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Discussion Starter #1
Since buying my 2002 Prius in December I've been on the interstate a few times and have been dissapointed with the handling above 55 mph.

I have had the alignment checked and front and rear wheels are both slightly toe in and the tech said he didn't think he could adjust it any better. I've also experimented with a myriad of tire pressure settings. Things are better at lower tire pressure but the problem still exists.

I've recently done some reading in the Prius New Car Features book and I've come up with a new theory. I've attached the relevant page from this book to this post.

The rear suspension of the Prius contains toe control links, trailing arms, an axle beam, and a series of bushings. The idea here is to correct rear axle toe when cornering. The book gives an example of how a right turn can cause the left rear wheel to toe out. This is a bad thing that should be prevented when cornering. Toyota has implemented rear axle toe control to solve this problem. Toyota says this problem is solved by a toe correct function that utilizes the lateral force during cornering.

I understand how this is beneficial during lateral forces encountered during cornering.

I'm thinking about what happens when lateral forces are encountered on the straightaway. Examples include crosswinds or the tires encountering ruts in the pavement. In these situations it seems like the rear axle toe control would be undesirable since we want the Prius rear wheels to continue tracking straight ahead. It seems to me the lateral force is going to confuse the rear suspension into thinking that the Prius is cornering when it has merely experienced a crosswind. I think this could be the explanation for the squirrelly behavior I've experienced on the interstate.

I can imagine a scenario where a gust of wind would try to blow the Prius from left to right across the lane of traffic. This lateral force will induce forces on the toe control links and trailing arm bushings of the rear axle such that the the rear wheels will tend to steer in the same direction that the wind is blowing the Prius. This has an effect of making the crosswind seem much bigger than it is in reality. One can come up with a similar scenario navigating the 18 wheeler ruts that are a standard feature of Idaho interstates and highways.

Does anybody know if the Prius rear suspenion toe correct function is adjustable? I'm guessing the answer is no.

Thanks,

Ed
 
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Discussion Starter #2
There are no rear toe adjustments. How were the rear toe settings when the alignment was checked? If it's not tracking properly in the first place, you are going to have problems.
 
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Discussion Starter #3
Prius Tek,

Thanks for your reply.

My rear axle toe settings turned out to be right on the money. However, now that I've spent a Saturday studying the rear suspension on my Prius I've come to the conclusion that rear toe measurements are meaningless. Why do I say this?

First keep in mind that each rear wheel is allowed to pivot around Trailing Arm Bushing No. 3. Imagine this scenario. Put the Prius on a lift, grab the rear of the right rear tire and pull it out away from the car, now pull the rear of the left rear tire and also pull it out away from the car. Lower the Prius back to the ground and make rear axle alignment measurments (don't move the car before taking the measurements). In this scenario you should get a toe in rear axle measurement.

Now lift the car and push in the rear of both rear tires and lower back to the gound. Repeat the rear axle alignment measurements. This time you should get toe out rear axle measurements.

Since each rear wheel is allowed to independently pivot around their No. 3 Trailing Arm Bushing it is my opinion that any rear axle toe measurement is a waste of time.

Keep in mind that the rear wheels do not pivot a great deal. This is because the No. 2 Trailing Arm Bushing limits how much the Toe Control Link can pivot around the No. 3 Trailing Arm Bushing.

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Now I can talk about my experience with Prius handling. On smooth pavement with no crosswinds the Prius is the best handling car I've ever owned. However, I live in Idaho where it is unusual to find a stretch of smooth pavement and it is also unusual to have a day without significant crosswinds. It is indeed a special moment when the pavement is smooth and there are no crosswinds.

The Prius does not handle nicely above 55 mph on interstate pavement that is lumpy with 18 wheeler ruts. I now believe I understand the source of this problem and it has nothing to do with alignment. The issue is that the lumps in the pavement induce lateral forces to the trailing arm bushings in the rear suspension causing the rear wheels to "steer" the car in the direction of the lateral force.

In my first interstate freeway experience I had a tough time maintaining control of my Prius. Here was my problem. I would feel the rear of the car begin to pull the car to one side. I would compensate by steering the other direction. I would suddenly be veering way too far the other direction. Sounds like oversteer? Yes, it does. The issue was that no corrective steering whatsoever was needed and so even the slightest correction with the steering wheel gives a sense of oversteer.

I decided to test this theory. I went back out to the rutted interstate where the speed limit is 75 MPH. I convinced myself that I would not use the steering wheel to correct for any side to side movement. I would instead focus on holding the steering wheel steady and keep it aimed at a fixed point in the distance ahead. I off course would correct if it became obvious that I was going to drift out of my lane.

