Editorial Type:
Article Category: Research Article
 | 
Online Publication Date: 04 Aug 2021

Elimination of Stray Forces from Tire Dynamics Measurements or Beware the Backpath

Page Range: 272 – 287
DOI: 10.2346/tire.21.20027
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ABSTRACT

The forces that enter the mounted tire spindle of laboratory-type tire dynamics test machines include the following items: (1) direct tire-generated forces, tire nonuniformities, and tread pattern vibrations; (2) direct tire-transmitted rough road surface or cleat impact forces; (3) direct machine resonance-amplified versions of items 1 and 2; (4) machine frame backpath-transmitted versions of items 1–3; (5) dynamic loadcell crosstalk; (6) external noise from foundation vibrations; and (7) adjacent load station vibrations traveling through the machine frame. Although items 1 and 2 are sought in spindle vibration measurements, items 3–7 are also included in the mix and confound the measurement, confusing the analyst into thinking that machine properties are tire properties. Not only do items 3–6 not exist in vehicle operation but also comparison of results from one test machine to another can be an exercise in comparing machine to machine, not tire to tire. Tire dynamics measurements should simulate tires in roadway operation, not create a whole new set of problems that do not exist in vehicles. Elimination of item 7 paved the way to developing a tire failure warning system that operates on tire endurance test machines and can be adapted for operation on passenger vehicles to warn the driver of tire trouble. This article develops the theory of stray force measurement, describes a method for eliminating stray forces from experimental tire dynamics data, and provides experimental verification of the effectiveness of these methods.

Keywords: tire; dynamics; uniformity; vibration; resonant frequency; resonance; failure warning; crosstalk; backpath; loadcell; accelerometer
FIG. 1 —
FIG. 1 —

LSU test machine ignores inertia force “ma,” resulting in resonance amplification of the data as shown by the loadcell plot in Fig. 3.

FIG. 2 —
FIG. 2 —

HSU measurement needs to include the inertia force m according to Newton's second law to find the force f(1)(t) entering the wheel from the tire.

FIG. 3 —
FIG. 3 —

Examples of tire test machine data when imparting the force measured by an instrumented hammer (top left), loadcell response (top right), DFMS response (bottom left), and overlay of hammer and DFMS forces (bottom right).

FIG. 4 —
FIG. 4 —

Summation of loadcell and inertia forces reveals the applied force.

FIG. 5 —
FIG. 5 —

Confirmation of DFMS's full-spectrum capability is shown in the frequency response function of the spindle as experimentally determined using hammer impact with a light truck wheel mounted.

FIG. 6 —
FIG. 6 —

Calibration of mass value is achieved by nulling the “tail” after hammer retract.

FIG. 7 —
FIG. 7 —

Red plot is DFMS with a best fit null tail; blue is the loadcell reported force.

FIG. 8 —
FIG. 8 —

Backpath force moving from initiation at the roadwheel rim, and then along with some external force, moving through the roadwheel and frame to the back of the loadcell.

FIG. 9 —
FIG. 9 —

DFMS backpath vibrations at the spindle sum to zero.

FIG. 10 —
FIG. 10 —

Roadwheel hammer impact force in blue and DFMS spindle response in red.

FIG. 11 —
FIG. 11 —

Hammer impact ringdown.

FIG. 12 —
FIG. 12 —

DFMS vs hammer impact (cleat) force.

FIG. 13 —
FIG. 13 —

Direct-path (dynamic crosstalk vibrations) Fz into Fx.

FIG. 14 —
FIG. 14 —

DFMS Fz vs hammer.

FIG. 15 —
FIG. 15 —

Two-station tire endurance test machine with electronic failure warning system.

FIG. 16 —
FIG. 16 —

Backpath cancellation being shown by the loadcell in blue, m + c in red.

FIG. 17 —
FIG. 17 —

Station 1 loadcell and accelerometer data.

FIG. 18 —
FIG. 18 —

Station 1 loadcell and DFMS data.

Contributor Notes

1 Corresponding author. Principal GRP Dynamics LLC, CEO (emeritus) TMSI LLC, 3422 Bancroft Road, Fairlawn, Ohio 44333, USA. Email: grpotts@grpdynamicsllc.com
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