Bogus frame flex "test?"

Scott Ehardt said:

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

Other flex. I was riding one of my bikes the other day and while
climbing, the computer magnet on my front wheel was hitting the pickup
thingy mounted on the front fork. The reason this was a bit remarkable
is that it only happened while out of the saddle and climbing. All was
quiet while cruising on the flats. I had never actually noticed wheel
flex until I realized what was going on. I didn't feel it as I was
climbing, but the noise of the magnet hitting the sensor was evidence
of wheel flex I had never acknowledged previously.

Jhas said:

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

Quoted message said:

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

Quoted message said:

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,

Quoted message said:

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

Quoted message said:

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 . . .

Other flex. I was riding one of my bikes the other day and while
climbing, the computer magnet on my front wheel was hitting the pickup
thingy mounted on the front fork. The reason this was a bit remarkable
is that it only happened while out of the saddle and climbing. All was
quiet while cruising on the flats. I had never actually noticed wheel
flex until I realized what was going on. I didn't feel it as I was
climbing, but the noise of the magnet hitting the sensor was evidence
of wheel flex I had never acknowledged previously.

Jhas said:

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

Quoted message said:

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

Quoted message said:

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,

Quoted message said:

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

Quoted message said:

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 . . .

Dave Thompson said:

Jhas said:

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!

Absolutely - that "test" has no more bearing on the actual performance
of your bike than the color of your tires (other current thread
nonwithstanding). Either the LBS guy buys into old wive's tales and
mythology, or he's trying to scam you into buying a new bike.

It would take an amazing amount of visual skill to determine the
minute amount of lateral flex in the frame relative to that in the
tires, rims, spokes, fork, crank and bottom bracket. I can easily
push any road bike an inch or two off-axis, knowing full well the
frame itself is probably moving no more than a few mm at most.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame

Dave Thompson said:

Jhas said:

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!

Absolutely - that "test" has no more bearing on the actual performance
of your bike than the color of your tires (other current thread
nonwithstanding). Either the LBS guy buys into old wive's tales and
mythology, or he's trying to scam you into buying a new bike.

It would take an amazing amount of visual skill to determine the
minute amount of lateral flex in the frame relative to that in the
tires, rims, spokes, fork, crank and bottom bracket. I can easily
push any road bike an inch or two off-axis, knowing full well the
frame itself is probably moving no more than a few mm at most.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame

David L. Johnson said:

I think any energy that goes into flexing the frame simply is

translated

Quoted message said:

to heat. When you climb out of the saddle, really thrashing the bike

from

Quoted message said:

side to side, that motion translates to the pedals by the fact that
pushing the bike to the side would raise the forward pedal slightly,

but

Quoted message said:

you are also pushing on that pedal keeping it down, giving an

additional

Quoted message said:

push against the pedal. The extend to which you flex the frame will

only

Quoted message said:

return a horizontal and sideways motion to the pedal when released

(near

Quoted message said:

BDC for the affected pedal stroke), not contributing to forward

motion. It

Quoted message said:

would have to push the pedal backwards at that point to help.

I think precious little of the frame flex is lost as heat within the
frame. The metal just doesn't have much in the way of internal
friction, IOW it has little hysteresis loss.

There is a way to recover some of the minute amount of energy lost in
flex. Consider: What your "losing" is work from your power leg. It
exerts force during a slight extra distance downward. The way this
energy (i.e. stored work) can be gotten back is by helping lift that
same leg when it's on the backstroke, by exerting a minute lifting
force upward.

Lots of people think they actively pull up on the backstroke. From
reports I've seen of measurements from instrumented pedals, they don't;
they may reduce the downward force during the return stroke, but their
rear leg is lifted partly by the force transmitted by their forward
leg. (BTW, altenative pedal mechanisms that rely on consciously
lifting the "rear" leg seem to do badly.)

Anyway, if the front (power) leg is partly helping to lift the rear
(passive) leg, then the recovered frame flex will be helping a bit,
too, as the frame helps "spring" the bottom bracket back toward center.
Minutely, of course.

I think the thing we all agree on is, it's VERY unlikely to be a
significant effect!

David L. Johnson said:

I think any energy that goes into flexing the frame simply is

translated

Quoted message said:

to heat. When you climb out of the saddle, really thrashing the bike

from

Quoted message said:

side to side, that motion translates to the pedals by the fact that
pushing the bike to the side would raise the forward pedal slightly,

but

Quoted message said:

you are also pushing on that pedal keeping it down, giving an

additional

Quoted message said:

push against the pedal. The extend to which you flex the frame will

only

Quoted message said:

return a horizontal and sideways motion to the pedal when released

(near

Quoted message said:

BDC for the affected pedal stroke), not contributing to forward

motion. It

Quoted message said:

would have to push the pedal backwards at that point to help.

I think precious little of the frame flex is lost as heat within the
frame. The metal just doesn't have much in the way of internal
friction, IOW it has little hysteresis loss.

