Many mountain bikers, myself included, have gazed absently at the complicated arrangement of tubing in a suspension linkage system, trying to understand how it works or why the manufacturer went with that particular design, before finally giving up and resolving to leave that business to the engineers. It’s not long after hearing words like “dynamic leverage ratio” or “fully utilized damper progression” that many of us begin to zone out. But in truth, linkage systems can be broken down in a comprehensible way, and in fact, understanding linkage systems could play a key role in making sure you’re riding the right bike.
A linkage system consists of tubing and pivots, comprising a bicycle’s rear suspension. Compared to other mechanical advancements in mountain bike technology, linkage systems are relatively young. After making their first appearance in the 1990’s, linkage systems have been the focal point of many engineers and designers, which is why they have changed so much in the 20-odd years they’ve been around.
A major hurdle faced by early designers was weight. Initial designs were comprised entirely of metal, as carbon fiber hadn’t become widespread in the bike industry yet. This, combined with an early emphasis on the importance of strength above weight, produced many bikes that weighed 40 pounds or more. And not only were these bikes heavy, but pretty inefficient as well. Issues like pedal bob and brake jack were common pitfalls of these designs.
Pedal bob refers to the bouncing up-and-down motion of a full suspension bike when the rider is pedaling. This motion steals power from the rider, making the ride more arduous. As the rider pedals, the energy produced by their legs is supposed to go directly to the rotation of the back wheel. But when a bike has a bad pedal bob issue, the energy expelled by the rider not only goes to spinning the wheel, but to moving the bike up and down as well.
Pedaling on an efficiently-engineered bike is like running on pavement, where the output of each step goes directly to carrying the runner forward. Pedaling on a bike with pedal bob, though, is like trying to run through deep sand. Much of the energy devoted to each step is lost by the foot sinking into and being pulled back out of the loose sand. This issue makes the ride much more tiring, and thus limits the distance and time that a rider can endure.
Another issue with early suspension designs was interference with brakes. Some suspension systems compress when the rider is braking, impeding on the suspension’s effectiveness. This problem is called brake squat. Adversely, brake jack occurs when braking causes the suspension to extend. Both of these problems diminish the bike’s maneuverability. Ideally, the rider would want the suspension to be unaffected by braking, and that’s exactly what the proceeding bike designers set out to do.
Pioneering New Designs
One designer who played a key role in the evolution of linkage systems was Horst Leitner, an Austrian immigrant and inventor of the aptly named Horst-link. The Horst-link is what’s known as a four-bar system. It consists of a rear pivot located below the rear axle on the chainstay, and a similar pivot located above the shock on the seatstay. This means there is no direct connection between the rear axle and the frame. The placement of these pivots allows the designer to combat brake squat and brake jack. By fine tuning the Horst-link, designers can make suspension that moves independently of braking, giving the rider a much smoother and more controlled ride.
As linkage systems continued to evolve, designers were left with the difficult task of designing a bike that absorbed the bumps of a rough trail, but without absorbing the momentum of the rider’s pedal stroke. Essentially they were trying to eliminate pedal bob. A bicycle’s resistance to pedal bob is called anti-squat. And as you could imagine, designing a bike with effective anti-squat is not an easy thing to do. But, luckily for us, the bike industry is filled with some pretty smart folks.
What they found was that the downward force of the pedal stroke—which is the cause of pedal bob—can be resisted by the tension of the chain. To achieve this, designers had to start with what’s called the Instant Center (IC) of a linkage system. On a single pivot bike, the IC is in the center of the pivot, because the rear axle rotates directly around it, like the hands of a clock.
On more advanced systems like the Horst-link and VPP, the IC moves as the suspension cycles through its travel. When multiple links are involved, the axle path changes, thus the center around which the axle is rotating also changes.
By drawing a straight line between the rear axle and the IC, you’re given what’s called the swingarm line. If the swingarm line is at an upward angle (meaning the IC is above the rear axle) then the driving force of the rear wheel combined with the tension of the chain will drive the frame upwards, thus counteracting pedal bob. The downward motion of the pedal stroke, which usually causes the bike to squat, is met with the upward force caused by the positioning of the angle and the tension of the chain, giving the rider a much more efficient pedal stroke. This is a major factor in many of the contemporary high pivot designs.
Looking at Things From a New Angle
Some bike companies have opted to rely on more than just pivot placement to ensure their bike has sufficient anti-squat. Linkage systems like Yeti’s Switch Infinity and the NAILD R3ACT system used by Polygon have added sliding shafts that are designed to separate pedaling efficiency from suspension performance.
In recent years, some designers have also taken the performance enhancements of a linkage system and applied them to the front wheel as well. Several linkage forks have made their way on to the market, offering a substitute for the standard telescoping suspension fork that has dominated the bike industry since the inception of suspension.
The idea behind the linkage fork is that if bars and pivots are added to the equation, the designer can manipulate the position of the front wheel to enhance traction and handling while the suspension is being compressed. Rather than the wheel moving in a straight line when the suspension is compressed, the axle path can be changed to give the bike better bump sensitivity and make it easier for the rider to get up and over obstacles.
In the coming years, we can expect to see many more experimental suspension designs, and it’s realistic to expect that the feel and performance of a full suspension mountain bike will continue changing.
But you don’t have to wait for the future to find a bike that performs well for your riding style. For example, if you’re a gravity-oriented rider, you’ll probably be happy on a four-bar design that has good braking capabilities. Whereas if you’re a bigger climber, seek out a bike that has an effective amount of anti-squat, so you get the most out of each pedal stroke. Because climbing is hard enough without your bike working against you.
By Jeff Walker, Content Writer, jans.com.
Shifting Trends is our latest blog series covering recent changes and developments in the bicycle industry. This is the fourth in the five-part series. Click here to read our last post, and check back next month for our final installment.