Damping Forces – How it Works

There are two damping forces we need to think about, bump (compression), and rebound (extension), both of these damping forces can be broken down into low speed and high speed damping. 

Bump (Compression)

         Slow speed bump fixed rate:

As you turn into a corner the initial suspension movement uses the slow speed damping force, this force adds value to the spring frequency (stiffness).  Too much slow speed damping will raise the spring frequency (suspension stiffness) limiting the weight transfer, which in turn lessens the grip to the tyre.  Not enough slow speed damping and the weight could transfer too quickly unbalancing the car (wallow).

Slow speed damping is often referred to as “slow speed leak” as, it is exactly that…oil leaking through the piston and or adjuster.

In a fixed rate damper the slow speed leak can be as simple as a hole or holes drilled through the piston allowing oil to bypass the shim stack.  The size of the hole or holes will dictate the amount of leak and therefore the slow speed damping force.

The “Shim Stack” can also play a part of the slow speed damping forces by the number, thickness and type of shims, which we will cover later.

 As the piston moves, oil passes through the bleed hole/holes, as the velocity of the piston rod (shaft) increases the bleed hole/holes will not be able to cope with the speed the oil need to pass through the piston, its at this point the bleed hole/holes “choke” (like putting you finger over the end of a hose pipe).  The pressure then builds up behind the bump shims until the shims bend back allowing the oil to flow through the piston bump ports (the beginning of high speed damping force depending on shim stack).

 As the damper compresses the piston rod (shaft) displaces a quantity of oil (like getting in a full bath…you will displace a volume of water), this oil needs to go somewhere.  A gas charged damper with a floating piston (separate piston) allows the displaced oil to compress a nitrogen filled gas chamber.

                Adjustable Slow Speed Bump

 A damper with adjustable slow speed bump (compression) controls the oil displaced by the piston rod (Shaft).  The volume of displaced oil will depend on the diameter of the piston rod.  As the oil is displaced by the rod, the adjuster regulates the flow of displaced oil by restricting a hole with a tapered pin before the floating piston can be move, therefore increasing or decreasing the pressure or slow speed damping force.  The range of slow speed adjustment depends on the size of the hole, the angle of the tapered pin and the amount of movement the pin can travel.

A bleed hole in the piston can be used along side a slow speed adjuster if you need to play with where the range of adjustment start and finishes.

So we have the flow of the oil through the piston and the oil displaced by the piston rod (shaft) affecting the slow speed bump damping force, with control over the displaced oil and setting the level of where the slow speed force starts by the size and number of the bleed hole/holes.

High Speed Bump (compression)

When the wheel hits a bump or kerb, the wheel needs to react quickly and get out of the way without firing the body into the air.  So the oil needs to pass through the piston and the high speed adjuster (if fitted) quickly. This lowers the high speed damping force, which in turn lowers the wheel frequency (stiffness), so not to upset the cars balance or control, not throwing the front of the car in the air.

To put it another way, if you jump off a six foot high wall your legs give way to your weight on impact and stiffen up as the movement slows down to a more controllable speed.  This is as complex as high speed damping needs to be.

The harder the bump the quicker the wheel needs to travel and more oil needs to flow through the piston.  This is controlled by the shim stack and the sizes of the bump ports (see figure) in the piston, as well as (like with slow speed) the oil displaced by the piston rod trying get to move the floating piston and compress the gas chamber.

As we said earlier, when the slow speed bleed hole/holes start to choke the pressure builds up behind the bump shims until the pressure opens the shims and oil flows through the bump ports.  This will depend on the cracking pressure of the bump shim stack.  The cracking pressure of the shim stack in turn depends on the number, thickness, diameter and shape of the shim stack.

The top plate diameter dictates how much the shims can bend up allowing more or less flow at high speed.  If the diameter of the top plate was the same as the largest shim, the largest shim would only be able to open as much as the height of the other shims in between and therefore restrict the flow.

So the bump shims and top plate will dictate the high speed bump damping forces, just like the way our legs react automatically to changes in compression forces when we land after jumping.  The bump forces change to different pressures on the damper, which we can pre-set into the shim stack by which shims we use (Shim selection will be covered later).

 Adjustable High Speed bump

Dampers with adjustable high speed bump control the displaced oil by the piston rod in a similar way to slow speed.  There are different methods of controlling high speed bump, but the principle is the same.  The pressure of the oil inside the damper will be allowed to reach a certain pressure before the oil is allowed to flow into the canister or move the floating piston and compress the gas chamber.  It’s a bit like a pressure cookers safety valve with a spring or shim stack that we can change the pre-load on to vary the cracking pressure (opening pressure) and therefore control the minimum and maximum high speed damping force.  It is very important that the bump shim stack on the piston (oil flow through piston) is set to the minimum high speed damping forces that are required, as restricting the displaced oil via the high speed adjuster can only add value to the high speed damping forces.

The pressurised oil hits the flat face of the high speed adjuster, the pre-loaded high speed adjuster spring must be able to hold back the oil pressure beyond the cracking pressure of the main bump shims on the piston.  Once the pressure increases (due to faster piston rod movement) the high speed adjuster opens allowing the oil to flow into the canister.  Additional pre-load on the spring will raise the high speed damping force by restricting the displaced oil.  As the floating piston moves the compression ratio on the gas will increase, making the high speed damping forces progressively higher with travel.  As the piston rod velocity slows down the damping forces react by closing the shims until the slow speed damping forces are enough to control the movement of the piston rod.

Rebound (Extension)

Firstly think about two different scenarios.  Taking off over a jump and landing after one.  As the wheel leaves the ground the suspension will open until it reaches full droop or full open.  The only weight acting on the rebound damping force is the weight of the Unsprung weight (the wheel, brake calliper, wheel hub etc) which is anything under the spring, and the energy stored in the spring (the difference of the springs fitted/loaded length and free length/open length). E.g. If the fitted/loaded length of the spring is 8 inch, and the free/open length is 12 inch, and the spring rate is 100lb per inch (linear rate), there is 400lb of  additional force opening the damper, which is acting on the rebound force.  The opening or rebound travel needs to be controlled, so the damper doesn’t “top out” and damage its self.  However, it needs to open quickly to be ready for landing so the full length of bump travel can be used.

Once you have landed and fully compressed the spring and damper (and possible onto the bump stops) the amount of stored energy in the spring (and possible bump stop energy) will be at its maximum.  This stored energy will want to explode, forcing the damper open, firing the sprung weight (body) into the air.  The initial movement from the fully compressed damper will need maximum rebound damping force (high speed) to control the rebound or opening.  As the spring reaches its natural fitted/loaded length the rebound forces needed to the control the rebound will drop as the stored energy is released.  The perfect rebound damping forces will allow the spring to return to its fitted/loaded length without “over shooting” or passing its natural ride height, which would result in the sprung weight (body) going back in to bump travel (bouncing).  So the dampers rebound forces needs to react with higher and lower damping forces depending on the energy stored in the spring.  In theory there is only one rebound damping force needed for a given weight and spring rate, the “critical damping force”.  However, the ability to adjust the rebound can have dramatic effects on the handling.

  

Adjustable Rebound Damping Force, Slow Speed

The rebound adjuster only controls the slow speed damping forces, as this is the critical adjustment needed to control the spring as it approaches its natural fitted/loaded length.

The jet and tapered pin control the flow of oil passed the shim stack, for slow speed rebound damping force.  The tapered pin is pushed down the piston rod into the jet by a 3mm diameter rod which is controlled by the adjuster, normally at the top of the piston rod.

The high speed rebound damping force is set into the design of the shim stack, by the number, thickness and diameter of the shims.

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