's Forum Archives - General

Archive Home >> General(1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 )

my bb flex theory confirmed(55 posts)

my bb flex theory confirmedishmael
Feb 24, 2003 3:30 PM
I've always thought that flex in the bb wasn't necessarily a bad thing. it makes sense, to me, that the energy pent up would just be released later in the pedal stroke, possibly helping to pull through the dead zone, or atleast making for a smoother out of the saddle ride. I never experinced this in my short riding career but I now know someone who has. He said he had an old cannondale and the stiffness made climbing out of the saddle less fluid. He prefers steel because of the flex, feels he can get into a better rythem.
Keith Bontrager.czardonic
Feb 24, 2003 4:07 PM

FWIW, I don't think you can assume that the "energy" will be released in a way that will aid your stroke.
Leave it to Keith to give us way more info and logic...greg n
Feb 25, 2003 7:49 AM
than one can digest in one sitting. That's why he's one of the best.
So this is how you confirm theories?Kerry
Feb 24, 2003 5:09 PM
Not with actual data, a theory supported by science, or an engineering calculation, but with a completely subjective comment from some random rider. Now we're on to something! Some other guy thinks I'm right (AKA - "It is generally known that . . ."). Not to be confused with "It is known that . . ." (I think). Clearly this is a better approach than studying the issue and digesting the existing knowledge.
Am I missing something here?js5280
Feb 24, 2003 6:05 PM
Keith Bontrager isn't exactly "some random rider." He was inducted into the Mountain Bike Hall of Fame ('94), started his own bike company in 1981, designed racing motorcycles prior to that, and is one of the world foremost experts in frame and component design. Pick any Trek, even Lance's, and you'll probably find at least one Bontrager component on it. Don't believe me, go here: I'm pretty sure he's got engineering degrees to back up his experience but don't have that info off hand.

He's also one of the nicest people I've ever met. I raced with his wife, Laura, in the 2001 24 hours of Moab and basically spent that weekend with whole Bontrager family. Nicest people you'll ever meet. Keith's Master team placed first so he's a hell of rider to boot. Did I miss the sarcasm or was your comment about someone else?
Keith & Engineering degree ...sacheson
Feb 25, 2003 12:14 PM
I think he fully admits to lacking any formal degree for Engineering, but I think you'd agree that his experience and success with his work more than compensates for doing endless calculations for a grade.
Yes, you are!Kerry
Feb 25, 2003 4:35 PM
If you look at the structure of the thread, I was responding to the original post, not to the Keith Bontrager comments. My reference was purely to Ishmael's typical lack of logic in his conclusions.
how bout respond to the fellow at the bottomishmael
Feb 25, 2003 5:51 PM
and tell him about his lack of logic. And who said I had to be logical. I'm just passing on a theory of mine that has unsolicited supporters and is obviously contestable. And if youre so logical, how bout you give me some of your logic.
what the hell are you talking about?the bull
Feb 24, 2003 5:46 PM
How is the release of torsion going to pull your bike forward while it is in the "dead zone".
re: my bb flex theory ... my 2c ... flame me laterMrDan
Feb 24, 2003 5:56 PM
You are contributing energy to laterally flex the BB/frame with each pedal stroke. That energy is completely lost. It does not matter that the frame is elastic and comes back to it's "neutral" position. That energy is lost forever as heat/friction within the structure. Your friend may indeed enjoy the flex and rhythm(sp?), but guaranteed it doesn't make him more efficient getting up a hill. You want all your energy to be used propelling the bike forward. It's perfectly fine that he/she prefers steel. There are also very stiff steel bikes I presume. It's all how they're engineered. You could make an incredibly stiff steel chainstay/BB/downtube combination, but it's likely to weigh alot more than an AL/Fiber setup. Steel was the original bike material because nobody had developed the technology of using AL or fiber for bikes. Progress shows us different ways to accomplish things. I'll rant more if you like...
re: my bb flex theory ... my 2c ... flame me laterHeron Todd
Feb 24, 2003 7:18 PM
Mr. Dan,

Two comments:

1. When a frame flexes at the BB, it is being loaded torsionally, not laterally. The force being applied is trying to twist the downtube.

2. Technology has indeed given us new materials, but that doesn't inherently mean that the products are better. AL is used today primarily because it is cheaper to manufacture with AL now than it is with steel. Notice all of the aluminum adult Huffys at the department store.

