How would you run a lateral acceleration test in a vehicle on twisty roads at no more than 40mph?

How would you run a lateral acceleration test in a vehicle at slow speeds
from 20mph to 30mph on twisty roads and at no more than 40mph?
The issue is discussed in this repair thread:
o Clare - are smaller car tires easier to balance than SUV tires?
formatting link
!topic/>Where I picked up a free graphical tool for testing acceleration:o Sensors Multitool, version 1.3.2, by Wered Software
formatting link
there are two types of acceleration graphedo acceleration
o linear acceleration
formatting link
I'm not sure yet how to construct the test experiment:
formatting link
what I want to measure iso lateral acceleration
Any advice for how best to set up the measurement test?
Reply to
Arlen G. Holder
formatting link
?&q=fundamentals+of+vehicle+dynamicsThe above link should get you to this book; Thomas D. Gillespie-Fundamentals of Vehicle Dynamics Society of Automotive Engineers Inc (1992)
You can then download the full PDF file and that should help you get started. I have the hardback copy of that book. The content therein should get you started in the right direction.
Reply to
Hi Xeno,
Thanks for that starting point, where, as you know, the goal is to figure out first exactly what is going on, geometrically, with the outside feathering on long very windy (steering lock to steering lock) turns at slow speeds.
Clare, as you know, had suggested testing lateral acceleration, where he was positing that the load on the inside tire, even at slow speeds (less than 40mph) on the downhill curves, could exceed the load range per tire.
The download worked after I gave it a bogus email and password (8 characters was enforced) where I also had to give it a bogus university and department and matriculation status.
Blue drawing number #8, on page 20 was particularly instructive in the book "Basic principles of vehicle dynamics", as was blue drawing #2 on page 16.
Reply to
Arlen G. Holder
Not a hope of it exceeding the load range of the tyre. If that were the case the tyres couldn't possibly grip at normal highways speeds where the lateral forces *and* weight transfer really come into play. Any lateral testing, whether at low speeds or high, will not take into account the effect of any weight transfer. It can be calculated and this is, IIRC, given in Fundamentals of Vehicle Dynamics by Gillespie. That said, at 20-30 mph, weight transfer is negligible in terms of the effect we are seeing on the tyre shoulder. It is the extra positive camber that is creating the effect and the longitudinal feathering is the result.
I used the college
formatting link
where I once taught even though I retired from there 18 years ago. It was happy enough with that. I was in the automotive department
formatting link
. Interestingly, of the 43 teachers in my department - motor mechanics - when I first started there nearly 40 years ago, only 2 or 3 remain, all others have since retired or shuffled off the mortal coil.
Yes, that was the point I was making with regard to tyre contact patch distortion. Thinking of the page 20 diagram, now imagine what the contact patch looks like with added positive camber. It'll be *curved* and the tread blocks will be distorting and *scrubbing* - more so at the outer extremity.
BTW, I have that complete book from which that chapter is excerpted. Lots of really good info in there though it doesn't delve into the mathematics as much as does the *Fundamentals of Vehicle Dynamics*.
Reply to
Hi Xeno, I much appreciate that you UNDERSTAND the complex geometric changes that occur during camber scrub low-speed lock-to-lock conditions, where we both agree the specific phenomenon I am troubleshooting is a specific very mountainous situation which isn't covered well on the net, but which does have pragmatic workarounds, as you & Clave have discussed.
Here's a shot from today with the vehicle parked at one of the curves. o Passenger tire at (static) steering-wheel lock, heading uphill:
formatting link
Driver side tire at that same wheel lock situation:
formatting link
's not easy to tell, but that inner tire (which is the one wearing themost in these slow speed lock-to-lock turns) should be taking on a morepositive camber, while the outer tire should be taking on a more negativecamber.
Even though the outside tire is taking on more of the force, the wear is happening more so to the outside shoulder of the inside tire (the tire with the more positive camber).
formatting link
you've explained it prior, at _slow_ speeds (30 mph nominally), thereisn't as much weight transfer to the outside wheel, and yet the insidewheel is at a tighter lock than the outside wheel due to Ackerman Angleeffects, where the more positive camber on that inside wheel causes theoutside tread area to longitudinally feather unidirectionally more so thanthe outside wheel, which takes on a lesser more positive camber. Since we're effectively riding on the outside tread blocks of the inside tire, those outside blocks are forced to break traction and slide, which is what's causing the longitudinal unidirectional feathering, particularly when traveling downhill.
