Frame flex and efficiency

[Tom Ace]

41 wrote:

> Actual measurements of what is the question, and this requires some
> thinking: think twice, measure once. He has done thinking but perhaps
> more is useful. For example, his method ignores flex of BB spindle, hub
> windup, brake flex / front tire compression at horizontal, and other
> such things.

The subject of this thread is "Frame flex and efficiency".
My measurement is of course affected by flex elsewhere as well,
so yes, I got a little off-topic -- but if frame flex can have
a bearing on efficiency, so can flex in the BB spindle and in
other parts. The question of whether good use is made of energy
returned when flexed metal springs back applies to other parts
as well as to the frame.

Of the effects you mentioned, brake flex and front tire
compression are irrelevant to what happens in riding--but
they are not responsible for much of the one-inch pedal
drop I saw in high gear. Sheldon Brown's gain ratio is
just the ticket for getting a handle on this; the high
gear I used has a gain ratio of about 7. That is, the
bike moves forward 7 times as much as the pedal moves.


> He tried with pedal
> at horizontal, it would have been better with pedal vertical (down).

A test with the pedal at 6 o'clock doesn't put tension in the chain.
That test wouldn't be tainted by brake flex, but it could leave out
some of the effects that are relevant in riding.

Tom Ace

 

[Philip Holman]

"41" <[email protected]> wrote in message
news:[email protected]...
>
> Dave Lehnen wrote:
>> 41 wrote:
>
>> > Roughly speaking: for a 60cm fram e, if the line connecting the
>> > seat lug
>> > to pedal/marker is at about a 20 degree angle from the plumb, and
>> > if
>> > the motion were such that the seat lug remained stationary, a 1
>> > inch
>> > drop would corresond to about a 7cm or nearly 3 inch lateral
>> > t ranslation of the BB. In bicycling magazine tests they get it to
>> > move
>> > 2-5 mm when they simulate such forces in a fixture.
>> >
>>
>> Your calculation completely ignores twisting or torsion of the
>> bottom bracket relative to the average plane of the fr ame.
>
> Uh, no it doesn't, even though your explanation is not entirely clear.
> As I said this is a rough calculation but it is what it is represented
> as. Give a calculation of your own of the translation if you don't
> like
> mine.
>
>> Tom has made actual measurements, not speculation based on
>> assumptions of how a frame reacts under load.
>
> Actual measurements of what is the question, and this requires some
> thinking: think twice, measure once. He has done thinking but perhaps
> more is useful. For example, his method ignores flex of BB spindle,
> hub
> windup, brake flex / front tire compression at horizontal, and other
> such things. That he could get a quarter inch difference just by
> changing gears show that these have a real effect. He tried with pedal
> at horizontal, it would have been better with pedal vertical (down).

How does one measure the advance of a pedal weighted in the down
position?

>I can't compare if only because I weigh so much less and my frame is
> heavier gauge.
>
> And as I said bike magazines which test for the same in jigs get only
> a
> few mm deviation at the BB.
>
Are you saying their measurements are more valid than Tom's? Was that a
right of left deflection? Right pedal loading always results in more
deflection due to chain tension. Tom provided 2 data points for the
deflection. From this, a zero torque defection can be calculated. Its
approx .87". I'll wager this was a right pedal deflection.

Phil H

 

[Jim Adney]

On Mon, 08 Aug 2005 22:25:03 -0600 [email protected] wrote:

>On Mon, 08 Aug 2005 23:13:14 -0500, Jim Adney
><[email protected]> wrote:
>
>>On 7 Aug 2005 21:43:41 -0700 "Tom Ace" <[email protected]> wrote:

>Dear Jim,
>
>[I asked}
>
>>> Does this drop take into account the flattening of the tires
>>> with the load?
>
>[Tom Ace replied]
>
>>The one inch doesn't include the tire squashage.
>>The pedal drops by a little over an inch relative to the floor.
>
>I couldn't resist repeating "squashage"--some coinings are
>too good to ignore.

