Why is Prius unstable in windy conditions?

They all have spoilers. It's that little upturned lip on the back of the rear hatch; the piece that feels like it will fall off when you use it to shut the hatch instead of grabbing the handle on the inside.

Tom

 

OK, I gotcha.

 

OT: One of the best tips I read on PC was to pull the hatch down and then push the hatch closed by pushing on the Toyota logo. Avoids grabbing the spoiler and possibly breaking it.

 

If you look at the front of the Prius its a rolling spoiler. The front has a very steep slope. Look at one sideways and you'll see what I mean. Why put a spoiler on the fback when the front has a huge one.

 

I've come to think it somewhat strange that PriusChat doesn't have an
Aerodynamics sub-forum in the Mods forum as a low Cd is a significant
element in the Prius' high MPG/FE. It's a fascinating and complex subject
for consideration.

The Ecomodder site does have an Aerodynamics forum. This is not a Prius
specific site. Nonetheless, there is much to be learned from the postings.
Take those little vertical walls attached to the bodywork in front of the
wheels. They be 'air dams," and despite their small size they divert air
around the spinning tires and contribute in a big way to the Prius'
outstanding Cd. It appears that if they were any bigger they'd screw up the
airflow and raise Cd.

The matter of low Cd cars tending to have some difficulties in crosswinds
has been discussed there too. (They too have a very useful search function.)

Aerodynamics -
Ecomodder.com Fuel Economy Forum

After lurking there for some two months now, I've come to believe that
there isn't much that can be done by us regular folks to significantly make
the Prius 'slippery-er' aerodynamically speaking. It seems that the Prius
body design team did a really good job.

Sure, there are some items that may help somewhat at the margin --
underbody pan, and just maybe rear wheel skirts -- but you've got as good
a chance of making things worse as better.

Oh yes, if you do make mods and want to get real numbers on their effect
on your Cd, you could rent some time at a real wind tunnel:

Cars

 

If you hadn't said it, I was about to. There's likely very little that we can do on our own to improve the aerodynamic characterstics of the car. Usually when people add things like spoilers and fairings (I've even seen vortex generators on a Civic), they just make things worse.

 

The front of the Prius attempts to maintain smooth airflow over the surface. The lowest drag would come from laminar flow over the entire vehicle, but that's not going to happed. You can stay fairly close to it at the front, but turbulence builds up as the air moves over the surface, and eventually the airflow separates. This separated airflow induces large amounts of drag if allowed to work on the back end of the vehicle, so a Kammback design, such as the Prius, has a chopped off rear end and a spoiler to induce a large bubble of rolling air behind the car. The rolling air effectively lengthens the vehicle and reduces the downward angle of the airflow, greatly reducing drag.

This explanation is a bit simplified, but all of the important parts are there to understand why you still need a spoiler at the back, even if the front is wedge shaped.

Tom

 

So back to what I said earlier, the low coefficient of drag and lack of downward force is what makes our car so affected by crosswinds correct?

 

The Cd of the vehicle has nothing to do with it. That drag coefficient is measured with zero crosswind, and the headwind component of drag has no effect on the lateral motion of the vehicle.

edit: Note that the drag of the profile of the vehicle does have an effect on crosswind stability. If it had a lower Cd, or more likely a smaller area, crosswind would have less of an effect on the vehicle.


Downforce would help keep the car in place in a crosswind, but the Prius (or any other passenger vehicle) doesn't travel fast enough to generate appreciable downforce via a wing on the back.

You can say "lack of downforce is at fault", but you could equivalently blame the Prius for being so light. If it were heavier it would exhibit increased crosswind stability.

 

The low coefficient of drag doesn't directly make a difference. Low or lower downward force, high cross-sectional area, narrow track , soft suspension, and no toe-in are what makes the crosswind handling what it is. The low coefficient of drag is indirectly related in that you could gain some downward force aerodynamically at the expense of drag. You could also increase the downward force by adding weight.

It mostly comes down to fuel economy. The narrow track, low drag, light weight, and zero toe-in all make for better mileage. It's a trade off.

Personally, I don't find the crosswind handling at all bad, but then I'm used to cars like the Prius. I've never owned a tank of a car, where you point it down the road and it just goes.

Tom

 

PC running slow and I got an error message, I didn't think this posted.

 

PC running slow and I got an error message, I didn't think this posted.

 

I think it is a combination of factors.
Prius is tall, and it is tall toward the front, over the driver and front seat passenger there fore the force of cross winds is toward the front of the vehicle pushing the front to leeward as well ast the wind pushing the whole car to leeward.
The curved tail lights allow the air to move smoothly into the void behind the prius but in a cross wind the different apparent wind over these curves creates lift behind the rear wheels to windward further exagerating a rotation of the vehicle to leeward.
This combined with low frontal resistance, low rolling resistance tyres and low to negative down-force will make the Prius feel like it is "being blown around" on the road at highway speeds.

My 2 cents worth about the spoiler.
I believe you can look at the upper and side surfaces of the Prius as you would the hull of a power boat. A power boat will get up on the plane, that is it will rise out of the water and leave a smooth indentation in the water which is much longer than the hull. The top of a Prius does this to the air. Boats have a theoretical top speed in displacement mode which is a factor of their length on the water line, once this speed is reached applying more power will either lift a planing hull onto the plane or a displacement hull will stick its bow in the air and create a deep hole in the water with little gain in speed. It is a requirement for a planing hull to have a sharp clean trailing edge that water can easily flow off of and create the hole in the water which is longer than the hull. The small "spoiler" on the hatch of a Prius is that clean trailing edge which allows the air to flow off of it in a smooth way to make a hole in the air much longer than the Prius. The sloping back window of the Prius reduces the size of the air space behind the Prius before the air flows off the "spoiler".

I'm not sure why it is a spoiler when it smooths air flow, it doesn't spoil it.