KTM 50cc Three-Shoe Automatic Clutch Setup and Maintenance Procedures
Text and photos by Mike Rosso
One of the most important - if not THE most important - performance procedures on any KTM 50 is automatic-clutch maintenance and setup. In proper working order with clutch shoe engagement adjusted to optimum RPM is your key to maximum performance.
A worn or improperly maintained clutch causes problems sometimes misdiagnosed as carburetor or ignition woes but are often due to a clutch engaging far too-early or too-low in the working RPM range. A clutch that engages too low in the RPM causes engine "bog" which is like taking off in too high a gear with a manual trans motorcycle or car. Optimum RPM range for clutch engagement on a competition KTM 50 is 8,000 RPM (+/- 100 RPM). This is how your KTM 50 was set-up before it left the KTM factory.
There are a few maintenance items that influence factory performance settings: clutch shoe wear, or clutch material, and heat generated by a working clutch. Remember, the clutch is a "friction" device and works by changing heat into kinetic movement. A clutch actually slips for several thousand RPM, from around 8,000 RPM, before making full engagement between 10,000 to 11,000 RPM.
We see graphic results on our dyno of several horsepower when a clutch is properly set-up which makes a significant difference on a 50cc mini. You may not have a "dyno", or access to one, but there are other ways to achieve proper set-up and maximum performance, in your home shop.
Start with the most basic maintenance: CHANGE THE TRANSMISSION OIL-FLUID before any competitive use or after every 3-4 hours of use. The engine holds about 250cc's of fluid (8.5 oz or 1/4-quart). Contaminated oil hinders performance more than any other factor. Contaminants get between the "Belleville" disc springs (washers) and minimize how far they compress - so much so that the clutch eventually never fully engages. Constant slippage further wears clutch shoes and results in higher temperatures and more rapid fluid breakdown. It's a costly vicious cycle when compared to a few ounces of new fluid.
A HOME DYNO FOR $151.
There are some nice small battery operated digital inductive tachometers that are available from KTM HEQ, part number 451.29.075.000 (rev meter). You can use this rev meter to determine the clutch engagement point.
With rider in seated position and engine running, hook up the inductive and note the RPM when the bike starts to move. This should happen at 8,000 RPM +/- 100 RPM. Rider should be in full-riding gear: helmet, boots, etc., - but you knew that! - and should not help the bike to move. Sometimes even a little resistance is a good thing, as clutch engagement will change for the better when it gets to race conditions.
An adult can straddle the mini and simulate rider weight and resistance to check the initial engagement RPM point, too. The key is consistency. You can also do this in reverse order if you feel clutch is working as desired, by checking RPM and noting it, before you disassemble it or the bike reaches a maintenance interval.
NOW YOU KNOW THE ENGAGEMENT POINT, BUT HOW DO YOU ADJUST IT?
To increase disc-spring tension and raise engagement RPM, remove the disc-spring stack from the center of the clutch shoe (6mm bolt with 10mm hex head) and add a 0.5mm shim under the stack. Result is an approximate 500 RPM increase of initial engagement point. A total of 1mm of pre-load is allowed.
To decease engagement RPM point, remove a 0.5mm preload shim or, if no shims are installed, rearrange original stack to eliminate one set of doubled disc springs. Be sure to maintain the same overall original stack height by facing the concave sides together.
Here's the basic disc spring layout on its side, starting with the base (BASE) disc-springs, in the clutch shoe well, and working toward the hex head (HH) of the bolt
BASE ( ) (( )) (( )) (( )) HH
Another important tip is to remove and measure the diameter of three assembled clutch shoes (approx., 82mm new). Measure inside diameter of clutch drum (approx., 84mm new). Difference between drum and shoes should be no less than 1.5mm, no more than 2.0mm.
To space the shoes out add a maximum of 1mm of shims (see parts info in engine section of your parts book) to underside of shoes, (between center hub and shoes) to maintain recommended 1.5 to 2.0mm clearance. If more than 1mm shim is required, clutch shoes must be replaced.
OK, LET'S REVIEW...
1) Keep oil /fluid clean and fresh. 2) Check engagement RPM on a regular basis. 3) Clean and check "Belleville" disc springs on a regular basis. 4) Measure clutch shoes and clutch drum and maintain a 1.5mm - 2.0mm clearance.
RULE OF THUMB
0.5mm of preload under the stack will increase the clutch engagement RPM by 500 RPM and vice-versa.
THE BOTTOM LINE
Don't overlook clutch set-up when dealing with engine performance issues. A well maintained and set-up automatic cutch system delivers the optimal performance your engine is capable of. -Mike Rosso
Next, disassemble disc-spring stack by removing 10mm hex head bolt in the center of clutch shoe friction material. NOTE ASSEMBLY ORDER! Clean and check Belleville disc-springs for wear and clean spring discs with safe cleaning solvent. Check each for wear by noting condition of the black coating - replace if worn. We recommend highly competitive racers replace with new, as they eventually heat fatigue and do not perform to there best potential.
Reassemble disc-springs, apply Loctite "Thread Locker 243" and torque to 96 lbs/inch - 11 Nm.
Reinstall drum and clutch shoes onto crankshaft and tighten clutch assembly nut to 22 lbs/ft - 30 Nm. WARNING: Do not exceed maximum torque specs or center hub will fracture.