Sloping top tube geometry

[Harry Phinney]

"Qui si parla Campagnolo" <[email protected]> wrote :
> ideally, as a rider gets taller, his femur gets longer and there is a
> need for slacker seattube angles. Just making the seatpost longer
> doesn't make up for that. Why frames of 1 cm increments also have
> decreasing seat tube angles as well.

But at a constant (non-90 degree) seat angle, the seat moves back as it is
raised. As a 6'2" rider with over 35 years of racing experience, I prefer
frames with 73 degree seat angles, as they allow me to keep the saddle
clamped about in the middle of the rails. My belief is that large frames
tend to have slack seat angles to allow builders to quote longer top tube
lengths, even though the length comes not from stretching the front-center,
but simply from moving the saddle back.

Harry Phinney

 

[Harry Phinney]

"Michael Press" <[email protected]> wrote
> Some people, including myself, think that torsional and
> vertical compliance are a bad thing. I prefer that the
> force I exert on the crank rotates the chain wheel
> rather than twisting the frame. At 155 lb, the only
> place I look for compliance is in fork blades thin at
> the ends with an exaggerated curve.

The difference in compliance will be negligble, but the torsional compliance
of the "compact" frame will (all else equal) be less than that of a
traditional frame. I'll also point out that virtually all energy that goes
into flexing a frame will be returned as useful work as the pedals approach
TDC, so there's no significant difference in mechanical efficiency between
flexible and stiff frames. I'd wager a large sum you flex frames a lot less
than did Sean Kelly on his (very) flexible small-tubed aluminum frames.

Harry Phinney

 

[jim beam]

Harry Phinney wrote:
> "Qui si parla Campagnolo" <[email protected]> wrote :
>> ideally, as a rider gets taller, his femur gets longer and there is a
>> need for slacker seattube angles. Just making the seatpost longer
>> doesn't make up for that. Why frames of 1 cm increments also have
>> decreasing seat tube angles as well.
>
> But at a constant (non-90 degree) seat angle, the seat moves back as it is
> raised. As a 6'2" rider with over 35 years of racing experience, I prefer
> frames with 73 degree seat angles, as they allow me to keep the saddle
> clamped about in the middle of the rails. My belief is that large frames
> tend to have slack seat angles to allow builders to quote longer top tube
> lengths, even though the length comes not from stretching the front-center,
> but simply from moving the saddle back.
>
> Harry Phinney
>
>
i agree. shallow seat angles suck for us larger guys - i like to have
my knees well and truly over the pedals and power up accordingly. too
laid back and all you can do is be a fred.

 

[jim beam]

Harry Phinney wrote:
> "Michael Press" <[email protected]> wrote
>> Some people, including myself, think that torsional and
>> vertical compliance are a bad thing. I prefer that the
>> force I exert on the crank rotates the chain wheel
>> rather than twisting the frame. At 155 lb, the only
>> place I look for compliance is in fork blades thin at
>> the ends with an exaggerated curve.
>
> The difference in compliance will be negligble, but the torsional compliance
> of the "compact" frame will (all else equal) be less than that of a
> traditional frame. I'll also point out that virtually all energy that goes
> into flexing a frame will be returned as useful work as the pedals approach
> TDC, so there's no significant difference in mechanical efficiency between
> flexible and stiff frames. I'd wager a large sum you flex frames a lot less
> than did Sean Kelly on his (very) flexible small-tubed aluminum frames.
>
> Harry Phinney
>
>
the disadvantage of excessive flex is unintended rubbing. as you're
saying, there's no hysteresis loop for flexible metal, but if that
flexibility allows a brake to contact a rim for example, you're worse
off with the flexible frame. and then there's the whole issue of shimmy....

 

[[email protected]]

On Apr 10, 6:31 pm, jim beam <[email protected]> wrote:
> [email protected] wrote:
>
> > Calling sloping top tubes a "technology" is evidence
> > that the software industry has triumphed and that
> > word no longer has any meaning.
>
> software industry? explain?
>
> as for use of the word, just because something is commonplace and taken
> for granted, doesn't mean it's not worthy.

The software biz calls every operation or algorithm
a "technology." (If your technology plugs and plays
with something, it's a "solution.") For example, Amazon
has a patent on "one-click technology." Leaving aside
whether this innovation should be patentable, it's an
algorithm somewhere in the front-end of their website
that lets you order a book with only one click. That's
not "technology," it's just a putting together of
various pre-existing algorithms. Something like the
TCP/IP protocol or Ethernet (or Amazon's entire
warehouse backend) is more like what it takes to rise
to the level of "technology."

