59 Tiger Cub Motorcycle

FRS 106, Michael Littman – Spring 2013


Andrew Hanna, Dan Taub, Danny Zirkel

What is it?

Clutch (n) — a device that enables two revolving shafts to be joined or disconnected as required, esp one that transmits the drive from the engine to the gearbox in a vehicle

Drive Train vs. Friction Plates

(Image is a friction plate)
  • Friction plate: connected to clutch basket
  • Driven plate: connected to inner hub
  • When clutch depressed: both spin freely
  • When clutch not depressed: surface friction prevents slippage (allows transfer of energy)


  • Thickness: 1/8 in
  • Diameter: 4.95 in

Cups / Springs


  • Compress the clutch plates together when clutch is not engaged.


  • Free length: 1.4 in
  • Compressed length (for testing): .77 in
  • Force applied during testing: 40lbs=177.93N
  • Spring constant: 761lbs/ft = 11,120.55 N/m



  • Utilize mechanical advantage to lower the force needed on the handle.


  • Distance from pivot to push rod: .25 in
  • Distance from push rod to applied force from cable: 2.5 in
  • Mechanical advantage: 5:1

Not listed here: Primary Chain, Push Rod

The Science Behind

How much force is required to depress the clutch?

(Measurements based on ’58)

Force to release clutch without any mechanical advantage: ~200lbs

Divide by mechanical advantage of lever (5:1): ~40lbs

Divide by mechanical advantage of clutch handle (2.5/.6 =4.2:1): ~9.5lbs (with no friction)


Symptoms: Engine revs but no corresponding acceleration.

How: Clutch slippage is when the clutch plates are slipping across each other, rather than engaging and transferring engine torque to the drive train.

Why: The cause can be a poorly adjusted clutch, or worn springs, or worn plates.

Our clutch slips at between 6 and 9 ft lbs with well-compressed springs.

Working With the ’58

Clutch was heavy, noisy

**During testing, clutch cable “snapped”

Solution 1: We used emory paper to smooth out the teeth on the driven clutch plates, then buttoned up and attached new cable

Consequences: “Oh, well that’s just awful.”


Solution 2: We lubed cable to stop noise; tightened adjustment screw for better push-rod connection

Consequences: “…still worse than it’s ever been.”

Solution 3: Run cable along a straighter path (reduce friction)

Consequences: “The best this clutch has ever felt!”

Reassembling the ’59