Note that the passenger tire of this rather heavy bimmer SUV is the original tire of only about a year and a half old (about 15K miles or so), where the outside edge counter rotational longitudinal feathering is almost worn away, but the driver's side tire had to be replaced a few months ago, where the counter rotational longitudinal feathering is easily felt on the outside few inches of the tread.
Am I correct that these are the possible ameliorations, bearing in mind that every change made has an effect somewhere else in alignment and that each change has to be made in the standard caster/camber/toe order? 1. First, increase tire pressure (to decrease tread squirm) 2. Second, potentially decrease positive caster (to the low end of spec) (where the goal is to change how SAI affects the camber angle under turns) 3. Third, possibly (increase?) static negative camber (within spec) (although increasing negative static camber "may" also decrease the SAI) 4. Set toe to spec last. [If I got anything wrong, please let me know as it's confusing!]
Obviously this is a compromise, as weight shift, self centering and steering forces may correspondingly change at speed, as you're well aware from this video clip you prior suggested
formatting link
Reply to
Arlen G. Holder
Sorry about the delay in this response; I have been giving the issue lots of thought. Far more thought than I have ever previously been required to put into it. I understand the specific phenomenon, up to a point. A lot of the heavy maths involved are beyond my pay grade. ;-)
Yes, start with caster setting reductions as they are the safest to dick with. Also, tolerances can be leveraged as well. For instance, if you have a caster range of 1-3 degrees positive with a half degree tolerance, you could arguably drop the caster to half a degree positive. You will have reduced steering return but SAI and pneumatic trail should still work for you as designed. Steering will be lighter, that is one benefit, but may be a little less stable at higher road speeds. It's a case of trying it out and seeing the effect on the highway.
Yes but, due to the direction of forces at the contact patch, the one with positive camber is getting the same force direction as the outer negatively cambered tyre. That's an important point which your link makes clear and, I must say, is the only place I have ever seen that *in print*. I had already deduced that much from observation but it is nice to see that others see it too.
By making the tyre stiffer with increased pressure, you may be reducing tread squirm. A little bit of *tyre* flex to help the tread blocks maintain contact as they pass through the contact patch would, I suspect, be a good thing. I wouldn't make overpressuring the tyres the first stage.
Note my point re tolerances.
Increasing negative camber will *increase* SAI. Remember, camber and SAI (inclination) are locked together because of design and the proof is in the *included angle*. Let's establish a few reference points here; Study the following diagram;
formatting link
salient point; SAI + Camber = Included Angle. Note that the included angle is designed into the steering knuckle and cannot be changed. That means the relationship between the camber angle and the steering axis *inclination* is fixed.
Now observe this one;
formatting link
's the same thing with an SLA suspension. This one is more relevant to us. A little aside here. If I want to change the camber to the negative, I would have to move the upper ball joint inwards. That is usually done with shims at the upper control arm inner pivot. If the adjustment is at the inner end of the lower control arm, I'd have to move the lower ball joint outwards to get camber more negative. In this case the adjustment likely would be eccentric plates or bolts on the lower control arm inner pivots. Since I cannot alter the *included angle* as it is designed in to the steering knuckle, what do you think is happening to the tilt of that pivot axis (line between ball joint centres) as I move the upper ball joint inwards while the lower ball joint remains where it is? You have to be making the steering axis tilt *away* from the vertical reference Try to imagine you're confronted with a vehicle where the static camber setting is 0 and the included angle is, say, 10 degrees. Given this; SAI + Camber = Included Angle - therefore 0 + 10 = 10. If I now want to change my camber's current 0 degree setting to the negative, say, by 2 degrees, and given my included angle is currently 10, my camber change has to increase the inclination of the steering axis by 2 degrees. It will be leaning further inwards at the top ball joint by 2 degrees from its original 10. It will now be inclined by 12 degrees with respect to the vertical reference point.
