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Spike on the graph of reaction force exerted by the ground on the rear wheel at the bump

asheshshrestha
Hi,
My two wheeler Adams model is subjected to run through the bump of height 10 cm approx. And I am interested in the ground reaction on the rear wheel at the bump when it moves with a velocity 25 km/hr. So I have plotted the ground reaction force on the wheel for a 6-second time span. I have made two graphs with different steps ( 400 steps and 600 steps ). I am pretty confused about which curve should I really trust. The curve of finner steps ( greater than 500) have spikes that show 25 KN ground reaction at the bump while in course one (less than 500 steps), it is smoother than the graph of finner steps ( no spikes at bump ) but of course, It has the effect of bump that shows 2500 N ground reaction force on rear wheel. The ground reaction at rear wheel is 1400 N at rest condition. Any logical suggestion might be helpful for me.
Thanks in advance
errortoask
errortoask2
!
  • 0
    what kind of bump exactly was it? If shorter than a speed bump, with length not much more than contact patch length, simple tire models like Pac2002 do not make sense then. Do not trust either of your results. I hate to say (no, not really Sunglasses): a point contact tire model cannot capture the complex highly dynamic and nonlinear force/moment mechanisms.
  • 0
    what kind of bump exactly was it? : It is the default Adams bump that I found in the Adams road library ( 3d_bump.rdf). The schematic of the bump is similar to :
    bump
    Yes, I too agree that there is a lot of variation in the result due to a slight change in different parameters when we use to contact between road profile and wheel tire. More importantly, there is a significant fluctuation in simulating impact fore mainly. I observe it works fine at rest condition. That's why I assume it will work fine at the dynamic condition on impact force too when a bike hits a bump.
     
    It is better to look into a graph of a standard tire from special force rather than the graph of vertical contact force.
  • 0
    well, under certain conditions (if speed is large enough) and if you are *not* interested in the exact force history during which the step is inside the contact patch, a step *down* simulation might be possible with a simple tire model (since the wheel will 'fly over' the step). It definitely does *not* make sense in the step *up* case, since here you will have tremendous bending in the tire belt/carcass and non-connected contact patch geometry. You'll need a physics-based tire model capturing these effects, like FTire.
  • 0
    You are surprised that you don't see spikes when you have longer time steps (and please talk time steps, not number of steps)? The solver might simply have taken a step over that event with the longer time step.
    ADAMS is a numeric solver. No numeric solver is exact. So there is a process to tune the solver settings.
    1. Do a simulation with default solver setting (ERROR and HMAX). Use regular GSTIFF I3. Save the results.
    2. Decrease ERROR an order of magnitude and run again.
    3. Plot some typical displacements from these two runs. (in your case, for example front fork stroke, swing arm angle, body heave and pitch). (ERROR mainly control accuracy of displacements)
    4. If they differ too much, go back to 2.
    5. Decrease HMAX an order of magnitude. Simulate.
    6. Plot some typical accelerations and forces from the last two iterations. In your case, tire forces, wheel and body accelerations. HMAX does control accuracy of accelerations.
    7. If they differ too much, go to 5.
    8. Now repeat this for different solver settings.
      1. GSTIFF SI2 is usually much more accurate for the same ERROR and HMAX. But also quite a bit slower.
      2. HHT is usually faster, if you have lots of contacts, much faster. Would not expect that to be the case here.
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