Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - External Mirror Removal (with tuft testing)

I started a blog recently; read the original article here.

Mirrors are an easy drag problem to spot: they stick out from the car body, disrupting flow. This is easy enough to guess, but how and how much do they adversely affect things? To answer that we need to do some testing.

The Problem with Mirrors

The problem with mirrors is twofold: first, in order for them to function they need a large, flat surface on the back side, which contributes to high drag for the mirror itself. Second, regardless of the drag of the protuberance such as a mirror, when it is brought close to the body of a car the total drag is more than the sum of the two. This phenomenon is called “interference drag.”

You can get an idea for why this is by looking at a tuft test of a window surface behind a mirror. Here’s the flow over my truck’s side window with the factory mirror—which is not large by truck standards—in place:

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Enjoy the soybean fields. If you follow this blog, you're going to see a lot more of them.

Notice how the tufts are disrupted all the way to the back edge of the window, far behind the mirror. There isn’t much body surface behind that, so I would guess the flow doesn’t clean up at all. Quite an effect from something as small and seemingly insignificant as a mirror!

Smoothing the Flow

Ultimately, we want the flow over a side window to look like this:

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That’s my Prius, with its nicely rounded A-pillar, laid back windshield, and no mirror. The tufts are nicely aligned and not flapping much, showing little turbulence. How does the truck look if we do the same? Well, not as good—it’s simply not possible to change that A-pillar and windshield geometry without rebuilding the entire cab structure.

But first, here’s the flow with the mirror folded in:

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Not as great as I had hoped! You can still see a large wake behind the mirror and the flow largely moves upward. Now, here it is with the mirror removed entirely:

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Ah—finally the tufts have been cleaned up considerably, and the disruption along the bottom of the window has been almost entirely eliminated, with nice, attached flow across it. That’s what we want to see, and after I test for the actual change in drag, I’ll likely make a closeout panel and remove the mirror permanently.

Is Mirror Removal Legal?

This is a modification that gives pause to many who might otherwise consider it. Is it even legal to remove the side mirrors on a car in the United States?

Certainly, removing the passenger mirror is legal; up until a couple decades ago, many new cars were sold without a passenger side mirror at all (they were often an extra-cost option). Now it is impossible to find new cars without two mirrors, even on the cheapest cars for sale today.

Whether it is legal to remove the driver’s side mirror is a bit of a gray area. To sell a new vehicle in the US, manufacturers must ensure that their automobiles comply with Federal Motor Vehicle Safety Standards; included in those standards is a requirement for an external mirror on the driver’s side (companies such as Tesla and Audi are fighting this because they want to fit cameras instead of mirrors for lower drag). Many people think that this means all cars in the US must have a driver’s side mirror—but this is not the case. After the car has been sold to a buyer, its equipment, use, and licensing are regulated by whatever state it is in rather than the federal government—and state regulations on mirrors vary widely. Some states, like my native state of Washington, explicitly require a driver’s side external mirror. Others, like my current state of Illinois, do not. In fact, there is similar language among nearly 30 states in their mirror regulations, e.g. “Every motor vehicle, operated singly or when towing another vehicle, shall be equipped with a mirror so located as to reflect to the driver a view of the highway for a distance of at least 200 feet to the rear of such motor vehicle” (ILCS 5/12-502; emphasis added).

Notice that the way this statute is worded, it does not require side mirrors at all—let alone on the outside of the vehicle! As long as the rear window is not blocked, the interior center mirror satisfies the requirements of the law. If you live in or operate your vehicle in one of these states, you’re good to go with no external mirrors (as I’ve been doing for ten years now on two different cars).

However, if you drive through a state that does require a driver’s side mirror, you will technically be in violation of the law without one. The way many states’ statutes are worded, the mirror requirements apply not just to vehicles registered there but any that operate on its roads—even if you’re just passing through. In this case, you might want to come up with a temporary mirror solution, or if it’s too much hassle for you then just leave the driver’s side mirror on. In the last decade I’ve been pulled over once for no mirror, on my way through Kansas several years ago, but did not receive a ticket. Your mileage may vary, but it’s good to plan ahead based on what you’re comfortable with.

Mirror Replacement

You could absolutely install cameras if you want, but I decided to go more basic. When I removed my mirrors, I picked up some brackets from the hardware store and a pair of blind spot mirrors; their convex surface and placement right next to the window means I have as good a view of my blind spots (if not better) as with the factory mirrors:

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I'll do something similar with the truck if I remove the external mirrors.