Getting back to bikes, butted frame tubes, bladder
molding carbon, TIG welding, these are technologies.
Sloping top tubes are not technology. They are a
design element. It doesn't take anything different
to make a sloping top-tube frame, you join the tubes
the same way. That's not pejorative; they are a
sometimes useful and functional design element. But
calling something "technology" that isn't serves to
make it sound more mystical and sophisticated than it
actually is. (This is of course why Amazon calls their
one-click thingy "technology," to make it seem like
retailing WMD that should be kept out of Barnes & Noble's
hands.)

Ben

 

[Qui si parla Campagnolo]

On Apr 10, 8:45 pm, jim beam <[email protected]> wrote:
> Harry Phinney wrote:
> > "Qui si parla Campagnolo" <[email protected]> wrote :
> >> ideally, as a rider gets taller, his femur gets longer and there is a
> >> need for slacker seattube angles. Just making the seatpost longer
> >> doesn't make up for that. Why frames of 1 cm increments also have
> >> decreasing seat tube angles as well.
>
> > But at a constant (non-90 degree) seat angle, the seat moves back as it is
> > raised. As a 6'2" rider with over 35 years of racing experience, I prefer
> > frames with 73 degree seat angles, as they allow me to keep the saddle
> > clamped about in the middle of the rails. My belief is that large frames
> > tend to have slack seat angles to allow builders to quote longer top tube
> > lengths, even though the length comes not from stretching the front-center,
> > but simply from moving the saddle back.
>
> > Harry Phinney
>
> i agree. shallow seat angles suck for us larger guys - i like to have
> my knees well and truly over the pedals and power up accordingly. too
> laid back and all you can do is be a fred.

What also sucks is having a big frame with such a forward seat tube
angle that you can't get your saddle far enough back to get you knee
over the pedal spindle. There are LOTS of zero setback SPs, few way
aft SPs.

 

[jim beam]

Qui si parla Campagnolo wrote:
> On Apr 10, 8:45 pm, jim beam <[email protected]> wrote:
>> Harry Phinney wrote:
>>> "Qui si parla Campagnolo" <[email protected]> wrote :
>>>> ideally, as a rider gets taller, his femur gets longer and there is a
>>>> need for slacker seattube angles. Just making the seatpost longer
>>>> doesn't make up for that. Why frames of 1 cm increments also have
>>>> decreasing seat tube angles as well.
>>> But at a constant (non-90 degree) seat angle, the seat moves back as it is
>>> raised. As a 6'2" rider with over 35 years of racing experience, I prefer
>>> frames with 73 degree seat angles, as they allow me to keep the saddle
>>> clamped about in the middle of the rails. My belief is that large frames
>>> tend to have slack seat angles to allow builders to quote longer top tube
>>> lengths, even though the length comes not from stretching the front-center,
>>> but simply from moving the saddle back.
>>> Harry Phinney
>> i agree. shallow seat angles suck for us larger guys - i like to have
>> my knees well and truly over the pedals and power up accordingly. too
>> laid back and all you can do is be a fred.
>
> What also sucks is having a big frame with such a forward seat tube
> angle that you can't get your saddle far enough back to get you knee
> over the pedal spindle. There are LOTS of zero setback SPs, few way
> aft SPs.
>
well, as the rider and tester of many large frames, my experience is
that i've never experienced a problem with the seat being too far
forward with a normal setback post, only too far back due to the frame
angle. even a 75 degree track frame is fine. 72 degree "road" frame is
awful.

 

[Qui si parla Campagnolo]

