Side mirror drag & effect on fuel economy - quantified
 

(Originally written August 11/06)

You often hear people talking about removing side mirrors to reduce drag. Is it effective or worthwhile? That's what this thread is about.

Still that thread seemed to run out of steam. I'd like this one to perhaps lead to more concrete numbers, maybe encourage an experiment or 2 to confirm...)

• Both the Civic VX and Metro XFi eschewed outside passenger side mirrors. Cost savings could have easily been the reason on both cars (mirrors on both sides weren't as commonplace in the 80's and early 90's as they are now).
  .
• Phil Knox ran his aero-modded CRX at Bonneville, and says removing his side mirrors made no measurable difference in his top speed (radar-measurement)
  Yahoo! Groups

Yet so many aero concept cars omit the mirrors. For example...



So there we have mirror drag quantified: "30 counts of drag" - which if I'm not mistaken means Cd ".03". Seems overly large to me, but there it is.

Anyone have their Cd formula handy for their vehicle want to translate that to FE savings at various speeds?

From the Chalmers University site:

EDIT:

.03-.06, is 3-6% of total drag.

According to those two aero sources, it's a pretty significan effect (at least in my books).

3 - 6% of total Cd for a typical car is low-hanging fruit, ripe for the picking.

Though: the base Cd isn't mentioned, and highway speeds aren't defined.

If it turns out it's really worth taking mirrors off, I may go for a combination approach...

- Probably a set of folding mirrors so I can retain full mirrors for city driving (my current pair are fixed, and I have a set of folders from the red Forkenswift)

- with one of those round convex mirrors stuck on the aft end of the driver's side mirror for use when folded.

- Probably an addition to the inside rear view mirror

- and more shoulder checks for the passenger side

I de-passenger side mirrored the Flea this afternoon, and picked up the folding driver's side mirror from the red Forkenswift. It's not a direct swap - I'm going to have to re-engineer things a little.

I would like to do a SG1 mirrors vs no mirrors experiment on my test course, but I have other things I still want to test too, and only so much time.

Using the aero/rr web tool I made based on Dan's spreadsheet, my car theoretically gets 56.7 mpg (US) @55 mph (Cd: 0.34, frontal area sq.M: 1.858, @ 25C, default fuel & engine eff. values). That's actually not far off my actual FE at that steady state speed.

Removing one mirror: Theoretically, using the more conservative GM numbers, taking off a mirror reduces the Cd by 0.015 and the frontal area by about 0.0375 sq. M.

That improves my 55 mph highway FE to 59.64 mpg (US), or +2.94 mpg (US) / +5.19%.

That's a gigantic improvement in my books, if it's true.

EDIT- I mistakenly used the least conservative numbers. The Chalmers report has the more conservative range. So I'm guessing the actual effect will be less than 5.19%

More tidbits from the Chalmers PDF:

Other notables from the PDF (it's definitely worth a read for anyone who hasn't seen it yet):

• "A very well designed mirror increases the drag of a car of approximately ΔCd=0.012. But this value can reach 0.025 to 0.030 for the worth [worst] designs. Actually it seems reasonable to think that the average value for ΔCd is of around 0.015 (ref. 9)."
  .
• "...[the mirror's] combination with the A-pillar contributes to the creation of trailing vortices at the side front of the car, which are highly drag-consuming."

Undertook a little project this afternoon: replaced my stock non-folding but otherwise aerodynamically shaped driver's side mirror with a folding but otherwise not very aerodynamic mirror from the red Forkenswift car.

http://images6.theimagehosting.com/mirror-before.th.jpg

Above image: before - the stock mirror. (click to zoom)

http://images6.theimagehosting.com/mirror-after.th.jpg

Above: after - the folding mirror. It took a little cutting and drilling to make it fit, and I had to cut a piece of black plastic (left over krazy karpet) to fill the gap since the mirror base is a different shape than the OEM mirror). But all the modifications are hidden & reversible. Should I want to revert to original condition, I'll be able to no problem.

http://images6.theimagehosting.com/m...-detail.th.jpg

Above: it became clear pretty quickly that the folded mirror might present less area to the wind, but the open hinge was an aerodynamic disaster. So I drilled a hole on the hinge to hold a shaped gap filler made from a piece of sheetmetal contoured with bondo. A "pin" (bolt) fits in the new hole on the hinge. I can easily install/remove this from the mirror while sitting in the car.

The plan is to fold the mirror for highway use. I'm going to stick half of one of those round convex mirrors to the trailing edge of the folded mirror, so I'll still have some functionality.

Is it better than the stock mirror? Only if I fold it - it's clearly less aerodynamic than the stock mirror was.

