Quote:
Originally Posted by Dimman
Can you explain the relationship of the damping curves to spring rates?
I was looking at adapting some QA1 dampers to my Supra, and they provide damping curves and re-valve kits for all their shocks. But the less-expensive non-adjustable ones you would have to rebuild and change shim-stacks instead of nifty clickers. But didn't know what starting point would be suitable. I get that high speed is more impact related and low speed is more weight-transfer, but what do the numbers mean and how do they relate to the spring rate/unsprung weight?
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Ok, so damper curves are kind of like spring rates, in that they aren't very useful all by themselves (eg, is a 200 lb/in spring stiff or soft? depends on what it's holding up). A spring/mass/damper system (your suspension) is governed by a second order ordinary differential equation:
m*(d^2 x)/(dt^2) = C*(dx)/(dt) + kx
Or, in more mundane terms:
m*a = C*v + k*x
It's the three forces in the system, force from the damper + force from the spring = the force acting on the sprung mass. The "ma" comes from Newton's Second Law, and the "kx" comes from Hooke's Law. C is the
damping coefficient. It's the slope of the lines in your chart. An important concept to know before I go any further is
Critical Damping (represented by Cc). Critical damping is a value of the damping coefficient that will bring the sprung mass back to equilibrium as quickly as possible after a disturbance without over shooting. Values of C that are less than Cc are considered
underdamped, and values of C that are greater than Cc are
overdamped
Cc = 2*sqrt(k*m)
Damping Ratio (similar in concept to ride frequency),represented by zeta = C/Cc with zeta<1 is underdamped and zeta>1 is overdamped. Zeta=1 is critically damped.
Being critically damped isn't actually a good thing, it means that the damper is actually causing the suspension to bind up a bit. At the very least it will reduce mechanical grip on less than perfect tarmac, and at worst could cause your suspension to pack down on rough roads. The flip side to that coin is the stereotypical beat up old Cadillac Deville with blown out shocks that never stops bouncing. Zeta = 0.7 is a good compromise and a good place to start. Getting a little more indepth, the forces acting against the damper are usually about two times higher in rebound than in bump. So you'll want more like zeta = (2/3*0.7) = 0.47 in bump and (4/3*0.7) = 0.93 in rebound. But then, you need the damping coefficient to be digressive to soak up big bumps. A good place to start is to halve the damping coefficients for high speed bump/rebound starting at ~4-5 inches/second.
So you end up with the ballpark figures of:
High speed bump: C = (1/3*0.7)*Cc = 0.233*Cc
Low speed Bump: C = (2/3*0.7)*Cc = 0.47*Cc
High speed rebound: C = (2/3*0.7)*Cc = 0.47*Cc
Low speed rebound: C = (4/3*0.7)*Cc = 0.93*Cc
Example:
Let's say your car's sprung mass weighs 1800 kgs, and you're a total badass w/ coilovers and kidneys of steel, so your super stiff springs are giving you a ride frequency of 4 Hz.
so: 2*pi*4 (s^-1) = sqrt (k/1800kg) .... k = 181,000 N/m for a wheel rate of 45,000 N/m. Let's assume you've got a 1.1:1 motion ratio, so your imaginary spring rate is 50,000 N/m.
That means that Cc= 2*sqrt(50,000 (N/m)*1800(kg)/4*1.1) = ~10000 (N*s/m)
--You need to include the motion ratio in here.
So you'd want start with roughly:
High speed bump: C = 2300 N*s/m
Low speed Bump: C = 4600 N*s/m
High speed rebound: C = 4600 N*s/m
Low speed rebound: C = 9200 N*s/m