Progressive Electroic Clutch

Why a Progressive Electronic Clutch?

Before getting into technical detail, I want to address why I think my Progressive Electronic Clutch (PEC) is even a good idea (at least for a trials bike) in the first place. A friend, who excels at geriatric motocross and rode trials in the past, was the most critical of the PEC concept. I respect his opinion. His criticism went along the lines of, why would you want this type of “clutch” if it's nothing more than an extra throttle? And if you can't store/release energy in a spinning flywheel, why complicate the operation of the controls?

It may come down to personal preference. I'll try to explain mine. One of my early trials mentors taught me to keep one (and only one) finger always, always, always on the clutch lever. It was probably the best trials advice I ever received and it transformed my riding. Now I'm completely dependent on the technique.

The throttle is a fairly coarse control on the 5.7 (it has just 72 degrees of rotation). You can certainly drive the bike with just the throttle, but by using my PEC you can achieve a much finer control. The throttle sets the maximum attainable and the electronic clutch modulates it. Of course, it's possible to completely ignore the PEC and use only the throttle if that's how you want to ride it.

Keeping a good grip on the bars is imperative in trials. (You don't grip the bike with your lower legs as in motocross since a wide stance is preferable.) For me, it's often difficult to alter the throttle setting during certain maneuvers while maintaining a good grip on the bars. Whereas, moving a single finger is much easier to accomplish. This is especially true if you have to “regrip” the throttle in order to achieve the required rotation.

Because the motor controller operates in “torque” mode, the rear wheel spins up quickly if you lose traction (as in a loose hillclimb). For me, small finger movements work better than temporarily closing the throttle to regain traction.

Whether or not a PEC is an advantage depends on the particular motorsport and the rider's skill. At the higher levels of trials, the clutch is used more like an on/off switch, but lower-level trials riders (and hard enduro riders) tend to slip it more.

Is my PEC as good as a mechanical clutch? No. Is it better than nothing? Yes.

KTR-10mm linear potentiometer zip-tied to impact pad

Avid Speed-Dial bicycle lever with custom cable

PEC Design

Electrically, the behavior of the clutch lever is opposite that of the throttle. With no human interaction, the throttle is at zero percent. Whereas with no human interaction, the clutch is at 100 percent.

The PEC interacts with the standard Magura throttle. As may be seen in the accompanying schematic, a dual op-amp LM358 is used as a pair of unity-gain buffers. Although this is a single-supply op-amp, it does not swing rail-to-rail. But that's not a problem with this design because the supply voltage is 5V and the maximum required output is only about 2.6 volts.

R1 forms a voltage divider with the throttle potentiometer. R1 is trimmed to about 3950 ohms. This produces a maximum of about 2.6 volts into the controller at WOT. Trimming is necessary to accommodate slight differences in the resistance value of various Magura throttles.

Unfortunately, as received, the KTR-10MM linear potentiometer would not work in this application. It would not pull down to zero ohms (which corresponds to zero volts when configured as a voltage divider). This is important because a zero-volts command signal allows you to kill the power completely - just as the clutch would on an ICE vehicle.

Luckily, it was a simple matter to disassemble the potentiometer and reverse the shaft's orientation. With the shaft reversed, it pulls to zero ohms (and thus volts) yet still has mechanical travel to spare. The spring provides a reasonable lever resistance and it coil-binds when the clutch lever is fully pulled in, thus protecting the electrical portion of the potentiometer from damage.

Progressive Electronic Clutch Schematic

Initial PEC Testing

After some static testing on a stand, the first rolling test was conducted in the driveway. The PEC exhibited excellent low-speed controllability. I could easily do nearly full-lock circles and figure-eights until I got dizzy. And when dumping the clutch at about half throttle, it did a nice wheelie!

However, about 3 out of 10 of those wheelies resulted in the controller shutting itself off immediately afterward. I then had to cycle the battery to restart it. The LED error light on the controller indicated it performed an “Internal Reset” which, according to the manual “May be caused by some transient fault condition like a temporary over-current, momentarily high or low battery voltage. This can happen during normal operation.” I guess the controller does not like high slew rates on its throttle input. I wondered if my unshielded wiring to the PEC could be picking up noise, but none of the other wiring on the bike is shielded, so that seemed unlikely. I wondered if there might be a setting in the controller I could alter to fix the issue.

Real-World Testing

Although the driveway testing was encouraging, the first real test was at our club's practice grounds. Overall it was a huge success! Not once did the controller reset itself as it had during the driveway testing.

The prototype worked without a hiccup. I was quite pleased the two original zip ties held everything in place for an entire day and are still going strong. (I brought a tub full of assorted zip ties, expecting the worst.) Although I would ultimately like to build something inside a normal clutch master cylinder, I could see just putting a box around this prototype for continued testing.

For me (and Cindy as well) the PEC made the EM more intuitive to ride. It was more like a normal ICE trials bike. Since we have a variety of trials bikes, it's easier to move from one to another (and perform competently) if they all behave similarly. This is in contrast to Tesla's goal of having “one pedal” control which arguably has advantages – just not for me.

One of the 5.7's inherent strengths is climbing hills. Throttle/clutch control during slow maneuvers is now excellent. But I still have trouble jumping logs with it. This is especially true on any non-perpendicular crossing of wet logs. Cindy crashed several times on obstacles that she could easily get over with her Sherco 200.

When jumping logs, I have to remember to hold the power on longer than with an ICE trials bike. Of course, you could get used to this but when riding a variety of bikes, it would be easier to maintain a standard control methodology. I have a possible solution to the problem that involves adding an inertial mass (flywheel) directly to the motor.

Tight turns are still not great, but a change to the front forks may improve that.

For the next electronic refinement, see the section on Intuitive Digital Control.