Why is Prius unstable in windy conditions?

We probably need more information to answer your question. In a crosswind, I'd wager that the relatively large cross-sectional area of the side of the car causes it to catch the wind. Downforce won't do much to change that.

I suppose you could mount a vertical stabilizer on the aft end of the car, but that would just get you pushed around more while adding some marginal directional stability.

 

Well, although I don't have that problem, I've heard that the BT Tech Stiffening Brace (available on the Priuschat shop) has helped that.

 

Car is rather slim and stock suspension is not real tight with some free no extra charge body roll. Lots of people here add Toms suspension parts that seem to help. The car is excellent for what it is except for that crosswind problem. You get used to it though and you'll be forced to slow down. I drive across a thin peninsula between 2 Bays everyday and its crosswind city.Gotta hang on tight!! Scared me a few times. Sorry, just gotta slow down or start working on the suspension. Search: Tom's. Oh and the front end is so low now I'm clipping some driveways so don't think a front dam would work.

 

The Prius is fairly high and narrow, with a large cross sectional area. It may be aerodynamic from the front, but not from the side. The power steering system on the Prius has very little dead band (feels like zero dead band to me), which tends to cause operator induced oscillations with new Prius drivers. It has to be driven with a light hand. The crappy OEM tires are a third factor. We swapped ours for good Micheline tires and the difference is like night and day, especially at highway speeds. The suspension is also fairly soft. To summarize:

1) Height, side area, and aerodynamics - Nothing you can do with this.

2) Steering - It works well, but you need to get used to it. Don't correct every little gust of wind. Give the Prius its head and it will track fairly well. Over control and you will be all over the road.

3) Tires - Give them away and get real tires.

4) Suspension - I like the soft ride. If you don't like it, you can add new suspension components for a stiffer ride.

Tom

 

When we first got ours, it was a bit bouncy in the wind.

After better tires and a BT stiffening plate, I no longer feel the wind.

 

I was having the same problem when I drove on highways and I thought it was my alignment. (NJ has the most pot holes especially after winter).
I will be getting better tires once I wear through these OEM's. Other than that I guess I will have to get used to it... Boo

 

Hi All,

It is typical for cars with low Cd (Coeficient of Drag) , like the Prius does, to have poor cross-wind maners due to their aero shape. But this is not the case with the Prius. It was designed for a low Cd and a small, but negative coeficienct of rear lift.

But, the stock Prii can be very unwieldy in cross wind gusting conditions.

This issue has been tracked down to allignment, and chasis stiffening, primarily, and improved tires (as well as higher pressures) help as well here on Prius Chat.

First thing to do is go get an the free alignment offered during the first year of ownership at your dealer. Ask them to insure that the front toe-in is positive. The specification spans 0, and sometimes the cars are negative which is a problem.

Second thing is to get a rear stiffening plate. BT Tech is popular, but there are other vendors too.

PIO (pilot induced oscillations) can make it seem worse than it is, but even if you deliberabately hold the wheel perfectly still during a sideways gust, the response of the stock car with one or both front wheels at negative toe-in, is scarey. Do the two things above, and you wont believe your driving the same car it will be so stable. The stock car is very hard to drive by the seat-of-your-pants, due to the back end wiggle in cross winds. The stiffening plate will damp down the rear-end wiggle to that typical of other cars, and the car becomes easy to cross wind counter steer.

 

That is the opposite of my experience, as well as the opposite of what my engineering education tells me should happen.

Higher tire pressures make the problem worse. With higher tire pressures, the tire sidewalls are stiffer, and the contact patch is reduced, making it easier to change direction. The car feels "darty". Running the front tires at 35psi tends to help dampen things out, eliminates the "darty" feel, and actually helps the car feel more stable in crosswinds.

Assuming the alignment is within spec, I believe a tire change and running at mfgr's recommended pressures offer the most benefit in reducing the sensitivity to crosswinds.

 

This is food for thought for all you "over inflaters" out there in Priusland:

Lets see now, the Toyota engineers were smart enough to design a very complicated and efficient car. Remember, the main goal for them was to design a fuel efficient car, and that includes the tire pressures. But from what I read on this forum, they were totally stupid when, after running hundreds of hours of tests of tires at various pressures, they decided to "arbitrarily" recommend 35 and 33 psi for the Prius. Come on now, folks, there is a lot of wrongheaded thinking going around. Sure, you want the best gas mileage, but there are other things to consider that the Toyota engineers probably figured in. For example, the extra wear and detuning effects on the suspension components and the rest of the car. As a fellow engineer, I resent the second-quessing by many untrained people in this matter. If the tire pressures are so totally wrong, the rest of the car must also be quite poorly designed and therefore you should not have bought it.