Similarly, manufacturers can now purchase carbon forks from Taiwan at a price quite competitive with a good, crowned steel fork. Incidentally, the Taiwanese CF forks are a fraction of the cost of Kestrel, Time, Look, etc. The growing popularity of carbon stays is also due to these decreases in manufacturing cost. A manufacturer today only needs to call Taiwan for some inexpensive stays and glue them into their frames. No R&D is required, and the cost is comparable to metal stays.

So, yes, there is progress being made in the bicycle industry, but that change is not necessarily to a *better* product. Often, it is simply to one that is more cost-effective to produce and market.

Todd Kuzma
Heron Bicycles
LaSalle, IL
alright, some flame ... not trying to start a religous war...MrDan
Feb 24, 2003 8:43 PM
Hi Todd,
Agreed that the bike industry is looking for more efficient "profit" machines... that's the nature of all free commerce. Ultimately though, what works well for the customer will succeed. Pound for pound, AL is stronger than steel or fiber, therefore for equivalent strength the AL should always be lighter. I know you don't want to hear this, but the Al/fiber combination is going to be very hard to beat unless it proves itself unreliable. You see the Italian bikes using Al/fiber all the time now... also the bike industry is like any other consumer market... new and different keeps people buying. I for one am still riding my 20+ yr/old retro Taiwanese Lotus, and I KNOW it's more reliable than the new shifter/brake combos. However I'm going to buy a new bike finally (I'm dovorced now the $$ is MINE again!) and it will be either a Zurich or Klein, but ultimately the decision will rest most on how I feel after a couple hours in the saddle of each including climbing, descents, rough roads etc.

I can't agree that the downtube major mode is in torsion, but there is a slight "twist" ... the major mode is shear I'd guess (ok flame me!). This is why you see the downtube of most Al bikes is ovalized along the axis of the BB to increase the shear moment which resists the lateral shear of the tube. ((I don't know why this wasn't done with steel bikes, but I'd guess it was asthetic heracy to do so, and would be diffucult with lugged technology. Ovalizing and welding/joing is probalby alot easier with Al?)) I say this from the standpoint that the tire patches on the ground are my anchors so that when I push down on the right pedal, the entire frame wants to twist clockwise (from the riders viewpoint) relative to the head tube. I could probably make the strongest bike by using I-beam and X-beam shapes, but it would be ugly as hell - I'd have to put cosmetic tubing over the things to make it saleable. Someday maybe I'll have the luxury of working out the stress properties of frames, but those days are behind me... I'd rather ride.
alright, some flame ... not trying to start a religous war...xcandrew
Feb 25, 2003 3:39 AM
The major stress mode for the down tube about 2/3 torsion with the remainder being out of plane bending. Seat tube about 4/5 out of plane bending, the rest torsion. See

That's why I've always admired the Ritchey frames, with the lower part of the seat tubes ovalized, and the down tubes round...
alright, some flame ... not trying to start a religous war...Heron Todd
Feb 25, 2003 8:19 AM
>This is why you see the downtube of most Al bikes is ovalized along the axis of the BB to increase the shear moment which resists the lateral shear of the tube.

Talking to a number of framebuilders I found that ovalization is done primarily to fit oversized tubes together. Downtubes are horizontally ovalized at the bottom so they can be welded to the BB shell. Top tubes are vertically ovalized so they can be welded to seat and head tubes.

This isn't done very much with steel frames because the tube diameters are generally smaller so it is not needed. However, the new True Temper S3 tubing is ovalized for this very reason.

Todd Kuzma
Heron Bicycles
LaSalle, IL 815-223-1776
A rebuttal.JS
Feb 24, 2003 8:59 PM
1. when a frame flexes a myriad of loads are being placed all through the frame, I've seen FEA plots, you can probably find some if you do a internet search.

2. I'm in the industry and I can tell you that it is still cheaper to make a low grade steel frame than a low grade Al frame. Al is used in adult Huffy's because it is a selling point, just like those lame low end suspension designs.

Asian composite products run the spectrum from junk to very nice, depending on how much you want to spend. I find it amusing that you compare Taiwanese carbon to Kestrel, Time and Look because all three of the companies sell composite forks manufactured in Taiwan. As an aside, it is actually proper to refer to "carbon fiber" products as composites because almost all contain more layers than just carbon fiber fabric.

