Clare - are smaller car tires easier to balance than SUV tires?

Hi Clare, I appreciate your advice as you and Xeno know this stuff whereas the rest just make it all up it seems.

If adding more negative camber is even better than reducing the positive caster, then that's easier, as you noted, and likely better on high speed straight-line driving (we don't do high speed cornering ever).

I will take your advice on the multi-side shoulder wear, by bringing the front tires of this RWD vehicle to 40psi or so, which I understand and where I appreciate that advice.

Since these tires were religiously rotated every 5K miles using a pattern I devised myself of H->X->H->X->H->X, etc., the tires ended up wearing "relatively" evenly overall, which is shown in this shot of the rears:

I was just about to mount & balance these rear tires as we type where you can see they're worn sort of kind of evenly except in the inside edge, where these were mostly worn when on the front axle.

I likely should flip them on the wheel at the 10,000 mile mark after the first two X->H rotations have been done, which will move the inside edge to the outside edge. These darn tires have a whitewall stripe, which I hate, so that's why I didn't flip them on the rims prior.

Not really. You are only driving in the one direction so the feathering will only be in one direction. If it were in both directions, it wouldn't be feathering, it would be *scalloping*, a diagnosis for a different cause.

You could experiment with it. After all, caster is not the only driver of steering returnability but you need to be very judicious in doing so noting that steering will be less precise, possibly more vague. Regardless, any caster reduction will be only part of the story since the primary cause, SAI, is well out of your control as it is a designed in feature.

I was going to suggest you run wide arcs around the bends but it seems that's not even possible.

It's what the tread is doing at the contact patch that is the critical issue here and that is damn difficult to visualise. Start with the forces acting on a tyre contact patch and you will see what I mean;

Once I started to understand the forces creating slip angles, then I began to get the bigger picture on tyre wear and, more importantly, handling.

I am presented with that same fait acompli with my car. My case is not as bad as yours as I get ~60,000 kilometres per set of tyres anyway, even with camber scrub issues.

You aren't Robinson Crusoe in that respect. It has taken me years to come to the level of understanding of steering I now have. I have a greater understanding of steering and handling now than had when I was teaching the topic at a technical college. I have found traditional texts on the topic aren't sufficient to give one the depth of understanding required - seek instead engineering texts on the topic and those devoted to motor racing. Those who work at the extremes of handling seem to have a better idea of what's happening at that contact patch.

I probably did. There is a whole section there on what actually happens when that tread passes through the contact patch. I cannot find a suitable diagram that provides, in and of itself, a decent explanation save for those on slip angle forces linked above because, as you have noted, it is difficult to picture in your mind what is going on.

I know what you mean. It has taken me a long time and a lot of reading about steering geometry before I had enough of an understanding how it all works.

A lot of cars have *money* wasted on them following that process. Far better to understand system operation and then experiment around the causes of the problem in order to effect a better *compromise* that mitigates your issue. As has been noted, you will not likely be able to cure your issue since it is a result of steering geometry compromise favouring highway operation but you now have a more precise start point.

You might want to discuss with him your issue and why you want to change the spec. Get him onside.

Adding more negative camber also increases SAI, one follows the other and SAI is not separately adjustable since it is designed in. That means your negative camber increase will increase SAI. As I have previously stated, steering and suspension geometry is one huge compromise so you need to think carefully about the unintended consequences of *any* non standard setting you use. That's why I said to ensure your caster spec was on the *low side* of the acceptable range. That will ensure the least amount of unintended consequences as it will still be within factory spec.

Just refresh my memory here, was it the *inside* or the *outside* of your tread that was wearing more and with the longitudinal feathering? Camber scrub tends to affect the *outside* edge of the tyre. For it to affect the *inside edge*, the camber on the tyre would need to be going well into the negative. It really can't be doing that. As I have stated previously, the camber of the tyre on the outside of the turn loses camber and goes more vertical whilst the tyre on the inside of the turn goes from slightly positive to heavily positive. The tyre on the outside of the turn could be heading slightly into negative territory but that would depend on static camber settings and would be minimal. Certainly the tyre at the high positive camber will be doing most of the shoulder wearing in those sharp slow speed turns.

Hi Xeno, I'm sorry for causing confusion.

The feathering is on the OUTSIDE edge of both front tires.

If I said otherwise, it was my mistake.

You can feel it on the edge of the tire closest to you when you stand next to the tire, which is the OUTSIDE edge.

I apologize if I said otherwise.

I need to learn more about camber scrub in slow speed turns!