There is a way to recover some of the minute amount of energy lost in
flex. Consider: What your "losing" is work from your power leg. It
exerts force during a slight extra distance downward. The way this
energy (i.e. stored work) can be gotten back is by helping lift that
same leg when it's on the backstroke, by exerting a minute lifting
force upward.

Lots of people think they actively pull up on the backstroke. From
reports I've seen of measurements from instrumented pedals, they don't;
they may reduce the downward force during the return stroke, but their
rear leg is lifted partly by the force transmitted by their forward
leg. (BTW, altenative pedal mechanisms that rely on consciously
lifting the "rear" leg seem to do badly.)

Anyway, if the front (power) leg is partly helping to lift the rear
(passive) leg, then the recovered frame flex will be helping a bit,
too, as the frame helps "spring" the bottom bracket back toward center.
Minutely, of course.

I think the thing we all agree on is, it's VERY unlikely to be a
significant effect!

Jhas said:

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 . . .

the flex is real. the question is whether you care. if you like what
you have, great. personally, i've come to like stiffer frames, but hey,
i don't like shimmy and stiffer is the way to go in that regard.

Jhas said:

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 . . .

the flex is real. the question is whether you care. if you like what
you have, great. personally, i've come to like stiffer frames, but hey,
i don't like shimmy and stiffer is the way to go in that regard.

In article <[email hidden]>,

David L. Johnson said:

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.
things, then release it, but you still get tired from doing it.

You're describing things as though force to flex the frame is
in addition to whatever force is being applied for pedalling, but it is
not. The force is the same, what adds are displacements. The situation
is not that different from having a spring present between your shoe and
the pedal spindle.

To see where the energy all goes, consider where and when it is
stored. A frame that's been bent has energy stored in it, yes, but how
much it's bent depends only on the force that's applied: how much
heavier you're stepping on one pedal than the other.

This force does not change suddenly. More importantly, it goes
to zero during the transition from the downstroke for one pedal to the
downstroke for the other side. There's no energy stored in the frame
at the top and the bottom of the pedal stroke! Energy from a downstroke
isn't recovered on the upstroke as there's already none left in the
frame when the upstroke starts.

This is not to say that the frame does not participate in energy
transfer. Energy gets put into the frame during the first half of the
downstroke, with maximum stored energy coinciding with the peak pedal
force.

But, what about during the second half of the downstroke? As you
lighten up on the pedal stroke, the bottom bracket flex "unwinds". What's
your foot doing? It's still going down by a certain amount, determined by
the rate at which the chain is moving, but the extra displacement from
flex decreases as pedal force now decreases - this decreasing flex
means that your foot is now moving _LESS_ than it otherwise would. The
work going out the chain during this part of the pedal stroke is more
than what the foot is putting into the system, the difference being
from energy that was stored earlier in the downstroke.

Frame flex does not do anything to consume energy. What it
does do, is act to smooth out the power that gets delivered to the
chain: when you lunge onto a pedal, some of the work you do gets
stored in the frame rather than propel you forward immediately, so
your peak acceleration is reduced compared to a rigid but otherwise
identical frame. But this energy is not lost, with the reduced
acceleration being sustained longer as the frame unwinds. This changes
the dynamics of how the bike moves around under you as you sprint out
of the saddle, but these changes all average out through the course of
a full pedal stroke and make no difference in the amount of work you
ultimately do. A stiff frame will feel faster, but it isn't.

-Luns

Mark Hickey said:

It would take an amazing amount of visual skill to determine the
minute amount of lateral flex in the frame relative to that in the
tires, rims, spokes, fork, crank and bottom bracket. I can easily
push any road bike an inch or two off-axis, knowing full well the
frame itself is probably moving no more than a few mm at most.

It's pretty easy to see on a trainer, it's more than a few mm.

Quoting David L. Johnson <[email hidden]>:

Quoted message said:

Zog The Undeniable said:

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.

I think there is a simple explanation, actually.

When you take the first stroke, it takes a lot of energy to push the BB
sideways. On subsequent strokes, it does not, because most of the motion
is accomplished with the energy stored by the previous stroke.

You never get the energy from the first pedal stroke back, but subsequent
strokes (with the same force) don't do a significant amount of work
flexing the frame.
--
David Damerell <[email hidden]> flcl?
Today is First Aponoia, April.

jim beam said:

Jhas said:

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 . . .

the flex is real. the question is whether you care. if you like what
you have, great. personally, i've come to like stiffer frames, but hey,
i don't like shimmy and stiffer is the way to go in that regard.

Probably the important thing to bring away from this thread (or should
I say the latest incarnation of this thread...) is that the force the
LBS guy is putting on the frame isn't similar to anything you'll
actually DO to your bike while actually riding it (with the possible
exception of those who play bicycle polo).

As Jim (errrr, s'cuze me, "jim"😉 mentions above - if the frame doesn't
flex enough to cause chain rubbing/spontaneous shifting problems and
shimmy, who really cares how much it flexes? I too hate lateral flex,
but realize that there's a point beyond which any further reduction is
pointless. That's why the tubes on Habanero frames are bigger than
steel but smaller than big tube aluminum.