Am I crazy? The evidence is mounting, isn't it.

Also, I have no way to definitively say that folded, this mirror will create less drag than the stock mirror. There's no question frontal area is reduced. But it will create its own unique turbulence, and I won't pretend to know that it will automatically be less drag inducing than the stock turbulence. I'm just guessing that it's going to be better.

While on the side-view mirror topic, we all know how to set them, right? No, probably not.

http://www.cartalk.com/content/features/mirrors/

Just picked up 2 of those round convex mirrors. Depending where you mount them, they could work fine.

One small problem with convex mirrors (particularly those round ones) is your vision has to adjust to their focal length. So on the driver's side, it takes a split second longer to check than glancing at a flat mirror (or even a mildly convex passenger mirror). Not a huge deal, but there you go.

Are external mirrors legally required?

This question prompted me to check, and lo and behold, in Ontario...

I wouldn't remove my driver's side mirror.

Occasionally folded back is a good compromise and will prove useful I'm sure when I need to use it to see behind (as opposed to beside, which the convex mirror mounted inside the car permits).

And knowing the law, now I don't have to re-deploy the mirror in a panic when I see the police (which I did today). :)

I had near perfect weather conditions for doing a little experiment: light north winds, which are blocked by a ridge and tree line that runs along the north side of my usual "test" road, leaving the road in nearly still air.

I did an A-B-A, where A = both mirrors on, and B = passenger mirror off (with the hole covered) and driver's mirror folded, with my aero hinge insert in place (see pics, earlier in this thread).

Speed was 88 km/h / 54.7 mph, cruise control set once, cancelled with the brake and resumed for each pass. (Yes, an odd speed choice. I tried to set the cruise control right at 90k, but didn't quite make it. :) )

55.25 mpg (US) - average of first A runs (2 bi-directional runs)

56.51 mpg (US) - average of B runs (2 bi-directional runs)

55.04 mpg (US) - average of last A runs (1 bi-directional run)

For an average difference of 1.27 mpg between all A and B runs, or an increase in FE of 2.3% at this speed, for this car. Not quite the 2.94 mpg 5.19% improvement estimated earlier in this thread (for just one mirror removed!). But also not insignificant.

So there you go.

FYI for those following this thread, I've posted a summary of the mirror information & road test on metrompg.com, correcting some of my sloppy pre-test drag estimates in the process:

Reflections on side mirrors: testing drag vs. MPG

http://www.metrompg.com/posts/photos/mirror-chart-s.gif

Ditto on the Prius....
http://www.cleanmpg.com/photos/data/...m/DCP_6377.JPG

I'm hoping Franklin at hybrid effects can make a A pillar cover to replace the OEM one that has the thingy sticking out.....

http://www.cleanmpg.com/photos/data/...m/DCP_6378.JPG

Although it takes time focusing, I can see headlights in both the rear and internal side view mirror simultaneously.

k
 

Taking notes ,drivers side mirror cover :thumbup:

I really wish I was not having such a hard time finding cars that I can prototype on.

Franklin

Actually, I was hoping for a passenger side mirror cover.:D

I wonder how much difference could be had from fitting more aerodynamic mirrors, rather than replacing them.

Reason I mention this is my 91 Celica GTS had/has very aero looking mirriors. Let me try and dig up a pic.

Removing a mirror will reduce frontal area as well as lowering the Cd. A more aerodynamic mirror would only lower the Cd.

OOP'ssssssssssssss


I ment passenger side mirror.

Thing is that there is really nothing saying that you can not remover both mirrors and put a tiny mirror on the drivers side.

You could mount cameras on both sides or one side,,,, and have a mirror the size of a golf ball on the driver side to pass state inspection laws.

Franklin

To pass NC safety inspection, you are required to have an inside mounted rear view mirror and and outside mounted drivers side mirror. It does not mention the size of the mirror. See page 49.

http://www.ncdot.org/dmv/vehicle_ser...nforcement.pdf

So it might be nice to have a small mirror molded into the drivers side mirror mount. Either that or cut a section out of the OEM mirror housing, glue it back together and install a smaller miror surface. It might be fun to try if I was willing to spend $160 on a spare mirror to make the change reversable.

krousdb -

That's why I was thinking of using a bicycle mirror. I do have smaller motorcycle mirrors on my todo list, but they're maybe only 50% smaller, not nearly as small as bicycle mirrors. I did just get this installed :

WRAPAROUND PANORAMIC WIDE-ANGLE MIRROR
http://www.jcwhitney.com/autoparts/I...D:100000218469
Attachment 362

I basically have zero blind spot now. As you can see in the picture, it even shows both front windows (it feels like I'm watching myself in a Sonic fast food commercial). There is a little distortion at the edges, but it's not an expensive mirror. Because of this, I feel 100% safe to install smaller side view mirrors on the car.

