Frame design question (no, not compact frames)

[Appkiller]

So, after a group ride the other day a dude was checking out
my ride and asked if it was stiff. Hell yes it is,
especially in the bb. He looked at my chainstays and
commented that the large profile of the chainstay was
responsible for this stiffness. Hmmm. These are lenticular
(Foco) in shape with the long axis in the vertical plane and
not exceptionally wide in the horizontal plane.

I was about to contradict him, but decided not to as it
really didn't matter whether or not he understood bicycle
design. AFAIK, the horizontal width of the chainstay will
determine the relative stiffness of the bb, kinda like the
concept behind "bi-ovalized" down tubes. I would agree the
addition of more material in the vertical plane will add a
wee bit of stiffness in the horizontal plane, but not nearly
as much as if the same amount of material was added in the
horizontal plane.

I would also assert that the stiffness in the bb had much
more to do with the advent of the super strong air-hardened
steels (thermacrom, 853, S3) than anything else.

Was I smart to not contradict him because:

A. I would have looked like an idiot and the above blather
is all wrong?

-OR-

B. Direct contradiction (even if correct) of an assertion of
someone you just met can alienate them?

Just wondering,

App

 

[Bruni]

You are right about the minor axis being paramount for
stiffness, which is why I "deovalize" such stays and make
them longer to get my tire/ring clearance back. If this
seems to contradict the shorter/stiffer trend, it does.
Shorter/stiffer is a direct inverse proportion, so 2cm
shorter only gets you 5%. OTOH, fattening an 18mm minor axis
to 22mm is 50% stiffer (measured, not calculated). As to
853, its elastic modulus is nearly the same as other steels
and since it is to my knowledge only 22.2mm ROR (as opposed
to the 24mm oval I prefer, it would have to be nearly round
to get the 22mm lateral dimension. This is why steel is
still a competitive mat'l., it packs a lot of stiffness into
the small space afforded a designer between tires and rings.
Low density alloys only have a stiffness/weight advantage
when diameters are large,and walls are thin.

--
Bruni Bicycles
"Where art meets science"
brunibicycles.com
410.426.3420
Appkiller <[email protected]> wrote in message
news:[email protected]...
> So, after a group ride the other day a dude was checking out my ride
> and asked if it was stiff. Hell yes it is, especially in the bb. He
> looked at my chainstays and commented that the large profile of the
> chainstay was responsible for this stiffness. Hmmm. These are
> lenticular (Foco) in shape with the long axis in the vertical plane
> and not exceptionally wide in the horizontal plane.
>
> I was about to contradict him, but decided not to as it really didn't
> matter whether or not he understood bicycle design. AFAIK, the
> horizontal width of the chainstay will determine the relative
> stiffness of the bb, kinda like the concept behind "bi-ovalized" down
> tubes. I would agree the addition of more material in the vertical
> plane will add a wee bit of stiffness in the horizontal plane, but not
> nearly as much as if the same amount of material was added in the
> horizontal plane.
>
> I would also assert that the stiffness in the bb had much more to do
> with the advent of the super strong air-hardened steels (thermacrom,
> 853, S3) than anything else.
>
> Was I smart to not contradict him because:
>
> A. I would have looked like an idiot and the above blather is all
> wrong?
>
> -OR-
>
> B. Direct contradiction (even if correct) of an assertion of someone
> you just met can alienate them?
>
> Just wondering,
>
> App

 

[Zog The Undenia]

Appkiller wrote:

> So, after a group ride the other day a dude was checking
> out my ride and asked if it was stiff. Hell yes it is,
> especially in the bb. He looked at my chainstays and
> commented that the large profile of the chainstay was
> responsible for this stiffness. Hmmm. These are lenticular
> (Foco) in shape with the long axis in the vertical plane
> and not exceptionally wide in the horizontal plane.
>
> I was about to contradict him, but decided not to as it
> really didn't matter whether or not he understood bicycle
> design. AFAIK, the horizontal width of the chainstay will
> determine the relative stiffness of the bb, kinda like the
> concept behind "bi-ovalized" down tubes. I would agree the
> addition of more material in the vertical plane will add a
> wee bit of stiffness in the horizontal plane, but not
> nearly as much as if the same amount of material was added
> in the horizontal plane.

True.

> I would also assert that the stiffness in the bb had much
> more to do with the advent of the super strong air-
> hardened steels (thermacrom, 853, S3) than anything else.

Sadly not. The stiffness (Young's modulus) of different
steels is much of a muchness - around 200,000 MPa [1]. It's
the strength that varies, which allows you to use thinner
tubing with stronger steels. Thinner tubing will actually be
less stiff as a manufactured tube. The way to get stiff
tubes, asusming we're talking about plain round ones, is by
making the diameter bigger or the gauge heavier. The former
is more acceptable for a bike.

[1] think of it as the stress that would stretch a rod of
the material to twice its original length (in the real
world it would break first). https://vault2.secured-
url.com/reynolds/compproperties.html

 

[Paul Kopit]

On 30 Apr 2004 10:41:49 -0700, [email protected] (Appkiller) wrote:

>I would also assert that the stiffness in the bb had much
>more to do with the advent of the super strong air-hardened
>steels (thermacrom, 853, S3) than anything else.

How do you measure the stiffness of the BB?

 

[Phil Brown]

>>I would also assert that the stiffness in the bb had much
>>more to do with the advent of the super strong air-
>>hardened steels (thermacrom, 853, S3) than anything else.

Nope. It has to do with the shell itself and the brazing
technique. A cast shell is stiffer and a good brazing job is
stiffer. phil Brown

 

[Appkiller]

Paul Kopit <[email protected]> wrote in message news:<[email protected]>...
> On 30 Apr 2004 10:41:49 -0700, [email protected]
> (Appkiller) wrote:
>
> >I would also assert that the stiffness in the bb had much
> >more to do with the advent of the super strong air-
> >hardened steels (thermacrom, 853, S3) than anything else.
>
> How do you measure the stiffness of the BB?

I measure it qualitatively and visually. I have the same
bottom bracket (chorus) and similar cranksets (one record,
one chorus) on two different bikes (one sano foco, one
schwinn paramount ti) and I have noted (repeatedly and
intentionally)during strong efforts that as I view through
the front derailleur cage, there is barely perceptible
lateral motion of the chain relative to the cage on the foco
bike while there is obvious lateral motion on the paramount.

Ergo, the one bike is relatively stiffer in the BB area than
the other. The reason for which, I will assert, is the
frames themselves.

Logical, reasonable? I think so.

App