Unstable at higher speeds

Sometimes on a straight section of the highway I would let go the steering wheel and see how long the car would stay in lane. Sometimes I find the car drifts ever so slightly to the left for a second or two, then corrects itself, then drifts to the right, and then corrects itself, ... back and forth. I wonder this may be that the car is tracking the shallow depression worn into the road surface by all the cars and trucks. Pure speculation, no scientific proof.

 

that would indicate that it is hitting something. to track straight means it will eventually start drifting to one side or the other. for it to change back means it hit something to force it to do so.

and on both my Pri's i could take my hand off the wheel and maintain my lane for a good 10 seconds or so. but i guess it was during good weather.

a few weeks ago (Easter Sunday) was driving home from Salem, OR and hit a very nasty squall. it rained hard enough that i hit one spot on the freeway and hydroplaned a bit. never really lost control but did feel the momentary loss of road contact. but was it Prius-specific? nope. was on the road with a half dozen other cars doing 65. hit that spot and WE ALL slowed down to about 40 mph for the next few miles so have to assume they experience something as well.

there was a bit of crosswind there and handling really wasnt an issue... now visibility was another story. at times, i could barely see 100 feet in front of me. it was the middle of the afternoon but dark enough that headlights were not just a good idea as a car without lights would be nearly invisible 50 feet away.

 

Yes it must have hit something, and my theory (OK, pure speculation, wild guess) says that the "something" is the side wall of the shallow tire track groove worn into the road surface by all the cars and trucks driven on the road.

 

Exactly right. I've had this happen with other vehicles with worn tires before.

If a vehicle has a constant pull to one side then it is out of alignment. If it is tracking well a slight nudge will either straighten it or overshoot. But having to hold a slight countersteer would mean it wasn't tracking.

As to the dobey's post about caster and such, the basis is incorrect, because shifting the center of gravity and warping the structure is indeed changing the effective caster, camber and the toe. Stiffen the structure and those won't change as much.

 

I can't see how a crosswind can significantly change the center of gravity of a car. The only things that can noticeably change CG is a big re-distribution of mass, such as passengers, cargo, fuel depletion and so forth. You'd have to warp the car frame enough like putty for a mass re-distribution and for CG to be affected. Toe-in is relevant to crosswind effects in that it impacts straight-line tracking stability. But, whether you have correct toe-in or not, you will still have crosswind effects.

Caster and trail can only be changed by changing the kingpin (steering pivot axis) alignment - since this can only be done in a plane parallel to the direction of travel, it's quite unlikely for a crosswind to be able to affect these angles. Besides, no amount of frame warping will ever eliminate camber angle or trail in your front wheels, and the bottom line is, as long as you have caster angle and trail, the car will always be affected by crosswinds. Only improved sideways body aerodynamics can significantly reduce the force experienced by the car in crosswinds.

But I'm happy to be corrected if required, if you can outline the fundamental reasoning and physics of your argument...

 

Dobey,

When the wind shoves the car, I (we) can feel it. We can feel the body roll and that definitely is shifting the center of gravity. The effective angles are shifting and there is deflection of the structure occurring.

If you were correct, then stiffening wouldn't help in cornering either. Keeping the body more rigid makes vehicles handle better in my experience. There is a big difference between expecting something to be a cure all, and anticipating incremental improvement.

Note that I used the word "mitigate" rather than "eliminate" when I started out talking about stiffening.

I didn't say anything about the force changing, but rather the vehicle and suspension's response to it. And the response to it will be altered by stiffening things. Good luck using the above to explain why vehicles behave better with strut tower braces, sway bars, stiffer springs, stiffer bushings, etc. They most certainly do handle greater forces by proper selection of such components in my experience.

When you can explain the real world physics, get back to me. At present your case for cornering with a brace is inconsistent with your stance on wind loading, another lateral force.

 

Coming back from vacation My 2005 prius 50/48 tire pressure. At interstate speeds (75) I get a lot of weave from line to line, I followed a light green prius ,no idea of year, that seemed to track much better than mine. What can I do? Tires are oem and good tread. Front was aligned by reputabe dealer. still has plastic rail thingy that goes from one rail to other. Help

 

Well, in my dynamics terminology there is a subtle difference between shifting CG and moving CG - shifting the CG means you are relocating the CG of the structure relative to the rest of the structure (as in mass re-distribution within the structure)... moving the CG is what happens when you move the structure, which is what I suspect you meant originally. I must have misunderstood that. Ok, let's proceed from that.

