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During 1997 I penned (typed) a number of articles encompassing suspension and
brake modifications carried out by you innumerable and fanatical Mini owners.
Throughout those discourses, I frequently alluded to the necessity of damper [shock absorber]
changes to maintain the status quo of the Mini's formidable 'handling'.
Consequently your collective curiosity has been pricked into action as can be
attested to by the number of enquiries I've had since. More interestingly these
enquiries have been developing into a singular nature - just what do dampers do,
and why do they need changing? Well, read on............

The damper's job is very simple to explain - to keep the tyre in contact with
the ground wherever and whenever possible. How it must achieve this is where the
complexity comes in, as it has to deal with a number of continually varying
factors/conditions thrust upon it by the car and it's suspension. To keep things
as clear as possible, I'm leaving out all suspension related details. Although
intimately related to the dampers performance specifications, it would only
cloud the issue to try and cover both in these all too few pages.

Remember 'slinkies'- those wound steel coils that 'walked' down stairs and
such? Picking them up was an art as they tended to 'boing' all over the place.
Well that's exactly what any spring does, no matter how stiff. When stretched or
compressed, then released, they all bounce up and down to some extent until this
natural energy dissipates and finally settles down. This bouncing up and down's
called 'oscillating'. The oscillations experienced when springs (rubber
'doughnuts' in our case) are fitted to a car are generally kicked off by the
wheels hitting bumps be they lumps or potholes - both have the same result. The
wheel's kicked upwards, causing the springs to contract (bump), then react by
extending (rebound) chucking the car about into the bargain. The combined sprung
(body, engine, passengers, etc.) and un-sprung (wheels/tyres, brakes, suspension
arms) weights affecting how much oscillating goes on, worsened by lumps being
hit consecutively - turning the car into a bucking bronco. Left un-checked the
ride becomes a stomach churning experience.

Worse than that, severe bumps can flick the wheels clear off the ground. Now
this can cause all sorts of excitement, as there is a number of very basic
functions to successfully pilot a car that rely totally on the wheels being in
contact with the ground. Steering. Braking. Accelerating. All impossible to
achieve if there's fresh air twixt wheel and ground! A serious situation
multiplied when encountering a succession of bumps.

The dampers job is to contain these oscillations in a controlled manner over
a wide variety of situations and conditions, so how does it do it?

Essentially the damper is a rod with a plunger attached to its base,
operating within a tube - bit like a bicycle tyre pump. Operating the pump is
easy when the orifice at its base is left uncovered. Sticking your thumb over
the end alters this dramatically - easy to operate in one direction, almost
impossible in the other. This is because of the resistance caused by the air
trapped between the plunger and now closed off end. Uncovering the hole a little
at a time makes it easier and easier to operate. Dampers operate in the very
same way, although being somewhat more refined.

The plunger's a valve operating in a tube full of oil. The oil gives
resistance, the valve calibrated to allow oil to flow past it in differing
amounts to regulate the amount of resistance in both bump and rebound. Combining
these two gives the damping needed to control the spring's oscillations. This
basic design is known as the 'mono-tube' damper. An alternative design is the
'twin-tube' type. With this type the pushrod valve deals with 're-bound'
resistance. The tube it operates in has another separate valve fitted to the end
of it that deals with 'bump' resistance, and is placed inside a bigger tube.
Contemplate the drawings at the end of this dissertation of each type to compare
their relative differences.

Intimated earlier, the dampers have to deal with a very broad spectrum of
performance in standard trim. Everything from solo pilot on smooth tarmac roads,
to fully laden with family on picnic outing into the out back. Consequently
'compromise' creeps well and truly into the equation, making the standard
fitment dampers somewhat on the 'forgiving' side, ensuring a 'comfortable' or
'soft' ride. Price also determines very basic design, component quality, and
indifferent assembly. The situation decaying dramatically when even cheaper,
standard replacement, non-genuine dampers are fitted.

A couple of years general usage is enough to see off the best performance of
a set of standard dampers. Cheaper units end up leaking their precious oil out
because of poor quality components, and most suffer from oil degradation - the
oil becoming thinner with use, so vastly reducing the effectiveness of the
damper. All this leads to loss of control of the spring with the horrendous and
potentially very dangerous scenarios outlined earlier - loss of traction, be it
for accelerating, braking, or steering. On a Mini you're often doing at least
two of these at the same time due to front wheel drive. Correctly functioning
dampers are an absolute must. Up-rated or high quality ones can dramatically
improve the cars handling, but which to choose?

