Optimizing Aerodynamics of a Truck: Part 4

So far, I’ve lowered the aerodynamic drag of my 1991 Toyota Hilux by removing its mirrors, blocking part of the cooling air opening, and fitting a large front air dam. Together, these reduce drag by more than 10%--which means I’m about halfway to my goal of cutting the drag of this truck by 20% or more.
Moving on from the front surfaces, I thought I would trial some modifications to the airflow around the wheels next.
Keep It Simple
Right off the bat, I have a conundrum. The front wheel housings have a metal inner fender and a narrow plastic cover which seals the gap between inner and outer fenders. But in front of the tires, there are effectively no wheel housings; it’s open to the bumper, stock valance, and now air dam. According to various textbooks, wheel drag is typically reduced when wheels are enclosed in housings, with drag decreasing the smaller the ratio of the wheel housing volume to wheel volume. This may or may not hold true on my truck; I have no way of knowing without testing it. But to build a cardboard mock-up wheel housing would be difficult, as there is very little structure I could even tape cardboard to:

Alternatively, I could see if blocking some more air around the wheels reduces drag, by taping cardboard extensions to the existing air dam. In both cases, I want to leave the middle of the air dam open, as there are louvers in the skid plate behind it that look like they’re intended to cool the front differential—so I don’t want to block off too much air in the middle of the truck.
Air dam extensions, needless to say, are much easier to build and test than a complete wheel housing in this case, even though air dam extensions may not reduce drag as much or at all. How do I decide what to do?
Well, take a lesson from car manufacturers: try the cheap, easy thing first. If that doesn’t work, then test the more complicated, more expensive thing.

So, today I’ll test smooth wheel disk covers, front air dam extensions, and rear air dams/wheel housing extensions similar to current production trucks:

As before, I’m interested in how much these devices change overall drag, so I’ll use throttle-stop testing—which will easily show that—rather than pressure measurement or tufts, since I can use this method on my truck (unlike the Prius, which has electronic throttle control).
I headed out to one of my usual testing locations on a day with clear skies and light winds:

The corn is as high as a...horse's eye, maybe?

Once again, I was surprised by the results of testing. I honestly did not think that the short dam extensions in front or the wheel covers would make any measurable difference.
The rear dams I thought would make a measurable improvement, but unfortunately, I did not get to test them after all since the tape wouldn’t stick to the dirty fender liner back there. I’ll have to come up with another temporary attachment method to test those another time.
However, the front dam and wheel covers stayed attached successfully throughout the test:

I've had multiple farmers over the last few months ask me what the hell I'm doing.

Here are the results, with percentages corrected to reflect the change from the original drag of the truck:



% Change

% Change, corrected


94 kph



+front dams

96 kph



+front dams, rear covers

96 kph



+front dams, front covers

97 kph



+front dams, all covers

98 kph



A sidenote: throttle-stop testing really feels like magic. Julian Edgar, who developed this method, gave me some pointers when I was first experimenting with this, and I remember him writing that when you get your technique dialed in the vehicle gets to its top speed and “just holds there” (or maybe he posted that on the Ecomodder forum? I can’t recall now). It’s true, though—I tested over a two-mile section of straight road, holding at 80 kph until a specific point where I pressed the throttle pedal to its stop position and held it there through the test section, verifying with the TPS voltage display that it was in the same position for all runs, and recorded the new top speed at the same point. I used two cornfields for my markers, but you can use really anything—a light pole, road sign, mailbox. Just be consistent and you should achieve usable results; check by testing windows up/windows down.
You may have noticed something curious: the rear wheel covers didn’t change drag by themselves but did when added to the front covers. I found something similar when I tested mirror removal and grill blocking; what might be going on is an interaction. That is, the changes to the flow on one part of the truck affect a change made to another part. In this case, there may be any number of reasons why the drag improvement increases when rear covers are added with the fronts when those same rear covers did nothing by themselves. Perhaps there is less turbulence coming off the front wheels, and the cleaner flow reaching the rear wheels makes those covers more effective? Or maybe the front covers affect the yaw angle of the flow behind them, and this change to the oncoming flow direction allows the rear covers to work better? Interactions can work front-to-back, like I found here, but also back-to-front, which you may find in your testing—such as a spoiler increasing pressure/reducing flow velocity far ahead of itself. This is just one reason why you can’t guess at aerodynamic changes, and why car aerodynamics are so complicated.
If I build permanent wheel covers and front dam extensions, I will have reached my original goal of reducing this truck’s drag by 20%--far too easily. Now I have options: target a larger drag reduction, forgo either or both modifications and test some others, or combine both of these options. The wheel covers in particular will be a challenge, since I will have to design something that not only allows me access to the lug nuts but also the front locking hubs on this old-school 4WD truck. So, I may hold off on those for the time being while I investigate other changes that would be easier to implement.


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