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Old August 26th 19, 12:51 AM posted to rec.bicycles.tech
Frank Krygowski[_4_]
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Default WTB Suntour CYCLONE BB Spindle

On 8/25/2019 7:10 PM, Tom Kunich wrote:
On Sunday, August 25, 2019 at 2:17:03 PM UTC-7, Chalo wrote:
Tom Kunich wrote:

Out of curiosity - why would you use an old and not very efficient
way to do things then you could buy a normal cartridge BB that will
last you the rest of your life since it is sealed to keep dirt out?


Sealed BBs don't last forever. They seem to last about as long as loose ball BBs, but with less maintenance along the way.

Square-taper sealed cartridge BBs do flex substantially more than traditional cup-and-cone BBs, because the spindle overhangs the bearings by a longer distance. The first time I saw cartridge BBs was in the new fleet of rental bikes at the shop where I worked in 1992. The first test ride I did after building one up made me think something was broken, because the pedals dove so far underneath the BB shell.


Just to get on the same page - the actual crank axle material has changed. It only requires a very small amount of chromium to greatly increase the strength of the axle. Moreover hollowing the axle out also increases the rigidity of the axle. This is what led to the octalink BB which FAR stiffer than any of the older ISO. Oh, wait, as a mechanical engineer Frank doesn't know anything about this.


OK, I let a _lot_ of Tom's nonsense go by without correction. He gets no
pass on this one.

Strength is one thing. Stiffness is another. They are not the same.

Chalo was talking about deflection of the bottom bracket and crank
assembly. Deflection for a given load is related to stiffness, and is
influenced by the shape and size of the relevant parts plus the relevant
modulus of elasticity, NOT by their strength. (In assemblies like bottom
brackets, of course any looseness in connections affects deflection, but
that's a separate matter.)

The modulus of elasticity (i.e. stiffness) is almost exactly the same
for all alloys of steel. Chromium in small amounts doesn't change it
measurably. Chromium (or other normal alloying elements) even in large
amounts barely affects it. So all steels have essentially the same
stiffness.

And - good grief! - can please we ditch the idea that a hollow tube of a
given material and outside diameter is stronger or stiffer than a solid
rod of the same diameter? It's not! How is that not obvious??

Take a hollow tube of a given material, given O.D. and given I.D. It
has a certain amount of strength and stiffness. Take a second such tube
with dimensions chosen so it just barely slides into the first. Fasten
them together appropriately. That combination has more strength and
stiffness. It _must_ because you've combined the strength and stiffness
of the two tubes!

Add another tube to the inside. Each time you do that you get more
strength and stiffness, until you've made the thing a solid bar.

Here's where a tube is better than a solid bar: If its dimensions are
appropriate, it is stronger and stiffer PER UNIT WEIGHT. That is why
bike frames and some bottom brackets are hollow. They can have more
strength and stiffness for the same weight, or less weight for the same
strength and stiffness.

(Now it's time for Tom to ask some rhetorical question that deflects or
changes the subject.)


--
- Frank Krygowski
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