Finally, steel is a viable material for bicycles but it's achilles heel is it's density. Because a bicycle frame has to be a certain size and shape, you have to connect the headtube to the bottom bracket and seat tube then to the rear dropouts, no matter how strong you make the steel the density never changes so it's gonna take a certain amount of material to make those connections. Lately manufactures have been producing rediculously thin steel tubesets that are more fragile than any carbon or aluminum frame. with all this they still can't get the weight into the 2 pound range where Carbon and Al go with ease. In cycling weight is always gonna be an issue( deal with it)and steel just can't compete in that area.
A rebuttal.koala
Feb 25, 2003 3:15 AM
Agree with much of what you say, but my ridiclously thin foco frame is no more fragile than a thin tubed aluminum tubeset like my riding buddies Giant. It probably will dent easily but I think thin walled aluminum will too. Carbon certainly has a threshold where it fails in such a manner that can be catastrophic, whereas when steel fails there is usually some warning. Seems frames in my size are lighter by one half to three quarters of a pound in al or composite, my body weight varies that much on a day to day basis.
A rebuttal.Heron Todd
Feb 25, 2003 8:22 AM
>I find it amusing that you compare Taiwanese carbon to Kestrel, Time and Look because all three of the companies sell composite forks manufactured in Taiwan.

Ah, yes. What I intended to say was that manufacturers can get no-name or house brand carbon forks much cheaper by buying direct from the Taiwanese factories. I was offered carbon forks as cheap as $30 at Interbike. I would not be able to get a name brand fork for anything close to that.

Todd Kuzma
Heron Bicycles
LaSalle, IL 815-223-1776
Todd you are biasedbigrider
Feb 25, 2003 7:11 AM

I am a big fan of steel but it is hard not to be biased toward steel when that is what you are working with day in and day out.

If you factored out all other variables and focused on comfort, weight, and stiffness I think the obvious winner is carbon in the road world. What I am saying if Price isn't a concern and you take the best designs and manufacturing of both materials carbon is the winner. I think steel can match everything carbon does with proper design and workmanship EXCEPT ONE WEIGHT. With all that said I don't care about the extra 1 pound of weight that much and that is why I have a two steel bikes and only one carbon.

From a financially viable angle, you can't ignore the carbon/aluminum setups that are being sold cheap, are light, and apparently ride very well.
Todd you are biasedHeron Todd
Feb 25, 2003 8:28 AM
I don't disagree that good frames can be built out of any material. There are certainly great aluminum and carbon frames available. I'm currently working on introducing a titanium product. All I am saying is that the industry is not just driven by offering the "best" product. It's driven by offering the product that is easiest to sell at the lowest price.

Sometimes, the product design is all about getting the lowest cost possible. Sometimes, the product will include some extra features, which might cost more to do, because it will help to sell/market the product. These extra features may or may not represent a real benefit to the rider.

FWIW, I offer an Easton EC90 carbon fork option on the Heron Road frames. I have the ability to add carbon stays at very little extra cost but have decided not to do so as they don't offer any real benefit to the rider beyond looking cool (which is a valid reason but not enough for me).

Todd Kuzma
Heron Bicycles
LaSalle, IL 815-223-1776
Thanks for your input, but quit SPAMMING. nmSpunout
Feb 25, 2003 10:17 AM
Not spam. A knowledgable person is discussingOldEdScott
Feb 25, 2003 11:58 AM
an issue of interest to the board.
some may be lostishmael
Feb 24, 2003 8:16 PM
but obviously not all. The frame has some potential energy that will pull the bb back to its original position. True? Some energy is surely lost, but how much? And since the human body is not designed to pedal cranks, maybe the drawback of lost energy is made up for by an improvement in ergonomics...think, lots of energy is lost when pedaling that could be better used if the bicycle was better suited to the human body or vise versa. those new cranks that at least one pro team uses, I think they're called Roter cranks, supposedly produces a ten percent increase in something or other.
ishmael knows everything...he worked at an LBS for a whole day! nmdavet
Feb 24, 2003 8:31 PM
thats right big guyishmael
Feb 24, 2003 8:45 PM
and I'm ready to take you on in the bicycle knowledge category. Or do you only say that sentence over and over.
thats right big, the gloves are off Izzy?davet
Feb 24, 2003 9:47 PM
You are a font of half-baked knowledge, with a smidgen of background. Reading your posts is as amusing as watching a monkey trying to f**k a football.
What you need is a good set of Biopace rings! nmrwbadley
Feb 24, 2003 8:35 PM
some may be lostMrDan
Feb 24, 2003 8:51 PM
Yes, the frame is elastic so it returns, but the energy YOU put into flexing it is wasted... gone forever, and hence your body will need to supply more energy going up the same hill at the same speed than you would if the frame did not flex.
re: some may be lostcyclopathic
Feb 25, 2003 6:34 AM
we're talking about very small fruction. Ssteel is very efficient, not that much enrgy lost by flexing steel spring.
Mr. Dan you are right but wasting your timebigrider
Feb 25, 2003 7:02 AM
We had this post several weeks ago and everything you say is correct and backed up by physics, logics, and experts.

