According to the Owner's Manual, the Falco chain slack should be set to 25 mm at mid-run, with the bike on the rear stand. As the chain stretches, slack needs to be taken up to keep the chain from whipping against the exhaust or frame, or worse yet, jumping off the sprockets. But running a chain too tight will quickly stretch it, will accelerate sprocket wear, and may damage your countershaft bearings too. It is always better to err on the slack side, and your chain will last much longer loose than tight.
As the swingarm travels downwards, the chain slack increases. So, when measuring chain slack when the rear wheel is hanging (such as when using a GP style swingarm pivot stand), if you set the slack to 25 mm it will actually be too tight by Aprilia's specifications. Also, if you've raised the ride height using an aftermarket shock, or with a Mille swingarm, the chain slack specification is no longer valid.
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| The swingarm radius (swingarm pivot to rear axle) and chain radius (countershaft sprocket to driven sprocket) are not concentric. The result is that the driven sprocket is farthest away from the countershaft sprocket when the three pivots are in line. As the swingarm is moved above or below this line, the chain slack increases. |
To set the chain slack without specs, put the bike up on jackstands through the swingarm pivot (unweighting the swingarm). Support the swingarm by putting a scissors jack under a swingarm button, then remove the lower shock mounting bolt from the linkage. Using the jack, raise the swingarm until the countershaft sprocket, swingarm pivot, and rear axle are all in line. This is the point where the chain will be tightest.
| Using a taut string as a reference, raise the swingarm until the countershaft, swingarm pivot, and rear axle are all in line. Be careful not to let the string touch anything. |
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With the swingarm and sprockets in line, tighten the chain so the chain is almost snug, but the rear wheel spins freely. At least one chain manufacturer recommends a half inch (12 mm) of slack. That seems to be a good target for a free spinning wheel and no suspension load on the countershaft.
After setting the chain slack, lower the swingarm and replace the shock bolt. Now let the swingarm hang free. The chain should not be able to touch the frame or exhaust at this point. Aprilia has been nice enough to dent the exhaust pipe for clearance. If the chain appear loose enough to cause damage, take in a little slack and line up the swingarm and sprockets again to verify that there is still slack there. If you can't satisfy both conditions, you have too much suspension travel (ride height). Dirt bikes have solved this problem by installing chain rollers to keep the chain from sawing critical components.
When you are satisfied that the chain will not jump off the sprockets or saw the exhaust when the rear wheel drops into a pothole, put the swingarm back on the rear stand and remove the jack stands. Check the chain slack at this position. Record the value, and this will be your new specification for chain slack with your ride height configuration. Chances are, its more than 25 mm, but will probably be less than 40 mm in order to clear the exhaust. On my bike with Penske set so the swingarm hangs freely about an inch lower than stock, 35 mm appears to be the right number.
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I can't read the swingarm marks very well, so I use a caliper to get both sides of the axle spaced the same. This technically isn't any more accurate than the marks, because it relies on the blocks and swingarm opening being machined accurately. To really be accurate, you need to measure back from the axle to the swingarm pivot. |
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