Is Clutching Magic?
Not really! First draw a large pentagram on the floor around your sled, get some candles and a virgin to sacrifice, no-no that would be a waste. Anyway, Olav Aaen's Clutch Tuning Handbook is the bible of clutch set-up. If you don't already have a copy, get one, read it, put it under your pillow at night, take it to the can with you, read it, read it, read it. Even though the clutches appear to be pretty simple, they can make a perfectly sane man sound like a babbling idiot, muttering things to himself like, wait a minute I added weight andÖ.No, if I increase the spring rate andÖ..No if IÖ. DAMN IT !!!!! The basic idea of clutch tuning is to get the springs and flyweights in your primary (drive) clutch to bring the engine to a specific rpm PEAK using different weight and spring combinations. These combinations are endless thatís why no one person has all the answers for every condition. Then adjust the spring tension and helix ramp angles on the secondary (driven) clutch to get the shift-out you want. These combinations are also endless. The shift "characteristic" is dependent on your personal preference and riding style. The theoretical "ideal" is a slight over peak at engagement, then flat at the peak HP out all the way though the shift-out with the rpm slowly climbing above the peak HP once you're shifted all the way out into overdrive .75:1. Remember thatís theoretical. The point of this being that generalizing clutch settings is impossible. Even if I tell you exactly what I run in my sled, it's no guarantee it will work for you. Most tuners wonít give away their "trick" clutch setups that cost them dearly in lost hours of sleep, endless parts runs and tuning rides, but they may be able to point you in the right direction. Clutch tuning is the one area where you can gain a competitive advantage.
Where do I start? Focus on the primary and getting your engine rpm right. That means right at PEAK HP with steady acceleration. You'll increase or decrease the weight and or profile (where the weight is distributed in the weight itself: heel, mid, tip) of the flyweights to achieve this. Lighter weights raises the rpm heavier weights lower it. Heel = bottom end, Middle = Mid-range, and Tip = top end. Make sure you're using the same profile of weight when you make changes or you'll end up chasing your tail. Do NOT change the secondary to adjust engine rpm. I've seen too many people increase the pre-tension on their secondary to bring up their engine rpm, myself included. You are shooting yourself in the foot !!!! It decreases the efficiency of transmission of power to the track while adversely affecting the ability of the secondary to keep the shift curve flat. Once you have your rpm where you want it, you next job is to get it to shift out the way you want. That's the job of the secondary. The combination of spring and helix is what determines this. Multi-angle is what I prefer. The secondary spring directly equates to side-force on the belt. The idea is that the more side-force you have on the belt, the less efficient it is because of belt drag. Secondary pre-tension (wind) should be around 16-24ftlbs. What I try to do is get to the minimum side-force possible without slipping the belt in the secondary. Too much side-force and the clutch will back-shift too fast and put you in too low of a ratio and over rev when you stab the throttle out of a corner. Too little and it won't back-shift fast enough and it will react sluggishly (bog) and you'll loose rpm climbing hills or trying to maintain high speeds. Lastly, Iím a believer in low gearing, its less work for the clutches and engine they will run cooler. Most sled heads donít realize that the factory gears sleds for speeds higher that the sled will ever reach. They never reach a true .75:1 ratio, also the track and engine combinations the owners and after-marketers add, wonít allow the motor to pull the tall gearing (pulls the rpm down on a long climb or high speed).
Common Pit-Falls: Clutches MUST be clean. Bushings MUST be serviceable. Springs MUST be replaced annually. Belt MUST be cleaned before break-in and every other ride (preferably) and within minimum width tolerance. If all of the following are complied with, see below. Only change ONE thing at a time. Take good notes, like snow condition, temp, elevation, terrain, what you changed and the effect it had. The primary (drive) and the secondary (driven) will directly effect each other, try not to over compensate one to "fix" the other, itís a BALANCE youíre looking for.
Note: Remember, what works for boon docking will not work for drag racing and vise versa.
Under target rpm.
Over target rpm.
The Original "Hammer"