Quote:
Originally Posted by Schulisco
This table I created from the data out of the Bilstein classic catalogue with all known suspension / shocks setups for the 911:
Columns: Setup name, front axle, rear axle. The numbers are rebound/compression in Newton.
"Sportlich Strasse" means sporty road and is the stiffest/hardest suspension for these cars and not recommended for non trackday drivers. And on Targas/convertibles it should be forbidden as the body suffers too much on that.
Thomas |
you gotta realize that a shocks behavior can't be defined by a single # pulled from a catalog
those #s are all off by a factor of 10
those #s only apply to a specific rod speed of .54 m/s, which is quite fast
when the catalog says at what will be measured by a consumer are usually at odds
here's a Rohrig shock dyno graph of the performance of Bilstein hd (green), Sport(yellow), and a custom digressive revalve of either.
Not that even this chart does not fully describe the behaviors of the shocks as the gas pressure component has been zeroed out of all of them.
what you learn from the chart is that stock hd &sport are linearly valved which means that the damping increases w/o bound, this run goes to a shaft speed of 10 in/s which is a bit less than 1/2 the Bilstein factory spec. and yes this is a factor contributing to discomfort, it would be much nicer if a high speed blow off was used as is done by Ohlins
the reason digressives are so much nicer is that the damping force levels taper off at high speeds plus of course the additional damping at low speeds which is where vehicle maneuver speeds are generated
back to gas pressure effects, stock Bilstein have a static force of ~42#s extending the damping rod, when not zeroed out for a computer display, this raises all the reactive damping force so that it oscillates around that static #, for instance here if the dyno was measuring the oscillations @4 in/sec the forces would be in the range from ~60# to ~20#s, and would cause additional friction in the motion, all this internal hysteresis leads to delay in reaction time. The gas pressure introduces a position dependent variable into a velocity dependent design
a non inverted monotube would be worse in this respect as they use damping rods that are larger larger which cause greater rod forces( Force = pressure *area) despite this drawback the monotube is generally considered to be superior to the twin tube design for performance use.
the twin tube on the other hand is general considered to deliver a more comfy ride using damping ratios that are ~.3 to .6 where for performance applications you want d/r ~.6 to ~1
the shock cannot be considered in isolation as it is only 1 component of the suspension system
1) the tire spring rate
2) the tire damping rate
3) the suspension spring rate
4)the suspension damping rate
5) the force on the road
6) the force on the sprung chassis weight
Mu is unsprung mass
Ms is sprung mass
and the front performance is not unconnected to the rear
ride frequencies are a very good measure of ride comfort
for comfort the rear r/f wants to be a bit higher than the front r/f, this imparts what is called a flat ride, but it is also slower to react than the opposite where front r/f > rear r/f which leads to pitching, most race cars will have this characteristic because performance is the goal not comfort. This also applies to tires where taller sidewalls are more comfortable than shorter, but if performance is the goal o short side walls are more desirable
sorry for the long winded digression I just got an a roll w/ nothing else to do on a cold windy day