I don't see that conclusion from my statement - we need to separate two concepts here: caster/trail (re steering sway/stability) and suspension support (re body roll). Stiffening the springs, or the swaybars, or the strut towers relative to each other, or the frame itself (or all of these) will indeed improve cornering and general handling (i.e. vehicle-initiated movements as opposed to external force-initiated movements like crosswinds) because these things affect body roll. However, front wheel caster angle (and hence trail) cannot be influenced by the springs, swaybars, shocks, or frame stiffness - these parameters are fixed by the geometry and alignment of the front wheel pivot joints, and the presence of trail will always mean the vehicle is always susceptible to crosswinds. Look back on the shopping cart example - you can make the cart frame as stiff as possible, and the suspension as stiff as possible by eliminating any springs, but it will still be easily swayed when you simulate a crosswind by pushing the cart from the side because of caster/trail on the front wheels.

Paradoxically, this characteristic is also what gives you your steering response when you initiate the vehicle movement through the steering wheel!

You need to be clear in your mind what you are actually referring to: response to body ROLL due to crosswinds (i.e the body structure tilting to the side) can indeed be improved by stiffening all those things mentioned above. However, no amount of stiffening will improve the tendency of the front wheels to move when subjected to crosswinds - you can have a solid brick as your frame, and eliminate any suspension in the wheels, but the presence of front wheel caster angle and trail will always cause the front wheels to sway in crosswinds, because there is an effective moment arm between the chassis and the wheel pivots. Look at a bicycle front wheel set-up for another analogy.

The only way you can affect steering sway induced by crosswinds is to alter the caster angle and trail, i.e change the geometry of the front wheel steering pivots. You can decrease steering sway (i.e improve steering stability) due to crosswinds by increasing caster angle, but it will also make steering the car harder!

Again, you need to be clear on what you are actually thinking of - body roll (which also affects cornering) induced by crosswinds will be improved by stiffening (as well as sidewise aero improvements). Steering sway induced by crosswinds, which is due to caster angle/trail, will not be improved by stiffening.

I hope the above clarifies the issues and provides you with a better explanation.

 

Centre of gravity, no. Centre of pressure, yes.

 

Which, by the way, are also continuously variable transmission driven!

 

The shoes for sure; the bikes not so much.

I'm looking into it, though. A fascinating invention of Leonardo Da Vinci's has been recreated in metal and a fluid that 'solidifies' under pressure. The NuVinci is a little heavy, especially for a race bike, but it's very smooth and infinitely variable.

 

Ah, I was thinking of the engine itself, i.e. you, the ultimate CVT propulsion! But then again, human power output isn't constant anyway....

There was another CVT concept for bikes which didn't seem to take off: http://cvt.com.sapo.pt/acvt/acvt.htm
The NuVinci looks good though, but I digress....

 

Dobey,

We'll just have to agree to disagree. I don't find your argument at all convincing. I've been clear enough in what I said.

 

It's no mere argument: the principles are quite demonstrable. Anyone can observe it next time on a trip to the supermarket.

Also, I have Tom's front strut brace installed, and while it does have a noticeable impact on cornering at speed and general handling response, it makes no noticeable difference to me in steering sway with crosswinds.

 

Check my thread on "it should have rolled" Car will over steer at highway speeds. This feels twitchy and only some race cars are designed to oversteer. Very few retail passenger autos come from factory with oversteer tendancies(public does not like oversteer). Run 2-4lbs more tire presure in front to help with issue(42f-40r). Stiff front sway bar will help. Stiffer than the TRD one(good luck finding one). Keep all luggage and heavy items in front or back seats. Not behind rear axle in trunk. When loaded down slow down. Have asked Toyota to explore and advise us, on this major issue.

 

I was trying to diplomatically say that I did not find your demonstration of "first principles" to be accurate and instead found them to be flawed and self contradictory as I pointed out. Considering it took you so many days to refine your argument and try playing semantic games, I was unimpressed. You've made some assumptions that I believe are incorrect. Applying first principles to incorrect assumptions often yields erroneous results.

I've got both the BT rear plate and front strut tower brace installed as of today (rear yesterday, other today). My first impressions were that the vehicle was considerably more solid and tractable with just the rear. I wasn't expecting as much as my butt told me was there.

I did get some decent crosswinds today and the car tracked noticeably better than it had previously. I'll be waiting for the opportunity to confirm the observation in 30 mph cross gusts on what has been the twitchiest strip of road for this car to serve as personal confirmation or denial of the initial impression.

