This project endeavors to develop, validate, and calibrate cost-effective field test equipment for measuring tire characteristics on vehicles while driving off-road. The four wheel force transducers are designed for use on the High Mobility Multipurpose Wheeled Vehicle (HMMWV or Humvee).

The components of a Wheel Force Transducer (left), and the assembled wheel force transducer, outside view (without cover).
Obtaining tire characteristics on offroad terrain for use in tire models, as well as suitable tire models, represents a significant research challenge. A first, but extremely important step in this research was to develop suitable tire test equipment. Due to the difficulty of simulating off-road terrain under laboratory conditions, field test equipment that can determine tire characteristics on vehicles while driving over these terrains is required.

The forces and moments between the tire and the road must be measured on a HMMWV vehicle. There are many derivatives of the HMMWV with large differences in mass. The M1165A1 w/B3 version was used. The gross weight of this vehicle is 12,100 pounds. The space envelope, and therefore basic dimensions available to fit the transducer, is dictated by the rim.

Due to the suspension components on the inside of the wheel, the transducer must be mounted to the outside of the standard rim. The wheel force transducer replaces part of the rim so that the rim is connected to the hub via the force transducer. To achieve this, the standard rim is modified. Modification is simple and involves cutting out the center portion of the outer part of the original rim to the same diameter as the unmodified inner rim.

Apart from finalizing the dimensions and layout, the detail design addressed two aspects; namely, sealing and strength. V-ring seals are used to seal the spaces between the different parts of the wheel force transducer to prevent foreign objects from entering the transducer. These seals make the transducer splashproof and seal against dust, sand, mud, etc. It will, however, not be waterproof when submersed.

Strength was analyzed by performing a finite element analysis (FEM) on two load cases that are considered to be worst-case. The applied load in each case is 50 kN (or 5000 kg). This means that the vehicle can land with the full vehicle weight on one wheel, and that only one of the six load cells in the wheel force transducer can carry the entire load. This resembles an extreme load case.

Finite element analysis results indicated that the load cell flanges can be manufactured from aluminum 7075 alloy in the T6 heat treatment condition. The use of aluminum also has advantages in terms of weight and corrosion. The mass of the wheel transducer is 21 kg and the HMMWV hub adapter is 8 kg, giving a total mass of 29 kg in the HMMWV application. The existing HMMWV tire and rim weigh around 70 kg. The change in unsprung mass caused by the wheel force transducer is therefore deemed acceptable.

Material test specimens were tested at different fully reversed tension-compression force values and the number of cycles until failures were counted. The initial fatigue life of the specimens was not satisfactory, but it was found that increasing the radius and polishing significantly increases the fatigue life. The original flange design was subsequently modified to reduce the stress concentration at the critical point by increasing the radius and adding more material. The results were again verified with finite element analysis. The flanges are now expected to have a life in excess of 1 million cycles at the extreme load case.

A test flange was manufactured and fatigue tested. Initial fatigue testing indicated that the load cells and load cell pins are not strong enough. The load cell pin diameter was subsequently increased from 12 mm to 17 mm, and the shoulder in the initial pin design was removed to eliminate the stress concentration. The 12-mm-diameter spherical bearings were replaced with 17-mm bearings. The load cell design was changed to reduce stress concentrations and the load bearing area was increased.

Fatigue testing on the test rig was stopped when the top aluminum test piece completed 1.1 million cycles and the bottom piece 1.3 million cycles without any visible cracks. This life is considered more than adequate as this means that the vehicle can drive, with the full vehicle load on a single wheel, for more than 3600 km. As the wheel force transducer is a piece of test equipment that will only be used under controlled conditions for dedicated tests, this life is more than acceptable.

This work was done by P.S. Els of the University of Pretoria, South Africa, for the U.S. Army International Technology Center – Atlantic. ARL-0140


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Wheel Force Transducer for Field Testing

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This article first appeared in the August, 2012 issue of Defense Tech Briefs Magazine.

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