Right, so the basics of how a damper works is a piston/valve moving up and
down in an oil bath. The oil affords a certain amount of resistance, the varied
complexities of control dealt with by the design of the valving (how much oil it
lets past and when). Nice and simple. So why are there so many different types
of damper made for the same fitment? Ignoring for the moment manufacturer
competition, let's have a look at methods used to up-grade the damper's
performance by the most common aftermarket damper suppliers for the Mini (i.e.
not race orientated exotica!).

Oil. Just like engine oil, there are many grades and qualities used in
producing dampers. And just like engine oil, the better quality the more
expensive it will become. No way round that. And quality is important. The oil's
an essential element - if it wasn't there the damper would be useless. It has to
maintain constant performance over a wide range of temperatures - typically for
standard production units between -20 degrees to +120 degrees C (-68 to +248
degrees F). It's also responsible for lubricating all the internal moving
components - again necessary to maintain performance over a long period. The
harder the damper works the quicker the oil degrades - constant use on rough
ground or high frequencies experienced when racing with hard settings. The
latter can work the damper so hard excessive heat's generated, burning the paint
off the body! You can't change damper oil like you can engine oil - practically
all dampers that you're likely to come across are sealed. So high-grade oils are
necessary, facility for periodic replacement would be ideal.

High damper speeds (very rapid movement up and down) can cause the oil to
become aerated, causing foaming. This vastly reduces the oil's resistance, and
consequently the damping efficiency. To limit aeration foaming, some
manufacturers use pressurised gas. When used in mono-tube dampers, the gas is
usually under high pressure. Where combined with a twin-tube type, low pressure
is usually used. The gas used is generally nitrogen, a floating piston
separating the gas and oil to prevent them mixing. This method has a couple of
extra bonuses - a permanent pressure is exerted on the oil, dramatically
improving response and quieter operation.

Valve design. Far too numerous and technical to outline here, but of great
importance. Generally speaking, the valve is made up of a stack of shim plates
that deflect under varying resistance pressures, covering/uncovering various
holes of differing sizes. Thus controlling the throughput of oil, and therefore
the damping rate. It is the development of this valving for each car, each
suspension/spring set-up, each discipline (road/race/rally/etc.), or the ability
to combine any of these that sets one manufacturer's dampers apart from the
others. The mono- tube can restrict effective valve area where space is limited,
i.e. making the tube very small in diameter. Performance increases are only
possible with highly developed and comparatively more expensive valve design.
Because of this, most adjustable sport/performance dampers are of the twin-tube
design.

Adjustability. It has become 'vogue' to have adjustable dampers on your Mini;
mostly because of bar stool bullsh*t requirement, as few who use them ever
adjust them. Or would be able to discern differences of small adjustments even
if they were made come to that. The fact that a few manufacturers are producing
'adjustable' dampers at purchase prices that can only be described as
ludicrously low suggests that they've identified this syndrome. The adjustment
leaning itself more to correcting damper performance loss caused by oil
degradation over time than actually contributing to the performance envelope of
the damper. Some are adjustable 'on car', others only adjustable by removal.
Whichever type of adjustment used they all achieve the same thing - restricting
oil transfer from outer to inner reservoir. Consequently only twin tube types
are adjustable.

Component quality. The adage 'quality isn't expensive, it's priceless'
springs to mind. It's absolutely impossible to produce an effective, efficient
damper for next to nothing. A truly effective damper will use fully and properly
developed, high quality components, produced to exacting tolerances to ensure
efficient and consistent performance. After all, it's no good having a set of
supposedly matched dampers doing different things at each corner - and this
happens, believe me!

The Mini's been around a long, long time. Many damper manufacturers have had
a stab at producing dampers for it - some more successfully than others. The
problem lies in the rubber cone spring. It works nothing like a normal spring,
so is extremely difficult to evaluate. Only those that have put the time into
development rather than copying some one else's design, or pure guess-ology,
have managed to produce dampers of worth. Strangely enough, it seems to be those
that have appeared on the scene more recently (say in the last 10 years or so)
that are winning the day.

Any changes made that deviate from the standard set-up such as wheels/tyres,
disc brakes, bigger disc brakes, or anything else that increases the un-sprung
weight at each corner will necessitate an improved damper. Retaining the
standard ones is folly, cheap replacements dangerous. When replacing the
standard damper on a totally standard car, go for a better grade.

