Pneuman Test Sim...

Load Mechanical Systems

Needs["Mech`Mech3D`"]

ground = 1;

base = 2;

wheel = 3;

SetBodies[
Body[ground,  
          PointList -> {{0, 0, -11}, {1, 0, -11}, {0, 
                1, -11}}], 
Body[base,  
          PointList -> {{-6.88, -6.88, 0}, {6.88,  -6.88,  
                0}, {-6.88,  6.88,  0}, {6.88, 6.88, 0},  {0, 0, 
                18}}], 
Body[wheel,  
          PointList -> {{-6.38, -6.88, -8}, {-7.38,  -6.88,  -8}},  InitialGuess -> Automatic]];

PointList[wheel, 2]

groundgraphic = Graphics3D[ground,  1, 2, 3];

Show[groundgraphic]

baseplategraphic = 
        Graphics3D[{Bar[Line[{0, 0, 0},  {0, 0,  .25}], 13.0, 30], 
            Bar[Line[{0, 0, 0}, {0, 0, 18}], 1.17, 5]}];

baseplatetext = Graphics3D[{LocalAxes[All, 20]}];

Show[baseplategraphic,  
      PlotRange -> {{-15, 15}, {-15, 15}, {-5, 
            20}}]

wheelgraphic = 
        Graphics3D[{Bar[Line[PointList[wheel, 1], PointList[wheel, 2]], 3, 
              20]}];

Show[baseplategraphic, wheelgraphic]

Retry...

Needs["Mech`Mech3D`"]

ground = 1;
base = 2;
wheel = 3;

bd[ground] = 
        Body[ground, 
          PointList -> {
  {0, 0, 
                0}}];

P1 is local origin.
P2 ,P3is LF wheel.
P4,P5 is RF wheel.
P6,P7 is RR wheel.
P8,P9 is LR wheel.

bd[base] = 
        Body[base, 
          PointList -> {
 (*P1*) {0, 0, 
                0}, 
 (*P2*) {-6.88, 6.88, 
                0}, 
 (*P3*) {-6.88,  
                6.88,   .25}, 
 (*P4*) {6.88, 6.88, 
                0}, 
 (*P5*) {6.88, 
                6.88,  .25}, 
 (*P6*) {6.88,  -6.88, 
                0}, 
 (*P7*) {6.88, -6.88,  .25}, 
 (*P8*) {-6.88, -6.88, 
                0}, 
 (*P9*) {-6.88, -6.88,  .25}}, 
InitialGuess -> {{0, 0, 11}, {1, 0, 0, 
                0}}];

P1 is local origin
P2 is steer axis.
P3 is L side of axis
P4 is R side of axis

bd[wheel] = 
        Body[wheel,  
          PointList -> {
  {0, 0, 
                0}, 
  {0, 0, 
                1}, 
  {-0.5,  0,  
                0}, 
  {0.5,  0,  
                0}}, 
InitialGuess -> {{-6.88, 
                6.88, -8}, {1, 0, 0, 0}}];

SetBodies[bd[1], bd[2], bd[3]]

cs[1] = 
        Revolute5[1,  Line[base, 2, 3], Line[wheel, 1, 2]];

SetConstraints[cs[1]];

CheckSystem[]

SolveMech[1]