But, Sheldon says you regain the flex, but my friend can't feel the loss, but the tubes don't feel hot, but , but , but

I know that you can't get the losses back, you can't restore energy in your legs, bobbing doesn't convert to circular pedal motion to propel you forward,

but if everyone believes the truth the post would be boring.
re:Mr. Dan you are right but wasting your timecyclopathic
Feb 25, 2003 7:35 AM
you can talk forever, but until there're some numbers given this is nothing but talk. Suppose steel frame looses 3 times more energy then Al but if Al frame wastes 0.5w and steel 1.5w who cares?

It is obvious that losses are miniscular. If they were in range 3-10w it would have been easily detected and we wouldn't have this deja vu all over again.
point takenbigrider
Feb 25, 2003 8:03 AM
Quantifying such a complex issue as this is not a straightforward or easy task. If the losses are minimal then you are right we should argue for fun only.

However one variable which is real in this issue is big riders like myself notice the flex much more because we have the potential to create more flex. We output more energy to move the greater weights up the hills. So what a light rider might pass off as neglible may be a bigger deal with larger riders. To add another non quantifiable experience into the discussion every rider I have talked to that switched from steel to carbon (trek 5200( and weighed more than 200 pounds raved about the gains experienced climbing ( the sample size was 4)

I didn't mean to restrict my comments to carbon only. There are some steel and aluminum designs out there that provide great stiff climbing rigs. I was comparing vintage steel 70s design to trek 5200 carbon. There is a huge difference in these two rides.

The other point that I wish to make that is sort of in agreement with what you said is if I am riding flats what do I care about whether the frame weighs an extra pound. That is neglible to me. If you are riding an aluminum rig that has shaped tubes and is light I am almost sure that you wouldn't feel a diffence in the energy loss because there probably isn't a difference.
I am light and I used to flex BB alotcyclopathic
Feb 25, 2003 8:45 AM
until I found that hamstring on one leg quad on other were ~20% stronger then corresponding counterparts. Didn't take long to correct it.

Yes some riders probably loose more then others due to muscle imbalance, diff Q factor, frame design possibly weight etc etc etc. Climbing will be affected by weight, mood, phase of moon and most importantly rider perception. Gee put me on nice new bike and I'd climb better.

Still if the difference was considerable riders and mfg would have addressed it long time ago.
re: my bb flex theory ... my 2c ... flame me laterxcandrew
Feb 25, 2003 3:12 AM
I disagree that the energy used to flex the frame is completely lost. Imagine what happens when you apply a load with the bike at a standstill (brakes on, or against a wall). The stress applied to the frame through the pedal and crank system will flex the frame so that the pedal will move a little bit, say downwards 5 mm from a horizontal position. When you remove the load, the elasticity of the frame PUSHES the pedal back up 5 mm. The pedal pushing against your foot is indistingishable from your foot pushing against the pedal (Newton's 3rd law), so that energy goes into propelling the bike. The same thing happens when the cranks are rotating, it is just easier to visualize stationary. There is some energy lost because no spring system can return 100% energy (or perpetual motion devices would exist), but much of that energy is returned. There isn't much damping in frame materials - frames behave like a spring and will vibrate, so not much energy is lost as heat and I would guess that more energy is returned than lost. Some more energy can be lost if there is no load on the pedal when the frame returns to "neutral" - basically, if you stop pedaling, just like if you load a spring and step off to the side before it springs back.
energy bits... I'm just a new guy... oh and Todd...MrDan
Feb 25, 2003 9:30 AM
Ok, I'm promise I won't post another thing regarding this after what I say here! You make a good point, but try this experiment to learn the difference between force, energy and work. Take your bike and flex the frame from 1/2 to 1 inch by pushing on the BB with your foot. Hold it there. Hold it there for 5 minutes. Your total work output does not increase after the initial flex movement, but in the interim 5 minutes or so, you feel your muscles beginning to strain. The muscles must use energy to create the force to keep the frame flexed. Ok, now take your foot off the BB, and yes the frame comes back to it's lowest stress position.
My point is that the muscle energy you use to flex the frame saps away from the energy you could use to propel the bike forward. The energy exerted in that 5 minutes is never coming back to propel you forward, or more relevant the hundreds/thousands of times you flex the frame going up a hill...

