Vital brake bracket
21 Oct , 2019
Traction: FULL STOP
Eminent Cycles’ floating brake bracket
In our previous article, we took a broad look at the leverage ratio, anti-squat, and anti-rise numbers of our new Onset 29er. This time we’re focusing in on anti-rise, AKA “brake jack”, in an effort to better explain its effect on suspension and how AFS virtually eliminates it.
What is anti-rise? To answer that we must first think of what happens during braking to slow or stop any bicycle. The rider’s body weight moves forward on the bike and unweights the rear end. There is a natural tendency for the rear end to expand due to the rear wheel unweighting, but this does not always happen with bicycle suspension. Mountain bike suspension designs which place the axle and brake mount directly on the chainstay (single pivot/faux bar) are heavily influenced by the force of the brake slowing the rotation of the wheel. This drives the suspension forward to compress or “brake jack”. This can be improved by moving the brake and axle mounting location to a separate portion of the linkage, not directly pivoting on the frame. Common examples of this are split pivot and Horst link designs where in both examples the brake mount is not on the chainstay, but the seatstay. By mounting the brake on the seatstay, the brake follows the seat stay motion and lowers (but does not completely eliminate) the transfer of compressive forces to the suspension.
At Eminent we took it one step further. We prioritized and scrutinized traction in all circumstances: rough open terrain, tight and twisty, pedaling in or out of the saddle, and finally, under all levels of braking forces. All of us have had a bike that was super planted in the ideal wide open situation, but shut down under braking. In our opinion, compromised traction in even one situation is overall compromised traction. Full Stop.
With these ideas in mind, we refined our suspension metrics and worked to come up with a design that would function in all situations. During the development process, we continually were challenged to get the numbers we wanted, as tweaking pivot locations to improve the leverage ratio curve or axle path made the anti-squat or the anti-rise worse. It was apparent that there was a relationship of compromise involved in trying to get the right leverage ratio, anti-squat, and anti-rise numbers we wanted without affecting the others. The only way to truly resolve this conflict was to separate the braking mechanism from the rest of the suspension by creating the floating mount. We modeled it and found that there was a good amount of independence between the anti-rise and the rest of the suspension metrics. Our resulting design mounts the brake on a separate link or “brake bracket” that rotates and slides around its mounting points. The diagram below illustrates the points of movement, where the axle location “A” rotates, and the forward location “B” slides and rotates as the suspension moves through its travel.
With suspension movement uninhibited by braking forces, we were able to achieve an anti-rise number that is not dependent on other suspension kinematics, truly decoupling brake and suspension forces. Putting it into numbers, AFS achieves an anti-squat value of 150% with an anti-rise number of 53.2% at sag (see below anti-rise curve of the Onset LT).
It is this low number that gives our bikes the planted and predictable feeling as a result of uncompromised traction both on and off the brakes. As discussed in the previous article, 100% gives a neutral effect while braking, and going below that allows the suspension to move freely. We have found that we if we design with a lower target, the resulting bike performance is great traction when braking in all body positions on the bike.
To put all this in perspective, visualize a very fast technical section, using all of the bike’s travel. At the end of this section is a very hard, loose, off camber turn. In a perfect world, you would hit the tech section as fast as possible, then immediately kill that speed to make that sketchy turn at the end. The problem with a brake jack prone bike is the moment you hit that brake the rear end sits deep into the travel, effectively reducing the amount of available suspension. You get bucked, lose your line, or worse blow the turn and eat it. Your only option is to ride the technical part slower, partially braking all the way through to ensure you enter the turn at a safe speed. This simply was not an option for Eminent. Our priority was and is traction, in all situations, wide open, climbing, sprinting, or… on the brakes.
Note: When we move around the bike, our center of gravity changes location and with this, so does our anti-rise calculations. So the lower the person the higher the anti-rise number, and if we are low to begin with, we don’t rise above 100% and always have good traction while braking.
Are we onto something here? These folks seem to think so…
Pinkbike: Richard Cunningham Braking while descending tricky chutes or while trail braking into rough corners does not unsettle the suspension, nor affect the ride height of the chassis
Vital MTB: Alan Davis The Onset is one of the most confident all mountain bikes we’ve ever ridden descending steep, sketchy lines. The standout impression of the Onset was how supple and planted the rear suspension feels.
Mountain Bike Action: Brandon Castelli Another feature that sets the Onset apart is its braking performance. Eminent mounted the rear brake on an independent track designed to better isolate braking forces. On the trail, it does that well, providing easy-to-control braking forces, ultimately resulting in more speed.
If you find this interesting, we invite you to take a test ride of our bikes today at your nearest retailer or demo location. Thanks for reading, the Eminent Team.
Please login and you will add product to your wishlist