TireComparison.pdf

7/27/2019 TireComparison.pdf

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Tech Tip: Comparing Tires

Comparing Tire Data

Comparing the force and moment characteristics oftwo sets of tire data is an important task. It is im-portant for tire manufacturers and also for vehicledynamicists. Differences between the sets of datacould include:

Constructions

Compounds

Rim widths

Inflation pressure

Tires made in different batches

Any other change that might make a difference

Simple comparisons usually focus on directly com-paring the force and moment response of the tire.Comparisons like "which tire can generate more lat-eral force" fall into this category. This type of com-parison is useful, but does not give you the wholepicture. Comparing more complex quantities whichcannot be directly measured, but are rather calcu-lated, can give you a better understanding of the

differences.In this tech tip, were going to compare two tires well simply call them Tire A and Tire B. Thedata well use was collected at Calspan by the FSAETTC (Tire Testing Consortium). We will start bydoing the simple comparisons, then move on to morecomplex comparisons.

We can start the comparison process by creatinga new project in OptimumT. We will create two TireItems. These are the way that you organize a projectin OptimumT. The Tire Item allows you to store in-formation about the tire size, construction, manufac-turing batch, test conditions and many other piecesof information. Any number of data files and tiremodels can be associated with each Tire Item. Sincewere comparing two tires, were going to create twoTire Items. Were going to import cornering datacollected for each tire using the Import Wizard. Thiswizard allows us easily specify the format of the file(see Figure 1).

Figure 1: Import Wizard

Once we have imported the raw data, we caneasily view it. A lateral force versus slip angle graphfor Tire A is shown in Figure 2.

Figure 2: Raw tire data (Tire A)

It is often useful to fit a tire model to the data even if we wont do any simulation work. Compar-ing two tire models instead of directly comparing theraw data allows us to more easily see differences themodel removes the experimental noise and test hys-teresis inherent in the raw data. Fitting a tire modeis a relatively simple task using OptimumTs ModeFitting Tool. For this example, we will fit PacejkaMagic Formula 96 models for both tires. For moreinformation about this, please see the OptimumTdocumentation.

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Figure 3: Lateral Force vs. Slip Angle for two tiresat three vertical loads. The curves are colored ac-cording to both the tire used and also the verticalload.

Simple Comparisons

When looking at the lateral tire characteristics, thesimplest comparison that we can do is overlaying thelateral force versus slip angle plots for the two tires(Figure 3). To aid in understanding a graph, Op-

timumT has powerful coloring functionality. In thiscase, we have colored the two models differently. Thedifferent vertical loads displayed are also colored dif-ferently to allow easy identification.

From this graph, we see right away that Tire B(blue) has much more grip than Tire A (yellow). Italso appears that at low loads, Tire A has a higherinitial cornering stiffness than Tire B, and this trendis reversed at high loads. We will quantify this laterwhen we plot the instantaneous cornering stiffness.Furthermore, it appears that Tire A reaches its max-

imum lateral force at a lower slip angle. We willquantify this later as well.

More Advanced Comparisons

While looking at the simple force and moment char-acteristics of two tires is useful, the best use of Op-timumT is to look at more complex quantities.

Earlier, we noticed apparent differences in the

Figure 4: Instantaneous cornering stiffness versusslip angle for two tires at three loads.

cornering stiffness of the two tires. We can use Opti-mumT to plot the Instantaneous Cornering Stiffnessversus slip angle (Figure 4). This is the instanta-neous slope of the lateral force versus slip angle curve(Fy/). By looking at this graph, we see that ourinitial observation was correct. At the lightest loaddisplayed, Tire A has a cornering stiffness (Fy/when = 0) of 492N/o while Tire B has cornering

stiffness of451N/o. At the highest load, this trend isreversed. At the highest load, Tire B has a corneringstiffness that is over 50% greater than that of TireA.

What we can gain from this graph goes far be-yond the initial cornering stiffness, though. We seethat at low loads, the instantaneous cornering stiff-ness of Tire A falls off a lot more sharply. This meansthat at low loads, Tire A will be more controllablebut less responsive. The shape of the instantaneouscornering stiffness curves is similar for the two tires

at the two highest loads.

We can also look at the cornering stiffness coef-ficient. This is defined as the cornering stiffness (atzero slip angle) divided by the vertical load. We plotthis against normal load for three different camberangles in Figure 5. We can see from this graph thatTire Bs cornering stiffness is roughly proportional tothe vertical load. Tire As cornering stiffness varies

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Figure 5: Cornering stiffness coefficient

less than linearily with load. In simple terms, if theload is doubled, the cornering stiffness of Tire B willroughly doubled, while Tire As cornering stiffnesswill not increase nearly as much. This can give usinsight about how the tire will affect the handing ofa car.

Another difference that we saw when looking atthe Figure 3 was that Tire A appeared to reach itspeak lateral force at a lower slip angle. We can use

OptimumT to quantify this. We can plot Slip Angleat Peak Fy versus vertical load. See figure 6. Fromthis graph, we see that at very low loads, Tire Areaches its peak lateral force at nearly 2.5o less slipangle than Tire B. At nearly 2200N vertical load,the slip angle at peak lateral force is equal. The dif-ference in the slip angle at the peak lateral force hasan effect on the wear of the tire, the power consump-tion when cornering and also the rate at which thetire builds temperature.

Lets take a look at the camber response of thetwo tires since this is an important factor in how thetire behaves on a car. Were going to plot the In-stantaneous Camber Stiffness (Fy/) against slipangle for three loads for the two tires. This is shownin Figure 7. In this graph, the instantaneous cam-ber stiffness is evaluated at 0o inclination angle. Thisgraph tells us how much of a change in the lateralforce will result when the camber is changed. Asan example, if Fy/ = 0, then changing camber

Figure 6: Slip angle at peak lateral force for two tires

would have no effect on the lateral force; ifFy/ =100N/o, then changing the camber by 1o would in-crease the lateral force by approximately 100N.

Right away we notice one thing: Tire B has al-most no response to camber when the slip angle isabove about 4o. As we saw before, the slip angleat the peak lateral force for this tire varies betweenabout 6.5o and 4.5o, so the peak lateral force forTire B does not depend strongly on camber. Tire

A has a much stronger camber response at all slipangles. This is important information to have, espe-cially when choosing camber curves for the car thatthis tire is fitted to and also when choosing the casterand KPI angles.

Finally, were going to look at the coefficient offriction of the two tires. In OptimumT, the coefficient of friction is defined as the peak lateral forceneglecting the camber thrust, divided by the verticaload. We remove the camber thrust so that the co-efficient of friction is independent of the sign of theslip angle. We can create a graph of the coefficientof friction versus the vertical load for the two tiresThis is shown in Figure 8. This graph tells us aboutthe load sensitivity of the tire. The more horizontathe curves are, the less load sensitivity the tire hasHere we see that Tire A has more load sensitivity(the line is steeper). This means that a change inthe anti-roll stiffness of one axle will have a greatereffect on a vehicle with Tire A than on one with Tire

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Figure 7: Instantaneous camber stiffness of two tires

at three loads (the inclination angle is zero degrees)

B. This graph also gives an alternative representa-tion of the observation that we made earlier: thatTire B has more grip than Tire A.

This tech tip only gives a brief example of whatcan be done when comparing the lateral character-istics of two tires. Similar analysis can be done tocompare the longitudinal characteristics as well asthe combined characteristics.

If you have any questions about OptimumT orany of OptimumGs other products and services, pleaseemail [email protected]. Also, dont for-get to visit www.optimumg.com

Figure 8: Coefficient of friction for two tires.

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TireComparison.pdf