Price wars abound, but does not mean to say you are getting the best deal. As
I pointed out earlier, quality cannot come cheap. If you don't really need
adjustable dampers, don't buy them. Especially cheap ones. Their performance may
well be less effective than decent gas pressurised, non-adjustable, mono-tube
types, which are even cheaper. If you really need an adjustable damper, it's
because you need the virtues the name implies, with re-buildability a bonus, so
expect to pay a little more.

A decent set of dampers will transform the 'handling' of your Min. Trust me.
To save a myriad of 'phone calls, e-mils, etc., my preferences are - Kayaba
Gas-a-Just non-adjustable, Avo adjustable. Amen.

Basic mono-tube damper design. The piston has two valves in it, one opens as
the damper compresses (bump - centre pic), the other when it extends (rebound).
Essentially, larger orifices allow greater oil throughput over small/low speed
bumps. Bigger/multiple high speed bumps actuate components in the valving to
make the orifices smaller, reducing throughput, giving more control, thus
keeping the tyre in contact with the ground. The design of these valves is very
complex, their development and quality makes one type function more effectively
than others.

Ubiquitous
twin-tube damper. Generally more widely used where limited working space demands
small diameter tubes. Valve area/effectiveness is increased as two separate
valves are used. The one fitted to the piston rod deals with rebound control,
the one built into the inner tube base deals with bump control. When in bump
mode, the oil's forced through the bump valve into the outer reservoir between
the inner and outer tubes. In rebound mode, it's forced through the
piston-mounted valve, and also drawn back through the base valve to replenish
the inner reservoir.

Low-pressure gas-filled twin-tube damper. Operates in exactly the same way as
that in diagram 2, except performance is improved by replacing the air above the
oil with nitrogen under low pressure - typically 2.5 to 5 bar. This helps reduce
performance-sapping aeration foaming of the oil, and slightly improves damper
response.

High-pressure gas-filled mono-tube. The piston mounted valve does double
duty, dealing with both bump and rebound control. It therefore has to be
extremely well developed, and made of quality componentry. In this case, the
nitrogen is introduced under high pressure - typically 25 to 30 bar - to the
base of the damper. A floating piston's used to separate the oil and gas. When
assembled, the air gap is completely removed by this design, vastly improving
damper response, and virtually eliminating aeration/foaming. Nitrogen is
slightly compressible. Combining the effects this has with superior valve
design, it's feasible to produce a 'self adjusting' damper. The result's a
damper that can out perform many cheap adjustable types. It's mono-tube design
also means the damper body is slimmer than the more conventional twin-tube. This
helps alleviate tyre-rubbing problems where wider/deeply inset wheels are used.
Although I have implied with-in the text that the mono-tube damper is at a
slight disadvantage to the twin-tube as far as valve performance goes - a
quality designed and manufactured mono can easily eclipse a cheap twin-tube;
adjustable or not.

• 552018
      Kayaba Gas-A-Just gas pressurised mono-tube - front
  
• 552019
      Kayaba Gas-A-Just gas pressurised mono-tube - rear
  
• 342001 Kayaba Up-grade standard gas damper - front
  
• 342002 Kayaba Up-grade standard gas damper - rear
  
• 442001
      Kayaba standard replacement damper - front
  
• 442002
      Kayaba standard replacement damper - rear
  
• C-STR1675
      Koni standard ride height adjustable damper - front
  
• C-STR1794
      Koni standard ride height adjustable damper - rear
  
• C-STR1717
      Koni lowered ride height adjustable damper - front
  
• C-STR1795
      Koni lowered ride height adjustable damper - rear
  
• NGM1 Spax standard ride height 'knurled knob' adjustable damper - front
  
• NGM2 Spax standard ride height 'knurled knob' adjustable damper - rear
  
• NGM11 Spax lowered ride height 'knurled knob' adjustable damper - front
  
• NGM12 Spax lowered ride height 'knurled knob' adjustable damper - rear
  
• NGM1A Spax special softer rate adjustable for standard road use - front
  
• NGM2A Spax special softer rate adjustable for standard road use - rear
  
• C-STR306 Spax original adjuster, standard ride height - front
  
• C-STR307 Spax original adjuster, standard ride height - rear
  
• C-STR308 Spax original adjuster, standard ride height - rear for
      van/estate/woody
  
• C-STR304 Spax original adjuster, lowered ride height - front
  
• C-STR305 Spax original adjuster, lowered ride height - rear
  
• GSA971388 Unipart standard front
  
• GAS971389 Unipart standard rear
  

Article Date: Feb 14, 2001
Car Accociations: MINI,
Hits: 11230