BMW Motorcycles (and applicable to many others):
Steering Dampers
Including a basic discussion of shock
absorbers and springs in general.
©
Copyright, 2012, R. Fleischer
article 54, section8A
section8stdampers.htm
First....a basic discussion of shock absorbers:
Shock absorbers do a number of things, including what
the name implies, 'absorbing shock' (bump) energy.
In almost all motorcycles, the shock
absorber is both hydraulic and air
operated, and is almost always combined with
the suspension spring. The energy captured by the spring in any
one direction
is moderated by the shock absorber,
otherwise the spring would allow the parts of the suspension called 'unsprung
weight' to 'pogo'
...or oscillate up and
down...and the more the oscillation, the less time the tire is on the ground.
Shock absorbers are often designed such
that their
effect in one direction is vastly different than in the other
direction; and some shocks are adjustable for it,
in one or both directions. Various names
are given to these two directions
and effects, but often 'bound' and 'rebound'
are used. The common shock absorber and spring units made by Koni
(now IKON)
and installed at the rear of the twin-shock Airheads, most of which use the
7610-1298 model, have an adjustment at the spring perch for ride height (as you
increase the loading, use a higher setting; and may have an adjustment via a
black toothed wheel under the rubber cup at the top, it is an adjustment for
REBOUND.
Some types of Ikon and Koni shocks do not have that wheel, but are adjustable by
rotating the upper part, and some models have NO adjustment.
Besides absorbing spring energy; shock units,
properly designed and set-up, will greatly
help keep the wheel in contact with the ground for longer
periods of time,
particularly during quite bumpy roads; and
most particularly, at least to the typical rider, during bumpy downhill
turns. Generally,
better-quality shocks are quite
good at this. The best shock units, which are usually very
expensive, will give better control over bumps and other
road irregularities ...no matter the size or frequency of such, and will not
'fade' or change characteristics very much,
as the absorbed energy is
converted to heat.
As a somewhat general
rule, cheap shock absorbers have characteristics
that may be adequate but are not the best, and ride quality and handling may
suffer...sometimes quite substantially. Except on very high-end motorcycles, the factory shock units are generally a
compromise between cost,
performance,
and average rider acceptability.
Although the front fork suspension shock/spring unit and the
rear shock spring unit may look totally different, they
both have to do a similar job,
and
the performance of one will affect the other.
Some steering dampers are hydraulic/oil operated and sophisticated.
There are various types of 'dampers'....and just the word 'damper' has somewhat
different meaning in various
Countries....typically though the difference is between suspension shock
absorbers and steering shock absorbers;
where 'damper' is more often used with steering (some use 'dampener').
STEERING DAMPERS (also
called Dampeners):
Some have used them to mask out some of the effects from other problems...such
as excessively loose wheel bearings and steering head bearings.
Some few still seem to think that they are 'the answer' to supposed tendencies
of bikes to have tank slapper's. Airheads with properly set up bearings
don't have those problems.
The /5-SWB models CAN have problems with FORK mounted
windshields, and the dampener CAN help with that. In fact, any
airhead with large fork-mounted windshields can have instabilities.
The R65 with the short wheelbase is much less susceptible.
Bikes can get increasingly close to instability with rear
trunks, side luggage, backrests, rear weight.....etc.....and in some instances
the use of a bit of dampener can allow 'more comfortable' higher speed cruising,
than without the dampener. There are obvious limits to the 'help'
though!....and that limit might well be very suddenly thrust upon you.
Use of steering dampeners/dampers always makes steering effort a bit...or more than a bit... higher.
They can, whether friction disc or hydraulic-type, do a good job at masking
certain types of road irregularities. They were often used by folks that
had mostly unfounded fears of loosing control of their bikes on the popular
rain-grooves being increasingly added some years ago to paved (especially
concrete) freeways. This is PARTICULARLY so with the
straight-ribbed tires....those old RB series Continental's, as a prime example.
Straight-ribbed tires are still made, .....most follow
rain-grooves....and when the rain-grooves were made by tipsy-road workers, they
are ...well....they can wake you up from your 'only one cuppa' your
morning commute.
They can be of help in very soft off-road work. Quite nice on irregular
cobblestones, rocky surfaces, and so on....and in any situation where some
really large irregularity might try to, seemingly, not
necessarily!... wrench the bars out of your grip.
On many sidecar rigs, the use of a dampener is a necessity, some
rigs are uncontrollable in the range of 10-25 mph, without a dampener.
When I install a damper on a sidecar rig, I make sure the wheel
bearings and steering head bearings are properly set up FIRST.
Since the type of front tire has an effect, that is taken into
consideration too (the flatter the tread, the wider that flat
tread, the LESS dampening needed).
In general, a hydraulic dampener is more effective than a
friction type,
but at a somewhat greater increase
in steering force, than a friction dampener.....some major portion of the reason
is in the mathematics of the velocity increase of the parts, and the STICTION
required to be overcome (OK...everyone
happy, I did not put any math formulas here?). SOMEtimes the reverse is
true, however! However, because
sidecarists use very large bars-inputs, some prefer friction dampeners, but not
all prefer them over hydraulic types. In many instances it
is possible to install a hydraulic dampener in a way that makes
its STROKE adjustable. This is a nice improvement, in that
you can use the minimum amount of stroke that works OK, without
adding the need for too much steering effort. In the case
of the stock friction dampers on some motorcycles, they are
sometimes not effective enough for sidecar use, and many times
one can modify it with perhaps another steel plate and one more
friction pad, and thereby come close to doubling its effectivness.
Unless your bike has a sidecar attached, or reduced trail for some other reason, or you
have a front fork mounted windshield or fairing.....or have a big dislike for
the 'feel' of the bike on soft or very irregular surfaces, or insist on a
ribbed front tire but dislike rain grooves...I would see no real reason for a
dampener.
The use of steering dampers has a real drawback, besides what has been mentioned above. They WILL reduce the 're-centering' forces, thereby making you work harder to steer your bike. This is true for 2-wheelers as well as sidecar rigs. I have seen sidecar rigs in which the steering was just plain UGLY, due to the wrong use of dampeners. AND, it is FAR FAR better to properly adjust the steering head bearings, and NOT use, or hardly, any steering damper, whether it is a friction or hydraulic type, and this is for 2 wheel bikes and 3 wheel sidecar rigs and trikes.
For a pdf file on the Ikon-Koni-springs, click this line
rev:
11/23/2004: clarity
04/08/2009: Add section at top on shocks in general
01/23/2010: clarify and add more on damper effects
05/12/2012: Add link to the springs pdf, inadvertently left out
previously.
© Copyright, 2012, R.Fleischer