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TECH: AFS: Part 1 – Introduction

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Eminent Cycles – The story behind AFS

Eminent Launched early December of 2017. Since then, riders have asked us what makes AFS (Active Float System) superior to other suspension designs in today’s market? The answer to that question is our combination of leverage ratio, anti-squat, floating brake, floating shock, and finally, adherence to good design practices concerning pivots and stiffness, resulting in a bike that has phenomenal traction. How did we get here and why is the design so unique? To answer that, let’s talk a little bit about suspension, its variables and then discuss our approach and how AFS keeps the tire on the ground for superior traction.

There are many variables in suspension design that come into play that make a suspension perform to the rider’s needs. The right leverage ratio (or shock stroke), the right leverage ratio curve shape, anti-squat, anti-rise, pedal kickback, and axle path represent the major factors taken into considerations when developing a suspension for a mountain bike.

 

Then there are the less calculated and more testing/feel factors, like frame stiffness, shock brand selection, air volume tuning, compression tuning, rebound tuning, bearing or bushing interaction at pivots/shock interface, that add to the design factors of a bike.

Put simply, there are as many design variables as there are testing variables that require tuning. This can be hard to make a selection without riding a bike, and the charts only tell half the story. We highly recommend trying our AFS system and other designs for comparison, get comfortable with what you like as a rider before purchasing as your next bike.

Ok, now that you have some understanding of the variables that result in the end product feel on the trail, let’s get into it and focus on a few design variables (as test variables are not so easy to compare) that we consider when creating a bike and how it influences suspension and ultimately, trail traction. Step one, understanding the leverage ratio curve. The shape can tell us a lot about how the shock should feel on the bike. I don’t think this has been said much, but our opinion is that the leverage ratio should be smooth and progressive so that a natural spring feel could be replicated. A natural spring gets firmer as it is compressed and we want this on the bike as we go faster and deeper into the travel, so it doesn’t bottom out on every bump. Not only is this important for perceived spring feel, it is equally important for the compression circuit in the shock to not see any up down changes in the middle of the stroke as this can cause the shock to slow down (due to an oil pressure surge in the compression circuit) and ultimately suspension stiffening and a loss of traction. In the chart below you will see an example of different leverage ratio curve shapes and what they are called for reference (Thanks to Pinkbike’s Dan Roberts for the graph). At Eminent we first start out with a progressive leverage ratio shape, and move it either up or down to get the desired shock stroke that we are interested in.

Step 2, understanding anti-squat. Before I discuss our approach, let’s first define it so that we are all on the same page. It is commonly understood that with both anti-squat and anti-rise, the measurement is based on percentage where 100% anti-squat or anti-rise means that the suspension will stay neutral with a person on the bike during pedaling and braking respectively (as a general analysis). I don’t mean to say they are the same thing, but to say that the scales for measurement are similar. In this case one will either squat or rise with chain tension or when pedaling and the other would either squat or rise with braking action. Anything above 100% will rise or expand the suspension and anything below will compress the suspension (current practice of 30% above or 10% below is considered to be generally neutral). For example, the Eminent Onset has 150% anti-squat (see below chart for illustration) at sag so that there is a slight opening of the suspension when pedaling to assist with traction.

Expanding on the anti-squat discussion, let’s focus on chain tension when the suspension is moving, and how it effects suspension compression when riding over bumps. If the suspension is opening when hitting a bump during pedaling (if above 100%), the suspension will slightly stiffen if the feet are not allowed to move (not the case where anti-squat is below 100%). If chain growth is also minimal, the feeling in the feet and suspension is minimal. If chain growth is larger, pedal kickback (feeling in the feet) and suspension firming are more easily felt. Taking the Eminent Onset again, chain growth is low and suspension feedback is minimal, so you get the improvement of climbing traction with little drawback. One last note on this topic, quick engagement free hubs add to the anti-squat/anti-rise equation when coasting as there is less “slack” in the system to account for chain growth during suspension movement and therefore anti-squat & chain growth can have a greater effect. Finally, keep in mind chain tension changes depending on what gear the bike is in.

Step 3, anti-rise and its effect on braking. The slang name for anti-rise is “brake jack”, which describes what happens when brake use causes the suspension to stiffen up, resulting in a loss of traction and erratic handling. We look at the anti-rise curve to better understand this behavior when braking, same scale as anti-squat where 100% is the neutral point. Anything above is going to compress the suspension and anything below is going to open/extend the suspension. It is common practice to have anti-rise below 100% as weight transfer during braking is forward and tends to unweight the rear end, so there is not as much of a need to compensate from zero (say towards 100% during braking). If the anti-rise is above 100%, then there is force pushing to compress the suspension and giving the rider a firming up the feel or brake jack. Lets take the Eminent Onset as an example, our floating brake enables for lower anti-rise values below 100% (see graph below) which does not compress the suspension, and gives the feel of plush suspension movement for increased traction when braking.

Lastly, from a good design practices standpoint, we decided to:

  • Mount the shock on the rear triangle “moving parts” (second part of floating system) to reduce off shock or off axis binding (shock flexes together in rear triangle) and improves ease of suspension movement
  • Use angular contact bearings to reduce friction in the lateral direction, improve ease of movement, increase durability and stiffness
  • Move the pivot typically near the seat stay pivot closer to rear axle resulting in a long rocker arm for excellent lateral stiffness and immediate shock response to a bump

If you put all the above elements together, you will find a consistent message: Eminent AFS strives to improve traction and control through optimized suspension movement. We invite you to give us a ride and see for yourself at any of our local dealers, demo events or just stop by our corporate office in San Marcos, CA.

AFS = MORE CONTROL, MORE SPEED, MORE FUN

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