Improving aerodynamics (of '87 Nissan Bluebird)

[landspeed]

Hi!

Just wondering if anyone had any ideas for improving the airflow over the rear of my car, as I think the shape is about the worst it could be (see attached picture!).

When driving in the rain today (after stopping so the rain had hit the rear window for a while), the water went into straight lines going down the window - does this mean there was attached flow?

[Peakster]

Just start experimenting! Does rain get on the rear window if you don't stop and let the droplets hit the window for a while? By the way, your car is really nice and low to the ground, so you got that aero-issue down pat!

Attachment 81

[MetroMPG]

What's the angle of your rear glass?

A 10 degree angle is considered optimal for the slope at the rear of a vehicle, with Cd worsening as you increase up to about 30 degrees at its worst. Of course, the sides should also be tapered in if possible as well (also following the 10 degree rule).

Peakster's drawing is the right idea, but I think the slope could be a bit more.

If you didn't want to make a rear structure to get that 10 degree shape, you could also accomplish something similar with a "wing" (not really a wing). See http://max-mpg.com/html/tech/main.htm

Other things to consider:

- partial grill block
- smooth wheel covers, and/or
- front/rear wheel skirts
- smooth under trays
- passenger mirror delete

There's more, but there are some to think about.

[Peakster]

Originally Posted by MetroMPG

A 10 degree angle is considered optimal for the slope at the rear of a vehicle, with Cd worsening as you increase up to about 30 degrees at its worst.

MetroMPG, what happens if the angle is more than 30 degrees?

[MetroMPG]

Cd starts to improve again.

30 degrees is approximately the transition (as far as airflow is concerned) between the definition of "notchback" (if it's got a trunk - perhaps "wagon" if it doesn't?) and "fastback".

Though beyond 30 degrees, the improvement in aero isn't equal when increasing vs decreasing. IE, a fastback of 15 degrees will likely be better than a notchback of 45 degrees, all else being equal. (The notchback would have a larger wake.)

[MetroMPG]

If you were to ask me what makes 30 degrees so bad for Cd, I'd have to say I don't know for sure. Should look that up tonight.

[Peakster]

Originally Posted by MetroMPG

Though beyond 30 degrees, the improvement in aero isn't equal when increasing vs decreasing. IE, a fastback of 15 degrees will likely be better than a notchback of 45 degrees, all else being equal. (The notchback would have a larger wake.)

That would make a lot of sense. I remember my Pontiac Fiero (90 degree angle from roof to rear window) having a giant wake when white smoke came billowing out of my engine compartment when a coolant hose ruptured on the Trans Canada Highway.

[MetroMPG]

I've got "Road Vehicle Aerodynamic Design: An Introduction (Paperback)" by R.H. Barnard

Wanna swap books, clencher?

[landspeed]

I was wondering about a Kamm-back wing - but it would look a bit strange.

When I am driving, no rain hits the rear window - is that good thing?

[The Toecutter]

Your rear angle is like the worst possible in regard to drag!

Build a Komm-wing for that *****, ASAP!

You could also do a partial grill block, rear wheel skirts, full underbelly, removal of passenger mirror(or better yet, cameras replacing mirrors), removal of exterior plastic trim pieces, lowered ride height, smooth wheel covers or hubcaps, front and rear wheel spoilers, shaved door handles, small plastic or lexan windscreen in front of the windshield and raked properly may help reduce front turbulenace, shaved door handles, side skirts, rear boattail or partial boattail, among other modifications.

They each have different levels of cost and difficulty to implement. If you want the car to retain a professional-looking finish, some of these aeromods could get very expensive(such as fiberglassing an entire rear boattail). However, if you're willing to settle for coroplast or cardboard tacked on, it can be very cheap.