After a few hours in the seat of the 1999 MXZ 600, the feeling in the seat of my pants confused me. When speeds increased and the trail got really rough, the front suspension was pure paradise and the rear was able to soak up the worst the trail had to offer. By contrast, when the pace slowed and the bumps were smaller, the sled tried to buck me off. All to often the sled found the seat of my pants. Regardless of speed, the washboard trails would get the sled kicking back and forth. No amount of suspension tuning eliminated the ‘kick back’ and ‘bucking’ of the sled.

A dealer told me the Accelerator and Control Modulator (ACM) was poorly adjusted and I should soften the rear springs. Adjusting the ACM would not have eliminated the problem since the ACM couples the rear swingarm to the front and its primary function is to limit weight transfer. Limiting the weight transfer does keep the skis from lifting, but does nothing to keep the rear end on the snow. Dialing in a softer setting for the rear springs caused my 220 lb. build to bottom the sled in the rough stuff. Adjusting the front suspension setting, which is based on rider weight, provided no relief from the tail kicking MXZ.

After a bit of Internet research, I found the ride quality issues of my MXZ are not specific to the 1999 MXZ, but to all SC-10 suspensions from Ski-Doo. The SC-10 design of a front swingarm and a rear swingarm is prone to generate a kick back effect over moderate washboard or stutter bumps. The simple solution to my problem was to couple the front and rear swingarms, limiting the range of motion in the rear swingarm.

Coupling Benefits

Coupling for the SC-10 ETS kit is basically linking the front swingarm to the rear swingarm on the SC-10 suspension. The ACM does just the opposite, it links the rear swingarm to the front swingarm, which does little to improve ride quality. To explain differences even further. When the front swingarm contacts a bump, the front to rear coupling limits the movement of the rear swingarm, which forces the rear of the swingarm of the suspension to rise so that the impact to the rear suspension is not as severe. That translates to less kick back. With the ACM, as weight is transferred, the front suspension is forced down. This is how the ACM controls the amount of ski lift during heavy weight transfer loads.

Understanding that coupling benefits will be maximized when the front swingarm’s travel is maximized is key to understanding why the kit performs so well. In the coupled suspension, you want the front suspension travel to be as great as possible to absorb more of the bump impact. Longer travel also translates to greater transfer of energy from the impacted bump and the generation of vertical downforce at the rear suspension because of the front to rear coupling.

Kit Installation

Installation is extremely simplistic and required about an hour’s worth of effort. Removing the skid was not a necessity, but I removed mine since I wanted to thoroughly inspect the suspension and make a few additional add-ons. If you do remove the skid, follow the steps in the Ski-Doo Maintenance Manual. The installation guide also gives specific suspension settings to get the most from the kit. (See the box below detailing the recommended settings. My differing settings are highlighted in blue.)

At this time, I installed Ski-Doo’s optional torsion spring bushings (Ski-Doo P/N 572086100) to prevent the torsion spring from bending over during a hard tail impact. This helps insure a consistent spring rate throughout the spring’s range of motion. Just be sure to follow the instructions and cut 1" off the end of the bushings to make room for the coupling arms.

Installing the ETS kit required the removal of the top carrier wheels and the rear boggie wheels along with the factory track tensioner. The kit provides a modified tensioner that acts as the lower mount point for the kit. With the kit installed, the rear of the track was about an inch off the ground. With a rider’s weight on the sled, this did disappear. Several posts to the rec.sport.snowmobile newsgroup have stated that the Boivin provided tensioner has incorrectly drilled mounting holes. Posters have pointed out that the bracket holes should be drilled ¼" towards the edge to prevent to rear of the track from lifting off the ground.

I also questioned Denis Boivin about the installed kit after I noticed the rear travel was reduced about an inch. In the article title photo of the installed kit, you can notice the rear of the sled is slightly lifted in the air. The nature of the coupling would limit the ability of the rear suspension to compress fully, but reducing travel would seem to be counterproductive. I was wrong. Denis’ response to my note is below. The drilling of the kit supplied tensioner bracket is also not recommended. To obtain the kits engineered efficiency, no modifications to the parts or sled are necessary. The product underwent extensive tests on a stock 1999 MXZ 600 and a 1999 ? MXZ 700. I considered a softer rear torsion spring, which I eventually did test before I added the ETS kit. The softer springs did make the MXZ more prone to bottoming on the rough stuff, so I switched back to the factory springs. Changing the front spring may also provide a big improvement in making the MXZ a smoother trail sled.

It's true that the kit seems reduce the rear travel of about one inch. But, this characteristic is essential to obtain 100% of the performances of the kit.

Don't worry about this inch and the one inch up under the rear wheel. In fact, this "inch" will disappear when you will seat on the sled. This effect is only there without driver on the sled.

The fact is that the link or coupling effect will be maximum when this "inch" is there. In other words, the front swingarm must be able to go in the negative of the bump and hit the bump with a maximum of stroke to permit a maximum of energy transfer to the rear and generate a vertical down force at the rear of the sled.

I hope this short explanaton will help.

Regards,

Denis Boivin

Conclusion

I see the kit as a must have for any owner of a sled with the SC-10 with ACM. The overall ride characteristics are noticeably smoother and controlled. A very slight reduction in ski lift is also a by-product of the kit installation. The kit enables the use of minimal spring preload settings without a mushy or bottoming out prone sled. It may be impossible to make the 1999 MXZ 600 a plush trail crusher, (who would want to?), but the A.D. Boivin SC-10 ETS kit goes a long way in improving an already fabulous sled. After almost 2000 miles, the sled is a more behaved trail champ will the ETS, and I should have installed it before it even left the dealer. To make the sled the absolute best, you’d need to look at Boivin’s Expert suspension, which we also plan to test on a future project sled.

AD Boivin design Inc.
700 rue Jean Marchand
Levis, Quebec, Canada G6V 9G6
418-838-3783 phone
418-838-3957 fax
adboivin@adboivin.com

Thanks for the bandwidth.

Matthew L. Baynard