Yes, an interesting coinage.

Having followed this thread further, I gather that the original
measurement DID include squashage. I'm puzzled by the further test,
however, where he did the same test while sitting on the bike and thus
removing squashage from the measurement. It seems he got the same
number.

I wonder if it's possible that the pen is recording the tire squashage
that occurred when he climbed on the bike, as well as the frame flex
when he pressed on the pedal? Otherwise, it's hard to understand how
the 2 measurements could be the same.

-
-----------------------------------------------
Jim Adney [email protected]
Madison, WI 53711 USA
-----------------------------------------------

 

[41]

Philip Holman wrote:

> Are you saying their measurements are more valid than Tom's?

They are more direct with fewer additional variables, if the question
is frame flex only. What I want is to reconcile the two results and
understand why they differ. BB spindle flex and motion in the bottom
bracket may be the most likely candidates. To make more explicit the
basis of my earlier calculation, it was based on the deflection being a
purely linear one from a fixed seat cluster. At the other extreme, if
it was a purely rotational one about the BB, no horizontal translation
would result. Clearly the reality is somewhere between these two
extremes; 1-1.5"? Still a lot.

Here's another possibility: the frame is about to break! No I don't
advocate that as the answer, the point is there are many potential
factors and "distributed experimenting" is a hard way to sort them out.

 

[Philip Holman]

"41" <[email protected]> wrote in message
news:[email protected]...
>
> Philip Holman wrote:
>
>> Are you saying their measurements are more valid than Tom's?
>
> They are more direct with fewer additional variables, if the question
> is frame flex only.

I recognize the difference between frame only flex and the deflection
measured by Tom. The subject is "frame flex and efficiency" but the OP
referred to the flex he was noticing when climbing on a steep grade.
Right or wrong, I have chosen to substitute "Frame" for "Bicycle". While
frame stiffness is a component, the flex he was experiencing was more in
keeping with Tom's measurement. Absolute deflection, force and
deflection at the pedal (load interface) are useful for calculating
overall efficiency. I assume your 2mm to 5mm measurement is lateral
deflection of the bottom bracket shell; a useful measurement for
comparing relative frame stiffnesses and estimating chain rub
probability. However it is not as useful for determining rider input to
calculate stored energy.

>What I want is to reconcile the two results and
> understand why they differ. BB spindle flex and motion in the bottom
> bracket may be the most likely candidates. To make more explicit the
> basis of my earlier calculation, it was based on the deflection being
> a
> purely linear one from a fixed seat cluster. At the other extreme, if
> it was a purely rotational one about the BB, no horizontal translation
> would result. Clearly the reality is somewhere between these two
> extremes; 1-1.5"? Still a lot.

Tom's 1" deflection at the pedal would need to be converted to a BB
shell lateral flex which would be a lot less. Like I've said in previous
posts, left BB deflection is greater, being assisted by chain tension
and conversely, right deflection diminished for the same reason.
Averaging the two would be more accurate.

>
> Here's another possibility: the frame is about to break! No I don't
> advocate that as the answer, the point is there are many potential
> factors and "distributed experimenting" is a hard way to sort them
> out.

I would speculate that frame flex accounts for 50% of Tom's measurement.
When this is translated to lateral deflection at the BB it is likely
that this will be further reduced to observed frame test results
(Captain Obvious at your service). Even so, Tom's frame does appear to
be on the flexible end of the spectrum.

The bottom line on all of this is power loss is roughly .3%. Like Jobst
has stated, handling is the most significant factor.

Phil H

 

[41]

Philip Holman wrote:

> Right or wrong, I have chosen to substitute "Frame" for "Bicycle".

? Bicycle for frame?


> overall efficiency. I assume your 2mm to 5mm measurement is lateral
> deflection of the bottom bracket shell;

Yes


> left BB deflection is greater, being assisted by chain tension
> and conversely, right deflection diminished for the same reason.