Here is what I found. In the deeper 18 wheeler ruts the tail of Prius "wags" as you cruise along at 75 MPH. I did not use the steering wheel to correct and focused on holding the steering wheel steady and straight ahead. As soon as you get to a smoother piece of pavement the wagging stops.

I've had my Prius a little over two months now and I think I've finally learned how to drive it. I think this "wagging" may take a little while to get used to. However, I believe there is no cure short of redesigning the rear suspension that will fix this problem. The rear wheels are merely pivoting on Trailing Arm Bushing No. 3 in response to lateral forces. They are doing exactly what they are designed to do.

When driving a Prius at highway speed focus on holding the steering wheel steady and don't try to correct for side to side motion that is caused by crosswinds or 18 wheeler ruts. The car is "self correcting" when these events occur. Any corrections that are implemented with the steering wheel will feel like oversteer because by the time you react to correct the car has also reacted to correct. If the driver and the car are both correcting you get overcorrection. A constant experience of overcorrection is worse than the "wagging" experience. From now on I'll just let the tail wag. :)

Ed
 
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Discussion Starter #4
Just came back from a trip to Vancouver Island. I found the handling gets better at higher speeds (had it up to 75 at one point). The car is also very sensitive to road ruts, probably because the track width doesn't match that of a semi trailer.
 
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Some personal opinions on high speed Interstate driving! I have about 2,600 miles on my Prius which i got middle of December. Uder 55 MPH the car handles beautifully however I do think a lot of the trouble with high speed driving is caused by the EL Cheapo tires! I just replaced the firestones on my 99 Buick regal at 40,000 miles with Good Year Intergra and car runs quieter and handles 100% better. Another thought is that I have a somewhat more difficult time getting used to the Electric Power steering on the Prius as it is entirely different than the Variable Power Steering on my 99 Regal. I especially notice the problem if I exceed 60 MPH on a very windy condition on the Prius. Any thoughts or comments would be appreciated.

Regards,
Don Good
 
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Discussion Starter #6
Some personal opinions on high speed Interstate driving! I have about 2,600 miles on my Prius which i got middle of December. Uder 55 MPH the car handles beautifully however I do think a lot of the trouble with high speed driving is caused by the EL Cheapo tires! I just replaced the firestones on my 99 Buick regal at 40,000 miles with Good Year Intergra and car runs quieter and handles 100% better. Another thought is that I have a somewhat more difficult time getting used to the Electric Power steering on the Prius as it is entirely different than the Variable Power Steering on my 99 Regal. I especially notice the problem if I exceed 60 MPH on a very windy condition on the Prius. Any thoughts or comments would be appreciated.

Regards,
Don Good
 
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Discussion Starter #7
I agree with the electric power steering. The steering wheel has no "play" and the car responds to even slight movements of the steering wheel.

I've also wondered about the tires. I've got Blizzak snow tires on my Prius right now. Ill be putting the factory Potenza's back on in a few weeks. I will pay close attention to any change in handling when I switch tires.

I've never paid much attention to rear suspension in the past. I think the Prius is the first car I've ever owned that has rear wheels mounted to a control link designed to pivot about a bushing. Have others driven cars with this type of rear suspension and noticed similar highway handling in crosswinds and ruts?

My new driving strategy is to focus on what I want the front wheels to do and ignore the side to side movement I feel from the back of the car. I'm starting to get the hang of it. I've finally figured out that there is no point trying to compensate for anything I feel from the back of the car. There is no way to correct for motion felt in the tail and attempts to control it with the steering wheel just seem to cause me nothing but grief.

I hope I don't sound like I don't like the Prius, because I really do like it. It's just taking me a little longer to "get the feel" than I thought it would. I spent my first couple months "fighting it" but I'm now starting to get the feel for it. Soon my Prius and I will zip down the rutted Idaho interstates and highways in harmony. :)

Ed
 
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I had suspected the same thing. So on my last trip I did some tests where I let the car run free (without steering at all) to see if the car was actually trying to swerve from one lane to another. I found that while the car sometimes FELT like it was swerving, it actually remained on a straight course. Now, I have not felt this whole wiggling thing as bad as some people have reported, it felt to me much like a crosswind shake, or a drafting shimmy, so my results may not apply to the severe problems others have reported.

Also, it took just a little bit of time to adjust to the power steering's responsiveness, but I like it a lot. I have a little bit of trouble switching between my two cars now, but not so much in the steering as in the braking. It was easy to get used to the prius brakes, but hard to adapt between that and a normal car. I think I like the Prius steering better than my van, but the van brakes better than the Prius.
 