At first I was disappointed when I got it. The original attachment seems to be designed for thinner (1970's style?!?!?) rear view mirrors. The other day I figured out how to remove the original attachment and retrofit my own attachment with zip ties.

CarloSW2

I've been keeping my eye on stuff like this "backup camera" on ebay.

I'm just wondering where I'd mount the camera. I need to figure out something before I have to get my car inspected again. Its down to the backup camera, or making a tiny mirror (about 4" x 1.5") for the drivers side.

In Virginia you have to have at least one mirror and all so the rear view mirror. The camera would be used to view what you you could not view if you had a tiny driver side mirror.

I think my post cunfused some people. The tiny mirror is realy just a way to pass inspection.

Here in Quebec you need only an exterior driver side mirror that must be adjustable from inside the vehicule and allow you to see a distance of 10 feet to the left of the vehicule 36 feet back of the driver's eyes when sitting normally with the seat furthest back. Nothing mentions the height of the mirror though.

Observations on side mirrors:

1) They have little effect on city driving where speeds stay under 40 MPH.

2) They are most usefull in city driving where traffic is heavy.

3) Folding isn't good enough for highway driving.

Solution:

1) Remove original mirrors.

2) Fabricate "stick on" mirrors to use in city driving.

With this configuration, you can literally "stick" the mirrors on in the city and "pop" them off for highway travel.

That's the best of both worlds. Also, I would permanently install inside mirrors.

If your car was made in the past 40 years or so, it seems you need to have at least one outside mirror in addition to the inside rear view mirror.

http://leg1.state.va.us/cgi-bin/legp...19VAC30-70-220

Originally, I was hoping to get by with just inside mirrors as well.

For reference, the full VA motor vehicle safety inspection rules and regulations are here:
http://leg1.state.va.us/000/reg/TOC19030.HTM#C0070

Rather than completely remove the mirror on the driver side and replace it with an uncomfortable in-side side mounted mirror, why not aero mod it? give it a boat nose and add wind spreaders to the top side.

http://img65.imageshack.us/img65/825...before1zy3.jpg
http://img54.imageshack.us/img54/981...rafter1qn0.jpg

Y'all are assuming removing the outside mirror improves aero and ya knows it might NOT be the case for every vehicle...

WaxyChicken -

I like how your thinking. On cars where the mirror is part of the door, other people have suggested a clear plastic cover that moves the air over the mirror while allowing you to still see the reflection.

CarloSW2

I think it's safe to say that most vehicles would be improved by a mirrorectomy.

The only way I could see it not helping is if the mirror is already fully involved in turbulent flow from, say, really unfriendly windshield/A-pillar design. Like perhaps the Jeep TJ style.

I've read the bubble Caprices' lose aero from the removal of mirrors. Where that knowledge came from I can only guess... perhaps the cops figured it out? Anyway it seems plausible to me.

Well, it sounds a bit odd to me, but I'm no expert. I'm just going by book stuff & what seems sensible to my mind. But I'll readily admit sometimes aero is counter-intuitive.

I heard that same thing about that same car. Don't remember where. However, I'd guess that in 99.something % of cases, it does add drag.

It does not make sense that removing mirrors on a car would add drag. Removing mirrors lowers the CD and the frontal area, it's a win win situation. The only way a mirror could reduce drag vs no mirror would be if it was in the wake of a bigger object.

I would like it a lot if you could link to some background information supporting your affirmation.

I've read dozens of papers and articles about aerodynamics. It was a small side-comment in one of those. Apparently Frank Lee has read that same one. I do not remember where it was.

All I could find was some guy over at gassavers saying that some other guy posted a reference to that effect a while back, but I wasn't able to find said reference.

One thing I can think of though is that this might be a confusion as in 1995 they changed the mirrors to more aerodynamic ones. Here's a comparison between a 94 caprice and a 96. See how the mirrors are like an extension to the A pillar in the 96 pic.

94
http://static.howstuffworks.com/gif/...3241990319.jpg

96
http://www.edmunds.com/pictures/VEHI...11-396x249.jpg

Kind of like Carl's quote, if there's anything I've learned over the years, it's that there's a lot in aerodynamics that is counter-intuitive.

All I'll say is that It's pretty bold to say something going against what is commonly accepted when you have no hard evidence to support your claim.

Aerodynamics can be counterintuitive, I concede, but it ain't magic.