by challenging the established axioms we can all further our own knowledge a little.
So anyway, rather than keep hijacking other people's threads, I thought I'd open my own whereby you can all have a dig back at me. It's a bit of a thesis so you'll all have plenty of ammo
My personal philosophy is similar to Z's, in that we tend to overcomplicate our designs, and don't pay enough attention to the stuff that really matters. Where I might stray from Z's philosophy a little is that I think our obsession with engine performance is a little unfounded.
Does more power always make us faster?
I started thinking about this in response to a comment made on another thread, when someone wrote that you can never have too much power. I replied that I disagreed, mainly because in principle I think that such outright statements don't really address the issues of balance we face when we design a car. EVERY benefit comes at some cost whether it be track speed at another part of the course, risk of failure, development time, financial cost, etc etc. It is our task as engineers to find the right balance, and to be aware of the compromises that relate to our designs.
In relation to the power argument, my assertion is that engine performance comes at the expense of weight. The more power/torque we have, the greater the loads we are placing on our drivetrain and cooling systems, and therefore the more material (read weight) we require to construct them (for a given safety factor). If we accept the premise of tyre load sensitivity, (i.e. that a tyre's coefficient of friction decreases with increasing vertical load), then this increase in weight has reduced our lateral acceleration capability our cornering speed. (There is a graph in the first chapter of Milliken to visualize the concept of tyre load sensitivity, page 29 or else if you have ever stiffened up the front / softened the rear to cure oversteer then you are aware of the effect). Effectively more power equals less cornering speed.
So a hypothetical argument. Imagine one team builds a car with a gun CBR motor, puts out 90hp. Everything built as light as it can to suit the power, the thing is a rocketship in a straight line and a damn nice little car. Another team detunes their CBR to 45hp and builds a car of the same geometry. Of course, they can downsize cooling system, driveshafts, CV's, chain and sprockets, diff and diff mounts, maybe rear tyre widths, even braking system to an extent. Weight savings? Maybe 5-10kgs? The thing will be a plug in a straight line, but if tuned properly the lesser weight should make it quicker around corners, (not necessarily much!). And somewhere in the middle there is a combination of straights and corners whereby the two cars will be equivalently as fast as each other. One problem, two solutions.....
Of course the above is a trivial example, but it's a philosophical argument to show that even with the same engine we can have less power and sometimes go faster because of it. (Remember I'm challenging the assertion that more power/torque is always better). A more significant effect would be seen when the second team takes the argument further and replaces their 45hp CBR with an engine that was designed for that power. And suddenly we have saved 30-40kg, and we get the situation where the "slow" car is starting to hold its own on tracks with longer straights.
I'm acting a bit as the devil's advocate here. But sometimes these mental exercises open up trains of thought that can end up working for you.
Our curious obsession with engine performance
So how much does our engine performance really help us? I get the impression that some engine junkies think that if their engine is 20% more powerful they'll be 20% quicker around the track. A more educated guess might be that if the track is 20% straights, then you will be 20% x 20% = 4% quicker around the track. When you work out the figures, it is not even that.
For the sake of simplicity, we'll consider that the car has constant acceleration. (If you have a reasonably flat torque curve within your operating rev range, it's not a bad assumption). The car's equation of motion down a straight can be calculated through the formula:
x = u.t + 1/2a.t^2
where "x" is the distance, "u" is initial speed (corner exit speed), "a" is the acceleration and "t" is the time taken. From the above we can see that for a given distance:
" There is a term (u.t) that is completely independent of acceleration
" The term containing acceleration is attached to a time squared function meaning any time saving will be related to the square root of the acceleration.
When we first started doing some research on the first RMIT single car in 2001/2002, we estimated the typical 600/4 might be around 20% better off on power-weight ratio, (given our conservative weight target for the first car). So imagining worst case enduro/autocross scenario for an underpowered car whereby you are leaving a low speed corner (say, 36kmh = 10m/s) and accelerating down a long straight (75 metres max according to FSAE rules, which we'll reduce to 60 metres given braking distance and corner entry and exit transients). We'll estimate slow car's acceleration at 10m/s^2 (just over 1g), and therefore the 20% more powerful car at 12m/s^2.