Todd, I understand where your coming from, but in reality it's quite often that cheaper ($ wise) does not mean inferior. The Al/fiber stay thing is here for quite a while, just like steel was/is. Who knows, in 5 years both Steel and Al/fiber bikes may be replaced by Beryllium(sp?) or something else. (Though I think Be is very toxic!) (I have no experience with Ti, but I hear it really isn't very good as a bike frame material, sure it's lighter than steel, but the steel is more practical, durable, stiffer, and CHEAPER! - You can sell anything to anybody if you can convince them it's better - as I said I'll either go with the steel Lemond, or the Al/fiber Klein. Any more $ than that is pointless for my own needs - and Lance can easily win the TdF on either of them. I personally don't like the fragility of the all fiber frame. Fact is the more road buzz a bike dampens, the less energy is spent on counteracting that buzz with clenched muscles and that's what fiber does best.

The beauty of what custom builders do is that those who can't properly fit a factory frame can get what they need and thats what you guys provide - there will always be a market/need for it. Personally I'll never need to go through that expense, I fit factory bikes perfect. Perhaps that makes ME cheap(er), but hopefully not inferior! Well, my spelling these days does leave something to be desired.. :) And my body can stand some more improvement as a bike riding powerplant... :)

ok, now go easy on me I'm the new guy ... I promise I'll be good now! This has been fun/interesting - really not trying to put anyones nose out of joint, and I hope I didn't do that!
energy bits... I'm just a new guy... oh and Todd...xcandrew
Feb 25, 2003 2:11 PM
I know very well the difference between force, energy, and work. My example was static just to help you visualize how some of that energy might (and does) return as a force against your pedals. I don't know why you bring up isometric work since that is not what happens when you are pedaling except at a virtual standstill.

Imagine that you are pedaling normally now. We should first agree that riders do not apply a constant force against the pedals, but rather a pulsating one with the greatest force coming when the cranks are approximately horizontal - if you graph the force against one complete crank revolution (position), you will get two more or less smooth peaks corresponding to the left and right horizontal crank position connected by a curvilinear line. Now if you were to measure the flex of the frame while going through one crank revolution with strain gauges (to measure BB out of plane deflection), you could graph it and get a smooth curve with peak strains on to the left and to the right in reaction to the force applied to the pedals. In other words, the frame does not flex all at once and it does not return to "neutral" instantaneously. Look at a bike from the right side. As the pedal moves from 12 'o clock to 3 'o clock, the strain on the frame (defined as the horizontal movement of the bb) gradually increases - you are loading the "spring". As you pass 3 'o clock and move to 4 'o clock, the force you are applying to the pedals in a normal stroke is decreasing. At the same time, the stress on the frame is also decreasing and the the frame is alreading moving back ("unloading the spring"). There is no delay in it moving back towards neutral - the strain on the frame is directly related to the force applied to it at that instant, like a spring. As the frame is moving back towards neutral from 3 to 4 'o clock, it is putting most of the force that was used to flex the frame the same distance in the opposite direction directly back against your foot on the pedal like the static example that I gave you earlier. As so it goes for the rest of the complete pedal stroke. As long as you are still pedaling (having not stopped pedaling when the frame is "returning"), that force against your foot goes into work propelling the bike.
ishamel shoots holes in phyics, gains professorship to MIT! nmdavet
Feb 24, 2003 9:50 PM
Don't worry Ishmael ...MrDan
Feb 24, 2003 10:27 PM
It's ok not to know physics. Engineers and physicists alike have their share of rude, opinionated, and insecure among their ranks like any other ...
Worry Ishmael ...53T
Feb 25, 2003 8:48 AM
It is OK to not be expert in Physics, if you have to do any deep thinking, you can hire me. However, the scientific method (observe, hypothesize, experiment, conclude) is very important in seperating modern humans from the dark-ages savages that we once were. Without the scientific method there is only religion to maintain order.
So, what you are saying is that Shimano IS BETTER than Campy...funknuggets
Feb 25, 2003 9:53 AM
A thought experiment....speed-chump
Feb 25, 2003 10:47 AM
So.... those hand excercisers - the ones with the
torsion springs that you squeeze.... it doesn't
take any exertion or effort to use those, because
they spring right back to where they started. After
all, they are made of steel which has negligible
internal damping. They don't really get hot when
you use them, so all the energy stored in them must
be coming back out, right? So if you get back all
the energy you put into it, it must be totally
efficient and not take any effort to operate. All
the effort you use to squeeze them comes back to open
your grip!

