MXA TEAM TESTED: PRO CIRCUIT KTM RISING-RATE SHOCK LINKAGE
WHAT IS IT? In 2016, KTM changed its rising-rate linkage. It was a major change, as the shock spring rate dropped from 57 N/mm to 48 N/mm on the 450SXF. This was a sure sign that the leverage ratio was dramatically stiffer. But, every MXA test rider felt that the 48 N/mm spring was too stiff for the rising rate. The backyard fix was to drop the 48 N/mm spring for a softer 45 N/mm spring, which KTM did in production later. Pro Circuit elected to fix the problem at the source with a completely new rising-rate shock linkage. Don’t confuse this with the typical longer linkage arms. Changing the length of the link arms does not change the rising-rate curve; it only changes where on the curve the shock begins to move. Pro Circuit’s system is a completely new linkage that changes the rising rate courtesy of a new eccentric bell crank and linkage arm.
WHAT’S IT COST? $349.95.
CONTACT? www.procircuit.com or (951) 738-8050.
WHAT STANDS OUT? Here’s a list of things that stand out with Pro Circuit’s KTM/Husky rising-rate shock linkage.(1) Concept. A motorcycle’s rising-rate shock linkage works by speeding up the shock as it moves through its travel. The shock shaft moves slowly at first, but picks up speed as it gets deeper into the rear-wheel travel. Because it moves faster as the rear wheel rises, the shock resists bottoming as it is forced to move a larger quantity of oil through the same-sized orifice.
By changing the shape of the cam, an engineer can feed in stiffness or softness. For example, in Supercross, bottoming is the biggest problem because of the size of the jumps. The engineer can change the linkage ratio to make the shock move even faster as it gets deeper into its travel, which would result in a massive increase in damping. Voila! No bottoming. Conversely, the cam’s shape can be changed to make the linkage ratio more linear, which translates into less rate of change in the speed of the shock shaft and a plusher feel.
(2) Pro Circuit rising-rate linkage. The previous generation (pre-2016) of OEM KTM shock linkages delivered an unwanted soft feeling. The 2016 to 2020 OEM shock linkage was redesigned to make the shock stiffer, but KTM went a little too far and got a sensation that was both harsh and stagnant. Pro Circuit’s rising-rate linkage utilizes some of the mechanical advantages of both designs to allow for the use of a more true-to-weight spring rate and linkage-ratio sweet spot. In essence, Pro Circuit split the difference between the soft pre-2016 linkage and the stiffer 2016–2020 ratio.
(3) Performance. There are two significant improvements to the Pro Circuit KTM linkage over the stock KTM linkage. One, there is a noticeable improvement in rear-wheel traction, which corresponds to a big reduction in deflection off of square-edged bumps. The mechanical advantage at the rear wheel is improved by an average of 7 percent across the full travel of the rear suspension. Two, most MXA test riders complained that the WP shock delivers a stagey feel in its mid-stroke, as though it is actually hanging up for a second before continuing on. These spikes in the curve are improved by 10 percent to allow the shock to feel more fluid through its stroke.
(4) Recommendation. We tested the Pro Circuit link on our 2020 KTM 450SXF. We started with the stock shock spring. It hated the new linkage. We replaced the 45 N/mm shock spring with a stiffer 48 N/mm spring and everything was rosy. The performance of the rear suspension improved considerably, the stagey feeling disappeared and the shock felt ultra plush throughout the stroke.
WHAT’S THE SQUAWK? When you add in the cost of a stiffer shock spring, this is an expensive mod.
MXA RATING: Once you find the proper spring rate (and for riders under 200 pounds, we would start with a 48 N/mm spring). The KTM suspension works better over small bumps and has a smoother transition through its full stroke. It loses the stagey feel that made the KTM rear feel as though it had three different phases to the shock stroke. This is a solid suspension mod.