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Old 09-15-2020, 04:49 AM   #1 (permalink)
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Basic car modification methodology

I get the feeling that people see car aerodynamic modification as a specialised, different area to normal car modification. But it isn't!

Just the same techniques as you would use in any car modification apply.

So, for cars of the last 20-30 years, the starting point is that to assume the manufacturer largely got things right. After all, they have spent vastly more in sophisticated resources than we, as amateurs, can. So in pretty well any case you can think of, it will be a case of building on, and tweaking, what the manufacturer did, rather than throwing it out wholesale and assuming we can do better if we start from scratch.

So if we are modifying the suspension, we should always start by working from the suspension the manufacturer had. Their springs rates (or, better still, natural frequencies), damping rates and sway bar rates are our starting points. So in a front-wheel drive, we might go 25 per cent stiffer in the front spring rate, 40 per cent stiffer in the rear spring rate, and a similar magnitude in sway bar rates. Damping would then be commensurate. We're starting with the manufacturer's rates and then going stiffer in the rear to dial out understeer (and also in doing so, of course, also making the car more treacherous in slippery conditions).

Or, if we are starting with engine management, we'd first assess what the manufacturer does in terms of air/fuel ratios, ignition timing advance and exhaust gas recirculation. In that case, we might choose to go a little leaner in full-load air/fuel ratio, a little further advanced in ignition timing, and add a little more EGR. (And so in turn run closer to engine destruction in many conditions.)

Of course, to do either of these things we need to have measured what the factory has done, and then build from that.

I've developed engine management maps from absolue scratch (with my Honda Insight) and it is truly a massive amount of work compared with just modifying what Honda did. Literally many, many hundreds (if not thousands) of hours.

And it's just the same in car aero mods. We measure what the factory has done (wool tuft testing, pressure testing, throttle stop testing, lift/downforce testing) and then build from that starting point. To assume that a 'one size fits all' in terms of aero shape is just as absurd as assuming that one size fits all in suspension rates, or air/fuel ratios, or EGR, or...

In fact, if I posted on any engine management forum that I have the magical air/fuel ratios for best power or best economy - all derived from Harry Ricardo's 1930s tests (that I have in my library) - I'd be laughed out of court. Why? Because these figures - and even in fact those from the 1960s - and now completely wrong in terms of modern developments.

The same with suspension. Using Lanchester's natural frequencies for suspension, from around the turn of last century, is interesting, but it wouldn't work now. (But absolutely still worth looking at.) Ditto with looking at the work of Maurice Olley (the father of modern suspension in cars) - he's a bit like Jaray or Prandtl in aerodynamics.

I would never denigrate these 'founding fathers' in any of these automotive technologies - but at the same time, I'd be very reluctant to blindly follow them. Too many things have changed! And car manufacturers, more than anyone, know about these changes.

In my thousands of hours in car workshops, listening to the best in the business, I have always been puzzled when a mechanic says, "Look at this stupid thing the [car manufacturer] has done!" Why have I been puzzled? Because you can bet your bottom dollar there's been a good reason for that approach*.

In contrast, when a mechanic says, "That's interesting; I wonder why they have done it like that?" I, too, am interested.


*Almost the only exception was when, in the early 2000s, I was working with a workshop that had a specialist hacker that had broken into one company's ECU software. His development tools were, I believe, better than even those of the OE manufacturer. Looking at engine management maps in the ECU, we could clearly see some the OE engineers hadn't bothered completing. That is, in areas where the engine would probably never go, you could see fuel and timing maps full of garbage. Fascinating!

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Old 09-15-2020, 06:15 PM   #2 (permalink)
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I realized a while ago that it's important, before modifying any aspect of a car, to understand why it was designed the way it is. Everything is a compromise, and an enthusiast modifying a car will have different priorities than a manufacturer trying to sell cars to the general public.
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Old 09-19-2020, 04:13 PM   #3 (permalink)
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So in pretty well any case you can think of, it will be a case of building on, and tweaking, what the manufacturer did, rather than throwing it out wholesale and assuming we can do better if we start from scratch.
I think it can be interpreted in terms of use case. The manufacturer build to the hypothetical broadest possible use cases. The result can be refined for more specific cases, like eco-modding, time attack, land speed racing, etc.
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Old 09-19-2020, 07:58 PM   #4 (permalink)
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It's a very good point, but the suspension design team usually doesn't have their work overturned (or directed) by an artist from Marketing to make the suspension look nicer in a commercial.

... Why have I been puzzled? Because you can bet your bottom dollar there's been a good reason for that approach*.

With aero bits that you can see (good or bad), that reason usually comes from Marketing.
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Old 09-19-2020, 08:10 PM   #5 (permalink)
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Originally Posted by Fat Charlie View Post

With aero bits that you can see (good or bad), that reason usually comes from Marketing.
I must see things differently. Cars now are slipperier, and have less lift, than ever before - that's engineering, not marketing.

I now rarely see cars that have major aero deficiencies because of styling.

In fact, I'd say it has been keeping production costs low that has had more of a negative impact on car aero in recent times (eg no undertrays).
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Old 09-19-2020, 08:41 PM   #6 (permalink)
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Quote:
Originally Posted by Fat Charlie
With aero bits that you can see (good or bad), that reason usually comes from Marketing.
Bucky Fuller said that humankind will defer to the computer. He never knew about adversarial generative AI.


https://www.greencarreports.com/news...-and-flaunt-it

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