Aerodynamics and stability

[aerohead]

Originally Posted by freebeard

What would this look like? I'm hoping Cybertruck chamfers.

Yep,the Cybertruck chamfer would do it.It's just a vertical edge to burst the flow,once the relative wind exceeds 10-degrees crosswind. Hucho reported that designers consider this condition such a short-term transient event,that the overall drag of the vehicle isn't really compromised by the mutilation.

[kach22i]

Post 3195 by myself quoting a scale model site...........regarding to the Pillbug.

https://ecomodder.com/forum/showthre...tml#post607916

And Aerohead again suggested a tail fin would have probably resolved the issue.

Now I have to remember the drag vortex quotation source. Something like relieving these forces, other forces become more apparent.

On my own vehicles, aero improvements increased stability, most notably in crosswinds.

[JulianEdgar]

Originally Posted by sgtlethargic

I believe I've heard on this forum that aerodynamic cars have stability issues. Is that true? How so? What can be done about it?

Thanks

Lots can be done about aero instability!

1) Reduce lift.

Most cars have a lot of aerodynamic lift - in some cases, hundreds of pounds. Furthermore, the latest research shows that aero lift has a disproportionate impact on vehicle stability. That is, what should theoretically have very little impact (considering the mass of the car and the magnitude of the aero lift force) does in fact cause stability issues. (This may be because the oscillations in lift forces match the resonant frequency of the suspension ie around 1-2Hz - and so the car is more easily unsettled.) It's pretty easy to reduce lift - on any modern car, best achieved by a smooth undertray and rear diffuser.

2) Move the centre of pressure rearwards.

This is easily achieved by rear fins. I see some other replies have also said this, but what they haven't said is how dramatic the impact can be of rear fins. If your car turns away from cross-winds, a rear aero rear fin (or two) will make it far more neutral. (If it turns towards crosswinds, more side area is needed ahead of the centre of gravity - not common, though.)

My Gen 1 Honda Insight (900kg - 1980lb in the form I drive it) is now amongst the most stable of any cars I have driven. (I used to be a car magazine new car test driver - I've driven lots of cars.) It develops downforce at 100 km/h (~60 mph) via its undertray, and uses two rear fins.

I think 'aerodynamic stability' is an area with so much potential gain in road car aero modification - it truly makes a radical difference, especially if you live in an area that has strong winds. (And even with 100 km/h / 60 mph speed limits.)

[JulianEdgar]

In fact, to continue the subject, if you love driving, I have become convinced that aero stability should be right up their with steering feel, and handling prowess.

When as a magazine driver I got to drive some high performance cars (twin turbo Porsches, Evo Lancers, etc) on real and challenging roads, I always felt two things most strongly - how well they rode and handled, and how 'rock solid' they felt on the road at speed.

A few decades later, I think 'rock solid' can be translated as having sufficient suspension travel and damping, and having good aerodynamic stability.

My little Insight will never be a twin turbo Porsche, but at speed it really does have that 'rock solid' feeling.

I was driving home the other day from my flat test stretch. The road I was on is now largely disused - it used to be the main highway, but now a freeway has bypassed it. The result is that you have the road to yourself. It's a very windy area (lots of wind turbines sited on nearby hills), and furthermore, as you come around some corners, the car can suddenly be exposed to the crosswinds.

I was going a bit faster than usual (about 140 km/h - 85 mph) around a long corner and the car was suddenly hit by a gust of crosswind as I came over a long crest. I could tell it had happened by the sudden increase in wind noise and the aero-induced body roll, but the steering correction - if any - was inconsequential.

As any car modifier does when they revel in something they've achieved, I was chuffed. It felt good.

(Footnote: it's technically likely that drag is also reduced by decreasing lift. I am not so sure about crosswind gust susceptibility: that gets pretty hard to figure out in terms of drag.)

[freebeard]

I was headed for the Un/Aerodynamic threads, but this fits here:


Just A Car Guy: I hope this is a courtesy shuttle, or luggage delivery