Reverse?


> The bottom line on all of this is power loss is roughly .3%. Like Jobst
> has stated, handling is the most significant factor.

I'm skeptical of being able to get a good number, but agreed that must
be low and that handling is the problematic area..

 

[[email protected]]

Tom Ace writes:

> In the stationary-bike demonstration, I can lower the outside of the
> left pedal by about one inch. This is with a Columbus SL frame
> (from the late 1970s) and 170mm cranks, and I weigh 85kg.

> The one inch doesn't include the tire squashage. The pedal drops by
> a little over an inch relative to the floor.

OK, so I did the same thing on my bicycle with SPD shoes and cleats,
the 50-16t gear engaged and the rear brake firmly held. Pushing down
on the right pedal and pulling up statically as hard as I can in the
seated position, I achieved a mark from a pen attached to the right
pedal on the fixed piece of paper 5mm long or 0.197". For me that's a
long way from one inch and also insignificant.

I think you have a soft frame, literally and it was that way when new.

Jobst Brandt

 

[Philip Holman]

"41" <[email protected]> wrote in message
news:[email protected]...
>
> Philip Holman wrote:
>
>> Right or wrong, I have chosen to substitute "Frame" for "Bicycle".
>
> ? Bicycle for frame?
Yes
>
>
>> overall efficiency. I assume your 2mm to 5mm measurement is lateral
>> deflection of the bottom bracket shell;
>
> Yes
>
>
>> left BB deflection is greater, being assisted by chain tension
>> and conversely, right deflection diminished for the same reason.
>
> Reverse?

No, try it.

>
>
>> The bottom line on all of this is power loss is roughly .3%. Like
>> Jobst
>> has stated, handling is the most significant factor.
>
> I'm skeptical of being able to get a good number, but agreed that must
> be low and that handling is the problematic area..
>
Phil H

 

[41]

Philip Holman wrote:
> "41" <[email protected]> wrote in message

> >> left BB deflection is greater, being assisted by chain tension
> >> and conversely, right deflection diminished for the same reason.
> >
> > Reverse?
>
> No, try it.

The reason I posed this is because what you said previously sounds like
the reverse:

#Right pedal loading always results in more deflection due to chain
tension.

Is it?a

 

[Philip Holman]

"41" <[email protected]> wrote in message
news:[email protected]...
>
> Philip Holman wrote:
>> "41" <[email protected]> wrote in message
>
>> >> left BB deflection is greater, being assisted by chain tension
>> >> and conversely, right deflection diminished for the same reason.
>> >
>> > Reverse?
>>
>> No, try it.
>
> The reason I posed this is because what you said previously sounds
> like
> the reverse:
>
> #Right pedal loading always results in more deflection due to chain
> tension.
>
> Is it?a
>
Well, right pedal loading causes left deflection of the BB. Confusing
isn't it

Phil H

 

[Tom Ace]

[email protected] wrote:

> OK, so I did the same thing on my bicycle with SPD shoes and cleats,
> the 50-16t gear engaged and the rear brake firmly held. Pushing down
> on the right pedal and pulling up statically as hard as I can in the
> seated position, I achieved a mark from a pen attached to the right
> pedal on the fixed piece of paper 5mm long or 0.197". For me that's a
> long way from one inch and also insignificant.

Did you try pushing and measuring on the left also?

In any case, thanks for taking the time to do the test.

5mm is less than 2% of the pedal stroke. I agree that's
not particularly significant. Of course, some people spend
big $$$ to get 1% sized effects, but that's their business.

> I think you have a soft frame, literally and it was that way when new.

I'd referred to the time of manufacture only to help identify
the vintage of material it was made from. I also doubt that
it has changed noticeably since it was new.

It's not the frame I do most of my riding on nowadays.

Tom Ace