Tech Professor Flashback: Push-Button Reverse

OK, longtime snowmobilers: Do you remember the era where it was actually a lot of work to back up your snowmobile? That’s right, relative newbies, reserves on snowmobiles hasn’t always been available at the push of handlebar-mounted button. In fact, most sleds of the 1990s and before didn’t come with reverse, and those that did had to have extra gear mounted on them.

Then, Ski-Doo launched its then-revolutionary Rotax Electronic Reverse (commonly referred to as RER) on the 1998 Touring SLE and Tundra R. It quickly worked its way through Ski-Doo’s lineup and – due to some “horse-trading” among the manufacturers – through the snowmobile industry. Snow Goer’s original Tech Professor Phil Mickelson (may he rest in piece) took a stab at explaining the technology and reasoning behind the engine-reversing process in his column for the the March 2004 issue of Snow Goer. It’s show in its entirety below. If you’re interested in having your own copy of the magazine that included the article, it’s available at the Snow Goer Store.

TECH PROFESSOR: It’s Just Magic!

When I was in college in the mid-1980s (yes, I’m over 40), I worked for a Suzuki motorcycle dealer in Duluth, Minnesota.

I worked in the service department and we had, on occasion, some strange service problems that came in.

One day, a customer came in with some bandages on his arms and a scrape on his forehead. He said his K-120 had started up backward and he wanted it fixed. It seems he had started the bike, dumped the clutch and it took off in reverse. It pitched him over the handlebars and left him bleeding and wondering what the heck happened.

It didn’t take long to discover the stator was loose and had rotated to a position that allowed the points to open close to top dead center. A wimpy kick on the starter allowed the engine to fire up in reverse rotation.

When I told my boss what was wrong with the customer’s bike, he asked me to set up a little 80cc bike with the timing on the other side of TDC. We pushed the bike backward to start it and my boss, who was one heck of a good rider, sat on the handlebars and rode the bike around, backward! We had a lot of fun with the trick bike and probably sold a few more Suzukis because of it, but I had learned a lesson that I would hear about many years later.

In my first year racing Ski-Doos for Halvorson Equipment Company, George Hayes, father of and tuner fur Hall of Fame racers Stan and Doug Hayes, told me of an early race he was at where one of the drivers on the starting line was frantically trying to get his sled started. Just able to get it started before the flag dropped, he gunned the throttle and promptly flew over the handlebars as the sled backed out from under him.

There were always questions about how to set the ignition timing of a sled in those days; what’s indirect timing vs. direct timing, do you hold the centrifugal advance ahead or not? Evidently this guy had his timing too close to TDC and in his frantic attempt to start the sled, it backfired and started the engine running backward.

A two-stroke engine, without mechanically driven valves, can operate in either rotational sense. It’s just a matter of stopping the engine, resetting the timing and cranking the engine in the opposite direction. The concept isn’t new and was used by Mercury Marine on its outboard engines in the late 1950s and early 1960s to reverse its engines. I talked with Geoff Burgess, an engineer working for Bombardier Inc. in Valcourt, Quebec, some time in the 1980s about future developments. He said they had a team in Valcourt working on a totally electronic system that would allow backing up a machine without any weight gain. It would reverse the engine’s rotational sense electronically rather than having to add a gear­box to the chaincase to allow reversing the sled.

I joked with Burgess at the time about our reverse rotation Suzuki and the poor devil on the snowmobile racetrack.

The first working prototypes of the system were developed by the research staff in Valcourt. Remember, it’s not just the engine reversing system that had to be developed, but a driven pulley that would allow reverse torque input also had to be developed. Rotax was also working on systems that needed special electronic controls and rath high computer power. The projects were merged so that several Multi-Purpose Electronic Modules (MPEMs) could be developed that would perform multi­control functions for the engines. The finished product, as we came to know it, was named Rotax Electronic Reverse (RER).

So, you ask, didn’t Ski-Dao patent the system? How did Polaris end up with its Polaris Electric Reverse Control (PERC) after a few seasons of use on Ski-Doo? Every manufacturer patents every piece of development they can. The patents allow the exclusive use of a design by the designer for about 20 years, but the patents also provide a bargaining position for the patent holders.

