Conspiring Design Features
When reverse gear is selected, a reverse-speed-limiter is energized which prevents the engine from operating in excess of a modest preset RPM level. This limiter prevents the vehicle from achieving significant speed in reverse, but also reduces power to the point that the ATV may not be able to negotiate small obstacles. In order to provide additional power in these circumstances, an override button is provided which bypasses the reverse-speed-limiter.
The engagement of the front drive hubs are controlled electrically. If the 4WD system is activated and the operator selects reverse gear, the front drive hubs electrically disengage, making the ATV a rear-wheel-drive vehicle. This feature distinguishes the ATVs under discussion here from others on the market, including units which have similar appearing reverse-limiter-override buttons, in that the four-wheel-drive system in other ATVs is not disabled when reverse is selected. Depressing the reverse-speed-limiter override button while in reverse reengages the front drive hubs.
If a rider is backing up a hill or obstacle, his weight shifts forward, as does the weight of the machine. The greater the machine’s angle, the greater this effect becomes. While the ATV is only driving the rear wheels, as weight on the rear wheels decreases, the traction available to pull the machine up the hill decreases, making it easier for the rear wheels to spin. Eventually, either the rear wheels begin to spin (traction limited) or the engine simply can’t pull the machine up the hill any further (power limited). Once the engine reaches the maximum RPM level allowed by the reverse-speed-limiter, pressing the thumb throttle further will change the sound of the motor slightly, but will not provide additional power. If the operator depresses the override button while the throttle is open, two things happen: first the reverse-speed-limiter is bypassed which allows the engine to spin-up to whatever level can be accommodated by the position of the throttle; and second the front hubs are engaged and begin powering the front wheels. Thus the vehicle backing up the hill goes from having reverse-speed-limiter limited power and less than half of its weight on driven wheels to having 100% power and 100% of its weight on driven wheels.
When the engine’s speed is being limited by the reverse-speed-limiter, an operator does not receive the feedback from the engine which would allow them to accurately assess the position of the thumb-activated throttle. Thus the throttle may inadvertently be pushed to near-wide-open condition.
Sitting in the operator’s position, I backed the vehicle up hills of various slopes with the 4WD system engaged. Once the rear tires began to spin (which typically occurred as the front wheels approached the base of the hill), the override button was depressed with the throttle open. Occasionally this resulted in the vehicle lurching backwards up the hill, but on several occasions, this resulted in the machine pitching forward back-over-front. The vehicle came to rest upside-down on several occasions, but when one front tire maintained contact with the ground significantly longer than the other the machine performed a "pirouette," rotating about the grounded tire and came to rest on its side.
In the second battery of tests, operator-controlled testing was conducted on the same hills as before. The ATV was backed up to the hills, and the override button was pressed while the throttle was released (closed). This provided four-wheel-drive and full traction from low power levels. It was determined that with moderate throttle application and power being delivered to all four wheels, the ATV was capable of climbing these hills backwards without significant threat of tipping over.
The rollcage was removed for the third set of tests. In place of a rider, a 22.5 Kg (50 lb) bag of potatoes was strapped to the seat, and a 9 kg (20 lb) bag of potatoes was strapped into each footwell area. The total center of gravity of the 40.5 kg (90 lb) potato cargo was located below the surface of the seat, and thus created a more stable system than any machine-rider combination. With the 4WD system engaged, the vehicle was backed up a hill of approximately 25 degrees, until the rear wheels began to spin. This was the steepest slope the manufacturer recommended be attempted by an operator. The throttle was held open with a zip-tie around the right grip while the override system was engaged remotely. The results were virtually identical to the earlier tests: on two occasions the vehicle lurched backwards up the hill, while on the third test the vehicle pitched back-over-front. The rear of the machine struck the ground with enough force to bend the rear subframe.
Polaris Design and Literature
Figure 1 shows the unaltered wiring diagram for the exemplar ATV. The portion of the diagram which was modified to include the three-relay system is indicated by a box. Figure 2 contains an expanded view of the wiring diagram’s modified region, showing the proposed modifications.
A working example of this 3-relay system was fabricated from components purchased at a local Radio Shack for about $30 retail. This circuit could be fabricated for much less given the scale of economy available to an original equipment manufacturer. The prototype 3-relay system was installed on the exemplar ATV, and found to work as expected, preventing the reverse-speed-limiter from disengaging while the throttle was open.
Conclusions and Recomendations
A simple three-relay circuit was presented which makes the warned-against actions, namely depressing the reverse-speed-limiter override button while the throttle is open, impossible. A prototype system was fabricated and installed on an exemplar ATV. The proposed circuit operated as designed, and did not hinder normal operation of the machine in any way.
This 3-relay modification, or its functional equivalent, should be included in the design of all current ATV models which have the above noted characteristics, and should be added to ATVs already in service.
Polaris Industries (2), 1995. 1995 Magnum Service Manual, P/N 9912961, Polaris Industries, Minneapolis Minnesota.
UPDATE, July 2010