Centrifugal Clutch Assembly & Tuning 101
Acceleration, punch and throttle response are some of the most sought after terms in racing today's 1/10 and 1/8 nitro powered vehicles. These elements are easily found and dialed in to perfection with today's centrifugal clutch systems. While most people over look the clutch and will depend more on their engine tune, nitro content and/or gearing selection – the top drivers depend on the clutch to control the “feel” of the engine as well as regulate fuel mileage and wheel spin. I'm going to run through the basic building instructions and tips as well as a few different set-ups.
#1 - Install the shim and flywheel collet onto the crankshaft. Depending on your engine and collet you may have to use a shim so the flywheel doesn't drag on the engine case. Install the flywheel and flywheel nut onto the crankshaft and tighten. Make sure it is very tight otherwise it can come loose and damaging the other clutch components as well as decrease the life of your engine.
#2 - Install the centrifugal shoes (clutch weights) into the flywheel. Make sure they are clean of any flashing or sharp edges, as this will ensure smooth and consistent clutch operation. Install the clutch shoe ring, lining up the holes with the flywheel pins.
#3 - Install the clutch shoe into the clutch shoe ring, lining up the holes with the flywheel pins. The clutch shoe and shoe ring should move freely up and down on the flywheel pins. If they don't, use the proper size drill or very carefully use a hobby knife to remove a small amount of material from the shoe. Usually, some material needs to be removed from the flywheel pinholes in the clutch shoe – it doesn't take much so be careful not to remove too much material!!
#4 - Install the spring cup into the clutch shoe making sure it is seated all the way. Install the clutch spring and the clutch spring adjuster. Tighten it down so that about .5mm of thread is showing. This will be our assembly setting, as we will do the final adjustment later.
#5 - Install the pinion gears onto the clutch bell and tighten them. Do not over tighten them or it will be extremely difficult to get them off later. The basics of your clutch are assembled now and this is where the set-up comes into play. How you build and set your clutch will be a key to improving your lap times.
#6 - First, we will set the “gap”. The “gap” is the distance between the clutch shoe and the clutch bell when it is at its farthest position from the shoe as possible. Install the clutch bell onto the crankshaft without the inner bearing – this will allow the clutch bell to make contact with the shoe without the bearing stopping it. Now, install the outer bearing, thrust assembly (including thrust retainer) and the 3mm screw. Tighten the screw down and check the clutch bell. There should be plenty of endplay that will give us the room to shim the “gap” properly. To shim the gap, place thin shims over the thrust retainer to the outside of the thrust assembly. Creating a smoother clutch engagement and feel usually yields a .35mm-.50mm gap setting. To create a more aggressive bottom end feel the gap usually yields a .55mm-.70mm setting. Setting less than a .35mm is not ideal as it can lead to clutch drag, especially as the clutch heats up. This can lead to excessive heat and a glazed clutch shoe. Setting more than .70mm is dangerous for several reasons. In a hard impact it is more likely for the clutch weights to be ejected from the clutch – seems impossible but it has been done!! Also, the .70mm can lead to what is called “spring bind” where basically the spring doesn't have enough compression travel so it won't allow the clutch shoe to come out far enough to fully engage the clutch – this can lead to premature wear and failure of the clutch. Generally, .35mm-.50mm is great for finals and controlling wheel spin on a slippery race track and .55mm-.70 is great for qualifying and high traction applications.
#7 - Setting the endplay is the next step. Remove the thrust assembly and clutch bell (be careful to keep the shims together for the gap). Install the bearing that goes into the backside of the clutch bell and slide it onto the crankshaft. When you do this your clutch bell should not drag on the clutch shoe. If it does then you need to add shims before sliding on the inner clutch bell bearing and clutch bell. This is important for two reasons – If the clutch bell is allowed to drag on the shoe it will lead to excessive heat, glazing and possibly wear as well as improper disengagement causing higher engine temps and a much harder job for the brakes to slow the car. You can also use this as an adjustment as a smoother feel is generated with a .1mm-.2mm endplay setting. A more aggressive setting is .2mm-.4mm. This is a driver preference and really doesn't make much of a difference between a qualifying and final clutch set-up.
#8 - The final adjustment is done to the clutch spring adjuster once the engine is bolted into the car and ready to go. Fire the car up and let the engine and clutch warm up by short throttle “blips”. Once the engine and clutch have warmed up, slowly open up the throttle (on the starter box) and listen/watch for when the clutch starts to engage. Usually it's about 1/10-1/8 throttle input when the clutch engages. If you see the wheels move and the clutch engage immediately upon throttle input then the clutch spring adjuster is too loose and it needs to be tightened. If the clutch spring adjuster is too tight you will hear the engine zing to a high rpm without the wheels moving (trust me, you will know that slipping clutch sound immediately!!). Once you are used to the sounds and feel you can do this on the bench very quickly and easily. Until you get that feel for your clutch and it's adjustments I recommend driving a few laps out on the track and bringing it in to make different adjustments and shimming to see what set-up you prefer. After a couple of tanks you will have a much better understanding of the adjustments and how they work with your driving style. A good clutch set-up will make your engine run cooler, brakes work better and can really improve your fuel mileage!!
Be patient and have fun!!!
Good Luck!!
Josh Cyrul
CEFX
Integrated New Media
