Repeatablitiy of Tyre Data

[BillCobb]

I will be open and honest in my belief that the Pacejka formulation is a convenient and thorough technology that clearly works for some tires. In fact, these tires are often designed and structurally configured to produce test data that conforms Pacejkally to the 3 regions of tire operation: Straight ahead ("linear range") driving on 2 or 3 lane roads at legal speeds and low bandwidth drivers. Then the midrange region with significant load transfer, softening cornering stiffness and significant second partial derivative changes to forces and moments due to slip, camber and vertical loads. Lastly is the region of operation seldom experienced by drivers still in control (or who believe they are in control) of the vehicle.

This is not a two way street. There are many tires with structure and tread materials which clearly do NOT fit the Pacejka paradigm either by design or by manufacture or by use. Work-arounds for this 'problem' include adjustment of the fitting algorithm's weighting function to downplay the area or poor representation. (You give up low g-level fidelity to get the best racing groove clarity). Or you give up the max-lat fidelity in order to meet a vehicle manufacturer's cornering coefficient specification. And of course there is the symmetry problem.

The 'best' data modeling in my experience comes from continuous splines that will clearly track all three regions and asymmetric characteristics. The weakness in them has to do with extrapolation and use conditioning factors (i.e. there is not practical way to adjust the splines for a change in air pressure or wheel rim width for example). The splines can be slow because of computational overhead and require large storage space in a tire library. You need to test every tire you use. No way to create clones of other tires because there is no structural basis for the coefficients produced. But if you have a good machine and a wisely chosen test procedure. You get data good enough to get a car successful right off the hauler, as they say. It's expensive to. You have to (or should) test every tire you use at each wheel position. Some of us have had the luxury of checking a box to get all this done, including the spare and also some non-sticker tires because the ones right out of the molds need to be cooked off in order to get stable results.

Let me also add that this whole thread also needs to have mention of a vehicle test program which is NOT related to track use or lap time generation. Instead, measuring control engineering parameters (gains and response times and natural frequencies and damping factors) for at a minimum of two conditions: Tires in a base position and also with fronts and rears switched. If the results are nearly the same, you can be pretty certain that you have a stable, consistent and manageable vehicle development program. If they are not, you probably have a 'flyer' at one position and so it is a waste of time to continue any further adjustments to the car (or truck, or golf cart or go-cart). This works well ever when the tires are different constructions on left and right sides.

I've looked at some other Hoosier non-TTC race and performance tire tests and am not surprised with what I've been reading here. They are not like globally available MacDonalds cheeseburgers all made from the same process book with identical ingredients, color, smell and taste. Since we usually tested three tires on of a given car for a model correlation comparison ( a front, a rear and a virgin extra), its nice to see a stable and consistent thin line for each tire (as I've shown in my previous rhetoric). But it isn't always the machinery, or the belt prep or the operator or the mounting and pressurization sequence. Sometimes its the Chef.

[Menisk]

Massive thanks to those that have put a heap of time into helping me come to an understanding here. I'm starting to get the feeling that the tyre data is something that needs to be used slightly differently to how I imagined. I went into this assuming that I'd be able to line up my models and do an analysis where I could say that for a given situation tyre A does what I want better than tyre B. I'm starting to get the feeling that the data is only good for giving me an idea of how tyre A works and an idea of how tyre B works, but that they can't be compared exactly. I assume that while on the belt tyre A might produce more lateral force than tyre B, I could end up finding the exact opposite on the surfaces we run depending on whether it relies more on mechanical or chemical adhesion, the ambient temperatures we run in and our ability to keep the tyre at it's operating temperature with our car.

In reality I think the only way we can properly compare tyres for our purposes is to head out to track with a set of each and put them head to head with a heap of notes and setup changes. I can see the tyre data being useful in the sense we can already have a ballpark setup for each tyre when we do this.

[Edward M. Kasprzak]

Two thougths:

All laboratory tire data should be correlated with on-car performance. Occasionally the two line-up very well and you're ready to go. Usually there is a process of understanding the differences between lab and on-car performance beforethe wealth of information in the lab data can be accessed. Adjustments in tire data handling and tire modeling are often required. Direct comparison of lab data to lab data is possible if the tests are specifically designed to minimize the influence of uncontrolled variables. For example, comparing two Round 5 TTC data sets would be reasonable, and comparing two Round 4 TTC data sets would be reasonable. But as we've seen in this thread, comparing Round 4 against Round 5 may not give good results as the tests are different enough to make a difference.

All tire models have different strong points and shortcomings. Pacejka models are not the only solution. As Doug mentioned earlier in this thread, MRA has (non-Pacejka) tire modeling technology, and there are other approaches. If you're using a model that does the things you care about well while doing a sufficient job on the things you don't care about too much then you'll probably be very happy with your tire model. Otherwise you won't be. One size does not fit all.

[Z]

Thanks to Doug for the link to the Edward's SAE 2006-01-3606 "FSAE TTC..." paper (back on page 2).

From Figure 4 of that paper it seems that the curves are in fact done with a very fine-point "airbrush". Especially so, given that each distinct curve in that figure supposedly represents 5 different camber angles! (Indicating that those particular tyres have very little camber sensitivity?)

However, the paper also has the comment, on page 2, that "... on road testing ... is not repeatable."

That gets to the gist of my feelings about this whole tyre testing issue, which is best summarised by Prof. Walter Lewin's much repeated comment from his series of MIT video lectures (use Search...)

