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Old 05-26-2010, 10:20 AM   #1
Apostolos OP
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Properly figured chain slack specs

I've finally been able to get the proper chain adjusting method for my F800gs accomplished.

I removed the top shock bolt and rotated the preload adjuster 90 degrees toward the front of the bike. Then I jacked up the rear wheel until the chain was at it's extreme tension point (almost perfect alignment between counter sprocket, swing arm pivot, and rear sprocket).

Then I adjusted the chain to just off taut, .25 - .5" slack. I tightened everything up, reinstalled the rear shock bolt and then took some baseline measurements of my chain slack while on the center stand.

It measured 1 - 1.25" between both tight and loose spots. It actually measured pretty much the same thing on the side stand and the difference was negligible.

I've found that the factory recommendation for chain slack 35-45mm or 1.4 - 1.8" is actually on the loose side.

My bike came from the factory with 1" chain slack (before i changed it to the manual spec), so it was actually correct from the factory. I guess the liberal spec in manual is just to ensure people don't over-tighten their chain stressing the drive components.

Another way to get the proper measurement is to remove the rear shock completely, but if you just remove the top bolt and rotate the preload adjuster so it will clear the frame when you lift the rear tire, you'll be able to get the right measurement.

I also had to set the preload almost to full before I rotated the adjuster toward the front. That made it easier to clear the frame.

I was concerned that I might damage the strut by rotating it, but I went for it anyway and it seems to be fine now that everything is back to normal.

This entire process was necessary to satisfy my need to know the exact specs for my chain (I have OCD ).

God Bless,
David
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Old 05-26-2010, 11:37 AM   #2
jaycommando
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maddness!

Thats the crazest thing I've heard of in a while, BMW R&D spent years working out specs and legal issues to make sure their chain tension is to spec and correct.
You disregard their opinion/math and formulate your own. Heres hopeing your chain doesn't fail else they may produce your site posting to void your warrenty claim....

But thanks for the info!
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Old 05-26-2010, 11:50 AM   #3
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Quote:
Originally Posted by jaycommando
Thats the crazest thing I've heard of in a while, BMW R&D spent years working out specs and legal issues to make sure their chain tension is to spec and correct. You disregard their opinion/math and formulate your own...
You're out of the loop. People have been figuring proper chain tension via the method I outlined for years.

By locating the chains extreme tension point is actually the only possible way to know for sure the correct slack. It's very simple mechanics.

The only challenge is getting to the point of extreme tension. Some suggest using ratchet straps or loading up the rear end with weight, but I always found that dislocating the shock allowing for unhindered suspension travel to be the eaiest. The challenge on the F800gs was the preload adjuster getting in the way and i didn't feel like taking the entire shock out. So I found an alternative.

God Bless,
David

Apostolos screwed with this post 05-26-2010 at 12:41 PM
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Old 05-26-2010, 12:19 PM   #4
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Originally Posted by jaycommando
Thats the crazest thing I've heard of in a while, ...
Perhaps I should have explained it a little differently for you.

There is a point of travel in the rear suspension where the front and rear sprockets will be at the longest possible mechanical distance from each other. The chain can never be stretched more than that distance.

The point of the longest possible stretch is generally when the counter sprocket, swing arm pivot and rear sprocket are in alignment or close to that. The chain can never be mechanically stretched more than that point.

Once you locate that point, you make sure your chain is adjusted to just off taut. If the chain was tight at that point with no slack at all, then drive components would be stressed when full travel of suspension was used.

However, if the chain is adjusted to just off taut at the point of extreme distance between the sprockets, then you are optimizing your chain slack so it's not too tight (stressing components) and not too loose (robbing power/torque and creating a safety hazard as it could jump the sprocket).

Hope this helps.

