Originally posted by catracer343
@Dec 20 2005, 04:22 PM
O-K, Here are some basics.
Heavier weights will produce more belt "grip" pressure and lower rpm's, because they will cause the primary to close, or "shift out" sooner.
The more added weight will cause more centrifical force and push the primary spring in "harder"
The stiffer primary spring will produce more rpm, unless you have heavier weights to squish it shut, then it just squeezes the belt more, and "loads" the motor.
There are two different # figures given with each spring (eg. 120#-310#)
The first figure (120#) is how much "force" is required to engage the primary.
The second figure (310#) is how much "force is required to fully compress the spring. (wide open, full shift)
The secondary spring is only calibrated with one figure (eg. 80#)
This is how much "force" is required to fully compress the spring at full shift.( I belive when it is placed in the "center" hole)
On a secondary clutch, you will see that there are several holes drilled in the cover that holds the spring.
If you move the spring into a different hole, let's say clockwise, you will be putting more per-tension in the spring, and adding more spring "force" and top end rpm's.
For each hole you move the spring, it is roughly a 100rpm increase or decrease, (if you loosen the spring).
Now we can talk about helix angles.
Some are what's called a straight cut helix (eg. 55*) while others are multi cut (eg. 60*-40*)
The steeper the angle (60* vs. 40*) means that it will open up the secondary clutch "faster" than a 40* helix.
When the clutch opens faster, the motor will get "loaded" more, resulting in a rpm decrease. This is why you will see more twins with a multi angle helix than tripples, because they make more torque at lower rpm's than tripples, although most all sleds now are running multi's. The more torque that a motor has, the "steeper" of an angle you can use.
I hope that this can help you find the right setup for your machine.
Just remember that there is no "perfect" set up that will do everything just right, otherwise I think that I would have found it by now!
Clutch tuning is somthing that takes countless hours of testing on the dyno, trail, or track, so don't be discouraged if you don't find the perfect combination for how you ride the first time...and remember to only change one thing at a time whene your testing. That is the best way to keep track of your progress. -Catracer343
Great explanation! Some other rules of thumb....
You tune your primary to control RPM's. That's it!! Pick a primary spring finish force first which will give you the proper shift. You can always pick the engagement later, but you need to get the shift pattern right first.
Helix is for controling the belt squeeze in the upshift and backshift. Secondary spring is a fine tuning device for the squeeze, ie: efficiency of the clutch. Usually you want to keep the secondary spring in the centre hole. Ideally you want just enough belt squeeze to keep the belt from slipping, but not so much that you lose efficiency. Belt slip or too much belt grip and you lose HP to the track. But, if you want alot of backshift, then you give up some efficiency to achieve this since you will be using helix finish angles and springs that will grip the belt hard.
If you need more or less RPM's from there, you change weights to adjust.
Each gram in weight is between a 100 to 200 RPM change.
Biggest rule....you want clutches that are cool to the touch after a good run. This means you are achieveing good efficiency.
Clutch kits are good for all around riding. There is nothing like setting up a clutch for your riding style, riding conditions, body weight, track size, engine mods, etc., etc.
Everything you read here is a simplification. Get Olav Aaens Clutch Tuning Handbook. Really great for understanding the theory. Then it's all about testing, testing, testing. It can be really frustrating, but alot of fun when things start getting close.