HTupolev wrote:
Tom A wrote:
The air spring of a tire as basically zero damping...the vast majority of the damping losses of the 1st SMD is in the tire casing and tread.Yes, I'm talking in reference to the Wheel Energy air cylinder representing the 2nd SMD load, not the air within the tire.
Aaah...I see. Yes the Wheel Energy load, unlike many other similar testing machines that use a simple mass somewhat "rigidly" attached to the wheel, employs an air cylinder to create the load force. So, unlike the nearly complete lack of damping in the 2nd SMD for those typical setups, the Wheel Energy setup as at least a small amount of damping present in that part of the system. In discussions with Josh Poertner on this subject, he noted that this type of air cylinder typically has only ~10% of the amount of damping as what one would need to simulate the damping if an actual rider was the "test mass".
HTupolev wrote:
Quote:
One could argue that the effect of the damping in the 2nd SMD system is demonstrated by the existence of the breakpoint pressure.I'm admittedly not a mechanical engineer, so I might be visualizing this poorly. I'm not entirely sure how you mean. Are you saying that, as damping in the 2nd system is reduced, the onset of inadequate suspension smushes out across a broader inflation range, experiencing a less-abrupt "breakpoint"? Or that the performance of tires as suspension simply matters less within the range of amplitudes and frequencies being discussed?
Here's some observations and conclusions that might help make things more clear:
-When roller testing tires on a setup with a rigid connection between the wheel and the test mass, Crr tends to continually decrease with increasing tire pressure. This is true for both smooth and rough roller surfaces, with the rough surface data showing basically an offset, or bias upwards. This implies 2 things:
- The lack of damping in the 2nd SMD system we discussed above results in the lack of a breakpoint pressure
- At the tire level, additional surface roughness results in additional flexing losses in the tire only.
- When field testing with a human representing the 2nd SMD of the system, there IS an observation of a pressure breakpoint. The location of this breakpoint, and the shape of the curve overall is highly dependent on the surface roughness and the speed encountered, and thus the energy input into the overall system through the tire. The fact that below the breakpoint pressure, especially with typical road pavement surface roughness, the shape of the curve matches that of the curves observed on the roller testing with "rigidly" attached masses points to the change of the 2nd SMD properties as the source of that breakpoint observation.
- When roller testing with a setup that DOES include an appreciable measure of damping in the application of the load to the wheel, such as the Wheel Energy setup, a breakpoint pressure IS observed for a given surface roughness, but typically at a higher pressure than what one would expect for the same tire, load, and roughness for field testing done with a live rider. Again, this implies that the amount of damping in the 2nd SMD system is important in the observation of the breakpoint.
- Lastly, all of the above observations are consistent with the concept of the 2 Spring-Mass-Damper (SMD) systems in series, like the model shown:
http://bikeblather.blogspot.com/