Adding negative camber will countderact the effects of SAI. Increased SAI (or "included angle" in the spindle) causes more change in camber as the wheel leaves the center position.. Changing camber DOES NOT AFFECT SAI. When turning in a downhill thrusting turn the outer tire will have a negative shift in camber while the inner wheel will have a positive shift. The more positive caster, the more pronounced this "tilt" or "corner carving" tendancy. If the tires are not stiff enough or are underinflated, the tires will wear excessively on the side facing the outside of the turn. If the tires ARE stiff enough or inflated hard, the wear will tend to move towards the side of the tire on the INSIDE of the turn - tending to even the wear across the face of the tire.

That's why my FIRST recommendation is to air up the tires, and to reduce the wear on the outside of the turn, possibly increase the negative camber. Setting the caster to the high side of the positive spec will give you extra negative camber on turns without any possible negative effects of negative camber on the straight and level. It's all a compromize. Dive and roll come into play along with road banking.

a good article on camber change is here:

That assumes the tyre starts out vertical with respect to the road. In the case of camber scrub, the tyre on the inside of the turn is going to extreme positive camber angles whereas the tyre on the outside *loses* camber and becomes more vertical. that means the outside tyre sits more flat on the road. The inside tyre, on the other hand, has gained camber and is riding hard on its outer tread edge. The above scenario, with the tyres *aired up*, keeps the tyre rigid and applies *more pressure (force)* to the outside edge that is already in heavy contact with the road. The centre of the tread is where most of the deflection occurs as you increase the pressure - most with crossply tyres, least with steel belted radials. Why do you think it is that radial tyres have minimal static camber angles? It is because they naturally sit flatter on the road with sidewall deformation rather than tread area deformation. Another aspect is that weight transfer is negligible in slow speed cornering and even if it were, that would still mean that the inside tyre still gets scuffing because the load gets reduced and tread slip occurs earlier than if the tread blocks were forced to maintain contact longer. The inside tyre is the one at the extreme camber angle. If it's a radial or, more so, a steel belted radial, it is designed to operate with minimal camber.

Hi Xeno, I agree with this special case, which most descriptions on the net ignore since they're looking at racing versus nominally around 30mph.

Today I counted the steering wheel degrees, where on many turns it was only

I also hung a redneck lateral 'accelerometer' on the mirror, which was simply the USB cable for the phone looped over the mirror, where it swung from side to side constantly, but at about 45 degrees or less most of the time.

While it was hard to snap photos since one hand had to be on the steering wheel, I had expected the loop of USB cable to swing further than 45 degrees but most of the time it seemed LESS than 45 degrees, I guess because the speeds are so slow?

Indeed, it is quite noticeable by its absence on the net. I suspect that's because the number of people affected as much as you are is quite minimal. I did once find a good description of it with pictures describing the type of feathering but that seems to have vanished as has a video that described exactly what happened during sharp cornering vis a vis camber angles. I could have used a video like that in my teaching days. That said, we had the teaching model that was used in the video, a steering knuckle setup mounted on a bench, so I could have made the video myself had I the time.

Yep, it's those full lock turns at low speed that does it. The vehicle manufacturer has had to make a compromise here and you are at the wrong end of his compromise.

If I had a need to "design" a suspension system I'd just ask my good friend Gary who has forgotten more than I ever knew about performance suspension and chassis. When it comes to figuring out what's wrong with an existing vehicle for a given application I've had a pretty good record at sorting it out.

Is there ever a "pure" anything issue???

You are worried about "snap oversteer" at under 30 MPH - - - -

Which is why I recommended tire pressure check FIRST.

The USA got the Opel - we got the Vauxhall. Whether sold as a Viva HC or a Firenza it was the same car. Also sold as the Magnum in the UK

And the sla suspension is designed to do what I am recommending to do

- but just not enough for THIS situation. By starting with a static negative you are just helping the factory design do a better job.

Which is what you want - and adding an extra 1.5 degrees or so can only help

"If I knew then what I know now I wouldn't know what I know now"

On 20/6/19 7:42 am, Clare Snyder wrote:

My point in referring to those books on performance chassis and suspension systems is that it gives you insight into why things are designed in the way they are on *road going cars* by comparing them to what the racers do. The design decisions are way different, not only because the environment is different but also because the race drivers are much more attuned to the handling of their cars. What is more, those drivers will know how to tweak their car's handling to suit their own personal preference.

