|
By: Jason Giacchino
March 2010 - Off The Pegs
Making Logic of
Linkages
Dear ATV Source:
I was wondering exactly what a quad’s shock
linkage does. Most sport ATVs have them but a
few (like KTM & Polaris) don’t. Why is that and
which design is better?
To answer this question, we have to travel back
in time mentally to the late 1970s just before
the idea of the “monoshock” rear suspension came
into being. Back then if you wanted rear
suspension on your machine, the only option was
a pair of short-travel shocks mounted at nearly
the far end of the swingarm (near the rear
wheel). In fact, some road cruisers still
maintain this configuration even to this day.
Anyway, to save space and weight, the idea of
reducing the pair of shocks into a single unit
began to circulate in motocross circles, but the
trouble was it was quite impossible to mount a
shock absorber directly where the rear wheel and
exhaust pipe were located. As such the only
feasible alternative was to move the shock way
up the swingarm, to nearly the area where the
swingarm pivot connects with the main frame.
It’s possible that this is where the story would
have ended except for one snafu discovered very
shortly after (like probably the first test
ride): The swingarm suddenly became a giant
lever! Think about it for a moment--much in the
way a pry bar or massive line bar can allow you
to lift previously immovable objects (or break
loose a rusty bolt that hasn’t been turned since
1989), this same principle of leverage
increasing force output was being applied to the
poor shock. What this meant in the real world is
that it took very little force (bump, stone,
rut, and so on) to put a whole lot of force on
the shock! As you might guess, the short-travel
absorbers at the time (with mere 110-lb. shock
springs) were getting pummeled thanks to the
lever/ fulcrum bit going on back there.
The remedy, it would turn out, was as simple as
bolting the lower shock mount not to the
swingarm itself, but rather to an extension just
below it made up of interconnecting rocker arms
to offset the radical change in leverage being
produced. Hence, the shock linkage was born. In
one incarnation or another, it has existed ever
since as a staple to the monoshock rear
suspension package.
However, as you pointed out in your question,
it’s clearly not the only way. To understand why
this is, let’s now turn our attention to how the
linkage actually works. What a rising rate
linkage (as it’s known officially) really does
is it increases the amount of compression
applied to the shock from the swingarm at
progressively greater amounts in accordance with
the position of the rear wheels. Say what? In
English think of it like this: The linkage
simply slows down the speed that the shock wants
to blow through its travel by keeping the amount
of force it receives the same regardless of how
fast your wheels are going up and down. The goal
of shock linkage is to prevent harsh spikes
along the curve and to make sure the shock
receives only enough force to remain active on
the small stuff, but to keep it from bottoming
harshly when you flat-land or come up short on a
jump.
With this in mind, the obvious question then
becomes, why doesn’t every quad with a monoshock
rear suspension design have a linkage? The
answer to this one is pretty much the same
reason tuners started experimenting with
single-shocks (rather than pairs) in the first
place: Ditching the linkage would shave weight
and simplify a fairly complex and crowded
design.
Fortunately, engineers discovered fairly early
on that fine-tuning the internal valving within
the shock body could actually duplicate the
rising rate effect without the need for the
external rocker arms and pivots. The math was
proven by simply experimenting with leverage
changes in relation to angles of a scalene
triangle. In the real world some trial and error
goes into perfecting the shock’s valving
characteristics, but once applied, KTM, Polaris,
and a few other manufacturers have mastered the
settings required to achieve the exact same
leverage characteristics that a rising rate
linkage would provide without the extra pieces.
So in conclusion, while the performance of
either configuration should be nearly identical,
a legitimate argument could be made that the
superior of the two designs could very well be
the less-popular linkage-less configuration on
account of five simple reasons: 1) Less weight.
2) Fewer moving parts/ less complexity. 3)
Allows for a larger airbox. 4) Allows for a
straight intake tract. 5) Lower production
costs.
|