 

Well, you didn't "point" anything out. You've lumped body roll, caster angle and trail all in the same category when they are distinct physical attributes with distinct impact on steering stability, and as I've outlined, improvement in one does not result in improvement in the other.

Hm, surprising, considering your backtracking "mitigate" versus "eliminate", and now the time it took for you to respond. So much for "agree to disagree"...

You've had your chance to show your argument, but you've absolutely failed to demonstrate your understanding of any of the dynamics involved. Your lack of a coherent response makes it impossible for anyone to reply with anything that will satisfy. Read up on trail and steering dynamics before advising others of suspension mods that you hadn't even tried yourself.

And my experience is that increased body stiffness does not improve steering stability in crosswinds. Your butt and mine are not reliable scientific instruments, and hence in the great likelihood of the presence of psychological effects, scientific scrutiny of the physical phenomena supports the qualitative observation. Unfortunately you have not presented anything near scientific understanding nor analysis of the dynamics and principles involved to support your proposition to the contrary.

What this clearly shows is your predisposition to a conclusion already made, even before the experiment has begun. See paragraph above on psychological effects.

 

I pointed out that when the frame is twisting the suspension alignment is not the same as it was before. The car will not end up pointing exactly the same as it was, nor will it track identically. Exactly how everything is changing in a complex system is not as important as realizing that your blanket statements that nothing is changing are flat out wrong--and demonstrably so when comparing one side to the other, the net will certainly change.

My Prius was twitchier (as in changing its track, requiring countersteer) before the rear stiffening plate and strut tower brace. So your theory has been disproven. As with any layman's report/review (or even a professionals) it is but one person's observation.

I could also have had a heyday with your shopping cart model since loading up the cart in various ways will indeed change the way that sucker tracks. Yes, I've tested it at the grocery store many times. You should have taken your own advice.

That is an intential lie by you. I didn't backtrack, "eliminate" was your term, not mine, as I pointed out. Unlike you I consider it unwise to make such an absolute statement and commitment to something I haven't tested. And when I made my statements I also considered tires a factor (as others have reported many times.) It gets back to the incremental nature of attacking problems.

As for time to respond, that was easy:
I didn't actually look the thread up again until I had the parts installed and tested. I had attempted to diplomatically disengage since I didn't have the parts and considered it unwise to overanalyze something when improving real world driving characteristics was the goal.

I had actually planned to do the modification sometime in the past few months, but kept putting it off. When Brian died, it appeared it might be my last chance, and I had to wait for parts. Rather than shooting my mouth off as you have been doing, I waited until I had parts and ACTUALLY TESTED THEM. And what did I discover? Much of the typical twitchiness is gone. Kaboom! There goes your operative theory. :target:

Funny, you should have directed that at yourself. You obviously have more book knowledge in the subject, but seem especially poor at applying it. My argument is still the same: your assumptions about the relationships of all the real world components remaining fixed as the frame flexes fails on several levels.

When someone like yourself tries to get into a cutesy argument over terminology (e.g. center of gravity) when the intent is clear your crediblity goes out the window.

Get some time in the seat with the mods before trying to apply book knowledge to things you don't understand nearly as well as you think you do. I'll take the real world over your slow, belabored constructions any day of the week. As it is I've now tried the mods and can report that you are wrong.

You couldn't be more wrong. I'll stick with what I can observe, and sometimes it is indeed counter to what I anticipate forcing me to reassess. So far what I've observed exceeded what I expected. It's really hilarious having you who lack the mods, tell me that my results are purely psychological. :lol:

If you (or anyone else) had the same mods and saw no effect I wouldn't conclude you/they were necessarily wrong. I've seen this sort of thing before with other modifications on many things, including cars. Some times it is the specific application/sample, sometimes it is some difference that is unaccounted for, some times it is the way the owner uses or interprets it.

At any rate, the toughest test still awaits. If it is as darty/twitchy on that same stretch of road in high crossroad gusts then I'll have to reassess the wind effect. If it isn't, then it will be obvious seat of the pants. One issue in that particular road has some fine grooving of the pavement. I've not eliminated the possibility that the combination of tires and grooving magnifies the problem in winds there.

