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Sine and Air Shocks Explained

// Designed for Perfection

If you’ve read about Alchemy’s mountain lineup, you likely know that the key to an Arktos is the Sine suspension system. Sine is interesting because it allows for a varied ratio of compression force from the rear wheel to the rear shock as the suspension moves through its travel. Technically, we talk about it in these terms: regression at the start of the travel in order to allow the Arktos to absorb small bumps and provide climbing traction, progression in middle of the shock stroke to get the bike into the meat of the travel, and regression again at the end to allow the bike to hit its full travel. 

But, let’s break that down into layman’s terms and then relate it to how an air rear shock functions to better understand the benefits. For starters, the main result of the Sine link is that it allows the rear wheel to move more easily through its travel at the start, harder in the middle, and again easily at the end of the travel. Well, you might be asking yourself, “Why would I want that?”

The answer is actually pretty simple, at least once you understand the fundamentals of an air shock design. Air shocks use air as the main spring component to support the rider on the bike. Because air is a small molecule and air shocks reach high pressures at bottom out (at least 5 times the starting static pressure) shock manufacturers have to utilize tight tolerance air seals or the air would leak out of the shock. That’s a good thing for sure, as no one wants to be stuck out on the trail with a leaking, bottomed rear shock. The downside of all of the sealing is friction. Air shocks, by nature, have more friction than coil shocks because of the seals required to keep the air in. And the most noticeable place that the friction is felt is the start of the travel. One way to overcome that friction is a design (like Sine) which increases the rear wheel to rear shock ratio at the start of the travel. With such a design, the rear wheel can move through small chatter without you feeling it and the wheel keeps locked to the ground so you aren’t getting tire spin.

Okay, so that all sound logical, right? Well, it is, but the problem with air shocks is that they are very linear through the middle of travel. That means, once you are past the point of initial friction, the shock quickly wants to keep that force moving to the very end of its travel, where all of those small molecules are highly compressed and can finally resist the forces coming from the rear wheel. Well, that’s where the varied nature of Sine comes into play. The Sine link physically changes direction after initial wheel movement, and the ratio of wheel to shock travel is reduced, increasing the force required to compress the shock. This IS what makes Sine so great, as it actually counteracts the air shocks linear nature and allows the suspension to stay high in its travel. Simply put, when you hit a bump, there is still some shock travel left to allow the bump to be absorbed.

So, we are through the first two parts of the travel, where we have the wheel moving easily at the start of it’s travel and the middle is effectively absorbing bumps. Now you are probably wondering what happens when you hit a big drop or bump, the kind that immediately sends the suspension to the end of the travel. As mentioned before, air shocks become very firm at the bottom of the travel, as the shock becomes fully compressed and the air no longer has space to displace. If you’ve ever experienced getting to bottom abruptly, you know it’s hard on both your bike and your body. Again, this is where Sine comes to your aid. As the Sine link physically moves back in the opposite direction, the leverage ratio on the shock actually reduces, requiring less force from the rear wheel to make the shock activate. The Sine link is counteracting the progressive nature of the shock (by making the suspension easier to get into the last part of the travel). It’s a yin and yang thing, but the result is that you get all of your suspension travel with an almost bottomless feel.

 

Okay, so you’ve just heard what sounds like more marketing-speak: “You are getting the best of both worlds”. The difference is that once you understand the basic design of Sine AND how it works in combination with an air shock, you can see that it’s POSSIBLE to achieve the best of both worlds. And as a result, you begin to understand how an Arktos can pedal well, absorb small bumps, provide excellent suspension throughout its travel.

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