Mark Hickey
Habanero Cycles
http://www.habcycles.com
Home of the $695 ti frame

Luns Tee said:

In article <[email hidden]>,

David L. Johnson said:

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.
things, then release it, but you still get tired from doing it.

You're describing things as though force to flex the frame is
in addition to whatever force is being applied for pedalling, but it is
not. The force is the same, what adds are displacements. The situation
is not that different from having a spring present between your shoe and
the pedal spindle.

To see where the energy all goes, consider where and when it is
stored. A frame that's been bent has energy stored in it, yes, but how
much it's bent depends only on the force that's applied: how much
heavier you're stepping on one pedal than the other.

This force does not change suddenly. More importantly, it goes
to zero during the transition from the downstroke for one pedal to the
downstroke for the other side. There's no energy stored in the frame
at the top and the bottom of the pedal stroke! Energy from a downstroke
isn't recovered on the upstroke as there's already none left in the
frame when the upstroke starts.

This is not to say that the frame does not participate in energy
transfer. Energy gets put into the frame during the first half of the
downstroke, with maximum stored energy coinciding with the peak pedal
force.

But, what about during the second half of the downstroke? As you
lighten up on the pedal stroke, the bottom bracket flex "unwinds". What's
your foot doing? It's still going down by a certain amount, determined by
the rate at which the chain is moving, but the extra displacement from
flex decreases as pedal force now decreases - this decreasing flex
means that your foot is now moving _LESS_ than it otherwise would. The
work going out the chain during this part of the pedal stroke is more
than what the foot is putting into the system, the difference being
from energy that was stored earlier in the downstroke.

Frame flex does not do anything to consume energy. What it
does do, is act to smooth out the power that gets delivered to the
chain: when you lunge onto a pedal, some of the work you do gets
stored in the frame rather than propel you forward immediately, so
your peak acceleration is reduced compared to a rigid but otherwise
identical frame. But this energy is not lost, with the reduced
acceleration being sustained longer as the frame unwinds. This changes
the dynamics of how the bike moves around under you as you sprint out
of the saddle, but these changes all average out through the course of
a full pedal stroke and make no difference in the amount of work you
ultimately do. A stiff frame will feel faster, but it isn't.

that's right for the frame, but if a flexing frame has slightly more
misalignment than a stiffer frame, you can argue about tire squirm,
subsequent hysteresis losses, etc. mostly academic i guess.

Quoted message said:

-Luns

Mark Hickey said:

As Jim (errrr, s'cuze me, "jim"😉 mentions above - if the frame

doesn't

Quoted message said:

flex enough to cause chain rubbing/spontaneous shifting problems and
shimmy, who really cares how much it flexes?

Actually, if you're going to do loaded touring on the bike, you might
care how much the frame flexes. I recall touring side by side with one
of my riding buddies, using identical packs, but me on my Cannondale
touring bike, he on his expensive custom-built steel frame touring
bike. He couldn't enjoy the downhills, because at anything over 25
mph, the entire bike shook badly.

I know shimmy is a tricky phenomenon. But I'm sure that a stiffer
frame would have helped.

"eflayer2" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

Other flex. I was riding one of my bikes the other day and

while

Quoted message said:

climbing, the computer magnet on my front wheel was hitting the

pickup

Quoted message said:

thingy mounted on the front fork. The reason this was a bit

remarkable

Quoted message said:

is that it only happened while out of the saddle and climbing.

All was

Quoted message said:

quiet while cruising on the flats. I had never actually noticed

wheel

Quoted message said:

flex until I realized what was going on. I didn't feel it as I

was

Quoted message said:

climbing, but the noise of the magnet hitting the sensor was

evidence

Quoted message said:

of wheel flex I had never acknowledged previously.

I had the same experience. It is disturbing and makes me wonder
about the long-term durability of low spoke count wheels. This
was the front wheel from my only set of botique wheels. I quit
using the rear wheel and will soon put the whole set on my wife's
bike, which she intends to sell. Probably fine for someone under
160lbs, but not for someone over 200lbs, like me. I have never
had this problem with a set of conventional wheels.-- Jay
Beattie.

That was my original point of this post -- I suspect the whole frame
flex "test" is bull____. The only test that counts is riding the bike.

that's the wheels!
stories like yours are unending
ever read the pirhana club in the daily comics?
i had a shop where i bought my main ride-
once i bought a few spacers and a shop saleswoman ran around screaming
that they had ripped me off as i walked out the door
they tried and maybe did vandalize my bike as i was buying it
i tried buying both headset bearings and they only had one then they
told me that they had screwed me on it and had two...
divinity expletive deleted

On 30 Mar 2005 08:54:19 -0800, "Jhas" <[email hidden]>

Quoted message said:

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.

Most frames a really quite flexible laterally. Here's a trick to try.
Stand next to your bike, with the near crank in the six o'clock
position. hold onto the bars and the saddle and push the pedal
sideways with your foot. You'll be amazed.

jeverett3<AT>earthlink<DOT>net http://home.earthlink.net/~jeverett3

That's exactly the situation I was describing in my original post. As
I said, I think that whole "demonstration" of frame flex is BS.