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R1100 belt noise

K7GLE

New member
After installing a rewired HES last week (thanks, GSADDICT!) I installed & adjusted a new alternator belt. Torqueing from behind was a PITA -- my clicker in-lb torque wrench doesn't click in the loosening direction, and my beam-type in-lb wrench is not the best tool for this adjustment...you have to be in the back of the bike to read it, and in the front of the bike to tighten the lockdown bolts, and it's easy to tension the belt too much. So I noted the twist resistance and deflection of the belt when the beam wrench registered 70(-ish) in-lb and locked down the belt tension there.

OK, whining time is over.

Test ride was fine, but I kept wondering whether the tension was really correct, so I bought an M8 acorn nut and re-adjusted the belt today. Much easier (and the twist- and deflection-tests looked about the same as before).

BUT - when I fired the bike up it made a real racket - a loud and kind of shallow knocking, obviously coming from the alternator area, at revs below about 3K. Medium-length story short, I re-read the maint. manual and Clymer, and "slackened off" the right side (looking from the front) lockdown nut, which I had torqued to 20 Nm like the other two holding nuts. Noise disappeared.

My question - WTF? Why would that nut, torqued to 20 Nm, cause that kind of noise? Does torqueing it twist the alternator pulley out of the plane of the crank pulley?
 
On reflection, it always made this knocking noise on startup, but it went away after several seconds.

I just restarted the bike and the noise was there but, again, only for a bit.
 
Just tried an around-the-block test, aborted at the end of the street -- knocking is there under load (starting from a stop, gently accelerating, etc). Belt too loose?

Maybe I should remove the belt to confirm it's really a crank-to-alternator noise.
 
On reflection, it always made this knocking noise on startup, but it went away after several seconds.

I just restarted the bike and the noise was there but, again, only for a bit.

Here's a dumbass, shot-in-the-dark question: Have you got the upgraded left-side cam chain tensioner? Your symptom indicates "no". The way to tell is if the tensioner sleeve is 17mm. If it is, you need the upgrade.
 
I've also had belts give me a severe knocking sound because the belt had oil on a small section. I learned that the hard way a long time ago when my brother had a Porsche Boxster serviced at the dealer. The knock when I heard it sounded like a spun con rod bearing. I was ready to pull the engine and when I grabbed a beer, like I do, I looked at the belt and there was a wet spot of oil about 1/2" long. Put a new belt on, knock was gone. That story has repeated itself many times.
 
Thanks all.

Yes, the left cam chain tensioner is a new one, but that was still my first thought.

I'll check the other two possibilities & check back.
 
When I first got my 1100 last August the alternator belt was cracked. Went to the trouble of making the adjusting tool as shown in the manual, including the spring on the end. What finally worked best was to just use two nuts other than the ones on there as they are too thick with the washer, and run one down till it touches the case, back off a quarter turn and then run the other nut down to it and lock them together. Now the alternator can be moved up and down from the front and then adjust per the book with your torque wrench.
 
Make our own adjusting tool, spring is not required.

I used an acorn nut and torqued that to 8 Nm from the front. That would serve the same function, right?

Seemed to result in the same belt twist resistance / deflection as torqueing from behind.
 
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As long as you can lift from front. I believe the springs purpose is to make sure the adjuster portion is pulled all the way to the front so it engages the teeth on that rack back there fully. if you make sure everything is positioned correctly you should be OK.
 
The nut alone didn't lift the alternator, but staking several washers behind it did the trick pretty neatly.
 
I haven't had time to revisit this yet, so I've been spending what spare time I do have by being confused about finishing the belt adjustment.

The BMW repair manual says that, while holding the belt at the appropriate tension, you "fully tighten the retaining nut at the top of the alternator, slacken the adjusting (tensioning) screw, and tighten screws fully."

Does the nut on the adjusting screw, which gets set at hand tight during the adjustment procedure, get "fully tightened" - i.e. torqued to 20 Nm - at this point as well? Following the instructions to the letter would leave it a lot looser, and I wouldn't want it falling off. I ask because earlier in the procedure it's described as a "nut", not a "screw." Did they mean to "tighten (remaining) screws and nuts fully"?
 
From the BMW service bulletin 12 001 02 (001)
Jan '02
 

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Here's what I thought was a very oddball, one-off cause for an alternator rattle but immediately after I posted it (over a decade ago) someone thanked me for solving their problem:

http://largiader.com/articles/altrattle.html

Yup - that was the problem all right...the top screw was loose, even with the nut tight. It wasn't even ungodly difficult to get at (tank off, of course). Rattle gone.

:wave Vielen Dank! :wave

By the way, as noted by others, it's impossible to get a socket, even with a U-joint, on the top alternator nut due to the shock absorber. However, the open end of a 13mm combination wrench comfortably accommodates a 1/2" socket drive. I put a 3/8"-to-1/2" adapter on the torque wrench, fit it into the open end of the 13mm combo, reduced the torque setting to adjust for the increased length, made certain the wrenches were collinear, and it torqued very easily from the right side using the box end.
 
Yup - that was the problem all right...the top screw was loose, even with the nut tight. It wasn't even ungodly difficult to get at (tank off, of course). Rattle gone.

:wave Vielen Dank! :wave

By the way, as noted by others, it's impossible to get a socket, even with a U-joint, on the top alternator nut due to the shock absorber. However, the open end of a 13mm combination wrench comfortably accommodates a 1/2" socket drive. I put a 3/8"-to-1/2" adapter on the torque wrench, fit it into the open end of the 13mm combo, reduced the torque setting to adjust for the increased length, made certain the wrenches were collinear, and it torqued very easily from the right side using the box end.

That's a really neat idea.
So factory torque is 8 Nm. How much did you reduce it to compensate for the length of the wrench?
 
So factory torque is 8 Nm. How much did you reduce it to compensate for the length of the wrench?

I didn't need to use an extended on the adjuster nut because I could reach it easily. The top of the front shock doesn't allow enough clearance to get a socket on the top "fastener" nut that wants 20 Nm.

The formula is:

Indicated Torque (i.e. wrench setting) = Required Torque at the nut x (wrench length / (total lever arm)) where total lever arm = wrench length + extension length

I needed 20 Nm, my torque wrench measures 12" from center of 3/8" drive to center of handle( the center of where you grab it, not 1/2 the length of the wrench)
My combination wrench measures exactly 6" from center of box end to center of open end so the lever arm is 12+6=18

So: Wrench setting = 20 x (12/18) = 13.3 Nm

(Check: Torque setting should be less than actual torque applied because lever arm is longer)

Note that this only works if the combination wrench and the torque wrench are lined up. If the combination is at an angle to the torque wrench the effective length of the total lever arm is reduced, reducing the amount of torque applied at the nut.


_______/ <-- cheesy ASCII illustration of 12" torque wrench with 6" combo wrench at angle
|----------| <-- effective length of lever arm now less than full 12" + 6"

If you know the offset angle you can calculate the new wrench setting by multiplying the length of the extension by the cosine of the angle, so if the combo is 60 degrees out of line with the torque wrench (cosine = 0.5) , you add 3" rather than 6", and set the wrench at 20 x (12/15) = 16 Nm to get 20 Nm at the nut. (Of course you could also just measure it along the line of the torque wrench, but where's the fun in that?)
 
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