MC Tested: Yoyodyne Slipper Clutch For KTM RC390

It's a shame that such a beautiful assembly has to live behind a cover, but even though I can't see Yoyodyne's slipper clutch I appreciate it every time I downshift and peel into a corner with the rear tire tracking smoothly.

The machined pressure plate and colorful finish may be attractive, but this setup isn’t about looks; it’s about mitigating back torque that can wreak havoc at corner entry.

My long-term KTM RC390—long since converted to a racebike—was suffering from severe back-end chatter at tip-in. Engine braking introduces disruptive forces to the rear suspension, and after exhausting the available suspension and tire-choice options I decided to squash the source of the problem with a slipper clutch, known to technical types as a back-torque limiting clutch.

It worked, and I was able to kill two birds with one stone since Yoyodyne’s Fred Renz had just finished a prototype clutch for the KTM and he needed it tested. The kit includes an inner hub with Renz’s ball-rearing ramps, a pressure plate, and two sets of spring caps so you can adjust the amount of slip. The setup uses the stock outer basket, springs, and clutch pack. At $850, Yoyodyne’s clutch is pricey, but it’s $150 to $600 less than other aftermarket slippers. Plus, it’s the only tunable option currently available for the RC.

Installation is easy, as long as you have a clutch-hub holder (a $25 workshop essential) and remember that the input-shaft nut has left-hand threads. After fitting this production clutch (I returned the prototype), I raced with it for half a season and it worked flawlessly. I didn’t touch the assembly again until I removed it to shoot photos for this evaluation, at which point I noticed excessive wear on the hub’s pressure faces.

Renz says that the wear is due to clutch slip under acceleration. As is the case with many high-performance components, this slipper runs tighter tolerances than production parts and requires more attention, and I failed to inspect the friction plates regularly. In my defense, the beta instructions I received with the prototype only mentioned measuring stack height at assembly, and I never noticed the clutch slipping. I did, however, lend out the bike several times for trackdays since I last rode it, and the latest person to flog the RC did mention that the clutch was behaving oddly.

In any case, this clutch is cooked, which drives home a critical point: If you run this clutch, you need to keep a close eye on plate thickness to ensure the clutch doesn’t slip under acceleration and wear prematurely.

Only first-year RC390s (2015) have a conventional clutch, so if you own a 2016 or later model, it’s already equipped with a slipper. It’s probably not as delicate as the Yoyodyne unit, but odds are that it’s not as refined, tunable, or beautiful either.