Shimming the Sachs 505 Clutch

From Moped Wiki
Jump to: navigation, search

Introduction

The Sachs 504/505 clutch is fairly well-known for its slippage issues. This guide will detail making and installing shims to ensure optimum performance and minimal slippage.

Operation

The transmission on a standard 504 or 505-1x engine is a simple unit, based off of a centrifugal clutch and reduction gears. The transmission only has one "speed", which may or may not be appropriate for the engine, depending on a variety of factors. In order to help prevent stalling and allow the bike to idle with the drivetrain disengaged, a centrifugal clutch is used. This differs from a traditional pressure-plate (automotive-style) clutch in several ways. First, the clutch has no external controls aside from the starting lever. Engagement and disengagement is triggered by engine RPM. The second major difference is that the clutch is of a multi-plate, wet design. This helps to improve cooling and adhesion of the clutch.

Terminology for this article

Clutch Basket-- holds the clutch pads, plates and donut. The "chassis" of the clutch; the gear on the back of the clutch basket engages the rest of the transmission.
Clutch Pads-- made of a friction material similar to brake linings. The pads are forced against the plates to transmit rotary motion. The outside of the pads have tabs which engage corresponding cutouts on the clutch basket, so when the pads turn, the basket does too. A 505-1x clutch has two pads.
Clutch Plates-- These solid steel plates are forced to rotate together by the plate hub. The plates rotate with the crankshaft of the engine. A 505-1x clutch has three plates, two thicker ones and one thinner one.
"Donut"--the "brains" of the clutch. Forces the pads and plates together when it expands.
Plate Hub-- also known as the "gear-shaped thing". Forces the plates to rotate together, preventing severe slipping and overheating. Also keeps the plates centered on the crankshaft.
Retaining Washer-- the odd-shaped washer under the clutch nut.

The central member in the Sachs clutch is the "donut" or flyweight. It consists of a circular steel spring, on which weights are strung. The donut spring keeps the weights firmly against each other. However, as engine RPM increases, the weights are forced outward by centrifugal force. The weights ride up a ramp plate mounted behind the donut as the donut expands. This allows the weights to push against the rearmost clutch plate. Pushing this plate compresses spring washers located in the middle of each pad and also pushes the entire clutch pack up against the washer and retaining nut. Since all free play is now taken up, the plates, which rotate with the central crankshaft, force the pads to rotate. Since the pads are in mesh with the clutch basket, the basket turns, turning the gear on the back and engaging the final drive. When RPM decreases, the donut contracts, allowing the spring washers to push the plates back apart and disengaging them from the pads. If the load on the engine is too great, the donut partially disengages as the engine RPM decreases, and the clutch is allowed to slip. This is analogous to slipping the clutch in a stick-shift car to prevent overloading the engine.

Shims

The above setup is great-- when it works. Unfortunately, as the transmission ages, the pads will begin to wear and the spaces between them and the plates becomes increasingly large, meaning the donut must expand more to engage the clutch. More expansion means more centrifugal force, which means the engine has to rev higher to engage the clutch. Conversely, less space means the clutch will engage at lower RPM's.

We have to have some way of ensuring that the clutch engages at roughly the same point in the rev range, even as it wears. We could theoretically weaken the donut spring, allowing the weights to move further, but this adjustment is fairly critical, so carefully calibrated springs would be needed. The donut is also hard to modify by nature of its design, so this really isn't a good option. However, it's fairly trivial to change the spacing between the plates and pads at rest. All we would need to do is put a thicker washer under the nut. This is exactly what shims do. With thicker shims under the retaining washer, the plates rest closer together and move less to engage. Less motion corresponds to less expansion of the donut, and a lower engagement RPM.

