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By: Jason Giacchino

March 2010 - Off The Pegs

Making Logic of Linkages

Dear ATV Source:
 

ATV Linkage

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.


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