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Scamdium strikes again!



 
 
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  #1  
Old July 25th 06, 04:20 AM posted to rec.bicycles.tech
Phil Lee, Squid
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Posts: 161
Default Scamdium strikes again!

http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...

--
Phil Lee, Squid


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  #2  
Old July 25th 06, 02:50 PM posted to rec.bicycles.tech
Tim McNamara
external usenet poster
 
Posts: 6,945
Default Scamdium strikes again!

In article m,
"Phil Lee, Squid" wrote:

http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...


No. Especially as the left side is more prone to breaking (subject to
both torque and bending), and it will break under highest load which is
while standing on it. Then you'll get dumped into the road, possibly
right in front of a large motor vehicle.

The only time I had this happen was in 1984 when I stood up at a stop
sign to accelerate from a stop, snapped off the left side of the BB
axle, and ended up on my ass in the road. Fortunately it was a low
traffic side street. Gouged the heck out of my left ankle, too.
  #3  
Old July 25th 06, 09:35 PM posted to rec.bicycles.tech
BigBen
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Posts: 68
Default Scamdium strikes again!

On Tue, 25 Jul 2006 08:50:30 -0500, Tim McNamara
wrote:

In article m,
"Phil Lee, Squid" wrote:

http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...


No. Especially as the left side is more prone to breaking (subject to
both torque and bending), and it will break under highest load which is


Forgive the ignorance: does that apply to hollow axle, tapered square,
XT b.brackets?

If so, which are the ones not made of aluminum?

TIA
jbr
  #4  
Old July 25th 06, 11:18 PM posted to rec.bicycles.tech
Phil Lee, Squid
external usenet poster
 
Posts: 161
Default Scamdium strikes again!

BigBen wrote:
On Tue, 25 Jul 2006 08:50:30 -0500, Tim McNamara
wrote:

In article m,
"Phil Lee, Squid" wrote:

http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...


No. Especially as the left side is more prone to breaking (subject
to both torque and bending), and it will break under highest load
which is


Forgive the ignorance: does that apply to hollow axle, tapered square,
XT b.brackets?

If so, which are the ones not made of aluminum?


No major manufacturer makes or sells aluminum-axle bottom brackets. Your XT
bottom bracket has a steel axle.

If you look at the pictures, notice that the break does not traverse the
90-degree inner corner for most of the circumference, where one might expect
it to break. Maybe the axle was notched during installation.
--
Phil Lee, Squid


  #5  
Old July 25th 06, 11:46 PM posted to rec.bicycles.tech
Tim McNamara
external usenet poster
 
Posts: 6,945
Default Scamdium strikes again!

In article ,
(BigBen) wrote:

On Tue, 25 Jul 2006 08:50:30 -0500, Tim McNamara
wrote:

In article m,
"Phil Lee, Squid" wrote:

http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...


No. Especially as the left side is more prone to breaking (subject
to both torque and bending), and it will break under highest load
which is


Forgive the ignorance: does that apply to hollow axle, tapered
square, XT b.brackets?


The left side of the BB axle is subject to both torque and bending
forces from the left crank. The right side of the axle is subject only
to bending forces from the right crank as the torque goes directly into
the "spider."

If so, which are the ones not made of aluminum?


Aluminum is a bad choice for BB axles period. Few are made from
aluminum, the vast majority are made from steel and a few from titanium.
  #6  
Old July 26th 06, 02:36 AM posted to rec.bicycles.tech
jim beam
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Posts: 166
Default Scamdium strikes again!

Phil Lee, Squid wrote:
http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...

they're fine if they're dimensionally appropriate for the job. aluminum
shimano axles anyone?

the problem comes when you use the same size in aluminum as you do in
steel given aluminum's much lower strength. presence of scandium in
this situation is pretty much irrelevant.
  #7  
Old July 26th 06, 02:48 AM posted to rec.bicycles.tech
Chalo
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Posts: 5,093
Default Scamdium strikes again!

wrote:

It's possible to design an aluminum bottom bracket axle that's safe for
even someone of Chalo's size, but it would have to have a larger
diameter than most (or all) current designs. Like not 19-22 mm but
rather something on the order of 50mm. Whether such a large diameter BB
is desireable is a separate question.


As a rule of thumb, a tubular structure made from aluminum can be made
equally stiff to a steel tube of similar wall thickness if its diameter
is increased by a factor of 1.4. Furthermore, if its cross-sectional
area is increased to accomodate the difference between the tensile
strengths of the aluminum and steel in question, then the aluminum part
will almost certainly prove to be as reliable as the steel part. The
principle is well illustrated by the aluminum frames that are
ubiquitous on bicycles today.

