On Mon, 24 Jun 2019 15:44:03 +1000, Xeno wrote:

> 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. ;-)

Hi Xeno,

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!

> Yes, start with caster setting reductions as they are the safest to dick
> with.

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.

> 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.

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.

> 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.

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.

>> 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:
>> <https://i.postimg.cc/T1HkcsX5/mount31.jpg>
>
> That's quite a bit of positive camber there.

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!
<https://i.postimg.cc/hGvsXBjK/mount34.jpg>

Given that front tire is clearly worn ONLY on the outside edge, that
positive caster of the inside front tire at steering wheel lock says it
(almost) all, does it not?

>> o Driver side tire at that same wheel lock situation:
>> <https://i.postimg.cc/KYXHVC3n/mount32.jpg>
>
> Man, does that ever look quite negatively cambered. I'd have expected it
> to be closer to vertical. It could of course be camera angles. Have you
> tried dropping a plumb bob next to the tyre to get an idea of what the
> camber angle actually is on both wheels?

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.

>> It's not easy to tell, but that inner tire (which is the one wearing the
>> most in these slow speed lock-to-lock turns) should be taking on a more
>> positive camber, while the outer tire should be taking on a more negative
>> camber.
> 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.

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.

> 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.

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?
<https://i.postimg.cc/YqHVb5gY/mount33.jpg>

If so, that came from this link, for reference purposes:
<https://www.quora.com/When-turning-I-see-there-is-a-plus-camber-in-a-vehicle-Why>

Which, I quote here for reference:
"The combination of these various angles affects the wheel camber when
steered. The inner wheel in a turn takes on positive camber because the
steering pivot is angled. By the same token, the outer wheel takes on more
negative 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"
<https://www.quora.com/When-turning-I-see-there-is-a-plus-camber-in-a-vehicle-Why>

What 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.)

> 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.

Interesting ... ok, so reducing positive caster to the low end of spec can
be the first step. Thanks for that advice.

>
>> 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)
>
> Note my point re tolerances.

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.

>> 3. Third, possibly (increase?) static negative camber (within spec)
>> (although increasing negative static camber "may" also decrease the SAI)
>
> 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;
> https://encrypted-tbn0.gstatic.com/i...sn-_-So5HKf2GM
> The salient point; SAI + Camber = Included Angle.

That's a nice diagram that shows the relationship between
o Steering axis inclination
o Camber
Resulting in
o 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;
> https://slideplayer.com/slide/463401...suspension.jpg

It looks the same to me.
o Steering axis inclination
o Camber
Resulting in
o Included angle

> It'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.

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).

> We know the prime cause of the camber gain on the inside wheel of a turn
> is because of the SAI angle.

This is the key point, I think, is it not?

> We also know that positive caster merely
> worsens the effect.

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.

> So, by moving the camber by any degree to the
> negative will worsen our SAI status by the same amount.

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).

> 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.

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.

> 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.

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.

>
>> 4. Set toe to spec last.
>> [If I got anything wrong, please let me know as it's confusing!]
>
> You are definitely getting there. As I said, it really does one's head
> in trying to visualise all these steering kinematics.

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.

> 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;
> https://www.youtube.com/watch?v=Qe9AS99o3o8

Some, but not all, the vehicles I'm seeking to align at home have no front
camber adjustment anyway, where I've made a conscious decision to stay
within OEM specs.

> There 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?

Yup.
I think I have my battle plan though, although no battle plan survives
intact after contact with the enemy...

> Steering and suspension design is one huge mass of compromises.
Yup.

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?