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Author Dampers FAQ
Adam
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Registered: 1st May 01
Location: Hurstbourne Tarrant
User status: Offline
12th Feb 03 at 21:55   View User's Profile U2U Member Reply With Quote

Dampers\Shocks FAQ


What do they do?

Isn't a gas shock better than an oil shock?


What about adjustables


Adjusting or tuning shocks, a customers example.


Mono Tube Vs. Twin Tube comparison


Dampers\shocks - What do they do?



A damper or shock absorber does not actually absorb shocks. Springs absorb shocks by compressing in response to vertical acceleration. The primary function of a shock absorber is to dampen the kinetic energy stored in the spring as the spring compresses. It converts this energy into heat which is one of the reasons we have gas shocks, but more on that later. When the force that caused the compression goes away, this stored energy releases and the spring extends with alot of force. Imagine a pogo stick.

Now, before we confuse everyone too much. We'll refer to a shock as a damper and vice versa through out this explanation. Technically speaking damper is more correct, but shock is fine as long we keep in mind what its actual job is. It's also important to note that a shock's rate needs to match the spring and the swaybar. This is why tuned suspension packages are a such a good choice.


Shock absorbers are inherently velocity sensitive (despite the marketing hype). That is, the faster the piston moves (increased vertical acceleration) the more resistance or damping will result. This is due to one of the laws of fluid dynamics, which states that a fluid's resistance to flow through any given orifice will increase directly as the square function of flow velocity. The bottom line is the harder the damper works, the harder it works!


Because a shock absorber is velocity sensitive by nature, it has to be load sensitive because the velocity is produced by an acceleration, which is composed of force and velocity.


In order to maintain sensitivity at low displacements, valving has to be arranged so that little damping takes places at low piston velocities and conversely substantial damping takes place at high piston velocities. All fluid filled shock absorbers are therefore velocity sensitive, a good shock absorber will also be frequency sensitive. And this is often what separates the good from the average. Naturally its difficult to get a real frequency sensitive solution for very little money.


The current state of the art in racecar technology is to lower the spring rate as much as possible whilst increasing the swaybar rate as high as possible. This allows the tyre to follow the road surface maintaining the maximum grip possible. If the spring rate or bar rate is too high then it will hop from bump to bump losing traction. The bar controls the roll, the soft (relatively) spring deflects for the bumps and the rebound on the shock can therefore be set at a relatively lower/softer level.


So, next time your asked by a salesman to "pull his levers" and see how stiff their particular shock is, remember that stiff ISN'T always better. And, you can't possible simulate the high frequency of road bump deflection with your hand. No matter how well practiced you may be.


Isn't a gas shock better than an oil shock?


Yes and no. A popular misconception is that a gas shock works on gas where as an oil (normal) shock works on oil. All conventional automotive shocks work by forcing oil through a programmed set of holes, however a gas shock will use compressed gas to keep the oil under pressure.


This is done largely to minimise aeration or "foaming" of the oil which would reduce the effectiveness of the shock as air passes through the valves rather than fluid. To see what this is like, tip a conventional shock absorber upside down and pump the shaft a few times. You'll notice the movement become jerky and uneven as oil and air intermittently pass through the valves.


The gas also helps to dissipate heat which keeps the oil cooler and maintains the viscosity and therefore the shock "rate". Gas shocks are ideally suited to long travel applications like rallying and off road. In fact, this is where the technology was primarily developed in the first place as lots of spring travel over big bumps really tests a conventional hydraulic shock.


There are many types of gas shock, twin-tube, mono-tube and remote canister combinations for super heavy-duty use like rallying and off-road racing. Most of the economical gas shocks are of a twin tube construction (low-pressure) where as most performance or race gas shocks use a mono-tube (high-pressure) system. There is no such thing as an ideal system, it really depends on the application as mono-tubes may have advantages in some respect but the high-pressure gas can act as a spring complicating the suspension design process.


The main disadvantage of a gas-pressurised shock is cost; more of it compared with a conventional hydraulic. Which leads to a very simple rule of thumb to help avoid confusion. If faced with a choice of gas or oil for the same price, it's unlikely that the real working part of the gas shock is of the same standard and level of sophistication as the oil. You get what you pay for. And, choosing gas shocks generally mean you'll need to design the rest of the suspension system around that fact with spring and bar rates being affected.


What about adjustables?


Adjustable shocks, why use them?


In simple terms, the adjustment of a damper allows for 3 things;



  1. Initial adjustment to allow closer matching of spring to damper rates.

  2. Fine tuning in response to changes to components like tyres or swaybar rates.

  3. Subsequent adjustment to maintain rate as the dampers inevitably wear.


With out going into too much detail as to the types and levels of adjustment, suffice to say that most adjustable dampers allow for changes to the "rebound" rate. This is the extension component of the dampers movement cycle and is the principal force that controls the spring's oscillations. That is, correct damper rebound rate is critical to the spring damper relationship.

Other types of adjustable dampers for the street use a gross bypass adjustment that simply controls the amount of bleed or bypass around the main valve mechanism. This is a very coarse adjustment that is not really suitable for performance tuning as it often forces you to change the bump or compression characteristics when you might only need some extra rebound and vice versa.


