Q. Which clutch springs should I use?
Let's get the job of the clutch springs straight before any discussion about selecting them. The clutch springs alter clutch engagement and disengagement. Once the clutch is engaged, the clutch springs have no bearing on CVT operation until speed and RPM drop enough that the clutch disengages.
As the clutch rotates, centrifugal force tries to force the clutch shoes outward against the inside of the clutch bell. Once the shoes contact the bell with enough force they will begin to lock together. There is usually a period of clutch slip during initial contact where the clutch is still spinning faster than the clutch bell because the outward force and subsequent friction between the clutch shoes/pads and the clutch bell are not great enough to overcome the sudden undertaking of trying to move the clutch bell, gearbox, wheel and tire. As the clutch rotates faster centrifugal force increases and increases the pressure and friction between the clutch and bell. Coupled with the gearbox and other components beginning to gain speed, this allows the clutch and bell to lock together so that the clutch and bell are rotating at the same rate and there is no more slip. At that point the clutch and CVT are fully engaged and engine power can drive the scooter.
Clutch springs provide resistance against the centrifugal force that causes the clutch shoes to move outward and engage with the clutch bell. Without clutch springs the clutch shoes would be pressing firmly against the bell even at idle or perhaps starting speeds. Essentially the scooter would always be "in gear" and trying to drive the rear wheel. Every time you started the scooter it would immediately start moving or spinning the rear tire rapidly. Every time you came to a stop it would try to keep moving unless you held the brakes. The scooter would be likely to struggle trying to accelerate because it would be asked to pull through low RPM ranges where it makes very little power and it could even stall out under such a load. This would all add up to an unsafe and under-performing scooter.
Adding a capable set of clutch springs delays clutch engagement by fighting centrifugal forces and holding the clutch shoes in and away from the clutch bell. Soft or stock springs often only provide enough resistance to allow the clutch to disengage at idle and slightly above idle. This makes the scooter safe to ride since it's not always trying to go, but it may not be the best for performance because the engine is still engaged at somewhat low RPM where it may not make much power. Stiffer clutch springs provide greater resistance and prevent clutch and bell contact until higher RPM. This can allow the engine to bypass it's weak low RPM range and cause the scooter to take off near or within the powerband for strong initial acceleration. If clutch springs that are too stiff are used, engine RPM may surpass the engine's peak power and make the launch weak. Much like any other CVT tuning, the goal of selecting the best set of clutch springs is to keep the engine within it's poweband as much as possible.
Now the questions becomes, "how do you know which springs will make the clutch engage when you want it to engage?". If you refer to the question about spring colors, you will see that many springs are labeled as +1,000RPM, +1,500RPM, or +2,000RPM. While these are not exact numbers, they can help you get an idea of what springs may work well for your setup. If you have a tachometer you can watch engine RPM as you launch and as the clutch slips and ultimately engages. If your engine reaches 6,000RPM during this process and you'd like to see it reach 7,000RPM then you should consider +1,000RPM clutch springs.
Again like other aspects of tuning, I feel that choosing clutch springs should be a trial and error process if you demand the best results. Swapping in springs based on a guess may work great, and it may show little to no improvement. If possible I like to have at least a couple of sets of clutch springs to choose from and I will try them all and observe their operation before deciding which I'll stick with. I'll observe RPM throughout the process of engagement at launch as well as how it feels, or you can even do timed runs or try seeing if you can improve the distance it takes you to reach a certain speed.
There's one other observation that is often overlooked while selecting clutch springs. You should also watch the clutch as it engages. This can be done on the center stand with the CVT cover removed or by looking through holes if your CVT cover is vented. Raise RPM until the clutch engages and watch the rear pulley and belt. If the belt starts traveling down into the rear pulley before the clutch engages you should consider softer clutch springs so that the belt is not beginning to "shift" before the clutch engages. The position of the belt dictates the drive ratio of the CVT, sort of like controlling which "gear" the CVT is in. When the belt is at the outermost portion of the rear pulley that is the low or 1st "gear" of your CVT. If the belt begins moving before the clutch engages you cannot make use of the most advantageous low ratio and instead the effect would be similar to taking off in 2nd "gear". You will not get the strongest launch out of your scooter if you ignore this point. HERE
is a video that may help to illustrate the idea.
Related Info :
Malossi Clutch Spring Info
Installing Clutch Springs
Clutch Removal Technique
Clutch Modifications And Upgrades
Clutch Spring Engagement Video
Full Service CVT Inspection
Lightening A Stock Clutch
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