Lost a drag race today, 800 Switchback Rush.

[DeerHuntr]

On my Polaris I had to switch out the entire weight, is Yamaha the same way? Thought I remember reading that they can be adjusted by adding rivets or something. How much do I add if that is the case?

 

[stingray719]

Put on an 8DN and test, mine and Tims dropped RPM with that belt I assume less slipping.

 

[DeerHuntr]

Aren't you going to loose top end with a longer belt?

 

[Mtnviper]

Yamaha uses tuneable weights with 3 (older weights have 2) holes in them. Yamaha sells rivets that are different length and material for different weight values.
It makes tuning easy since you can buy just the rivets instead of complete flyweights!
In the service manual for the sled, there is a clutching chart that will list what the sled came with, as well as part #s and the recommended settings for your elevation. Normally the clutching chart is pretty close and I will use it as a starting point and then fine tune from there.
If you don't have a Viper service manual, usually the dealer will photo copy the tuning chart page for you.

As far as how much weight to change? It depends on the profile of the weight, but there is a general rule of thumb of 1 gram (per weight) = 100 rpm change.

Also here on TY over in the tech pages under clutching, there are copies of the charts for the rivet part numbers as well as other useful clutch tuning info!

 

[FAMILYMAN]

For the best clutching efficiency, it's almost always best to tune engine rpm with the primary and the desired shift out/back shift with the secondary.
Basically its the primary clutch's job to control engine RPM. While the main job of the secondary clutch is to provide enough belt side pressure to prevent slippage and control the overall desired up shift/back shift characteristics.
There are exceptions of course, but this is the general rule of thumb when clutch tuning.

For example if you are over-reving and soften up the secondary spring to try and bring the RPM down, you may be able to do so. However because you are using a quicker up shift at the secondary to control RPM, at the same time back shifting (down shifting) will be slower and the clutching may seem "lazy or slow to respond" when getting back in to the throttle again after a corner.

When adding or subtracting rivet mass in the primary weight, the hole near the pivot (heel) is for tuning slower speeds, middle hole for mid-speed or shift out and the hole in the tip is used for tuning top end or full shift out.

For example, say that your over-reving on the bottom end, but higher speeds your under-reving. In this case you would add rivet mass to the heel and subract rivet mass from the tip. The goal here is to try and accomplish a "straight shift pattern", where the RPM stays fairly consistant through out the shift out when you have the flipper to the bar.

Normally I will tune the secondary for the snow or type of riding conditions.

For mountain or deep snow riding typically you will want a shallower angle helix and or a tighter secondary spring. This puts more side pressure or squeeze on the belt for less slippage and also speeds up the back shift so that the CVT drive system is quicker to repsond to the changing load conditions.

For drag racing only, normally you will run a steeper helix and or a softer secondary spring. With this set up the back shift will be slower, however because your not letting off the throttle during a drag race we don't care so much about back shifting!

There are lots of ways to clutch a sled and usually it comes down to what your priorities are.
The best drag race set up absolutely sucks at normal riding due to the poor back shift! While a good corner to corner set up will back shift quicker and be more responsive for that type of riding, but may give up a little in a drag race due to the slower upshift.

Nice!

 

[779cookies]

mtnviper. Kudos, one of the best explanations of clutching and effects on performance I have ever read. Well done.

 

[Mtnviper]

Thanks guys!
Bill