When the designers get it wrong, they do it spectacularly. The A Class Benz, when it was first released, was just such a cockup. In the Moose Test, it *fell over*. The natural roll resonance frequency of the rear suspension in the A Class near enough coincided with the 4 yaw reversals in the Elk Test so that each directional change *added* further to the toppling force until it became large enough to tip the car beyond the fulcrum of its outer tyre contact patches. It takes very precise counter-steering to correct such a thing *in real time* and the Benz solution was a number of modifications. The first was to modify the suspension by uprating spring and damper frequencies which increased the natural rolling frequency. That reduced the yaw enhancement coincidence effect but, because of extremes of loading between empty and full, didn't entirely remove it under all circumstances. Remember, the A Class did the Elk Test with a *full load*. The rear track was widened, this then required a greater side force to tip the car over. A stiffer front anti-roll bar was fitted which increased understeer but also reduced the tendency to tail swing in lane changing manoeuvres. This increased front tyre slip angles and rolling resistance thus causing the car to slow more quickly. The tyres were standardised at 195/50-15 instead of the original 175/65-15. These had a 10mm smaller rolling diameter lowering both the roll centre and CofG but also, to the negative, increased slightly the critical overturning moment. There were most likely other kinematic changes made to the front and rear suspensions and, though I can take an educated guess as to where these might have been made, I have seen no further detail concerning them. As a finale, stability control was fitted *as standard*, which Benz call ESP, and the precise braking reactions that the ESP gave, which in turn provided precise counter steering, finally solved the issue.

I was trained as a mechanic, not a design engineer, so I could quite easily have modified a car with different springs and created such a scenario as befell the A Class. This book;

No but, as stated, that particular feathered tread effect was pretty unequivocal as to its cause.

No but the car you correct for 30 MPH behaviour can easily become a pig at 60 or 70 MPH. That's when the snap oversteer becomes a concern.

This camber scrub issue comes about because people want steering stability at speed. Can't blame them for that and can't blame the manufacturers for biasing their designs that way, especially when only a very small percentage of drivers are going to see the extreme effects that Arlen is seeing. My Toyota has it but not to any extent that I'm going to swap tyres around on rims or anything like that. I'll just replace the tyres a tad earlier than I usually do. I managed to get only 60,000 kilometres out of the first set where I would have expected my more usual 80,000 kilometres that I was getting from previous Toyotas. It's no big deal. What I did do is change my route slightly so I travel a little further on my daily run but don't have to negotiate more than one tight roundabout and rather more gentle bends. That little bit of extra distance sure aggravates my wife though! ;-)

Yep, badgework engineering. In that era we had Isuzus dressed up as our local GM products.

I thank you, Xeno (and Clare) for UNDERSTANDING my particular issue.

I'm not actively participating simply because I'm embarrassed that I can't add any additional value, in that I can _read_ what Xeno (and Clare) say, and I can understand what they say, but when I've been driving the road daily (multiple times), I keep trying to _imagine_ what is happening, while I'm taking those turns with steering wheel inputs of o Mostly 90 degrees each way (some less, some more) o Some almost 360 steering wheel inputs o Some even 90 degrees more than 360 degree steering inputs All at around 20, to 30, to 40 (but no higher) speeds.

I did air up the tires, in an attempt to keep the tread more "firm", where I will check front camber & caster separately (I need the tools first).

I picked up what seems to be a great free accelerometer graphical tool o Sensors Multitool, version 1.3.2, by Wered Software

Here's a screenshot of the output on my Android device but I need to test it further before making any "lateral acceleration" claims ... where I'm not sure exactly yet how to test other than to put the phone on the seat as I drive downhill (uphill seems to have lesser forces for some reason).

I'm not sure yet how to interpret the difference between o acceleration o linear acceleration Given what I want is o lateral acceleration

Just turn your wheels to the full lock whilst parked on a flat surface. Then get out and look at the steering angles, in particular the camber on the front inside wheel. Then imagine what is happening at the tread blocks at that inside wheel noting that they do not follow a straight path when driving with the wheel at that steer angle, camber and lateral acceleration. BTW, page 348 of the above linked book depicts the type of tyre deformation I am on about. It applies to zero camber events however. Try to imagine what it will look like if the tyre is up on one (outer) edge and pressing hard on the tread shoulder.

This link, forgot to put it in.

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:

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.

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).

As you've explained it prior, at _slow_ speeds (30 mph nominally), there isn't as much weight transfer to the outside wheel, and yet the inside wheel is at a tighter lock than the outside wheel due to Ackerman Angle effects, where the more positive camber on that inside wheel causes the outside tread area to longitudinally feather unidirectionally more so than the 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