Now, being a hydro-mechanical device, the damper eventually wears out. The piston rubbing on the bore of the damper eventually wears the seals, the valve springs get softer allowing more oil to bypass the piston and valves, which control the oil flow and give the damper its "damping" characteristics. The more wear, the less "rate" you're left with.


With an adjustable shock we can compensate for this wear by increasing the rebound rate to bring us back to square one. Now before you start going off about your brand shock never wearing out, accept the fact that any hydro-mechanical device MUST wear out as its operation depends on friction, and friction is a natural wear component by definition. The only issue is how long it takes one damper to wear versus another.


The answer to this is almost directly proportional to the money spent. That is, the cheaper the shock, the quicker it will loose it's rate. This also means that an adjustable damper costing a little more than a non-adjustable will often be the superior choice for road use as it can be made to perform more consistently for longer.


Adjusting or tuning shocks, a customers example.


Copy of a customers email:


I actually got a bit carried away playing with the Konis and made a guage so I could get precisely comparable settings at each corner (piece of cardboard with a hole in the middle to put around the adjustment knob, with angles marked on it in 22.5 degree / 16th turn increments - high-tech!). I went through a whole range of adjustments (about 15 sets up and down, separated by test drives through the same set of corners) and ended up with what to me was a good compromise for demanding road, with minimal ride jiggling along the straights at the same time as minimal body roll / max steering response through corners.


What really blew me away was the impact of the front / rear balance in shock settings. The settings I ended up with were + 90 degrees front and + 135 degrees rear, with degrees = amount of turn on damper adjustment knob to increase damping. At this setting, turn-in was great and cornering attitude very responsive - neutral / subtle oversteer / very slight understeer, depending on small throttle movements and very controllable. Great fun - really a 'pleasurable full-body experience' (for me) to feel the whole car working like that.


But getting back to front /rear balance, a 16th of a turn up or down from this at the rear (while keeping the front the same) resulted in a very noticeable loss of responsiveness and controllable corner speed, due to either increased body roll or decreased reaction to throttle / steering. So much so that it felt like a totally different car.


Mono-tube
vs Twin-tube - ride characteristics.


We've assembled some graphs showing the varying performance characteristics of mono-tube and twin-tube gas shocks. You may have seen arguments from time to time about how mono-tubes ride worse that twin-tubes or vice versa. There's also a perception that a mono-tube will tend to "bounce" more at low speed delivering a jiggly ride that some find uncomfortable. Whiteline argue that any high-pressure mono-tube design gas shock will deliver a certain "bounce" in the ride due to the inherent design of a mono-tube compared with a twin-tube. The accompanying information helps to explain why this is often the case.


The following graphs show a force/velocity trace line using a computerised shock dynamometer. Each pair of shocks within each graph was for the same application, using commonly available performance brands. They are "sport" valved for road use and are a good example of their "type". Please note the units of measure used and how they relate. Take particular note of the comment regarding the "Comfort Zone" which exists in the 0.0 to 0.15 m/s velocity range. It is also useful to note the relatively high amount of force in extension vs compression and that initial compression (bump) response is very important in analysing perceptions of ride comfort.


The actual vehicle application and shock brands used are irrelevant for this example and will not be published. The mention of any specific shock brand and design configuration within this article is purely for illustration purposes and may not accurately reflect the brands actual performance across all models. (Sorry, standard disclaimer required.). We also make some generalisations on pressures used and it must be understood that the same manufacturer will use different pressures for different models within the same general configuration.


The first graph (blue vs red) shows a "lower-pressure" mono-tube design vs a twin-tube design. A "low-pressure" mono-tube is a relative description as we are still comparing a gas chamber directly behind the hydraulic chamber with a pressure of between 35-50 psi with a twin-tube with between 10-20 psi as a light pressure blanket in the outer tube. Examples of lower pressure mono-tube shocks include KYB, Tokico, Sachs and Boge. The second graph (green vs red) shows a "high-pressure" mono-tube vs a twin-tube. Pressures here are considerably higher with ranges between 75-110 psi.


In either case, with any mono-tube you will see a relatively sharp and significant increase in the amount of force required to get the piston moving at slow speeds such as larger road surface irregularities at low vehicle speeds. The classic "anti-example" to this would be the sort of ride you'd feel in an old Cadillac or similar. However an increase in vehicle speed will increase the piston velocity over the same given road surface with a relative "softening" of the initial sharp bump trait felt at slower speeds. As you can see, this symptom is much more pronounced in a higher pressure mono-tube.


Its obvious that the green trace represents a shock with an overall higher base rate that would be felt by any driver however the initial step is the key in how the shock will "feel". In the case of the blue line, the ultimate bump rates of the 2 shocks meet but it is the nature of the beginning of the blue curve that give this shock its particular "ride" character.


These graphs can't deliver a conclusive answer for something that is largely perceptual like ride comfort but an understanding of the differences will hopefully help the driver appreciate what they are actually feeling on the road.


 
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