Krazan
Automotive
- Feb 22, 2007
- 5
I have a question regarding gas pressure in a monotube shock/damper.
Conventional shock dynos measure the rod force due to gas pressure, and subtract that force from the measured force values as the shock is being cycled.
The reasoning (As explained to me) is that the rod force is essentially a spring force, and not damping, and so should not be considered.
Here's the catch (Or question)
Let's say for instance that I wish to valve a damper with a linear curve that provides 50% of critical damping of the sprung mass for rebound.
Just to put some numbers on this, let's say that I need a damping constant of 25 lbs per inch per second.
Let's also say that the rod force due to gas pressure is 50 lbs.
If a damper was built with these specs, I could compress the shock, and when released, the rod/shaft would extend at a rate of 2 inches per second.
Once attached to the suspension, would I effectively have zero rebound damping at a velocity of 2" per second, and "negative" damping below that velocity?
Would my effective total rebound damping force be reduced by 50 lbs at all velocities?
On to the compression side, would my effective damping force have 50lbs added to it at all velocities?
Let's look at this another way. If I were to valve a non pressurized shock so that the forces were 25 lbs per inch per second, the spring mass damper system should respond more or less as the mathematics would predict.
If I were to valve a pressurized shock to have the same damping characteristics as measured on a conventional shock dyno (With rod force subtracted from the measured results) the response of the SMD would be different.
So on to the big question. Should the gas pressure/rod force really be ignored when making damping calculations?
Does anyone take this into account when valving shocks?
Sorry this was so long. Any input would be appreciated.
Conventional shock dynos measure the rod force due to gas pressure, and subtract that force from the measured force values as the shock is being cycled.
The reasoning (As explained to me) is that the rod force is essentially a spring force, and not damping, and so should not be considered.
Here's the catch (Or question)
Let's say for instance that I wish to valve a damper with a linear curve that provides 50% of critical damping of the sprung mass for rebound.
Just to put some numbers on this, let's say that I need a damping constant of 25 lbs per inch per second.
Let's also say that the rod force due to gas pressure is 50 lbs.
If a damper was built with these specs, I could compress the shock, and when released, the rod/shaft would extend at a rate of 2 inches per second.
Once attached to the suspension, would I effectively have zero rebound damping at a velocity of 2" per second, and "negative" damping below that velocity?
Would my effective total rebound damping force be reduced by 50 lbs at all velocities?
On to the compression side, would my effective damping force have 50lbs added to it at all velocities?
Let's look at this another way. If I were to valve a non pressurized shock so that the forces were 25 lbs per inch per second, the spring mass damper system should respond more or less as the mathematics would predict.
If I were to valve a pressurized shock to have the same damping characteristics as measured on a conventional shock dyno (With rod force subtracted from the measured results) the response of the SMD would be different.
So on to the big question. Should the gas pressure/rod force really be ignored when making damping calculations?
Does anyone take this into account when valving shocks?
Sorry this was so long. Any input would be appreciated.