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Discussion Starter #9
I think many of us are forgetting the orgins of the Prius... it was intended as a metro car in the Japanese market first. In fact, to sell it abroad they had to increase the hp for the American freeways. I certainly need two hands on the steering wheel above 75 mph.
 
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Good point Doug.

The Toyota Prius was designed to navigate through downtown Tokyo rush hour traffic using a minimum of fuel and producing a minimum of pollution. When the Prius was on the drawing board the engineers were probably not thinking about stability and tracking while travelling 75 mph on a rutted interstate while being buffetted by the wind from the Suburban that passes you because you're only barely going the speed limit. I won't go complaining to Toyota about this. :)

By the way, I find myself with both hands on the steering wheel all the time. Since the Prius does not have a convenient place to rest an arm I just leave both hands on the steering wheel all the time. I wonder if this is some kind of subliminal safety feature. :)

Ed
 
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Hi all- I also had my '02 realigned to correct specs for the same reason all the previous posts have. Squirley. I have had the car to 87 mph and scared myself (after alignment.). I am going down to my local SCCA club affiliated people (race car drivers) and asking them on what mods I can do short of - big tires (bad mileage) - or redesigning suspension. Maybe tighter sway bushings, or heavier sway bar, or Caster adjustment to more negative. (trailing.). If anyone has tried any of these options, please post the results.

I will probably lean toward bushings and caster adjustments(front wheels). I will go away from the factory specs on that one if needed.

Steve Dickerson
NYS '02 super white 10,000 miles.

love it..

[%sig%]
 
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Steve,

I'll be curious what you learn. You mentioned the possibility of adding a heavier sway bar. I don't think the Prius has any true sway bars. I think a true sway bar has one end that mounts to the frame or unibody. The Prius does have front and rear stabilizer bars and also stabilizer links on the front. If you feel the need to modify the suspension to give the car a "stiffer" sports car feel be sure to let us know how it goes. The rear end of the Prius does feel "loosy goosy" to me but I'm getting used to it. So far I think I'm happy with the feel and control of the front wheels.

Make sure you get good advice before starting on your endeavor. The Prius is a unibody car. This means that the Prius does not really have a frame upon which the body mounts. Unibody cars are more like a skeleton. One way to describe a unibody car is that in a unibody car the roof holds up the floor (or at least keeps the floor from sagging down). In a car with traditional frame construction the body is mounted on the frame and the frame carries all the weight.

Unibody cars are very popular because they do quite well in crash tests. They protect the occupants much more than a flimsy body mounted to a solid frame. I've read that the vast majority of new cars and about 1/3 of new minivans have unibody construction.

One problem with upgrading the suspension on a unibody car with stiffer components is that stresses that would have been absorbed in the suspension are now passed on to the unibody. These stresses may be too much for the unibody which will result in the car body bending and twisting instead of the suspension components absorbing those stresses. In the cases where people want to upgrade the suspension of a unibody car for racing purposes they sometimes find they must mount the unibody onto some sort of subframe support in order to get the handling they desire.

My guess is that the Prius pivoting rear wheels were a way to reduce stress on the unibody and allowed Toyota to make the unibody lighter. In order to reduce unibody weight without sacrificing crash test safety Toyota may have decided to take weight out of the floor and underbody. The downside is that now you don't have anything solid on the bottom of the car to mount a stiff high performance type suspension.

Be sure to have the race car drivers take a look at the interesting rear axle and the manner in which the rear wheels are mounted on toe control links that are allowed to both pivot and deform (bend). I would be interested in hearing their thoughts.

Ed

PS. Remember, the Prius was designed to be happiest in stop and go urban traffic. The Prius is a bit out of its element on the race track. :)
 
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Hi again Ed Davis - By the way - did anyone see a recent TV show (I cannot remember the name), which is about 2 thieves working for the Government now (as in older shows). Anyway in a recent episode there was a chase scene and our heroes (thieves), were being chased through the city streets of LA I think, and they were driving a Prius!!. Anyway I think the other car ran out of gas or something (ha ha), and our heroes won the chase..

So much for my comment on race tracks.....

As per private emails to Ed Davis, I will investigate the possiblity of adjusting the caster to a bit more negative setting if possible to see what the wind effects are then..

Steve Dickerson
NYS '02 super white @10,000 miles since November '01.

Love it!!!!

[%sig%]
 
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Discussion Starter #14
There are no caster adjustments. You may get some adjustment by loosening the front sub-frame and moving it back to make the caster more negative. I don't think you would gain enough to make a difference.
 
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