BBS01 crank arm installation

[Lurkin]

On the way to work today, the LHS crank arm decided to be loose and it's lunched itself.

When it was installed, it was pushed on, then donked a couple of times, then the inner bolt was done up very tight to hold it on there.

A workmate has suggested pressing the replacement on with a vice or press, then appling the inner bolt with locktite.

My understanding is the supplied bolt already has loctite on it (the blue stuff) and pressing the arm on is overkill and can possibly lead to cracking the spindle.

Thoughts?

 

[Warren]

Do not bang on the end of a BB spindle....ever. You will be pounding the bearing balls into the bearing races.

A shallow tapered fit will be terrifically tight. It doesn't require a hammer or a press. Put grease on the backside of the head of the bolts. The bolt acts as a puller, and the back off the head acts as a thrust bearing. Put a foot of conduit tubing over the end of your hex wrench to increase leverage. Snug up the bolt until the wrench flexes/springs slightly when you apply pressure. That is tight enough. Recheck with the wrench and tube after each of the first several rides. You could do this with a torque wrench and hex bit, if you have them. After a decade of doing this, as described, I don't feel the need for one.

http://www.parktool.com/blog/repair-help/torque-specifications-and-concepts

I have never installed a Bafang setup. I can't say how deep the square tapered holes in the cranks are. If they are made too deep, so that the spindle comes flush with the end of the counterbore in the crank arm when properly tightened, then they have a problem. If a crank arm taper is that deep, no amount of bolt tightening will help. You could take the bolt out after installing the crank arm and check for yourself. It should look like this.

http://www.parktool.com/uploads/images/demonstrations/ccp22_square1.jpg

 

[Lurkin]

Do not bang on the end of a BB spindle....ever. You will be pounding the bearing balls into the bearing races.

Not exactly. If the other side is solid/supported, it has no effect on the bearings as the force is not being applied to them. But I get what you mean - and no, that isn't what happened.

So in summary - just push them on and do up the bolts without pressing etc. The rest of your explanation mimics the way it was put on in the first place.

That raises the question - why did it become loose?

 

[jtrops]

Ive seen good evidence that suggests the crank moves a bit further onto taper as it is used for a brief period after installation. This effectively loosens the bolt that secured it in the first place. If the bolt isn't snugged up after the crank creeps onto the taper it can loosen a little, and give the crank room to move further off of the taper. This is the closest I've come to what sounds like a plausible explanation for why they loosen outside of old cranks with a rounded taper. So after you put a few miles on the cranks you should tighten them again, and after that they should be good for the long haul.

 

[Lurkin]

Yep, pretty much.

Pretty explanation here: http://sheldonbrown.com/brandt/installing-cranks.html

Definite match to what has happened. I had not checked it after the first 1,000 kms. Grr.

Nevermind have obtained a replacement, which will be installed today.

 

[Lurkin]

Actually it's interesting. Elsewhere, Sheldon advises that a cottered crank cannot be installed sufficiently without press or hammer? Or am I looking at something different?

http://sheldonbrown.com/cotters.html

 

[melodious]

Double check the square of the arm now. I had a similar situation which resulted in the square getting deformed due to the looseness. Solution was to replace the arm which is easy peasy as its a common bike part (I used the arm from the original bike).

I too use a rubber mallet to "bang" the arm further in, ensuring a tight fit. Then finish it off with a loctited bolt. :?

 

[Warren]

Lurkin,

Yup. Like I said, recheck the bolts after the first several rides. If the taper in the crank arm is not an exact match for the taper of the spindle, it will take several tightenings before the aluminum conforms to the spindle.

A cottered crank is not a tapered pin fitting into a matching tapered hole. Cotters are an antiquated method of attaching cranks. The hole in the crank arm is straight sided, the flat on the spindle is not tapered either. Only one side of the pin is tapered. They are a real kludge. When you remove them, most have metal shaved off the tapered side.

jtrops,

The spindle is supposed to be case hardened. If it was worn down by an aluminum crank arm, it was not hard enough.

 

[jtrops]

I said "cranks with a rounded taper,". I was referring to the fact that the crank arm flats can get rounded by riding loose, and thereby destroying the fit.

So, yes i agree, and it sounds like you too agree with me.

 

[amberwolf]

Actually it's interesting. Elsewhere, Sheldon advises that a cottered crank cannot be installed sufficiently without press or hammer? Or am I looking at something different?

http://sheldonbrown.com/cotters.html

Yes, you are looking at a much different, older system.

Your cranks are square-taper cotterless cranks.

There is yet another newer system, ISIS, using splined surfaces instead. Might be even more I haven't worked with.

A cottered crank is not a tapered pin fitting into a matching tapered hole. Cotters are an antiquated method of attaching cranks. The hole in the crank arm is straight sided, the flat on the spindle is not tapered either. Only one side of the pin is tapered. They are a real kludge. When you remove them, most have metal shaved off the tapered side.

Some don't even have tapers at all. The ones I had on a "PanWorld" bike simply had a round shaft same diameter all the way to the shoulder the cranks slid up against just before the BB shell. Then they had the flat for the cotter pin. They took a hell of a lot of work to get the freakin' pins out, too. Thought I was gonna break the shaft!

 

[Lurkin]

Great thank you all for clearing that up.

Installed the replacement last night, away laughing! thank you!