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F-Twins Tech - F650/700/800 - Setting Proper Chain Tension


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The articles, posts and comments in this tech section are posted by individual members and reflect their personal thoughts and experiences with repairing, maintaining, and generally working on motorcycles. This information may require specific knowledge and skills, may or may not be correct or current to model.

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Final note: The motorcycle's two wheels are the only thing keeping you upright. When in doubt, take your motorcycle to a reputable dealer.


Setting the proper chain tension on your BMW F-Twin with a chain drive

Even with proper lubrication, over time our chain and sprockets will wear out. The chain will "stretch" as various portions of the chain wear down, and the sprockets will deform as the metal wears away. This creates chain slack. There are many variables that go into how long a chain/sprocket will last and how much slack is created throughout the life of the chain. Lubrication and cleaning schedule, O-ring/X-ring life, time/miles spent in the rain, if you live in an area with a lot of sand, or if you ride off-road a lot will all have an effect on the chain/sprocket life. You probably can't see this slack from one pin to the next, but multiply it by the 100's of pins in a chain, and you can end up with a lot of slack. Too much slack will cause excess wear on the chain sliders, rubber dampeners, and can cause the bike to ride a bit rough. If the chain is too tight, you can create excess wear on the sprockets and drive system.

It's generally recommended to check your chain slack every 500 miles and probably a bit more frequently if you ride a lot off-road. I have noticed that as your chain gets towards the end of its life, you require more frequent chain slack adjustments.

For the BMW F-Twins, we check our chain slack with the bike on the side stand.

There's a sticker on your swing arm that notes how much slack there should be.
For the F800GS - BMW recommends a chain slack of 35 to 45 millimeters. (1.4 to 1.8 inches)
For the F700GS - BMW recommends a chain slack of 30 to 40 millimeters. (1.2 to 1.6 inches)
For the F650GS - BMW recommends a chain slack of 30 to 40 millimeters. (1.2 to 1.6 inches)
For the F800R - BMW recommends a chain slack of 30 to 40 millimeters. (1.2 to 1.6 inches)

We can also check to see if it's time to replace the chain/sprockets by measuring how much distance there is between 9 chain pins. BMW states for all F-Twin models that the max distance over 9 pins is to be no more than 144.30 mm (5.7 inches).

Note 1: The specific bike shown is a 2013 F800GS, but there doesn't appear to be any difference between models or years (other than the above slack differences)
Note 2: The tools and materials used are by personal choice and are not due to any affiliation with any brand

Tools Required:
- 24 mm socket
- Ratchet that fit the above
- Breaker bar (optional, but may be needed as the axle nut is quite tight)
- 13 mm open ended wrench
- Torque wrench
- Measuring device
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Make sure the bike is on the side stand and that the chain is at its maximum sag (you may want to roll the bike forward and back a few feet to see if the chain is indeed at the lowest sag)

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Position your measuring tool at the midpoint of the swing arm. On the GS models, you'll notice a small indent in the swing arm, I like to use the middle of that indent as a reference point. For the R model, the end of the chain slider is about the mid-point of the swing arm. I also like to use the bottom of the chain as our measuring point.

Pull the chain down so it's taught and take your first measurement. In this example, the bottom part of the chain appears to be at about 2.75 inches.

Note: We are using a carpenter's triangle for this demonstration, it's not perfect because of the large amount of space between the measurement markings, but it works well to show the basic purpose of this maintenance item.

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Next, push up on the chain and take another reading. In this example, the chain appears to be at about 1.5 inches.

Chain slack is the difference between the two readings. For this F800GS, we are looking for a chain slack of 1.4 to 1.8 inches (35 to 45mm).

Our high reading of 2.75 inches minus the low reading of 1.5 inches is about 1.25 inches of chain slack.

Before adjusting anything, I like to roll the bike ahead a couple feet and make sure the chain slack is even. If I find big differences between the slack numbers, I know it's probably time to change my chain and sprockets.
If you need to adjust your chain slack, first use your 24mm socket and breaker bar to loosen the axle nut (don't remove it, just make sure it's loose enough to allow the axle to move).

Using your 13 mm open-ended wrench, loosen the lock nut of the chain adjusters (as indicated by the red arrow) on both sides of the swing arm.

If your chain is too loose, remove a little slack in the chain by extending out the adjusters with your 13mm wrench. If your chain is too tight, add a little slack to the chain by turning the adjusters in. Generally, a quarter or half turn is all that is needed for small adjustments.

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Make sure that you're turning each side the same amount, so that the rear wheel stays aligned. Line up the hash marks that are engraved into the swing arm.

Using a tape measure or a caliper to measure the distance between the slack adjuster and the swing arm cutout is an easy way to make sure that both hash marks are at the same distance.

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When your happy with the chain sag and alignment, use your use your 24mm socket and torque wrench to tighten the axle nut to 100 Newton Meters (73.8 Foot Pounds).

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Using your 13 mm open-ended wrench, tighten the lock nut of the chain adjusters (as indicated by the red arrow) on both sides of the swing arm.

Torque settings for the lock nut for the chain slack adjusters is 19 Newton Meters (14 Foot Pounds or 168 Inch Pounds). This isn't easy to torque properly since you'd need a crowfoot wrench that is set correctly. Good and tight is probably fine, but you should check it periodically.

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Just to be sure that nothing has shifted during reassembly, take a moment to check the sag again.

Finally, double check all your work and then ride off into the sunset!