 

I agree with you in principle. The engineers at Toyota had all of the information in hand when deciding upon the recommended tire pressure. However, it's possible that the weighting they assigned to various issues is different from the weighting a particular driver might use. Some people here would assign a weight of "0" to ride comfort if it would give them better fuel economy. Do you have data to show that higher tire pressures lead to accelerated suspension fatigue?

 

I have a one word reply with an explanation below:

Alignment



My car was a bit unstable in crosswinds when I first got it. It turned out the the rear toe settings were out of spec. It made a big difference in crosswind stability when I got this straightened out.

Gory detail in this thread if you are interested:

http://priuschat.com/forums/care-ma...3-2007-model-alignment-issues.html#post440934

 

As a fellow engineer, I'm surprised you have a such a simplistic view of the world. If engineers ran the world, it would be a very different place. (I don't know if it would be better, but it definitely would be different.)

However the world is not run by engineers. The world is run by marketing, accounting, and legal, all of whom regularly ignore what those silly engineers are saying. To conclude somehow that all of the design decisions for the Prius were made by engineers is naive, to say the least.

 

So, here's a question. Do you ever get tired of saying the same thing over, and over?

http://priuschat.com/forums/prius-m...ius-unstable-windy-conditions.html#post583693

http://priuschat.com/forums/care-ma...ng/30092-over-inflating-tires.html#post581417

http://priuschat.com/forums/prius-technical-discussion/44327-max-oem-tire-pressure.html#post580758

http://priuschat.com/forums/prius-modifications/43920-tire-pressure-2.html#post579158

I think we get your point. If we don't, then repeating it exactly the same way probably isn't going to make a difference.

 

I think the words everyone needs to keep in mind are "trade off" and "compromise". 35psi was probably the figure they came up which offered the best compromise for the average driver when considering all of the factors affected by tire inflation pressure. Someone who wants higher fuel economy and who is comfortable with the trade offs of higher tire pressure will prefer to run higher inflation pressures.

I'd like to run my tire pressures at 42psi for the improved fuel economy, but I do not like the darty feel, nor do I like how the ABS activates more frequently when braking on bumpy surfaces (tire patch loses contact with road due to stiffer tire sidewall). So 37-38psi is all I run. Others who are less sensitive to these effects won't have a problem with higher inflation pressures.

 

Hi priusenvy,

Stiffer sidewalls reduce roll. I believe Bridgestone actually markets, or did at one time, there special stiff walled tires. Reducing roll reduces camber forces. Which should help keep the car on-line. Reducing roll reduces the sideways area presented to the wind, as the car does not tip up on the side towards the wind.

I do not have engineering education in this area, however.

Your making an assumption about the patch being optimum at the door panel pressures. This has been proven false by tire life experience. If the patch has equal pressure across it, the tires wear evenly, and traction is maximized. At what pressure does that happen in a Prius with 185/65 R 15 tires ? Somewhere around 42/40 PSI. At pressures well below 42/40 the outside edges of the tires wear away faster than the center. Dramatically so at 35/33 PSI. Indicating the patch is not properly tuned. Michelin has a video which shows hydroplaning problems with tires that have low pressure in the center of the patch. You can see the water between the tire and the clear surface the camera is shooting up through with a low center patch pressure.

Yes, if you overinflate the pressure, say to something like 50 PSI (like one of our TPS moderator does), yes, the car will become darty. At 42/40, with a stiffener plate, the car is very nimble. I have had some near mis incidents, where the car was made to tip-toe around various obstacles, quite nicely with this pressure at speed.

If you want to address this topic in an engineering manner, then go get some of that pressure sensative film, and do some testing.

But hey, for everybody else who does not want to inflate to 42/40 psi, just do the alignment and the stiffener plate, and a great improvement will be experienced.

 

After 60,000 miles, in canyon freeway driving between Salt Lake City and Utah, I've come to a couple on conclusions(that have been mentioned):
1. Toyoda deflated the OEM tire to give the car better mileage and stability... the tire is still unsafe in any high speed condition.
2. Get a set of excellent all season tires; they can be inflated higher than the OEMs with no effect on stability; in fact, I have no reason to buy a stiffening plate with my Mich. hydroedges, inflated 42/40.. they are great

 

Yep,
My electronic engineer father-in-law often said that he was the only one in his department that would leave a bald spot on the circuit board for the junk that they all forgot.

 

Drive it for a few weeks. Your reflexes will learn to accommodate it.

 

:usa2:I have owned a 04 Prius since November 03, and now have an 07 and have not had anymore of a handling problem than any other vehicle in high wind situations. The only change I have made to the vehicle is ComFort Tred tires.