For my part, I find cranks with 3/4" (19mm) solid CrMo spindles with
fine-pitched splines to be perfectly adequate for my needs. This
suggests that an aluminum spindle of approximately 27mm diameter with a
9.5mm wall would be adequate for me, as long as it were made of some
very strong aluminum alloy like 7075-T6, 7068-T6, 7055-T77511, or a
Scandium-doped alloy. Such a spindle could easily be fitted into an
American BB shell along with some reasonably stout bearings.

Chalo Colina

  #8  
Old July 26th 06, 02:58 AM posted to rec.bicycles.tech
Phil Lee, Squid
external usenet poster
 
Posts: 161
Default Scamdium strikes again!

jim beam wrote:
Phil Lee, Squid wrote:
http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...

they're fine if they're dimensionally appropriate for the job. aluminum
shimano axles anyone?

the problem comes when you use the same size in aluminum as you do in
steel given aluminum's much lower strength. presence of scandium in
this situation is pretty much irrelevant.


It was tongue in cheek, of course.

I'm curious to know what proportion of high-mileage Ksyrium SSC SL riders
get broken spokes compared to standard-spoke riders.
--
Phil Lee, Squid


  #9  
Old July 26th 06, 03:13 AM posted to rec.bicycles.tech
jim beam
external usenet poster
 
Posts: 166
Default Scamdium strikes again!

Phil Lee, Squid wrote:
jim beam wrote:
Phil Lee, Squid wrote:
http://weightweenies.starbike.com/ph...ic.php?t=19036

Aluminum BB axles are not a good material selection...

they're fine if they're dimensionally appropriate for the job. aluminum
shimano axles anyone?

the problem comes when you use the same size in aluminum as you do in
steel given aluminum's much lower strength. presence of scandium in
this situation is pretty much irrelevant.


It was tongue in cheek, of course.

I'm curious to know what proportion of high-mileage Ksyrium SSC SL riders
get broken spokes compared to standard-spoke riders.


well, those spokes are 3x the size...

interestingly, when those wheels first came out, it seemed like i'd see
someone shipwrecked with a broken aluminum spoke once or twice a month.
the last few years though, i can't say i've seen one.
  #10  
Old July 26th 06, 03:30 AM posted to rec.bicycles.tech
[email protected]
external usenet poster
 
Posts: 14
Default Scamdium strikes again!

Chalo wrote:

wrote:

It's possible to design an aluminum bottom bracket axle that's safe for
even someone of Chalo's size, but it would have to have a larger
diameter than most (or all) current designs. Like not 19-22 mm but
rather something on the order of 50mm. Whether such a large diameter BB
is desireable is a separate question.


As a rule of thumb, a tubular structure made from aluminum can be made
equally stiff to a steel tube of similar wall thickness if its diameter
is increased by a factor of 1.4. Furthermore, if its cross-sectional
area is increased to accomodate the difference between the tensile
strengths of the aluminum and steel in question, then the aluminum part
will almost certainly prove to be as reliable as the steel part. The
principle is well illustrated by the aluminum frames that are
ubiquitous on bicycles today.

For my part, I find cranks with 3/4" (19mm) solid CrMo spindles with
fine-pitched splines to be perfectly adequate for my needs. This
suggests that an aluminum spindle of approximately 27mm diameter with a
9.5mm wall would be adequate for me, as long as it were made of some
very strong aluminum alloy like 7075-T6, 7068-T6, 7055-T77511, or a
Scandium-doped alloy. Such a spindle could easily be fitted into an
American BB shell along with some reasonably stout bearings.

Chalo Colina


Thanks for the input, Chalo. Your math makes sense to me.

FWIW, I secretly hope that a new BB standard emerges (the American (AKA
BMX) spec would work fine) such that we could use 25-30mm aluminum
bottom bracket axles with appropriately oversized bearings. One
advantage of an Al:Al interface is that both the BB and crank would
have similar stiffnesses, reducing the incidence of creaking.

People like Peter would hate it, but I suspect we're headed that way;
Cannondale, Specialized and Pinarello already offer such BBs. I guess
my secret hope is no longer a secret.

Jason

P.S. Peter, I mean no offense by this; "people like Peter" is meant to
mean "those skeptical of change qua change" rather than "people who
hate anything new."

 




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