By Gerald R Potts Ph.D

 

Prior to investing in a fully equipped wear test system, as above, it may be desirable to distill the essence of such a test machine by implementing the most sensitive variables, such as tire load and slip angle into a simpli-fied wear test machine

 

Figure 1: Roadwheel Type Force & Moment Testing Machine

Laboratory tire force and moment tire test machines from the 1950’s, as shown in Figure 1, have been adapted and optimized to provide wear testing of tires. Serious development of such machines has been underway for the past 40 years with increasing precision and sophistication, leading to commercially available wear test machines, as is shown in Figure 2. Such machines can control tire load and lateral force, speed, inclination angle, driving/braking torque,and inflation pressure in order to simulate actual driving conditions of a tire in-use on a road vehicle. Variable grit surface textures can also be applied to the road-wheel to simulate such road variations. Laboratory wear testing can thus be performed by repeatedly playing braking and driving torques and either slip and camber angles or cornering force time histories onto the tire spindle inside a controlled environment, while eliminating on-road test expenses.

Fig 2: Commercial Laboratory Tire Treadwear Test Machine

Indeed such machines were initially used to reproduce tire forces and moments encountered in real driving conditions by recording such operating forces, moments and contact attitudes of tires operating on vehicles while on wear test courses. Later, such recordings have changed into “Drive Files” that characterize typical vehicle maneuvers and may be built by sequencing more or fewer of these standard maneuvers to make drive files for given test severity and then to compare wear results between tires within a group built with programmed variations in structural and compounding differences in order to identify tires with maximum wear potential.

Figure-3: Endurance Machine with Dynamic Center-point Steer and Camber Motion Attachment

Simplified Wear Test Machine

Prior to investing in a fully equipped wear test system, as above, it may be desirable to distill the essence of such a test machine by implementing the most sensitive variables, such as tire load and slip angle into a simpli-fied wear test machine, as shown in Figure 3.
A primary assumption is based upon the notion that the wear rate is largely proportional to the rate of work done by lateral slip forces generated by the tires during cornering. This then leads to the ability to cross-relate road test, tire test machine, and vehicle operating variables in a first principles theoretical development, as follows.
Cornering Power is a term used during the 1950–60’s to describe Cornering Force; however, it can be legitimately defined as: Rate of energy loss per unit time due to the resultant lateral force Fy of a cornering tire being side-slipped at a lateral velocity Vy, as

Substitute the following for the right-hand terms:

 

where α is the slip angle in radians, valid for small slip angles, say less than 10°, and since 99% of driving is done with α< 1°

 

 

All the variables on the right side of Equation (3) are easily measured during road testing. It was realized some time ago that the value of accumulating a Cornering Power history of a road course, say the San Angelo wear test course, was to lay-out a road course with a similar Cornering Power histogram near to one’s home location so that wear tests might be performed more conveniently with quantitatively similar driving severities. Of course, the road surface abrasion and weather conditions would be different, but wear testing can be expected to correlate with what would be found at the modeled road course. This also extends to the ability to translate the road course to an indoor wear test machine; however, the recorded variable ay is not a directly programmable variable on a wear test machine, so some more transformations from the Cornering Power metric to machine variables are in order.

Once again, starting with Equation (1) and substituting for the quantities on the right side,

 

Equation (4) allows the Cornering Power history recorded on the road to be played directly onto a wear test machine in the machine variables V and α.
Another interesting transformation is to express Cornering Power in terms of driver’s variables to identify and quantify the important variables in driving severity. Lateral acceleration and vehicle speed are easily recorded during road testing and slip angle is easily programmed for a wear test machine, but of these variables, only speed is directly controlled by
the driver. How can Cornering Power be expressed in terms of the driver’s variables of path (1/R), and speed?
Starting again from Equation (1), substitute as follows.


Back-substituting these relations into Equation (2) yields

and teaches that to minimize Cornering Power, a driver should not drive a heavy car around sharp turns at high speeds. That fact is well known; however, Equation (5) quantifies the relationship and shows how important speed is in influencing Cornering Power. It also demonstrates another reason, besides weather conditions, why over-the-road wear tests are highly variable, for if a driver misses his programmed speed, even a little, with a fifth power variable, it will make a big difference in the Cornering Power, and introduce wear variation.
Once slip angle has been evaluated as a source of wear, other operating variables, such as inclination angle and driving/braking torque may be activated on the simplified machine. Force and moment load cells are also available to mount between the above steer/camber attachment and the tire loading carriage for full force and moment measurement and control capability leading to fine-tuning of wear for a particular vehicle model

CONCLUSION

Indoor wear testing has progressed far beyond the simplified example given above, into recording forces and moments at each wheel of a vehicle along with suspension motions for playback on laboratory wear test machines. Such cases have been detailed by Stalnaker and Turner [1] and Knisley [2]. A number of technical papers and patents detail further work in correlating indoor wear testing with wear test driving course results and should be studied to understand all aspects of the science and also to assure compliance with legal restrictions that the patents may present.
In addition to the tire-holding and loading aspects of a wear test machine, it must be mentioned that a footprint dusting and collection system needs to be included in order to minimize the caking of rubber dust onto both the tire and road-wheel surface. Such systems typically spray talc in-between the tire and road-wheel and also collect talc and tire dust by vacuuming the enclosed tire wheel unit during the test operation.

REFERENCES:

Stalnaker, D. & Turner, J.L. Vehicle and Course Chara-cterization Process for Indoor Wear Simulation, Tire Science and Technology, Vol. 30, No. 2, Apr – June 2002.

Knisley, S. A Correlation Between Rolling Tire Contact Friction Energy and Indoor Tread Wear, Tire Science and Technology, Vol. 30, No. 2, Apr – June, 2002.