Re: [CR] Stronglight Bearings Question

(Example: Framebuilders:Alex Singer)

From: "Andrew R Stewart" <onetenth@earthlink.net>
To: John Hurley <JHurley@jdabrams.com>, <classicrendezvous@bikelist.org>
References: <249DDD9704676C49AE6169AE3D2D9F4ECDB93E@Exchange-SVR>
In-Reply-To: <249DDD9704676C49AE6169AE3D2D9F4ECDB93E@Exchange-SVR>
Date: Tue, 15 Dec 2009 11:22:02 -0500
Subject: Re: [CR] Stronglight Bearings Question


John- While I don't totally agree with Michael's claim that misaligned cups will produce noise I also take stronger issue with your understanding of BB shells and cups.

Traditional cupped BBs are quite dependent on the faces of the shell for their finial alignment. There can be enough slop between the cup and the shell so that a cup can rock a bit within the threads. As the cup's flange or lockring seats against the shell face the cup will try to move to the face, all the way around the edge. Usually this is a very small amount of misalignment and facing minimizes this.

A simple experiment would be placing a slotted screwdriver between the shell's face and a cup's flange while tightening the cup. You will see and feel a cocking of the cup away from the screw driver's tip. You will be able to get the cup tight in the shell but it won't be seated up against the face fully around the face. The looser the threads are the worse the effect. (And some of the bikes we talk about on this list were not too well finished in the factory, or the tools were dull and the job rushed because once the cup's in and tight with a crank arm attached who will know at the time of sale).

Additionally the shell can distort from the heat of joining the tubes to the shell. Especially since the heat and mass is not evenly distributed around the shell. It is not uncommon to have a few MMs of shell face distortion, as measured along a straight edge parallel to the DT or ST, after brazing up the tube. The shell contracts after cooling, more dimension change where the heat was the greatest or the masses greatest (the same in this example). (The HT undergoes the same distortion from the TT and DT brazing by the way). This distortion effects both the face and the threading centerlines. So a piloted chasing (not a tapping) will cut some of the threads' material in the attempt to reestablish the threadings' center axis to that of the pilot. A facing after the chasing will correct the face to the threads.

One of the reasons that the traditional cup and cone (sometimes called an angular contact bearing design) has been used for so long on bicycles is that this design is much more tolerant of these misalignments then the now popular (and mostly off topic, MaxiCars, Phils, Bullseyes and their siblings excepted) radial contact cartridge bearings (not to be confused with "sealed bearings", most all bike bearings have some sort of seal like what we normally call dust caps).


----- Original Message -----
From: John Hurley
To: classicrendezvous@bikelist.org
Sent: Tuesday, December 15, 2009 10:36 AM
Subject: [CR] Stronglight Bearings Question



> Michael suggested the rattling ball bearings in Ben's bottom bracket
> might be due to poorly aligned bearing cups:
>
> "On a production bike it is almost a certainty that the BB shell was not
> faced and/or tapped. Because brazing heat causes the BB shell to expand,
> and then unevenly contract after cooling, the BB cups might be /
> probably are slightly skewwed to each other and this might be causing
> the bearing noise you hear."
>
> "The noise you hear might be lessened or eliminated altogther after
> machining the BB with a good quality PILOTED facer and taps."
>
> This explanation doesn't seem to agree with what little I know about
> bottom brackets, so I'm looking for insight. First, I thought all
> bottom brackets were threaded to receive the cups, so I can't understand
> bikes leaving the factory without having been tapped. Second, the
> orientation of the cups is determined by the threads and nothing else.
> If the threads are straight, the cups will be straight. If the threads
> are skewed or misaligned, the cups will be off no matter what you do
> with facing. Hopefully the curved surface of the cup affords a certain
> tolerance for error in the cup's alignment, so the balls can find their
> own correct line even if the cup is a little crooked.
>
> I have heard of the importance of facing the bottom bracket, but as I've
> said, I don't see how it would have much effect on the alignment of the
> cups. I would think the importance of facing would be in helping
> prevent the lock ring or the fixed cup from working loose, by providing
> a nice flat surface to tighten against. If the facing was poorly done,
> I could see it being improved by re-facing.
>
> However, if the bottom bracket threads were not cut correctly, how could
> this be repaired? Just retapping would make a mess wouldn't it? Would
> you first re-fill the old threads with brazing material and then re-tap?
> Can you re-tap oversized? Replace the entire bottom bracket?
>
> Could Ben's clicking noise be the result of one or more balls having
> escaped into the interior of the shell during assembly so he doesn't
> have the full eleven in one side?
>
> John Hurley
> Austin, Texas, USA
> _______________________________________________

Andrew R Stewart
Rochester, NY