thread800-179294
Hello,
In reference to the above thread: I appreciate that this thread is years old but I stumbled on it a while back while trying to find answers on this subject, and having pondered it and many other sources at length with the ‘what’s happening, when and why’ method, I’m certain the OP was correct in his questioning of what appears to be the ‘standard ’ of subtracting the gas force from dyno curves in order to arrive at the actual damping forces. (although this is half-right it appears).
Now, if we were talking about the damping being done by the valving inside the damper, then this would be wholly correct, but we want to know the damping values as applied to the car, and when fitted to the car the gas pressure remains, and some of the damping provided by the valving is ‘used up’ controlling the gas spring. It therefore has a large effect on the rebound damping applied.
My reason for researching this was that I realised that it has to be the case that a damper that extends on it’s own at speed 'x' due to gas pressure cannot exert any damping to the vehicle it is fitted to until that speed is exceeded.
The effect is also there to see on every gas-pressurised dyno curve i.e. there is no rebound damping until you exceed the natural extension speed (in the OP’s hypothetical example, two inches per second). Below that speed, the damper is actually assisting the rebound of the suspension - vastly different from a non-gas damper. And in my experience you really feel this on the road in the form of better ride, traction and grip.
Until now I’ve only mentioned rebound, and that is because apart from extra seal friction/stiction, the gas pressure only affects the rebound response. It took me a long time to see this but if you follow a typical rebound curve, all of the gas force is accounted for by the time the damper has slowed to a stop in the rebound direction, (resulting in an increased static and dynamic ride height, compared to without gas pressure) meaning that from there, in compression, the only force is pure compression damping (after overcoming seal stiction as mentioned). A dyno curve shows the static rod force as the starting point in compression but that is not the case. Well, it is the case as measured (total force) but is misleading as a) it has already been accounted for, and b) even if it wasn’t, the gas force is not a damping force and should not be factored as such.
In ‘no mans land’ between natural extension speed and zero, the energy of the gas spring is divided proportionately, depending on speed, between ride height increase (potential energy) and damper valving dissipation.
So in other words, to obtain the actual damping values, as applied to the vehicle, the gas force should be subtracted for compression, and should remain for rebound.
As the rod force causes a ride height increase it is easy to see why comparisons have been made to main spring pre-load. After all, the gas spring inside the damper is of course a highly pre-loaded, low rate (in my experience) spring. But, it operates in parallel to the main spring and therefore unloads it, not pre-loads it, and therefore treating the rod force as main spring pre-load does not seem correct. Also, as described above, it has very real effects on the rebound damping forces as applied to the vehicle and therefore cannot be ignored?
I realise this is contrarian but I’ve tried my best to argue with it and cannot!
Maybe you can…..?
Cheers,
Tim.
Hello,
In reference to the above thread: I appreciate that this thread is years old but I stumbled on it a while back while trying to find answers on this subject, and having pondered it and many other sources at length with the ‘what’s happening, when and why’ method, I’m certain the OP was correct in his questioning of what appears to be the ‘standard ’ of subtracting the gas force from dyno curves in order to arrive at the actual damping forces. (although this is half-right it appears).
Now, if we were talking about the damping being done by the valving inside the damper, then this would be wholly correct, but we want to know the damping values as applied to the car, and when fitted to the car the gas pressure remains, and some of the damping provided by the valving is ‘used up’ controlling the gas spring. It therefore has a large effect on the rebound damping applied.
My reason for researching this was that I realised that it has to be the case that a damper that extends on it’s own at speed 'x' due to gas pressure cannot exert any damping to the vehicle it is fitted to until that speed is exceeded.
The effect is also there to see on every gas-pressurised dyno curve i.e. there is no rebound damping until you exceed the natural extension speed (in the OP’s hypothetical example, two inches per second). Below that speed, the damper is actually assisting the rebound of the suspension - vastly different from a non-gas damper. And in my experience you really feel this on the road in the form of better ride, traction and grip.
Until now I’ve only mentioned rebound, and that is because apart from extra seal friction/stiction, the gas pressure only affects the rebound response. It took me a long time to see this but if you follow a typical rebound curve, all of the gas force is accounted for by the time the damper has slowed to a stop in the rebound direction, (resulting in an increased static and dynamic ride height, compared to without gas pressure) meaning that from there, in compression, the only force is pure compression damping (after overcoming seal stiction as mentioned). A dyno curve shows the static rod force as the starting point in compression but that is not the case. Well, it is the case as measured (total force) but is misleading as a) it has already been accounted for, and b) even if it wasn’t, the gas force is not a damping force and should not be factored as such.
In ‘no mans land’ between natural extension speed and zero, the energy of the gas spring is divided proportionately, depending on speed, between ride height increase (potential energy) and damper valving dissipation.
So in other words, to obtain the actual damping values, as applied to the vehicle, the gas force should be subtracted for compression, and should remain for rebound.
As the rod force causes a ride height increase it is easy to see why comparisons have been made to main spring pre-load. After all, the gas spring inside the damper is of course a highly pre-loaded, low rate (in my experience) spring. But, it operates in parallel to the main spring and therefore unloads it, not pre-loads it, and therefore treating the rod force as main spring pre-load does not seem correct. Also, as described above, it has very real effects on the rebound damping forces as applied to the vehicle and therefore cannot be ignored?
I realise this is contrarian but I’ve tried my best to argue with it and cannot!
Maybe you can…..?
Cheers,
Tim.
![[bigglasses]](/data/assets/smilies/bigglasses.gif "[bigglasses] [bigglasses]")
