Wandering Steering at highway speeds

Small changes in the car's suspension geometry can affect stability. These can come from the chassis flexing, you might gain stability by installing some bracing.

I have both of the under chassis braces from member Rude person's (front and mid chassis) that help keep the Prius on track.

Additionally, others report noticeable improvement after installing a strut tower brace that keeps the front strut towers from flexing and causing changes in steering geometry.

SCH-I535

 

Everything I mentioned are all aftermarket parts. It will do you no good to mention it to a Toyota service representative.

SCH-I535

 

I had the exact same issue after I purchased my used 2008. It had some flavor of Bridgestone tire on it at the time, and they were only about halfway through their useful life. The first thing I decided to fiddle with was inflation pressures. Settled on 42 front, 40 rear for the best mpg. But the handling continued to be poor. Took it in to a Toyota service shop for an alignment check and everything was within spec. So I decided to just drive it until the tires needed to be replaced. Just like you are experiencing, any little anomaly in the road surface would upset the tracking. Crosswinds were also a problem, as well as the turbulent wake behind 18-wheelers. I have driven many other cars that handled poorly in the wind and have developed over the years my own built-in "steering compensator". I'm sure other drivers also acquire this ability. As I continued to drive the car, white-knuckled at times, I learned that you sort of have to disable your quick-reaction response to a perceived motion transient. A little less technical sounding, resist the temptation to correct. This was especially difficult in a crosswind situation. The car will give you directional change inputs that your old tried-and-true compensator would want to react to. As hard as it might seem, if you just keep the steering wheel steady the vehicle will stabilize. My corrections were actually causing most of the problem. Now, of course, don’t go flying off into a ditch trying to follow these suggestions. It took me several months and several thousand miles to unlearn this behavior and finally relax behind the wheel. The electric power steering on the Prius is very sensitive to steering inputs. Along with a certain amount of flex in the suspension system, things can feel a little edgy at highway speed. You said that your tires have been replaced and the alignment checked. Were there any adjustments made? Was the alignment check done by a Toyota dealer? I assume that along with the alignment check the suspension was inspected. When I finally replaced my tires (with Goodyear Assurance Comfor Tred) I had unlearned the compensation enough to really enjoy driving again. The road irregularities are still out there, and the wind continues to blow. But I’m not bouncing all over the lane anymore. Hope this helps.

 

Yes I noticed that since the day I bought my car new. There is zero dead band in the steering so you cannot relax and just truck along. Very tight. Short wheelbase and small width don't help. Steering needs constant input to stay on the road. My biggest gripe about the car.

Worse with the original Goodyear Integritys. Second set of tires and alignment is perfect and still wanders.

What does really help is inflating the tires to 42 front and 40 rear or even more. Of course the car rides like a stage coach then and hits rather hard in my opinion but you get better gas mileage and it lessens the steering issue.

 

There was a long thread about this. IIRC part of the reason was that the alignment spec for our Prius was in favor of MPG and sacrificing stability. The toe-in spec is right at 0 deg - borderline of instability. (2004 Prius Alignment Specs) It would be interesting to see if there is little toe-in or even toe-out on OP's car and if adding a small amount of toe-in would help stability.

 

Doesn't really wander it just tracks every little anomaly in the road as previously stated.

Thin car short wheelbase and zero dead band steering all adds up to a twitchy little car. You stop noticing it if you drive only that car for a while but if you bounce between different cars all the time its really noticeable.
My wife mainly drives the Prius and likes the car so and doesn't notice it.

It's a guy thing. But higher the tire pressure the more it seems to help a little but anything over 44 and the car hits way to hard for me.

 

There are many older threads on this issue with Gen IIs. I experienced your identical problem and found that rear wheel toe ("non-adjustable") was out of spec. as delivered. Some owners pestered Toyota into a new rear suspension beam. Others (like me) used shims to correct the problem. For me, it was a near-miraculous recovery - one or two fingers instead of two tight fists of white knuckles. I also bought the stronger mid-section brace, but I'm not sure how much it really helped compared to the toe correction. As others have stated, "the Prius is very sensitive to wheel alignment". Don't settle for "in spec." - try to get "nuts on" numbers as much as possible.

