Shock compress ratio - linear?
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Racer
Joined: Mar 2003
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Shock compress ratio - linear?
Is the compress ratio of the stock shocks and springs linear or not? If not, after we lowered a Z, even we do not bottom out, the ride would be stiffer in theory. If the compress ratio is linear, the ride stiffness should be the same before the lowering, right?
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Re: Shock compress ratio - linear? (atz06)
It's pretty linear. The springs change less than the shocks, as their angles don't change at all, just the length of the adjusting links. What make sthe ride stiff on many lowered cars is they start hitting the bump stops on the shocks a lot earlier.
Racer
Joined: Sep 2002
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From: Austin Tx
Re: Shock compress ratio - linear? (atz06)
There are a variety of issues involved here:
Springs: linear with respect to compressed height
Spring geometry: nonlinear (but with small nonlinearity)
Shock geometry: same as spring geometry
Shock: nonlinear with respect to the velocity of travel; invarrient with respect to compressed height.
The typical shock uses a oil valve to allow oil on the highere pressure side to move to the lower pressure side. As the oil travels through this resistance, it absorbs energy. This porcess itself is nonlinear, and infact pretty close to quadradic (X**2) with respect to velocity of movement. This is what makes shocks feel too light at low speeds and too hard at higher speeds when running over the same bump.
Better shocks use a series of washers that progressively open up bigger and bigger holes so that the high speed damping is equal to the low speed damping. Compression is handled by one whasher pack, rebound be a different washer pack. Sometimes springs are added to fine tuen the damping.
Even better shocks have adjustments to the spring preload on the washer packs, some of the best have multiple adjustments.
And then there are shock 'bumpers' rubber devices used to change the spring rate as the shock compresses to the bump stops in a progressive manner. Racers use plastic washers and sometimes shape the bump stops to optimize the feel of the car under high cornering loads.
So: to answer your question: shocks are do not change as you lower the car, so do the springs. However, you are changing the geometry of the control linkages (camber gain, roll center, instant centers,...) and this geometry change can change the effective spring rate and effective shock rate, even though the springs and shocks are unchanged. Generally, the lower the center of gravity, the better a car handles--UNTIL the suspension bottoms, or the camber (and/or toe) gain impeede the utility of the lower center of gravity.
Springs: linear with respect to compressed height
Spring geometry: nonlinear (but with small nonlinearity)
Shock geometry: same as spring geometry
Shock: nonlinear with respect to the velocity of travel; invarrient with respect to compressed height.
The typical shock uses a oil valve to allow oil on the highere pressure side to move to the lower pressure side. As the oil travels through this resistance, it absorbs energy. This porcess itself is nonlinear, and infact pretty close to quadradic (X**2) with respect to velocity of movement. This is what makes shocks feel too light at low speeds and too hard at higher speeds when running over the same bump.
Better shocks use a series of washers that progressively open up bigger and bigger holes so that the high speed damping is equal to the low speed damping. Compression is handled by one whasher pack, rebound be a different washer pack. Sometimes springs are added to fine tuen the damping.
Even better shocks have adjustments to the spring preload on the washer packs, some of the best have multiple adjustments.
And then there are shock 'bumpers' rubber devices used to change the spring rate as the shock compresses to the bump stops in a progressive manner. Racers use plastic washers and sometimes shape the bump stops to optimize the feel of the car under high cornering loads.
So: to answer your question: shocks are do not change as you lower the car, so do the springs. However, you are changing the geometry of the control linkages (camber gain, roll center, instant centers,...) and this geometry change can change the effective spring rate and effective shock rate, even though the springs and shocks are unchanged. Generally, the lower the center of gravity, the better a car handles--UNTIL the suspension bottoms, or the camber (and/or toe) gain impeede the utility of the lower center of gravity.