We know the prime cause of the camber gain on the inside wheel of a turn is because of the SAI angle. We also know that positive caster merely worsens the effect. So, by moving the camber by any degree to the negative will worsen our SAI status by the same amount. This is the conundrum, one of many, that face the suspension designer. The increased SAI, in and of itself, will increase camber gain - worsening the very problem we are trying to solve. The manufacturer believes, with the current settings they specify, that they have the middle ground covered. The difficulty is that your situation is at the extreme boundary of that middle ground.
Have a look under the car in question and see what the camber adjustment actually does. If it changes the relative *lateral* location of one ball joint to the other, you are effectively screwed. That means the included angle cannot be changed without some bending or a different steering knuckle with the appropriate included angle.
You are definitely getting there. As I said, it really does one's head in trying to visualise all these steering kinematics.
It gets a bit more messy with MacPherson struts, especially when you move to superstruts. Given the distance between the two pivot points on the steering axis, getting any decent degree of camber and caster change is rendered more difficult. That said, depending on the manufacturer's method of attaching the steering knuckle to the strut, there are ways to adjust camber without *adversely* affecting SAI. For that purpose you use what are known in the trade as camber bolts. These are eccentric bolts that allow about ~1.5 degrees of camber adjustment. You can get camber kits that *extend* the limited factory adjustment as this video shows;
formatting link
may be some SAI effect depending on the system used but, compared to the *limited* options (bending, replacement knuckle) available to you on SLA suspensions, there is hope. Note, I think that camber kit as shown in the video will have some effect on SAI but not as much, in terms of degree change, as it has on the camber itself. Confused now?
> > Obviously this is a compromise, as weight shift, self centering and > steering forces may correspondingly change at speed, as you're well aware > from this video clip you prior suggested >
formatting link
Steering and suspension design is one huge mass of compromises.
Reply to
I appreciate the time, effort, and care for accuracy, as very few people would understand my common statement that the main reason most people can't do alignments at home isn't that they can't measure or tweak, but that the math would make most people's head explode.
As you and Clare are well aware, it's a LOT more than just trig! o Suspension geometry inter-relationships are engineeringly complex!
Thanks for confirming that the first step (whether or not I choose to go by airing up the tires), is to lower positive caster to the lowest angle within spec.
I agree with the tactic of using the tolerances, inevitable in any suspension geometry result, which is why the spec is always a range.
The good news is that it's possible to measure with sufficient accuracy, I believe, at home, using the home measuring equipment we've already stated, all of which, as always with tools, end up being free: 1. Camber bubble gauge & wheel clamp jig 2. Toe plates (usually an extension to the camber wheel clamp jig) 3. Tape measure and trigonometric calculator
In addition, we've discussed these "niceties" in terms of free tools 4. Steering wheel lock (which can be rednecked) 5. Turn plates (which can be rednecked)
Since the home alignment tools are free, the problem is simply that the math involved makes most people's head explode; which is why I very much appreciate your astute step-by-step advice.
I agree with all your statements that I need to realize alignment spec ranges are a mix of compromises, where, luckily, I drive like a little old lady on the highway, so "at speed" isn't more than about 75mph or 80mph at most, even on Highway 5 which has a 70mph speed limit, as I recall.
I thank you for cluing me in to LOOK for the high positive camber at steering wheel lock, which is, in reality, the REASON for the contact patch unidirectional feathering on ONLY the outside edge of the INSIDE tire!
formatting link
that front tire is clearly worn ONLY on the outside edge, thatpositive caster of the inside front tire at steering wheel lock says it(almost) all, does it not?
Thanks for pointing out that this outside tire, at static steering wheel lock, shows what appears to be negative camber, but where the tire, which has a few thousand miles on it, is STILL worn more on the outside edge, with the unidirectional feathering easily felt when running my hand over the tire in both directions.
My main plan for measurement is to obtain the camber gauge and wheel jig first, and then I can check all sorts of things for a DIY alignment o Camber at tires pointed straight ahead o Camber with tires at plus and minus 15 degrees (to calculate caster) o Camber at full steering wheel lock (to assess effects of camber scrub) etc.
