Bogus frame flex "test?"

[Jhas]

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 . . .

 

[Zog The Undeniable]

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.

 

[Zog The Undeniable]

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.

 

[Dave Thompson]

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.

 

[Dave Thompson]

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.

 

[Jhas]

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]

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.

 

[Dave Thompson]

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!

 

[Dave Thompson]

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!

 

[David L. Johnson]

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]

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
(_)/ (_) |

 

[Peter Cole]

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.

 

[Peter Cole]

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.

 

[David L. Johnson]

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

 

[David L. Johnson]

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

 

[[email protected]]

can one seperate frame flex and wheel flex and tire flex?
goes on all the time! the frame flexes when i ride over the train
tracks at 85 mph
ok but the wheels...?
expert expert!!

 

[[email protected]]

can one seperate frame flex and wheel flex and tire flex?
goes on all the time! the frame flexes when i ride over the train
tracks at 85 mph
ok but the wheels...?
expert expert!!

 

[Scott Ehardt]

"David L. Johnson" <[email protected]> wrote in message
news[email protected]...
> On Wed, 30 Mar 2005 19:46:18 +0000, Scott Ehardt wrote:
>
>> The question is, where does the stored energy go?
>
> Heat.

Yes, some is lost to heat. However, the amount is minimal. Consider two
actions with similar hand motions:

Stirring cake batter - all your effort goes to heat (the batter will not
"un-stir" itself)

Turning a hand crank to lift a bucket out of a well - in this case some
energy is lost to heat, but most is used to increase the potential energy of
the bucket. This energy could be re-captured when the handle is released
(it will "un-lift" itself.)

>> 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?

The bending of the frame is caused by pushing on the pedal (an action which
we agree makes the bike go). Therefore as the power stroke is completed
(and weight on the pedal reduces), the frame would still be pushing the
pedal "up" (towards the foot) and (like the stretched rubber band),
contributing to the forward motion of the bike.

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

 

[Scott Ehardt]

"David L. Johnson" <[email protected]> wrote in message
news[email protected]...
> On Wed, 30 Mar 2005 19:46:18 +0000, Scott Ehardt wrote:
>
>> The question is, where does the stored energy go?
>
> Heat.

Yes, some is lost to heat. However, the amount is minimal. Consider two
actions with similar hand motions:

Stirring cake batter - all your effort goes to heat (the batter will not
"un-stir" itself)

Turning a hand crank to lift a bucket out of a well - in this case some
energy is lost to heat, but most is used to increase the potential energy of
the bucket. This energy could be re-captured when the handle is released
(it will "un-lift" itself.)

>> 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?

The bending of the frame is caused by pushing on the pedal (an action which
we agree makes the bike go). Therefore as the power stroke is completed
(and weight on the pedal reduces), the frame would still be pushing the
pedal "up" (towards the foot) and (like the stretched rubber band),
contributing to the forward motion of the bike.

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

 

[David L. Johnson]

On Wed, 30 Mar 2005 22:19:51 +0000, Scott Ehardt wrote:

> The bending of the frame is caused by pushing on the pedal (an action which
> we agree makes the bike go). Therefore as the power stroke is completed
> (and weight on the pedal reduces), the frame would still be pushing the
> pedal "up" (towards the foot) and (like the stretched rubber band),
> contributing to the forward motion of the bike.

No. When the power stroke is completed, the relevant pedal is nearing
bottom. In order to contribute to making the bike move forward, the
restoring force of the spring would have to push the pedal back. It
pushes the pedal sideways.

--

David L. Johnson

__o | As far as the laws of mathematics refer to reality, they are not
_`\(,_ | certain, and as far as they are certain, they do not refer to
(_)/ (_) | reality. -- Albert Einstein