Bogus frame flex "test?"

I know the subject of frame flex has been discussed ad infinitum in
this group, but I wanted to throw out this recent observation. On a
recent visit to a LBS, the owner took great pains to demonstrate for me
how flexible my current ride is by - you guessed it - squeezing the
brake and then pressing on the pedal. Low and behold, the BB appeared
to deflect sideways quite a ways. Here's the thing, though. When I
actually RIDE the bike, I never get any front der. rub, even on the
hardest out-of-saddle climbs with the der. trimmed close to the chain,
and I've never had any unintended rear der. shifts or hihg-speed
shimmy either. In short, this frame works perfectly for me (plus,
unlike the new frame the LBS guy wanted to sell me, this one is bought
and paid for!). So my question is, what's actually flexing during
the frame flex "test" described above? I suspect it's mostly the
tires.

Regards to all . . .

Jhas wrote:
I know the subject of frame flex has been discussed ad infinitum in
this group, but I wanted to throw out this recent observation. On a
recent visit to a LBS, the owner took great pains to demonstrate for me
how flexible my current ride is by - you guessed it - squeezing the
brake and then pressing on the pedal. Low and behold, the BB appeared
to deflect sideways quite a ways. Here's the thing, though. When I
actually RIDE the bike, I never get any front der. rub, even on the
hardest out-of-saddle climbs with the der. trimmed close to the chain,
and I've never had any unintended rear der. shifts or hihg-speed
shimmy either. In short, this frame works perfectly for me (plus,
unlike the new frame the LBS guy wanted to sell me, this one is bought
and paid for!). So my question is, what's actually flexing during
the frame flex "test" described above? I suspect it's mostly the
tires.

They all do that - even my fat-tubed Cannondale MTB. The rear wheel and
the frame are flexing all right, but you get most of the energy back on
the return pedal stroke (frame metals are pretty good as springs).

On the other hand, if you're getting brake and/or gear rub, then your
frame *is* too flexible.

Jhas wrote:
I know the subject of frame flex has been discussed ad infinitum in
this group, but I wanted to throw out this recent observation. On a
recent visit to a LBS, the owner took great pains to demonstrate for
me how flexible my current ride is by - you guessed it - squeezing the
brake and then pressing on the pedal. Low and behold, the BB appeared
to deflect sideways quite a ways. Here's the thing, though. When I
actually RIDE the bike, I never get any front der. rub, even on the
hardest out-of-saddle climbs with the der. trimmed close to the chain,
and I've never had any unintended rear der. shifts or hihg-speed
shimmy either. In short, this frame works perfectly for me (plus,
unlike the new frame the LBS guy wanted to sell me, this one is bought
and paid for!). So my question is, what's actually flexing during
the frame flex "test" described above? I suspect it's mostly the
tires.

Regards to all . . .

Yer right, mostly tires and wheels with probably a teensy-tinsy little bit
of frame and crank flex. Most frame flex you don't feel while riding because
the bike moves slightly sideways, with the tires and wheels absorbing most
of that energy.

That 'standing-on-the-pedal-with-the-brake-on' trick is an old one that's
misused and misunderstood. Your LBS may be well-intentioned (or not). Did he
do the same "test" on the bike he proposed selling you? I would suspect it
would do exactly the same thing as yours.

Well, this particular LBS tends to want to upsell everybody to high
zoot brands. My frame does flex somewhat more than most in the
"standing-on-the-pedal" trick, but it works great for riding.

Jhas wrote:
Well, this particular LBS tends to want to upsell everybody to high
zoot brands. My frame does flex somewhat more than most in the
"standing-on-the-pedal" trick, but it works great for riding.
If you're happy with what you have, ride on!

On Wed, 30 Mar 2005 18:08:21 +0100, Zog The Undeniable wrote:


They all do that - even my fat-tubed Cannondale MTB. The rear wheel and
the frame are flexing all right, but you get most of the energy back on
the return pedal stroke (frame metals are pretty good as springs).