God Bless,
David

Apostolos screwed with this post 05-26-2010 at 12:35 PM
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Old 05-26-2010, 12:40 PM   #5
The Griz
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I just adjust as necessary until the chain slack is measuring 40-45mm (manual says 35-45mm, but I'd rather be on the looser side than tighter) while the bike is on its side stand. Seems pretty simple to me! My people back in the Motherland already did the math for me. I just follow their results.
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Old 05-26-2010, 12:52 PM   #6
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Originally Posted by The Griz
I just adjust as necessary until the chain slack is measuring 40-45mm (manual says 35-45mm, but I'd rather be on the looser side than tighter) while the bike is on its side stand. Seems pretty simple to me! My people back in the Motherland already did the math for me. I just follow their results.
That's cool, but I don't know why it's hard for folks to conceptualize the mechanical properties of factoring chain slack.

There is only one way to do it whether it's done at the factory in Germany or in your garage at home.

According to my observations, the manual is at the liberal end of the tolerance spectrum. I'm sure it won't hurt anything.

I can see why they would keep it at the liberal end. They obviously don't want people flirting with too tight. The manual spec is a general measurement that keeps the chain on the loose side.

Even at 35mm (the tightest spec in the manual 1.4") you're going to have almost 1" of slack at the longest distance between the front and rear sprocket.

That's not bad. It's also not necessary to have that much, but of course, I have OCD, so it drives me crazy.

God Bless,
David

Apostolos screwed with this post 05-26-2010 at 01:04 PM
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Old 05-26-2010, 01:05 PM   #7
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Thanks for the write up. Nice to know that once I've locked everything down that if I have a tight spot that is hair under spec i can just ignore it.
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Old 05-26-2010, 01:15 PM   #8
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Originally Posted by Motoriley
Thanks for the write up. Nice to know that once I've locked everything down that if I have a tight spot that is hair under spec i can just ignore it.
Indeed.

You definitely have more latitude going by the specs in the manual, which I'm sure is precisely why they listed them as such.

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David
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Old 05-26-2010, 02:11 PM   #9
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Originally Posted by ADVExplorer
Thank God for people with OCD!
Except if they live in the same house as you do.



David
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Old 05-26-2010, 02:16 PM   #10
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Except if they live in the same house as you do.



David
My poor wife. We'll be married 12 years this December. What do they say, "behind every good man there is a good woman rolling her eyes."

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Old 05-26-2010, 02:19 PM   #11
SlowRide13
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I have a question? When measuring the chain slack, how much pressure should you put on the chain (moving it up and down)? Light finger pressure? As hard as you can push with one finger? Jumpin' up and down on it with both feet? I've always done a "hard one-finger push", but have no idea if this is right.
Thank you,
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Old 05-26-2010, 02:35 PM   #12
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Originally Posted by SlowRide13
I have a question? When measuring the chain slack, how much pressure should you put on the chain (moving it up and down)? Light finger pressure? As hard as you can push with one finger? Jumpin' up and down on it with both feet? I've always done a "hard one-finger push", but have no idea if this is right.
Thank you,
Pete
My DID chain is rated at 9000lb tensile strength. You're not going to stretch it too much with your finger.

Using one finger I just push up on it enough to take the slack out.

I actually pull down a little bit first and start my measurement there and then push up.

The easiest way for me to measure is to place a tape measure flat on the ground and hook the end of the tape over the chain. Pull down a little bit (to start my measurement) and lift up with one finger on the chain to take out the slack and batabing-bataboom, you've got your chain slack measurement.

God Bless,
David
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Old 05-26-2010, 06:33 PM   #13
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mental improvement

I stand educated by your ability to do this. But I shall still do it as stated on the swingarm.
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Old 05-27-2010, 04:10 AM   #14
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Sounds like way too much work to adjust a chain. I think I'll just stick to the old method. Can't see my self going to that much trouble if I'm on a trip and need to adjust. Also I'm lazy and wrench for a living so I'm a firm believer in don't make it any harder than it has to be.
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Old 05-27-2010, 06:06 AM   #15
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Originally Posted by ADVExplorer
That's how I'm going to look at it too. Now that is. Thank God for people with OCD!
There's more of us out there than you know...

Since the center-stand is optional equip. BMW can't tell you to check tension on the center stand (which would give a more consistant reading)... Of course they could list it as an alternate... anyway...

Thanks David!
Yeah, in not as rigorous a way, I found the same thing - tension measured on the side stand vs. the center stand was the same number within my measurement error range of probably 1/4".
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