On Apr 11, 7:09 am, jim beam <[email protected]> wrote:
> Qui si parla Campagnolo wrote:
>
> > On Apr 10, 8:45 pm, jim beam <[email protected]> wrote:
> >> Harry Phinney wrote:
> >>> "Qui si parla Campagnolo" <[email protected]> wrote :
> >>>> ideally, as a rider gets taller, his femur gets longer and there is a
> >>>> need for slacker seattube angles. Just making the seatpost longer
> >>>> doesn't make up for that. Why frames of 1 cm increments also have
> >>>> decreasing seat tube angles as well.
> >>> But at a constant (non-90 degree) seat angle, the seat moves back as it is
> >>> raised. As a 6'2" rider with over 35 years of racing experience, I prefer
> >>> frames with 73 degree seat angles, as they allow me to keep the saddle
> >>> clamped about in the middle of the rails. My belief is that large frames
> >>> tend to have slack seat angles to allow builders to quote longer top tube
> >>> lengths, even though the length comes not from stretching the front-center,
> >>> but simply from moving the saddle back.
> >>> Harry Phinney
> >> i agree. shallow seat angles suck for us larger guys - i like to have
> >> my knees well and truly over the pedals and power up accordingly. too
> >> laid back and all you can do is be a fred.
>
> > What also sucks is having a big frame with such a forward seat tube
> > angle that you can't get your saddle far enough back to get you knee
> > over the pedal spindle. There are LOTS of zero setback SPs, few way
> > aft SPs.
>
> well, as the rider and tester of many large frames, my experience is
> that i've never experienced a problem with the seat being too far
> forward with a normal setback post, only too far back due to the frame
> angle. even a 75 degree track frame is fine. 72 degree "road" frame is
> awful.

The I think you are one of those who has shortish femurs for your
height, perhaps leg length predominately in your lower leg or a long
torso. Merckx/DeRosa type geometry, IMO, suits taller riders, with
seat tube angles in the 72 degree range for 60cm-ish frames. 72.5 for
a 58cm. Colnago, on the other hand, don't with steep seatutbe by
comparison and short top tubes. 'A 75 degree track frame' being fine I
think explains a lot.

 

[Steve Gravrock]

On 2007-04-10, Joe Riel <[email protected]> wrote:

> On the other hand, if the seattube length is continuously reduced, at
> some point things have to get worse. It's not clear where that point
> lies.

The smallest Fisher 29er hardtails are about far as you can push the
compact frame concept and still have a diamond frame:

<http://www.panix.com/~sdg/usenet/mtbs.jpg>

Other than handlebar height, which has changed since I took that
picture, those bikes fit similarly. I actually have more standover
clearance on the upper bike. As you can see, the geometry of the lower
frame is a bit extreme to accomodate the tall front end and still
provide semi-reasonable standover.

The bike rides well and feels plenty stiff. It's plenty heavy, but the
wheels and fork account for far more of that than the seatpost. The only
drawbacks I've noticed as a result of the steeply sloping top tube are
that the bike looks a bit odd, can only carry one water bottle, and sits
awkwardly on the kind of car rack that cradles the top tube.

 

[[email protected]]

On Apr 10, 7:38 pm, "Harry Phinney" <[email protected]>
wrote:
> "Michael Press" <[email protected]> wrote
>
> > Some people, including myself, think that torsional and
> > vertical compliance are a bad thing. I prefer that the
> > force I exert on the crank rotates the chain wheel
> > rather than twisting the frame. At 155 lb, the only
> > place I look for compliance is in fork blades thin at
> > the ends with an exaggerated curve.
>
> The difference in compliance will be negligble, but the torsional compliance
> of the "compact" frame will (all else equal) be less than that of a
> traditional frame. I'll also point out that virtually all energy that goes
> into flexing a frame will be returned as useful work as the pedals approach
> TDC, so there's no significant difference in mechanical efficiency between
> flexible and stiff frames. I'd wager a large sum you flex frames a lot less
> than did Sean Kelly on his (very) flexible small-tubed aluminum frames.

If the statement that the energy is returned useful work is 100%
correct, then the smoothing of the power delivery curve is in fact
beneficial.

dl

 

[Michael Press]

In article <[email protected]>,
"Harry Phinney" <[email protected]> wrote:

> "Michael Press" <[email protected]> wrote
> > Some people, including myself, think that torsional and
> > vertical compliance are a bad thing. I prefer that the
> > force I exert on the crank rotates the chain wheel
> > rather than twisting the frame. At 155 lb, the only
> > place I look for compliance is in fork blades thin at
> > the ends with an exaggerated curve.
>
> The difference in compliance will be negligble, but the torsional compliance
> of the "compact" frame will (all else equal) be less than that of a
> traditional frame. I'll also point out that virtually all energy that goes
> into flexing a frame will be returned as useful work as the pedals approach
> TDC, so there's no significant difference in mechanical efficiency between
> flexible and stiff frames. I'd wager a large sum you flex frames a lot less
> than did Sean Kelly on his (very) flexible small-tubed aluminum frames.

I never believed that the energy of elastic flex in the frame will
be transmitted into turning the rear wheel. The system is too
complicated for that to be accepted on the face of it. It would
take a meticulous laboratory study of the greatest exactitude to
persuade me that frame flex energy is not mostly lost.

--
Michael Press