Screeching clamping screws

[jamais]

No sexual connotations here, I've really had this happen on the bike. I'm referring to the two bolts, one on each slider tube that act to clamp the front axle when installing the front wheel.

When I loosened the bolts for a wheel removal, one of the bolts gave a screeching sound and was slightly-only slightly-harder to start backing out. I carefully looked at the bolt and the slider's threads and I couldn't see any deformations, stripping indications, etc.

When I reinstalled the bolt, it threaded on normally. But when I torqued it to the 22 Nm per the manual, it again screeched just as the 22 Nm was reached.

I guess I should replace the bolt, but I'm curious as to the reason this is happening.

[Boxer]

jamais, did you tighten those screws per the manual when you re-installed that front wheel? I don't know if it might have caused your problem...Or why they do it...But there's a "do-the-hokey-pokey-turn-yourself-around" procedure requiring bouncing the front fork up and down before tightening the second of those two screws.

[dallara]

I wouldn't worry too much about it, but you can "fix" it so it never happens again.

The "screech" is caused by the corrosion deposits up in the threads. You have a steel bolt threading into aluminum. Over time, and due to differing expansion rates of the two metals, etc... The two surfaces (steel and aluminum) get a thin layer of oxidation between them, causing them to "seize" to each other... The "screeching" is these two layers of oxidation sliding against each other and causing a resonance.

Often this "seizing" of the two dissimilar oxidation can become so severe that the two pieces literally become "welded" together over time, and the bolt breaks before it comes out... and out come the drill and the ez-outs, and heli-coils or time-serts...

This actually quite common with steel bolts in aluminum threads, and why a wonderful product called, oddly enough, "anti-seize" compound (sometimes called "assembly lube") is available.

Simply dab a bit of anti-seize compound on the tip of the bolt before you thread it in and the screeching will go away, and you will insure that you never sanp that bolt off trying to get it out. You will also get much more accurate torque readings...

If you don't have true "anti-seize" compound, a good, thick grease will suffice.

My personal favorites for anti-seize and assembly lubes are Bel-Ray Moly Assembly Lube and Loctite's Silver and Copper anti-seize sticks. You can find the Bel-Ray at most good motorcycle shops, and the Loctitie products at just about any auto parts store or tool supplier.

Hope this helps!

Dallara

[NoRRmad]

A problem with lubricating bolts during assembly is that the torque requirements change drastically. The amount of torque measured is dependant on the sliding friction between threads, and reducing the friction means that a given torque will result in a much tighter bolt, possibly stretching the bolt or breaking it. So be careful!

[dallara]

But that's why true anti-seize compounds are better than using plain grease. They are formulated to give the proper coefficient of friction to maintain proper torque values.

Dallara

[darthrider]

Re Dallara's post on anti-seize compunds.
To take it a step further, many techs recommend using either anti-seize or Loc-Tite on all or most fasteners, depending upon the application. I generally use anti-seize on non-critical fasteners (like all the small 'misc.' fasteners found all over the bike, fender mount bolts, exhaust fasteners, etc.) and blue Loc-Tite on critical fasteners like the front axle pinch bolts, brake caliper bolts, handlebar bolts, etc.
Both products will protect the metal and prevent the "screeching" problem.
Caution: Don't use red Loc-Tite or the even stronger versions unless it is actually called for in a permanent or semi-permanent application.

Re NoRRmad's post on lubricants changing the torque values. I wish I knew better how to address this for my own bikes as it is a real problem. And BMW has often made it more problematic by providing torque values that are not specific to dry or lubed threads, assuming that some fasteners will be lubed.
I'll hang this out here knowing I will be proved wrong, just to (hopefully) get a more complete discussion.
I *think* I'm right that BMW's spec for the four "lug bolt" fasteners for the rear wheel are dry torque values. But the expert techs I know lube them a bit since they are steel bolts threading into aluminum.
I use a smear of good anti-seize and the torque values in the owners manual, 'POS' that it is.

I have a stainless steel fasteners catalog at work and they provide a torque values table for all the fasteners. It says all values are for dry threads and it you lube them you should reduce the values by 10%.
Is the effect of reduced torque values from lubed threads the same with anti-seize, grease, oil, and Loc-Tite?

So, what's the real poop? We understand the problem what's the solution?

[NoRRmad]

That's tough to figure. You need to know the coefficient of friction for dry and lubed threads, divide one by the other, and multiply the result by the recommended torque value to get the 'amended' torque. If say the recommended torque is 22 newton-meters, the dry coefficient of friction is .8, the lubed coeficient is .4, then the new target torque is 11. (Just examples, I don't know the real coefficients.)

[Rootin55]

I remember an old mechanic telling me that the bolts on a wheel hub (car) were tightened properly when they squeaked.

[jamais]

Many thanks to all. Just a touch of anti-seize to the tip of the bolt surely won't hurt. By the way, I did do the front strut compression procedure before tightening the bolts.

[bakernks]

The book that answers all mechanical questions, folks, is called Machinist's Handbook. There is a table for torque conversions listing lots and lots of lubicants and the % of change their use requires to equal the true torques value. For fine threads, for regular threads, everything. It isn't black magic. Now this book is expensive, like $75, or on CD even more. So, a trip to the library for photocpoies of the pages you want, and the index is very well laid out to find them, may be in order. I have a copy at work and will do a chart here next week for our use. It will have to be, for copywrite issues, a somewhat shortened, adhoc version rather than a scan.
Roughly speaking, a lube will change the torque values from 12-25%. Meaning if you lube the threads, install the bolt with that percentage LESS torque to arrive at the actual installation torque figure. Right? there's less friction, so when you torque the bolt, it moves easier for a given value, and is actually tighter than you think it is at the wrench. If it's a critical bolt, like these axle pinch bolts in discussion, then having the actual value would be very important. A bolt holding the handlebar, very important. A bolt holding the side cover, perhaps much less so.

[new2BMW]

Guys,
Be very careful adjusting torque values without knowing some basic facts about your fastener.

I was preparing a write-up for this thread when I came across a really good one on the internet. This expains it all very well.

http://www.rockcrawler.com/techreports/ ... /index.asp

Again, be very careful. If you don't know all the data to make the calculation, you are making a very wild guess. Over torquing is bad as you can imagine, but under torquing can be just as bad.

[darthrider]

I was hoping we would hear from you on this, Mr. MT-01!
I will read that piece as soon as I get time and keep it...since you recommend it.

[new2BMW]

Thanks Dave. I'm not sure I deserve that kind of respect. I tend to forgo many engineering rules for the advice I can get from the experienced guys like you, Dean and Allan. However, in this case, I would hate to have some biker brother lose his front wheel over a mistake in torque application. Fasteners were my specialty back when my responsabilities didn't go far past my cubicle walls. (I wish I could go back to those days!)