 

What points the car is the direction of the front wheels, which is dictated by the steering alignment, which is controlled by the steering wheel. What changes the direction of the front wheels in a crosswind is the force of the wind on the body transmitted to the pivot joint, which has a moment arm relative to the wheel, due to its caster angle. Suspension struts may be depressed unequally in a crosswind on either side of the vehicle, but that by itself does not change the direction of a car. If you don't believe it, just take note when you're driving your car on a flat road without passengers - the driver side is heavier and yet your car will track straight, everything else being to spec.

Nowhere did I point out that "nothing is changing". You again fail to understand the distinct physical phenomena involved.

This makes absolutely no sense.

Or you are experiencing psychological effects. Your "proof" isn't so until it is reproducible by everyone.

So in the very next sentence you also admit it's only your (one) person's observation. Great.

Once again you fail to understand the distinct phenomena. Loading up the cart in various ways (which changes the location of the CG of the cart) is a different phenomenon to experiencing a crosswind. While I am heartened by your attempt to back up your statements with some physical experimentation, you should have tried pushing the cart on its side to demonstrate the effect of caster angle/trail, and how frame stiffness does not impact it. The topic of interest is steering stability in a crosswind.

Yet you made pronouncements and exhortations about suspension mods and steering stability without demonstrating any understanding of the facts and worse still, without even having tried any mods yourself. I'll leave it to you to name-call yourself.

Irrelevant, but whatever.

Which was too late, since you've already gone ahead to try to discredit statements contrary to yours and advised posters of the effectiveness of body firming mods with respect to steering stability in crosswinds. We can still see your previous posts, you know.

We can all see your posts proclaiming things beyond your grasp before you had the mods. Resistance is futile.

As opposed to you, I had the Tom's brace well before I posted in the thread, and my observations are based on my testing. We can all look at the dates of your posts, and bringing up the demise of your source to add to your incoherent argument is low cry-baby stuff. You should be totally ashamed of yourself.

And you can be as acerbic as you want, but it doesn't change the fact that you had spoken without knowledge, without understanding, and without testing. Now that you have some components, you are retroactively applying your preconceived notions as if these are facts, when in reality they are your biased and dubious observations only, subject to scientific PROOF.

Don't delude yourself, your observations are merely that... and you've already amply demonstrated your lack of real world physical phenomena, when you can't even outline the most basic scientific tenets of your "arguments". Your name-calling and insults don't detract anyone from this fact, and I'm not going to bother myself with it.

The bottom line is, you were (and still are) unable to scientifically explain your postulations on steering stability in crosswinds before your mods, yet you've made pronouncements testifying to your preconceived (and baseless) notions, even going so far as to reject offhand statements opposite to your beliefs. Post-mod, you exhort your preconceived conclusion and take it as gospel, when anyone with a basic scientific background knows that only double-blind tests or scientific measurements can absolutely verify the mod effects. And no, your butt is not a scientific instrument. While this also applies to my observation, as I've indicated before I'm open to scientific explanations to the contrary (which you do not have).

If you're looking for redemption, you might try another thread, or you can answer in sufficient technical detail this simple question: how can improving frame stiffness affect caster angle and trail? However, if you only have insults as in your last few posts, stick a fork in this thread and call it done.

 

Dobey,

How hoist on your own petard must you be to tell us we don't observe what we observe? Here is a thought for you: instead of arrogantly trying to tell folks it's all in their head, try to figure out what could be wrong with your disproven theory. What are you missing? Just because you don't understand or can't explain something doesn't make it so.

One of the things I learned early on with operators and mechanics (and sometimes other engineers) was not to discount their contrary observations out of hand just because I understood the theory/process better. When the observations didn't match the theory being applied, it often meant something was missing from the model of the system. I was never fully comfortable until I could explain the contrary observations.

I and others with the mods have noticed greater steering stability, tractability, etc. Since you can't explain it, that's your failing, not ours. It makes sense to me, while your explanations are lacking. It's easier to hold a line (straight or through a corner) and there is less body roll evident. I've driven three 08/09's and they all felt quite similar, these mods change the driving feel of the car.

I've done plenty of rationalizing/adapting computer models with real world processes and understand how to separate my emotion/desire from the observed behaviour. It's unfortunate you haven't developed the same skills.

So you've tested a different version of the mod and gotten a contrary result that reinforced your own theory going into the test? Do you not see what an arrogant hypocrite that makes you??? :lol: You are accusing folks of the very thing you are guilty of.

Better still, looking at your own previous post you can't even get your own story straight:

(Italics mine.)

So you are telling me you are seeing the same sort of effect as me, but I'm not seeing them. :lever: Wth? That's friggin' hilarious! Congrats on proving my point, idiot.