Riders who are familiar with the traditional torsion springs found in most rear suspensions are often thrown for a loop when it comes to setting the rear preload on a Yamaha fitted with the Monoshock rear suspension. Instead of torsion springs, the Monoshock suspension has a single coil spring on the single shock absorber.
Most of the sleds fitted with a version of the Monoshock suspension are set from the factory for a fairly light rider, under 200 pounds. In our experience the single coil spring does not have the range of adjustability of traditional torsion spring suspensions, meaning that you might not be able to get the proper ride height and preload setting for your weight by simply making adjustments; you might find the need to install an optional heavier coil spring.
How do you check for proper preload on a Monoshock suspension? Yamaha uses what is called a spring sag adjustment, much like what a dirt bike does. Here’s how;
1. Hang or tip up the rear of the sled so the rear suspension is completely unloaded, that is with no weight on any part of the track.
2. Measure the (unloaded) length of the shock spring, from end to end.
3. Set the sled back down on the ground and have the rider (with riding gear) sit on the sled.
4. Measure the spring length again (loaded length).
5. Calculate the difference between the two measurements. It should be 40-45 mm.
6. If required, adjust the spring preload to reach the proper 40-45mm setting. ON sleds with threaded adjusters instead of cam adjusters, pay special attention to the minimum installed spring length (you do not want the spring to coil bind). Most Yamaha coil springs can only be set 10mm shorter than the factory setting.
7. If required, heavier riders might require the installation of an optional heavier spring.
Generally, you will want to also increase the compression damping along with a spring preload increase. Also realize that as you increase the rear spring preload there will be a tendency to cause the steering to become heavier. If you installed meaty carbide runners to get the thing to corner better, you might find it all to come back into balance once you get the preload set properly. Always verify proper preload settings before making changes to the front end to get the sled to handle better!
The main take away from all of this is that many riders are using their Yamahas with an improperly adjusted rear suspension preload, if you do not weigh 175 pounds and have not made any adjustments to the coil spring length. The Monoshock rear suspension is primarily a comfort suspension and less capable of resistance to bottoming, thus the torsion spring versions in the RTX models which are more capable to withstand harder riding and resisting bottoming.