Post by tony811976 on Mar 18, 2007 8:40:15 GMT
Clutches
Two basic type of clutches exist in Minimoto, two shoe and three shoe. Both operate on the same basic principal which rely on engine speed to throw the shoes outwards to engage with the clutch drum and provide drive to the transmission, (sprocket, chain and wheels) The clutch sits within the clutch drum and housing and is attached to one end of the crankshaft. The engine spins and rotates the clutch throwing the shoes outward.
For racing purposes backplates of various weights, heavy, medium and light, exist. Heavy are generally used on tuned Minimotos or “Supers” Medium are general all purpose and light are for junior riders. The light backplate can be used by seniors but are not generally considered reliable due to the soft material (Ergal Alloy) they are made from. The purpose of the varying weights is to alter the flywheel affect. Heavy backplates slow down the engines ability to pick up revs when accelerating but also prevent the engine from losing revs too quickly when the throttle is shut off. Adding weight to the flywheel is a common trick used in Motorcross to slow down the rev pickup and increase traction. The light backplate does the opposite and the medium something in between. It doesn’t necessarily follow that any of the backplates increase or decrease engine power, they alter the way the engine behaves, and marginally at that!
The various manufacturers produce clutch shoes with in either ‘standard’ or ‘racing’ spec. The difference is in the lining material used. The ‘standard’ has a non sintered material and ‘racing’ a sintered material . After market producers can supply ‘Kevlar’ lined shoes. ‘Standard’ and ‘racing’ have similar life expectancy and require careful set up and running in. ‘Kevlar’ is a hard wearing long lasting alternative, the drawback being that they don’t provide the optimum grip on the drum, some live with it……..some cant and opt for the less hard wearing but better performing alternatives.
Clutch Drums are available in various sizes ranging from 80mm to 78mm and more recently from Polini in a kit which includes Drums in 82mm to 80mm sizes. The idea of the varying size drums is that a smaller drum can be utilised as the shoes wear.
Clutch springs provide the resistance to the shoe engaging the drum by holding the clutch shoe in place until the required engine revs supply sufficient centerfrugal force to overcome the resistance the spring provides. Springs come in three sizes (material diameter) and are colour coded. Light, medium and heavy springs are 2.1mm, 2.3mm and 2.5mm respectively. Colour codes for Polini springs and Bi-Zeta springs are different and are as follows:
Polini:
Blue = 2.1mm
Yellow = 2.3mm
Red = 2.5mm
Bi-Zeta:
Yellow = 2.1mm
Blue = 2.3mm
Red = 2.5mm
The lighter gauge springs are generally used where early or low rev engagement of the clutch is required,. medium springs where medium speed clutch engagement is required and heavy when later engagement is required. As a general guide light gauge springs are used on Junior 4.2, medium on Proddy (8.5) and heavy on supers.
Clutch settings are different for different power output bikes as the torque peak occurs later in the rev range on more powerful engines. As a general guide 4.2’s require the clutch setting to engage at 7500rpm, Proddys 8500rpm and Supers at 9000rpm. The aforementioned settings are a guide only, different riders have different ideas but all set their clutches will be set somewhere around the figures mentioned.
Setting the clutch to engage at the revs required is achieved by applying more or less tension to the springs. Early engagement is achieved by applying less tension to the spring and later engagement vice versa. Tension in the spring is adjusted by screwing out the 7mm nut on the end of the spring for early engagement and screwing in for later engagement. Approximate adjustment can be made by counting the ‘flats’ of the nut turned (one flat equals 150rpm. One full turn equals 900rpm) Accurate clutch setting requires the use of Vernier Calipers and a Rev Counter. The verniers are used to measure the springs to ensure both (or all) are of equal length. The Rev Counter is attached to the HT lead and a reading can be taken therefrom. The usual method to gauge at what revs the clutch is engaging is to place the left hand on the rear wheel, with the bike on its stand, and slowly increase throttle opening until the wheel starts to move (CAUTION IS REQUIRED) Once the wheel starts to move take a reading off the Tacho.... That’s the ‘clutch setting’. This requires practice and probably several attempts, even by the accomplished. Bear in mind that clutches get HOT and heat causes expansion therefore a cold clutch will engage slightly later than a hot one and some allowance for this might have to be made, I usually allow 200rpm!
All clutch linings require bedding in both to allow the lining to glaze and also to build up heat gradually and allow the glue to ‘cure’ A new clutch lining has a non shiny surface which once bedded in will turn black and shiny. During bedding in you may notice The shiny black finish on small areas only, which are high spots on the lining. The bedding in process can be speeded up by removing the clutch and filing the high spots down. The shoe isn’t truly bedded in until the lining has a ‘glaze’ over the whole surface. I have found Kevlar linings to be less sensitive to bedding in however care and the gradual heat cycle build up are still required. A clutch that isn’t bedded in with care will quickly disintegrate the lining. A new clutch requires a low RPM setting (4000-5000rpm) to begin with. No less than 2 sessions are required before inspection and filing. A further 2 sessions, or until the shoe is showing the majority of the lining to be glazed, will be required. After this a higher RPM (6000-7000rpm) setting can be made. Once the whole shoe is glazed you’re good to go!
The various parts of the clutch, springs, guide blocks, washers, bolts and pivot pins, require periodic replacement to keep the clutch tip top! All of the moving parts are prone to wear and can cause problems with reliability and therefore careful inspection on a regular basis is required.
A common trick in racing circles is to add weight to the shoes by screwing bolts or grub screws into threaded holes and hack sawing the ends off as necessary, loctite does no harm. Bi-zeta shoes come with a threaded hole already installed. Polini shoes don’t have the threaded hole however Vaughan supplies Polini re-lined Kevlar shoes with this modification already made and bolts in place. In addition Phospher Bronze inserts are installed in the pivot holes as these are prone to wear on long lasting Kevlar linings.
this has definatley solved my problems i think
and cleared up a lot of confusion for others.