How do you think you get that energy "back"? Unless that spring return
actually contributes to pushing down the other pedal, which it doesn't,
there is no way you recover that energy expended in flexing the frame.
You also get the energy back when you squeeze one of those hand-exerciser
things, then release it, but you still get tired from doing it.

--

David L. Johnson

__o | When you are up to your ass in alligators, it's hard to remember
_`\(,_ | that your initial objective was to drain the swamp. -- LBJ
(_)/ (_) |

David L. Johnson wrote:

On Wed, 30 Mar 2005 18:08:21 +0100, Zog The Undeniable wrote:


They all do that - even my fat-tubed Cannondale MTB. The rear wheel and
the frame are flexing all right, but you get most of the energy back on
the return pedal stroke (frame metals are pretty good as springs).


How do you think you get that energy "back"? Unless that spring return
actually contributes to pushing down the other pedal, which it doesn't,
there is no way you recover that energy expended in flexing the frame.

Without a full analysis of the forces, I'd hazard a guess that the
wind-up in the frame helps at TDC and BDC. If all that stored energy
gets lost in tyre scrub or elsewhere, that's quite a lot of wasted power
- possibly of the order of 50W when climbing out of the saddle. Are
bikes really that inefficient?

"David L. Johnson" wrote in message
news
How do you think you get that energy "back"? Unless that spring return
actually contributes to pushing down the other pedal, which it doesn't,
there is no way you recover that energy expended in flexing the frame.
You also get the energy back when you squeeze one of those hand-exerciser
things, then release it, but you still get tired from doing it.


The question is, where does the stored energy go? Does it go into the
motion of the bike?

Consider a fairly light book being pulled across a carpet by a fairly large
rubber band. If you pull the book lightly, the rubber band does not stretch
much and most of your work goes directly to moving the book. However, if
you pull it quickly and then stop moving your hand (but don't reverse), the
rubber band will continue to exert a force on the book until it is no longer
stretched. I'm not sure, but I think this is a better analogy to a flexing
bike frame than the hand exerciser.

--
Scott Ehardt
http://www.scehardt.com

Jhas wrote:
I know the subject of frame flex has been discussed ad infinitum in
this group, but I wanted to throw out this recent observation. On a
recent visit to a LBS, the owner took great pains to demonstrate for
me
how flexible my current ride is by - you guessed it - squeezing the
brake and then pressing on the pedal. Low and behold, the BB
appeared
to deflect sideways quite a ways. Here's the thing, though. When I
actually RIDE the bike, I never get any front der. rub, even on the
hardest out-of-saddle climbs with the der. trimmed close to the
chain,
and I've never had any unintended rear der. shifts or hihg-speed
shimmy either. In short, this frame works perfectly for me (plus,
unlike the new frame the LBS guy wanted to sell me, this one is
bought
and paid for!). So my question is, what's actually flexing during
the frame flex "test" described above? I suspect it's mostly the
tires.

There may be some frame flex, but so what? Lateral stiffness is mostly
a matter of personal preference. As for performance, there's no
significant difference, races have been won with either extreme. I've
ridden both kinds and prefer a stiff frame, but that's just me.

On Wed, 30 Mar 2005 19:46:18 +0000, Scott Ehardt wrote:

The question is, where does the stored energy go?

Heat.

Does it go into the
motion of the bike?

Only if you can see how that sideways push back is used by your body to
push the other pedal down. It looks to me to be totally wasted motion.

However, if you pull it quickly and then stop moving your hand (but
don't reverse), the rubber band will continue to exert a force on the
book until it is no longer stretched.

But in this case the release of the spring directly translates to moving
the book. Hold the pedals of a bike still, push the bottom bracket to one
side. Does the bike move forward? How?

--

David L. Johnson

__o | What is objectionable, and what is dangerous about extremists is
_`\(,_ | not that they are extreme, but that they are intolerant.
(_)/ (_) | --Robert F. Kennedy