Aurrgh. This suspension stuff is so confusing it makes my head explode! :)
I "thought" that the outside front tire in a slow speed tight turn downhill carries most of the weight, but, that outside tire has a flatter contact patch due to the effect of decreasing positive camber, so it's the inside front tire that wears most because, while it's not carrying most of the weight in the steep tight turn, it's far more tilted onto it's outside edge, such that the weight it does carry is disproportionate carried by a small portion of the outside tread of that inside tire.
Hmmmm... I'm confused so can you clarify which link you're referring to?
Since it's an important point that both of us know better than almost anyone on this thread, which is that what we're seeking to explain is not "in print" on the net in very many places, if any - is _this_ the link?
formatting link
so, that came from this link, for reference purposes:
formatting link
, I quote here for reference: "The combination of these various angles affects the wheel camber whensteered. The inner wheel in a turn takes on positive camber because thesteering pivot is angled. By the same token, the outer wheel takes on morenegative camber.
But actually both of these things are desirable. The purpose of negative camber is to increase grip in a turn. As a car turns, the tyre has to resist the force which is causing the car to turn. The outer wheel does most of the work, but without negative camber, the tyre would tend to roll away under the wheel and reduce contact with the road. The camber counteracts this effect so that the tendency of the tyre to roll under actually increases the contact, and hence grip, just when it¢s needed. But think about the inner wheel in the same turn. While it carries less of the turning force, what appears to be positive camber is actually negative camber with respect to the turning force. So that tyre is also gripping harder than it would do without camber.
*So it only looks like positive camber - in fact it¢s really negative* *camber, because it¢s the inner edge of the wheel that is leading in a* *turn*"
Aurrgh. This constantly changing camber stuff is making my head explode! :)
For example, I've re-read this sentence a hundred times, and I still can't make any sense out of the camber mechanism ... can you? "But think about the inner wheel in the same turn. While it carries less of the turning force, what appears to be positive camber is actually negative camber with respect ot the turning force"
formatting link
does _that_ mean? How can positive camber be negative camber at the same time? (I realize force vector diagrams are involved - but it's hard to fathom.)
Interesting ... ok, so reducing positive caster to the low end of spec can be the first step. Thanks for that advice.
Yup. It can actually be below spec, if I could measure to better than tolerance, given the spec includes worst case tolerance already.
Thanks for that observation.
Resulting in o Included angle
Resulting in o Included angle
As always, you're way ahead of me, so I'm reading (and re-reading what you wrote), where it all makes sense while I read it, but I have to ABSORB it to make sure it makes sense intuitively to me (sort of like how I have to constantly shift my mind when thinking of gravity as a curvature in spacetime as opposed to a simple force).
This is the key point, I think, is it not?
Yup. This need to lessen positive caster is something you've finally worked into my brain so that it's now "intuitive", much like how I'm trying to work the fact that there are, in reality, something like 10 dimensions to the universe, which takes a while before it becomes intuitive.
Ah. This is important!
It means I don't want any more negative camber than the spec (plus tolerance) allows for, most likely (at least for this one purpose).
Yup. Luckily I don't care much for high speed stability, if they define high speed as over about 80mph, which I almost never go.
I care about tire wear on the outside edge of the front tires mostly.
Each vehicle is different, where my bimmer sedan, for example, only has camber adjustments in the rear, and where aftermarket camber plates are needed on the front struts, which is going too far, IMHO.
I'll stick to the low end of spec, within tolerances, as you suggested.
Yup. Of the half dozen jobs I want to do that most people NEVER do in their entire lives, the alignment is singularly different not because it's hard to measure at home (it's not), nor because it's hard to adjust (that's easy) - but - because the knowledge needed is of an amount that makes most people's brains explode.
I think the MAIN takeaway is to reduce caster to the low end of spec.
Yup. I think I have my battle plan though, although no battle plan survives intact after contact with the enemy...
One thing is whether we can accurately measure camber "well enough" with a cellphone, where I use a $100 level but a cellphone "might" be accurate enough by now... do you think?
Reply to
Arlen G. Holder
Hi Xeno, BTW, one statement I can't yet wrap my head around is this one, which says that the high positive camber on the inside tire on turns is actually a NEGATIVE camber with respect to the force vectors.
formatting link
What the heck does _that_ mean?
Reply to
Arlen G. Holder
I thought that one would stump you. ;-)
The clue is in the wording - *with respect to the force vectors*.
