MC Tested: Yoyodyne Slipper Clutch For KTM RC390

Ari track tests Yoyodyne's slipper clutch on his KTM RC390 racebike.

yoyodyne slipper clutch
A premium slipper for bikes that didn't come with one from the factory. Just make sure you inspect the stack height frequently. - Buy It Now!Julia LaPalme

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.

Yoyodyne clutch
Buy It Now!Julia LaPalme

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.