KTM 50, 3-Shoe, Automatic Clutch Set-Up and Maintenance
»One of the most important, if not THE most important, performance items on any 50 is the automatic clutch maintenance and set-up. Having the auto clutch in proper working order and the clutch shoe engagement adjusted or “tuned” to the best RPM is the key to maximum performance.
»A worn or not properly maintained clutch will cause problems that sometimes are misdiagnosed as carburetion or even ignition problems and really may be due to a clutch that is engaging far too early or low in the working RPM range.
»A clutch that engages too low in the RPM will cause the engine to “bog”. This bogging is similar to trying to start in too high of a gear, for example, on a multi-gear, manual transmission like in a motorcycle or car.
»The optimum RPM range, for clutch engagement, for a competition KTM 50 automatic is 8,500 RPM, (+/- 100 RPM) and this is how the KTM 50SX-Pro was set-up at the KTM factory.
»There are a few maintenance items that influence or affect the original performance setting and these have to do with the wearing clutch shoes, or clutch material and the heat that is generated by the working clutch. Remember the clutch is a “friction” device and it works by changing heat into kinetic movement. The clutch actually slips for a period of several thousand RPM, from around 8,500 RPM, before making full engagement at or between 10,000 to 11,000 RPM.
»We see the results “graphically” on our dynamometer of several horsepower when the clutch is properly set-up and this is a significant difference on a 50cc mini. The fact that most of us don’t have a “Dyno” or access to one, there are still ways to achieve proper set-up and to achieve maximum performance, even in the home shop.
»Let’s start with the most basic maintenance and that is CHANGE THE TRANSMISSION OIL / FLUID before any competitive use or after every one to two hours of use. Contaminated oil does more to hinder the performance than any other single point.
»The contaminants will get between the “Belleville” disc springs (washers) and minimize how far they compress. The clutch will eventually never fully engage. This constant slippage will further wear the clutch shoes and result in higher engine temperatures and a more rapid breakdown of the fluid. It’s a vicious cycle that can be costly when compared to a few hundred milliliters or ounces of new transmission oil or fluid.
»The next important maintenance is to remove the clutch shoes and measure the diameter of the three assembled clutch shoes. The three assembled clutch shoes are around 82.00 mm when new. Measure the inside diameter of the clutch drum. The drum is approximately 84.00 mm when new. The difference between the clutch drum and clutch shoes should be no less than 1.5mm and no greater than 2.0mm.You can add a maximum of 1.0 mm of shims (see parts information in the engine section of your parts book) to the underside of the shoes, (between the center hub and the shoes), to space the shoes out, to maintain the recommended 1.5mm to 2.0mm clearance. If more than 1.0 mm of spacing is required the clutch shoes should be replaced.
» Next disassemble the disc spring stack by removing the 10mm hex head bolt in the center of the friction material of the clutch shoes. NOTE THE ASSEMBLY ORDER! Clean, and check the Belleville disc springs for wear. Clean the spring discs with a safe cleaning solvent. Check them for wear, by noting the condition of the black coating. Replace them if black coating is worn. We would recommend that highly competitive racers to replace them with new, as they will eventually heat fatigue and not perform to their best potential.
»Reassemble the disc springs and apply Loctite “Thread Locker 243” and torque to 96 lbs/inch - 11 Nm.
»Install the drum and the clutch shoes onto the crankshaft and tighten the clutch assembly nut to 22 lbs/ft – 30 Nm. WARNING: Do not exceed maximum tighten torque spec or the center hub will fracture.
»Now for the under $100 home dyno; There are some nice small battery operated digital inductive tachometers available from some of the lawn and garden power tool manufactures for less than $100.00. You can use these tachs to determine the clutch engagement point.
»To do this, hook up the inductive and with the rider in the seated position and engine running note the RPM when the mini starts to move. This should happen at 8,500 RPM +/- 100 RPM. The rider should be in full riding weight (with helmet, boots, etc., but you knew that) and should not help the mini to move. Sometimes even a little resistance is a good thing, as the clutch engagement will change for the better when it gets to race conditions.
»An adult can straddle the mini and simulate the rider’s weight and resistance to check the initial engagement RPM point too. The key here is consistency. You can also do this in reverse order, if you feel the clutch is working as desired, by checking the RPM and noting it, before you disassemble it or reach a maintenance interval.
»Now that you know the engagement point, how do you adjust it?
»To increase the disc spring tension and raise the engagement RPM, remove the disc spring stack from the center of the clutch shoe (6mm bolt with 10 mm hex head) and place a 0.5 mm shim under the stack. This will result in an approximate 500 RPM increase of the initial engagement point. A total of 2.0 mm of preload is allowed.
»To decrease the engagement RPM point remove a 0.5 mm preload shim or if no shims are installed, rearrange the original stack to eliminate one set of doubled disc springs. Be sure to maintain the same overall original stack height by facing the concaved sides together.
»Here is the basic disc spring layout, laid on its side, starting with the base disc springs, in the clutch shoe well, and working toward the hex head (HH) of the bolt; Base ( ) (( )) (( )) (( )) HH
· Keep the oil/fluid clean and fresh.
· Check the engagement RPM on a regular basis.
· Clean and check the “Belleville” disc springs on a regular basis.
· Measure the clutch shoes and clutch drum and maintain a 1.5mm – 2.0mm clearance.
· Rule of thumb; 0.5mm of preload under the stack with increase the engagement RPM by 500 RPM and vice versa.
· Don‘t overlook the clutch set-up when dealing with engine performance issues.
» A well maintained and set-up automatic clutch system delivers the most performance your engine provides.