That diagram you linked to above, I'll use that in an attempt to explain what is meant.
For a start, dismiss the middle wheel, ignore it.
Now imagine the left hand wheel is on the *outside* of the turn. You can see the effect of the lateral forces acting on the tread. The arrow indicates the *force vector* direction.
Lets now deal with the other wheel. Imagine it is the wheel on the other side of the car and the axle is coming out of the left side rather than the right. It has positive camber with respect to its position on the car but, naturally, it is leaning the other way. The lateral forces, however, are acting in the same direction as the outside wheel but the effect on the inner wheel is different. *To these forces*, the inside wheel looks to have negative camber because they aren't referencing the vehicle as being left or right. The forces see two wheels trying to turn in the same direction but are leaning in opposite directions.
This picture will assist you in imagining the scenario.
formatting link
One wheel, the outer, is showing a positive camber to the lateral force. The inner, on the other hand, is showing a negative camber lean with respect to the lateral forces. It will look like this;
formatting link
above pic shows what the inner wheel likely looks like on the road in a sharp turn. Just imagine that red arrow is the turning force at the contact patch. It is not hitting the outer tread edge *first* like the opposite wheel is. The outer tread is not touching the road so the force is acting on the outer half, or less, of the tread. That will cause some distortion of the sidewalls as the mass of the car tries to pull against the contact patch and flatten the tread somewhat. But, and it is a big but, modern radials don't have a lot of ability to flatten the tread, steel belted radials even less so. Overinflated tyres really aren't going to mitigate that very much. The pic really does show what you are up against. Have you looked under your tyres whilst parked on a flat surface? I think you would be enlightened.
Whilst on the topic, have a look at this book;
formatting link
was browsing through my dead tree library here and came across the following. Go to page 181, section 3.5.3. If you don't get the meaning of the text, Fig. 3.50 should make it clear. The kinematics of the double wishbone suspension (the SLA) make it clear that the KPI (same as SAI) *angle* alters on bump and rebound due to the different radii in the arc of each arm. That same angle alteration of KPI angle also affects camber. It is in the text description of the diagram; Construction determination of the kingpin inclination alteration angle on double wishbones which is *equal* to the camber alteration.The highlight is mine but what that diagram is telling you is that as you go into *bump*, your KPI increases and your camber moves commensurately to the negative. The same applies to McPherson Struts as can be seen in Fig 3.51
This backs up what I was saying with regard to the camber angle change on an SLA suspension following the SAI/KPI change, and vice versa. If you alter the camber, you have also altered the SAI/KPI. So, by moving camber to the negative you are increasing the SAI angle, with all the possibilities, both negative and positive, that entails.
What does this mean to us when cornering? Well, weight transfer will force the *outer* wheel into bump and give you a more negative camber on the *outer* wheel. The problem *for you* is that weight transfer works best *at speed*. At low speed, the outer wheel is probably retaining some positive camber. Have a look at your outer wheel when your steering is at the full lock. I'm guessing it may well still be at a positive camber angle. Not as much as the inner wheel but it too could be contributing to the edge wear.
Anyway, the first pic on this thread led me to this article which I think you will find interesting.
formatting link
relates to him switching from a strut with no camber adjustment provision to one that did have that provision. If you recall, I said that a strut suspension, depending on where the adjuster is, can have a camber adjuster that doesn't affect SAI much, if at all.
In summary, if you want to understand the kinematics and elastokinematics of suspension systems, this book will do it for you;
formatting link
linked to it previously. I have dead tree versions of the first (English) edition 1996 and the second (English) edition 2001. I'll respond to your other post shortly. Right now I've done my head in again cogitating over all this stuff. ;-)
Reply to
It did.
It's hard to figure out where the abnormal tire wear is coming from when the suspension geometry of Gillespie indicate high positive camber on the inside wheel in a tight turn while the force vectors indicate (high?) negative camber on that same wheel!
