The difference tube diameter makes

On Wed, 13 Aug 2014 11:30:31 -0400, Frank Krygowski
wrote:

On 8/13/2014 7:04 AM, Rolf Mantel wrote:
Am 13.08.2014 12:54, schrieb John B. Slocomb:

One wonders why skinny tube racing bikes were used for so long a
period. It is certainly not because engineers didn't know that a thin
large diameter tube is stiffer then a thin small diameter tube.

I think there's always been a tremendous amount of bike "engineering"
that was really tradition. And come to think of it, that's true of
other fields as well. Look at the 100 year history of auto design, for
example.

the most important aspect has always been minimal weight. For a given
minimal thickness of tubing, a small diameter tube is lighter than a
large diameter tube.

Large diameter tubes only started coming up when technology reached the
stage that the minimum thickness of tube that could be produced and
welded became significantly thinner than the minimum thickness necessary
for structural stability (I believe this was reached for aluminium
frames in the 1980s and for steel frames a lot later).

IIRC, the first I heard of a significant design choice based on
oversized tubes was in an article about Gary Klein. Supposedly, he was
an MIT engineering student who was excited to visit an exhibit of a
super-light bicycle; but was very disappointed to see that it had no
engineering innovation. They'd simply drilled and whittled conventional
parts down to stupid-light thicknesses.

He then started thinking about using aluminum frames, and it occurred to
him that aluminum's lower density would allow thicker (non-denting) tube
walls at larger diameters. So we got light & rigid Klein frames.
Cannondale and others copied the concept.

Tandem tubing has long been oversized. Our 1979 tandem is oversized
Reynolds 531. But the wall thickness is no less than standard, probably
to allow brazing by any guy with a torch. So back in those days, the
extra stiffness from oversized steel tubes came with a weight penalty.

Since then, there have been improvements in steel alloys and welding
techniques, so oversized steel works better than it once did.


Err, Frank! Aluminum bike frames predated Gary Klien by about a
hundred years. The first commercially made aluminum frame was marketed
in the 1890's I believe.
--
Cheers,

John B.

On 8/13/2014 6:46 PM, James wrote:


An interesting analogy with the springy pole and walking. I can imagine
that higher peak loads are more fatiguing.

On a bike however, I don't think we have to behave the same, regardless
of the frame stiffness.

When your weight lands harshly on the pedals at the bottom of the pedal
stroke while standing and pedalling up a very steep climb, couldn't you
start to press down with the other leg a little earlier to start to
unload the leg that's near the bottom of the cycle?

Honestly, I have tried to smooth out that motion. Sometimes I can. But
it most often comes up when I'm fighting to stay alive and in motion on
the steepest hills, and when I'm most tired.

And the older I get, the more it comes up! :-/


Perhaps it's another reason to pedal in circles rather than just
stomping down on the pedals - all the way to the bottom where downward
force does nothing to propel you forward.

Yeah, that makes sense to me.


--
- Frank Krygowski

On 8/13/2014 7:14 PM, James wrote:


I think oversized steel tubes will not break so easily as 1" tubes,
because the stresses should be lower. Frank might correct me if I'm wrong.

Probably depends on the particular stresses causing the potential break.

For bending and for torsion, and for a given load and tube weight, the
elementary equations claim lower and lower stress if that material is
made into bigger and bigger diameter tubes. There's no theoretical limit
to that simplistic benefit.

But obviously, you've got practical limits imposed by the ever-thinner
tube walls. So at a certain point, the tube is vulnerable to denting,
to buckling, to fatigue at stress concentrations at ends (or wherever
loads are applied), etc.

That stuff is hard to quantify. So I think most manufacturers have
gradually and slowly experimented, to see what they could get away with.
This is probably one of the reasons that bike design has tended to be
conservative. In a sense, anyone on light enough equipment is a beta
tester.

Thank goodness for guys like Jay, doing their part to produce data and
advance the state of the art!

- Frank Krygowski

On 8/13/2014 8:42 PM, John B. Slocomb wrote:

Err, Frank! Aluminum bike frames predated Gary Klien by about a
hundred years. The first commercially made aluminum frame was marketed
in the 1890's I believe.

I know that. I was talking about the idea of greatly increasing the
aluminum tube diameter to increase stiffness, yet keep weight low.

As I've mentioned before, I've ridden a friend's Alan frame, early
1980's vintage, IIRC. It was (and is) very flexible. I was able to
scrape both sides of the chain on the front derailleur cage during a
sprint. That's just not possible with a Cannondale. At least, not for me.

--
- Frank Krygowski

On Wed, 13 Aug 2014 22:02:01 -0400, Frank Krygowski
wrote:

On 8/13/2014 8:42 PM, John B. Slocomb wrote:

Err, Frank! Aluminum bike frames predated Gary Klien by about a
hundred years. The first commercially made aluminum frame was marketed
in the 1890's I believe.

I know that. I was talking about the idea of greatly increasing the
aluminum tube diameter to increase stiffness, yet keep weight low.

I've always wondered about whether it was the chicken or the egg?
If you make a tube out of aluminum and duplicate the wall thickness
and O.D. of a steel tube you have a pretty flimsy tube.

Of course you can over overcome that by making the aluminum tube
thicker... and heavier. But if you have to use more material why not
just make the tube larger in diameter which will make the tube
stiffer...

As I've mentioned before, I've ridden a friend's Alan frame, early
1980's vintage, IIRC. It was (and is) very flexible. I was able to
scrape both sides of the chain on the front derailleur cage during a
sprint. That's just not possible with a Cannondale. At least, not for me.

The latest carbon frames I've seen had the lower end of the seat tube
about the same width as the bottom bracket. I assume a super stiff
frame.

Somewhere I read some power figures and it seems to me that the
Japanese Keirin riders are capable of very high power output but I
believe that the keirin bikes are - or at least they were - still the
old steel tube frames. And given that the crucial part of the Keirin
race is that last sprint lap after the pacer pulls off the track It
sort of makes you wonder.
--
Cheers,

John B.