 

I found this helpful, it is the same idea as Mikes brace, just a different style.

g7 plate prius | eBay

 

UPDATE: Given that I have 3 hours a day in commuting time and the opportunity to drive over a varied road surface in all sorts of weather conditions, I've been giving some thought to the handling issues I have experienced and have also been reported by other owners. I am wondering now if some of these "departures from normal handling" might not be due to the vehicle stability control (VSC). We are all aware of the other intrusions into our driving day that this system makes, cutting electric motor power when wheels slip in gravel or snow, dropping out of cruise control on slippery roads. But the VSC is doing quite a bit more all the time, monitoring steering wheel inputs and comparing these to vehicle attitude several thousand times a second. The part of the system that applies counter-rotational braking to bring the vehicle back in line is what interests me most. I think this is what I am experiencing when crosswinds deflect my vehicle. I usually know what the wind direction is when this happens, but the reaction of the vehicle is counter to what I would normally expect. A gust from the left should deflect the vehicle path to the right, but the vehicle seems to angle over to the left. If you hold the steering straight the net effect is to cancel the deflection, in nearly real time. So perhaps even more reason not to use our normal human reaction and attempt to "turn" into the wind when this happens. Just some thoughts and observations from the open road.

 

I would repeat that the undercarriage brace solved this for me.

g7 plate prius | eBay

Sure beats blaming something you can't fix.

 

I've been wondering about that undercarriage brace. The stock brace looks a little twisty. Just might be worth $50 to give it a try. I'll report back when I get it fitted and do a little more driving. Thanks for the link.

 

I can only report a negative, before I installed it there were times I felt bad effects in cross winds. (passing semis)

After I installed it, I never felt that way again. (real improvement?, placebo?, I just get used to it? I can't say)

 

Ok, there's a few suggestions I can bring to the table here in my experience with this type of issue.

#1, alignment alignment alignment! Get it checked and get a printout! I know my dealership has a "Quick Check" machine that we do free and with all services to be performed. Rear toe should be as close to 0 degrees as possible. New "beams" aren't necessary if a shim can get you there. Nothing wrong with shims. The front, should be Toe-in close to .03-.05 degrees (Repair manual says 0 degrees +-.02) to help with stability as stated above. The main factor in any modern Toyota after any alignment angle has been changed is to do a "Zero Point Calibration" for the steering angle sensor. Some places, and I bet some Dealerships, won't do this. It makes the computer learn "Center" and if not done, the EPS will feel like it's searching for center because it lightens up at center. If it thinks center is actually 1-2 degrees off center, you'd get this feeling. Camber split should be NO MORE than .3 degrees (Spec is .58 degrees) to help eliminate pulls. If not within the specs, camber bolts can be installed or even the original upper knuckle bolts ground down. I don't like grinding on the bolts as I believe it does more harm than good to the strength, but it's an option.

#2, Tires tires tires! Mainly pressure. At least make them even side to side. I see most people agree that 42f 40r is the butter zone. Set them when the tires are COLD. Check them in the morning as this will likely be the coldest time of the day. A static balance is not a good practice for efficient tires. Dynamic balancing along with a road force will not only make a smoother ride, but most machines that do road force balancing can do tracking also to see if the tire pulls. The balance alone can come out true but when a road force is done, it'll determine the tires actual heavy spot. If it's too high, they'll have to phase match the wheel and tire so the heavy spot of the tire is on the light spot of the wheel (usually where the valve stem is). I've actually seen people purposely put patches on the inside of a tire on the light spots to even out the road force, but this is extreme.

#3, Body work! As you might already know, the CD of the Prius was a design feature for economy not looks. But, if you have damage to panels, including underneath the vehicle, can affect how the wind travels over, around, and under the car. Any difference side to side can cause abnormalities with tracking. While I doubt this alone would cause any issues, if multiple tolerances are close to the edge but still within, you get what we call "stacking tolerances".

Which leads me back to alignments where we see this most relevant. Just because every angle is "Green" doesn't mean the car will drive straight. I've seen it happen. Toyota had a problem when the last body style Tundra that came out. The plant workers were just making the alignment angles on the screen turn green. It wasn't until Toyota paid their Techs for hundreds of warranty alignments did they catch on because owners were complaining about pulls and SWOC. The big factor here with any of these topics is uniformity, left to right.

 

I named my Prius the "USS Wanderer" because it handles like a battleship and has the steering feel of one. In fact, the Merc land yacht S550 with 1500 lbs more has better steering feel than my Prius. The 2014 Avalon I rented recently has better feel too. Alignment is within specs, struts/shocks are good, tires are in good condition with proper pressure.

But me stops crying when me sees 50mpg.

 

Hard to give any advice without knowing what generation Prius you have. Are you sure "Other, nonhybrid" is serving you well?

 

Hadn't update my profile.... it's a '05 Prius. Got it a few months ago as my daily beater. It's been years since I last owned or drove a Toyota on a regular basis. But the ones that I drove were generally bland in the steering/handling department. I just accept the Prius for what it is - great gas mileage and practicality. Wish Koni and Bilstein made struts/shocks for the Prius but they don't. My current non-hybrid is a Mini Cooper S with LSD diff... it's a totally different animal.

 

For a Gen 2, the single most effective mod is the mid chassis brace.
G7 Performance Stiffening Plate 04 09 Toyota Prius P N 9000 | eBay