(For reference):
formatting link
throws me off is that the left wheel is positive camber while theright wheel is negative camber, where, on the vehicle, in a turn, I'm notsure _that_ is what happens since they both started out with similar staticcamber, and when the steering wheel was turned to lock, they each took on a"more positive" camber, didn't they? It's really just "one" wheel, in three different situations, right? a. positive camber on turns (with a small force vector to the right) b. zero camber on turns (with a larger force vector to the right) c. negative camber on turns (with an even larger force vector)
Heh heh. I never _understood_ what it was doing there anyway! :(
It seems like this is the _outside_ wheel, if we assume the car body is to the right of each of the three wheels, and that the center of the turning circle is also to the right.
So that diagram seems to be indicating the _outside_ wheel in a turn, does it not?
Yes. The tread in the lefthand wheel is firmly on the pavement while the weight of the vehicle is forcing the tire to the left of the tread, making the tread 'squirm' due to the lateral forces acting toward the center of the turn radius (I think).
Like this, right?
formatting link
It has positive camber with respect to its position on the > car but, naturally, it is leaning the other way. In the updated diagram above, BOTH wheels now have positive camber if we look just at their suspension geometries. Is that what you mean?
This is a good point that, if we assume _both_ wheels "start" out with positive camber, then the lateral forces are pushing both footprints to the right in that modified diagram, toward the center of the radius force vector.
My problem, I think, is that I _thought_ the geometric camber of the outside wheel, in a turn, is nearly neutral, while it's only the inside wheel which has high geometric positive camber.
However ...
With regard to the force camber (so to speak), I easily comprehend that it's obvious that, if we assume the two wheels in the modified diagram are _both_ positive camber (as in the modified diagram), then for sure, it's easy to see that the radius force vector for the left (outside) wheel is to the right, away from the direction of lean, and yet, in the case of that right (inside) wheel, the radius force vector is toward the direction of lean.
That is, if we assume both wheels start with positive camber... o The left wheel force vector is _away_ from the direction of camber lean o While the right wheel force vector is _toward_ the direction of lean
But what confuses me is Gillespie says, I thought, that the geometric camber _changes_ on a steering-wheel-lock turn such that the left (outside) wheel _changes_ geometric camber (on that one outside wheel) to be almost neutral?????
That is, my confusion arises from the fact I thought the situation was o Both wheels can start out in a straightaway with positive camber o But when you turn hard right, the steering geometries were such that o The left (outside) wheel takes on an almost neutral camber, o While the right (inside) wheel takes on a decidedly positive camber
So my confusion stems from the question of isn't it unrealistic to assume both wheels have positive camber in the middle of the tight turn?
ooooooooooooh.... I think I get it now.
If I put my hands with the pinky on the table out in front of me, and the thumbs up toward the ceiling in a vertical and then lean both hands outward to simulate high positive camber on both, and then, I imagine a turn to the right, BOTH hands tilt to the right.
However, what was a positive camber in the left hand, starts to look more and more like less positive camber and then neutral camber and then maybe even slightly negative camber in the left hand, since the force is to the right.
Meanwhile, what was a positive camber in the right hand, which only gets more and more and more positive as I lean both hands proportionally to the right, such that the right hand (inside wheel) decidedly looks vastly more positive in the right hand, since the force is to the right.
This part I completely understand now, and agree with you, as the wear pattern is extremely consistent with exactly that situation!
formatting link
'm working on responding to the rest of the post, but I have to go to ameeting a few hours away so it won't be until tonight.
Reply to
Arlen G. Holder
On the picture, yes. I was wanting you to think outside the square.
The article is intended to represent the different possible positions of a wheel on *one side* of a car. The middle wheel is representative of a wheel with 0 degrees of camber. It is flat on the road and has more grip than a wheel with positive camber *on the outside of a turn*.
That is why I added that pic of the old race car - you can see the positive camber on *both wheels* and their relationship to each other. What I wanted you to do was look at the force vector arrows that, in a turn, run in one direction. If you superimposed those arrows under the race car in the appropriate direction, you get closer to my point.
Yes, it is limited in the amount of camber roll compared to the inside wheel. It would look like the wheel you erased, the centre one. It would still be less effective than the one *presenting itself* to the force vector as a negative camber. That said, there is a limit to the benefit of negative camber.