The equations for the two cars become:
60 = 10t + 5t^2, which solves as t = 2.605sec (slow car)
60 = 10t + 6t^2, which solves as t = 2.437sec (fast car)
The car with 20% more power/weight has in the order of a 6.5% advantage along the straights. For an Oz or FStudent track with only around 15% full throttle time, the effect is in the order of 1% time based over the whole track. If Rotor's throttle pot figures are correct, then maybe for the US track the time advantage stretches out to around 2-2.5%.
Remember the above is worst case, assuming all the straights are long with a stop corner before them, and not taking into account that a lighter car should be able to get out of a corner quicker, thereby diminishing the effect of the 1/2a.t^2 term even further.
Effectively with the first RMIT single we only had to be around 0.2% quicker around the rest of the track for our single to be competitive, and we "bought" ourselves a 40kg weight saving to do it with.
The guts of it all? The contribution of 20% better engine performance may be as little as 1-2% towards lap times and that is before it is balanced against the negative effect on lap times due to the extra weight. And if your engine development team is only delivering a couple of horsepower here and there, you have to wonder whether it is all worthwhile.
As a real-life validity check of the estimates, in 2003 the best 600/4's had around 20% power to weight advantage over the first RMIT single. In the Acceleration event, the "u.t" term in the equation above disappears, so the time taken should be directly related to the square root of the acceleration, i.e around 10% quicker for a car of around 20% better power to weight. UQ were doing around 4.0/4.1 seconds, whereas RMIT was doing around 4.5 seconds. So the estimates seem pretty valid.
The question your team honestly has to ask itself is, are we really close enough to the Cornells, UoW's and UWA's of this world to be concerned about one percenters? Especially when we pay for it with trade-offs elsewhere on the track? Maybe, if your team is consistently finishing the whole event, and you are continually scoring over 800 points, then that last little bit of straight line speed might be worth it, might not. But if you are scoring less than 800 points each comp then I think you have much more pressing concerns than outright engine performance. Get the thing running smoothly and make sure your hot start works and direct the bulk of your resources to more important things.
What should we be concentrating on? I'm going to agree with Kevin Heyward here, in that I think the true key to track speed in FSAE is in the handling dampers and tyres. If you can build up your corner speed, it helps you everywhere on the track, even the straights (see u.t term in the straight line equation above).
If engine performance is so unimportant, why do pro race teams spend so much time and money on it?
I'm pre-empting the above question. FSAE is a completely different game to actual wheel-to-wheel racing, and that is due to the fact that we don't have to pass our opponents outright on the track rather if one car is lapping faster than another, the slower is flagged out of the way. This is a big difference. Under standard track racing conditions, most passing is done under brakes and into corners, which means that to be competitive you need to keep up with the opposition down the straights. Higher corner speed at the expense of straight line speed doesn't work too well in a wheel to wheel situation your competitive advantage under cornering is lost when you get baulked by the cars that blew by you down the straight.
If anyone is interested I can relay the story of Aprilia's 500GP entry of a few years back as an example but this post is dragging on enough already!
In FSAE, we are a lot more open to experimentation with where we can chase track speed - and can more easily trade straight line speed for cornering.
Intent of the competition
The good people who designed the rules for this event were well aware that this competition needs to be cheap and safe for it to survive the test of time. Everyone in professional motorsport knows that power is expensive, and to design a competition whereby power is the deciding factor would be disastrous at a student level. The regulations are, quite cleverly in my opinion, framed to make us think further than outright engine performance. Think about the 20mm restrictor, short straights, slaloms and lots of corners, extra points for fuel economy, speeds not high enough for significant wind drag, lack of wheel to wheel racing as mentioned above. Everything is there for us to be a bit more adventurous in our designs and yet most still cling tight to the premise that the first thing to consider is to have a greater engine performance to weight ratio than the opposition. I think it is a wasted opportunity to be a little innovative, that's all.
Now I've made a right nong of myself fire away ladies and gents
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