Doin' the Deed: Shimming your Clutch on the Cheap

(This is intended as a guide only. I am not responsible for any damages or injuries that may result from following this tutorial.) You're going to need:
-a flat-head screwdriver or equivalent
-a 17mm and a spark plug socket
-a piece of thin rope (I personally use an old vacuum cleaner electrical cord) or other piston stop
-an adult or non-adult beverage of your choosing, so long as it's in a can.
-a sharpie
-a pair of scissors

1. Open and consume the adult or non-adult beverage. The author personally prefers Budweiser when doing any clutch work. While you're completing the above...
2. Remove the five screws on the clutch cover, and remove the cover. A crescent wrench clamped onto the screwdriver handle can help in removing these if they're too tight and you're worried about stripping them. Try not to disturb the gasket, as you can re-use it.
3. Reach inside the transmission case and unhook the end of the coil spring with a screwdriver.
4. Unscrew the large screw from the top of the case. Remove the spring, screw and bushing. Move the starter actuating arm out of your way. Remove the starter cup.
5. Using your feeler gauge, determine how many shims the engine needs. Push the plates in as far as they will go. Then, measure the gap between the plates (or any existing shims) and the retaining washer. It should be .016 to .024". If it is, your clutch is set up properly.
6. Unscrew the spark plug and put your piston stop in its place. If you don't have a piston stop, shove some rope or cord in there (with enough tail hanging out so that you can retrieve it, of course!). The idea is to fill at least part of the cylinder with some sort of soft, noncompressible material. This may take a few tries depending on what you're using as a stop.
7. Unscrew the clutch nut, and remove the nut, retaining washer, any shims, plates, spring washers and pads. Inspect for any cracks or heavy discoloration.
Hopefully by now, you've finished drinking your beverage and you haven't lost any parts.
8. Rinse the can out, and cut off the top and the bottom and cut a slit down the side, making a rough rectangle. Flatten it out as best you can. These cans are aluminum, so they won't damage your scissors too badly.
9. Trace and cut your shims. 12 oz. cans are about .004" thick. So if you have a total clutch free-play of .032", you need to cut two shims to make up .008" to give you .024", in addition to any shims you already have in there. If you can trace a factory shim, that's the best way to do it. Otherwise, a circle about 1 1/2" in diameter with a 1" hole in the middle will work.
10. Reinstall everything. Make sure to put the two thicker plates on the outsides of the pack, and remember to put the spring washers in. Tighten the clutch nut (the torque spec is 25-28 ft-lb. I don't use a torque wrench on my bike, since I have a small 1/4" drive ratchet that you can't apply much more than 30 ft-lb. with. A torque wrench is always a good idea, though.)
11. Remove the piston stop. You may need to rotate the engine a little with the clutch nut to get the cord out if you went that route.
12. Reinstall the starter mechanism as follows: Place the cup on the crankshaft, slide the screw in part-way, install the bushing, slide the screw through the starter arm, install the spring, tighten the screw and then use a screwdriver to put the spring end inside the case.
13. Reinstall the clutch cover using a criss-cross pattern to tighten the screws.
Take the bike around the block to test. If the bike is still revving out too much (and assuming your pads and plates are in good condition), add one shim at a time until it engages properly. If the bike won't accelerate properly, stalls at a stop, or spins the rear wheel on the stand, you have too many shims; remove them one at a time until the clutch engages properly.

Notes

-Burnishing the clutch plates and pads while you have them out is a good idea. 180 grit sandpaper with a swirling motion works well. You're just trying to remove some of the glaze, not sand down the plates and pads. This allows you to eliminate glazing as a potential cause of your clutch slipping. The refinished clutch may be grabby at first, but will break in within a few miles.
-The starter cup does exactly the same thing as the donut does, but from the other side (i.e. the donut pushes the plates AWAY from the engine to engage; the starter cup pushes them TOWARD the engine to engage.
-Changing your transmission fluid? The sump takes just about a cup of Type F. A little extra fluid (maybe a tablespoon or two over "rated" capacity) may add some protection against overheating.
-There is a very definite "sweet spot" for clutch engagement. You should be able to feel the difference in how the bike performs. This is very much a trial and error job.