If one manufacturer has a patent on an item that another manufacturer wants to use, the patent holder can either sell the rights for use of the patent or trade the rights for its use to other manufacturers. It happens in many industries, including the snowmobile industry.

In 1972, Bombardier purchased rights from Polaris to produce the then-exclusive Polaris clutch design for use on the Blizzard and free air race sleds. Ski-Doo and Polaris have traded rights to patents for years, as have all the manufacturers. The appearance of PERC on Polaris models is the result of an agreement between Ski-Doo and Polaris and an exchange of patent agreements. Whether the agreement has an exclusivity clause in it will determine how soon we might see the magic reverse system show up on other brands.

While the systems used by Ski-Doo and Polaris are made by different manufacturers, they are essentially identical in design. While I shall discuss the system as RER, remember the same thing applies to PERC.

Electric Reverse Explained

RER is a system that electronically manipulates the ignition timing to reverse the rotation of the engine. On all but the SDI engines, the system utilizes one forward and one reverse trigger coil and the output of each trigger pulse is manipulated by its own timing curve in the RER brain box, or MPEM.

The system is obviously digital and when it comes to manipulating the trigger signal, this system really manipulates it. The MPEM has been programmed to recognize a signal sent by the reverse switch on the handlebar. When the switch is pushed, the module cuts off the ignition and the engine speed drops to approximately 550 RPM. The speed varies depending upon the engine equipped with RER. The MPEM then fires the ignition at about 60 degrees before top dead center (BTDC). Normal ignition timing is about 20 degrees BTDC. The huge advance forces the burning mixture to push the piston back down and start the engine in reverse rotation. A second trigger coil takes over and controls the ignition advance while the engine is in reverse rotation.

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In the case of the SDI engines, the magneto is equipped with a 34-pole stator, which keeps precise location of the crankshaft position for the injection system. Its signals are also used to control RER –  dual trigger coils are not used.

To eliminate any change of someone pushing the reverse switch while the sled is moving, the MPEM is equipped with a device that prevents it from reading a signal from the reverse switch when the engine is running at more than 3500 RPM. Also the MPEM will not read a signal from the switch less than 900 RPM or so. This means the machine must be started in normal rotation and be idling before the reverse program can be read.

When an engine is idling and the reverse button is pushed, the system takes over and slows the engine to the point where it can fire the engine in the other direction. The reverse timing curve takes over, but if a driver tries to back up too quickly it will retard the timing to the point it loses power and slows down.

RER is a clever system but it is probably part of the reason we no longer see rotary valve Rotax engines in Ski-Doo sleds. Unless the valve timing could be changed when the rotational sense of the engine changes, it would be hard to apply RER to its rotary valve engines. I should note, however, that the original design team in Valcourt had addressed the need to re-time the intake valve and had preliminary designs that could accomplish the task.

One other vital link makes RER work – the driven pulley. Most drive pulley designs will function normally no matter which direction they spin, provided there are torque transfer buttons on both sides of the transfer points. Centrifugal forces generated will be the same no matter which way the pulley spins.

The normal driven pulley design, however, will work properly only in one rotational sense. If the direction of the torque input to a standard driven pulley was changed, the reverse torque would disengage the cam from the buttons in the pulley and the belt would pull down in the pulley in a hurry.

Ski-Doo developed a new driven pulley design to use with RER. The new pulley utilizes a cam with a profile on both sides and two opposing sets of sliders. On the “forward” side of the cam, a normal profile exists while on the “reverse” side of the cam is a very steep profile. Engineers do not want to let the pulley upshift while backing up. The spring used in the RER driven pulley is a very high-rate compression spring. The spring is not engaged with the cam or pulley halves and no torsional load is placed on the spring.

Editor’s Note: Every issue of Snow Goer magazine includes in-depth sled reports and comparisons, aftermarket gear and accessories reviews, riding destination articles, do-it-yourself repair information, snowmobile technology and more! This story is from the November 2021 issue’s Cold Tested department. Subscribe to Snow Goer now to receive issues delivered to your door or your computer for a low cost.

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