Whilst on the topic of camber, what you need to get squared away in your mind is the *effect* of camber, as a separate entity, before you look at its place in a car. This article is a good start and introduces an important term:
Camber Thrust;
formatting link
describes it well. Moving right along, in a vehicle, the camber thrust of one side is balanced out by the camber thrust of the other, through the *steering linkage*, the net effect being the thrust forces of each side are balanced. You might ask, "Why do we have *static* camber at all?". The effect of each wheel trying to deviate from the centre line of the vehicle in opposite directions puts the steering linkage under tension. That helps reduce the effect of any free play that could potentially create a *shimmy* effect. That's one effect of camber, be it positive or negative, that has an influence on straight running. SAI. caster, mechanical trail & pneumatic trail also have influences on the straight running of a vehicle - primarily keeping the vehicle travelling straight or returning the vehicle to straight running after a turn. When you want to turn, however, you end up *fighting* these forces.
let's delve a little deeper. The camber thrust I mentioned above leads to a phenomenon know as *camber steer*. When a vehicle's wheels are inclined with respect to the vertical, the rolling radius is shorter on one side of the tread than the other. This link shows it best;
formatting link
tyre forms the frustrum of a cone and tries to rotate around its apex. This causes the wheel to deviate from a straight path to produce the effect known as *camber steer*. Positive camber will, therefore, make the wheels turn away from each other (toe out). Negative camber however will make the wheels turn towards each other (toe in). This is one of the reasons why the wheel track has to be set to match the design of the suspension to counteract the inherent tendency of the wheels to either move towards or away from each other. One advantage, as I stated before, is that it keeps the steering under tension. It is also the primary reason why camber angles must be matched side to side. Now, consider this, when you want to turn a corner you are fighting against caster, SAI, mechanical trail and pneumatic trail, all of which are trying to return the steering to the straight ahead position. On a nose heavy car, that can be quite a challenging task when you realise that, through the effects of SAI you are actually lifting the front of the car.
Now, knowing the effects of camber steer in the straight ahead position, what happens when we *turn* the steering? The inside wheel *increases* positive camber. That means the steering now has an imbalance in camber steer. The camber thrust at the inside wheel has now increased and is *assisting* in turning the car against the forces that are trying to straighten it. What about the outside wheel? Well, it has gone from having a matched opposite lean to leaning the same way as the inside wheel though, as you have noted, not as much. Regardless, it is still assisting through its increased camber steer, in turning the vehicle in the same direction as the inner wheel's camber steer is going. This is one of the reasons I showed you this pic;
formatting link
is what your car is trying to do, face the force vector resisting vehicle inertia with a negative camber but, at the same time, use camber steer to assist in the turn. On some cars, camber steer can be utilised effectively in aiding steering ease but, on others, and the Toyota Yaris is one, you can feel the self centering effects of SAI, caster, etc. up until about 2/3rds steering lock. After that point the increasing camber steer effects take over and dominate to the point where, when you want to straighten up, you have to physically turn the steering back to the centre until you reach the point where the self centering forces again dominate. Even powering forward (it is FWD) doesn't provide sufficient self centering forces to counter the camber steer. It is a weird sensation and it feels as if the steering has hit some *over-centre* point. It tells you how powerful camber steer can be but it also indicates that the manufacturer of the Yaris, and similar cars with the issue, hit a compromise point and you can see, or feel, that it was a middle of the road option. It also tells you that, on the more heavily loaded *outside* wheel, too much negative camber can create unforeseen issues. The manufacturer, in this case, has decided that high lock situations will only occur in very low speed events. Therefore the camber steer issue at high lock wasn't an issue and the focus was placed on low steer angle, high speed situations.
Yes. Suspension design however plays a significant role here. Some designs can travel from positive to negative and end up back at positive by the time full lock is achieved. Some strut types, IIRC, have this issue.
Yes, depending on suspension design.
In some case, that is the assumption, in others, not. I saw a car today that had a decided negative cant on the outside front wheel. Not as much as the inside runner, of course, but enough to be visually noticeable.
Spot on. Looks quite horrible, doesn't it? The wider the tyre, the worse it gets. What's more, pumping the tyre up more stiffens the sidewalls of the tyre and likely will aggravate the issue. Low pressure = more sidewall flex, high pressure = less.
Take your time, lots of info to digest and more in this post.
Reply to

Site Timeline Threads

MotorsForum website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.