"Any measurement is MEANINGLESS (!!!!!), without knowledge of its UNCERTAINTY!!!"

(Prof. Lewy also likes to shout a lot , and apologies to him if I didn't quite catch his emphasis correctly above.)
~o0o~

So, getting back to tyre-curves, my feeling is that if I was in the students' position, then;
1. I would FIRST want to know just how accurate (or not?) the curves are going to be.
2. Only SECOND would I want the actual curves.

This seems to be the exact opposite to the (typical) students' approach of wanting curves with Matlab-like 5 significant digit precision, but never showing any concern for just how uncertain those curves might be. (Well, hardly ever. OP and Bill and Edward and others above have qualitatively pointed it out, but still no quantitative "tolerance bands", or even estimates of such...).

So, again, "Mutter, moaaan, ... failure of the Education System, ... but at least Lewy is trying to make the point...". So once again,

"Any measurement is MEANINGLESS (!!!!!), without knowledge of its UNCERTAINTY!!!"

Z

(PS. Here is link to Prof. Lewin's MIT Lectures. See at 4:30 "Any measurement is MEANINGLESS...".
Lewy Lecture 1.)

[Flight909]

Thanks to Doug for the link to the Edward's SAE 2006-01-3606 "FSAE TTC..." paper (back on page 2).

From Figure 4 of that paper it seems that the curves are in fact done with a very fine-point "airbrush". Especially so, given that each distinct curve in that figure supposedly represents 5 different camber angles! (Indicating that those particular tyres have very little camber sensitivity?)

How do you arrived at this conclusion? Looking at the graph I see almost 10% difference in lateral forces for the different camber angles (for sure the markings on the graphs are quite difficult to see). In you're experience, how much does force vary with camber?

[Z]

How do you arrived at this conclusion? Looking at the graph I see almost 10% difference in lateral forces for the different camber angles...

Flight909,

Following are some quotes from Edward's paper, and my comments.
~o0o~

"Typical lateral force versus slip angle raw data is shown in Figure 4. This plot contains 5 loads and 5 inclination angles worth of data, which can be sub-divided into 25 individual curves."

"Slip angle sweeps to ±12 deg. at all combinations of loads and inclination angles.
* Inclination angles: 0, 1, 2, 3, 4 deg.
* Loads: 350, 250, 150, 50, 450 lb. (1557, 1112, 667, 222, 2002 N)."

My guess is that the five reasonably distinct curves seen in Figure 4 are for the five different Fz LOADS. Each of these distinct curves would then contain the five different inclination angles (~cambers). I base this on Fz loading being a first-order (ie. big) influence on lateral force, while camber angle is usually only a second-order influence (ie. somewhat smaller).
~o0o~

BUT also (!),

"Second, there is temperature variation in the data, as seen in Figure 5. As any tire is steered the surface temperature increases—the slip angle to sweep from zero to a large slip angle often produces somewhat lower lateral force than the return sweep to zero because of the higher temperature of the return sweep."

And,

"As normal load increases the tire gets shorter, and as the tire is steered the tire also gets shorter. This is most noticeable at very high loads. A careful look at Figure 7 shows that there is some leftright asymmetry. This is due to a phase lag in the test machine’s ability to maintain normal load. These small, low spring rate FSAE tires proved to be a challenge for the test machine. The effect of the normal load variation about the target is also present in the data of Figure 4." (my emphasis)

So at any given nominal normal load (Fz) there is some "thickening" of the airbrushed curve by the above temperature effects, and also by the test machine's difficulty in maintaining a constant Fz.

So that suggests that any lateral force (Fy) variation due to camber is very small indeed, since this variation might be hidden inside the above "thickenings" of the curves. And most interesting, IMO, is that this applies over the range of cambers (Edit: INCLINATIONS!) given above of 0 to 4 degrees. Quite a lot! (Edit: So from -4 to +4 degrees of camber! See PS.)
~o0o~

I would guess the Figure 4 results are from a highish-profile, bias-ply tyre, with quite a rounded cross-section? I would also guess that a low-profile (<~45%), radial-ply tyre, as commonly used on sporty production cars, would be much more camber sensitive. Perhaps losing a third (or more?) of its lateral grip over a camber range of -4 to +4 degrees???

Corrections to above welcome from anyone who knows more about these things.

Z

(PS. "Camber" and "inclination" angles are the same numerically, but may have opposite signs (+/_).)

[exFSAE]

I would guess the Figure 4 results are from a highish-profile, bias-ply tyre, with quite a rounded cross-section? I would also guess that a low-profile (<~45%), radial-ply tyre, as commonly used on sporty production cars, would be much more camber sensitive. Perhaps losing a third (or more?) of its lateral grip over a camber range of -4 to +4 degrees???

Rule #1 with tires: Do not make unbased speculation / assumptions / guesses. Even after years of working with gigabytes of data, you can be wrong more often than not.

[DMuusers]

Rule #1 with tires: Do not make unbased speculation / assumptions / guesses. Even after years of working with gigabytes of data, you can be wrong more often than not.

I agree. In my experience with designing tires, camber sensitivity is much more dependent on internal structure than shape or dimensions (though they do contribute). From tire data you can only say how stiff or soft a tire is, what its lateral stability is, camber sensitivity, etc. in trend like behaviour (not actual numbers). Linking those parameters to the actual size or structure type or whatever physical properties is in my opinion impossible (without seeing the tire).