A bike can be thought of as a conservative mechanical
system, but the human body is NOT a conservative system.
Add a bike to it, and it still isn't. Just because the
energy stored in the frame is given back doesn't mean it
is given back in such a way as to provide propulsion.
The energy spent swinging the BB from side to side could
just as profitably been spent squeezing a hand excerciser.
As far as both the bike and the excerciser are concerned
all the energy IS released. But nobody has yet convinced
me that it is released in such a way as to do anything

Consider the timing and phasing of the actions, and remember
that the human body is NOTHING like an engineer's mass/spring/
dashpot model of a dynamic system. Anybody in the house
in biomechanics? I'd like to know how the body IS modeled
in dynamic systems.
speed chump you are an example geniusbigrider
Feb 25, 2003 10:57 AM
You picked the perfect example to prove the point. I am soooo jealous I didn't think of it first.

The only way to better it is to say the guy is using the hand springs while he is running and when he squeezes his right hand the left hand is opening which is shooting energy into his right leg and propelling him forward.
BB flex is badjustina
Feb 25, 2003 1:00 PM
it's a rotation in a plane orthogonal to the plane of peddling motion and centered at the same point as the peddling rotation, at no point in time can it transfer energy to the peddle stroke. Even were it somehow possible to violate this physical absolute, at the dead point in a pedal stroke, when the BB is momentarily unweighted the BB returns its energy with a linear vector that is perpendicular to that of the peddling force. That said, what some people may experience is the fact that the flex back does possibly aid in shifting the rider's weight to the opposite foot which is now poised for the down stroke, creating the sense of rhythmic efficiency some feel. Since cyclists tend to lack upper body strength relative to leg strenght, storing up a little energy in the power part of the stroke to give a little extra bump to the weight transfer might make the whole process of out of the saddle climbing feel easier.

BB flex is badxcandrew
Feb 25, 2003 1:29 PM
>it's a rotation in a plane orthogonal to the plane of >peddling motion and centered at the same point as the >peddling rotation, at no point in time can it transfer >energy to the peddle stroke. Even were it somehow >possible to violate this physical absolute, at the dead >point in a pedal stroke, when the BB is momentarily >unweighted the BB returns its energy with a linear vector >that is perpendicular to that of the peddling force.

This is not true. If this was so, the frame would not flex in the first place. If you simplify the result so the the frame flexing back cannot put force back into the pedals, you must also simplify that the frame cannot be flexed by pedaling in the first place. This is Newton's 3rd law (equal and opposite forces).

You are also over simplifying the motion of the frame flex. It does not return when the "BB is monentarily unweighted". It is constantly moving in reaction to changes in force applied to the pedals. See my replys to MrDan above.
what about this exampleishmael
Feb 25, 2003 1:15 PM
what about jumping on a trampoline? the energy put in to the tool(the trampoline) allows you to jump higher than you would be able without the tool. Energy may be lost but the energy that is used is directed in a better way than the unasisted body could use it.

about being a bigger rider, the guy who was telling me how he liked the flex in the bb is 6 foot 7 and not skinny. He said he noticed lots of flex, but the flex seemed to work with his body better.
Put toestraps on the trampoline.....speed-chump
Feb 25, 2003 1:51 PM
Anyway, for what it's worth, think about this -
How do you get maximum height jumping on a
trampoline? You exert a DOWNWARD force by extending
your legs (jumping) while the bed is springing UPWARD.
Now at some point you have to bend or retract your
legs to prepare for the next jump. What happens if,
on the next jump, you leave your legs extended upon impact, then RETRACT them as the bed is springing upward? You
come to a dead stop, don't you? Where did all the
energy go? Your BODY absorbed it, not the trampoline (or

My point here is that TIMING is crucial. In both
instances the trampoline releases all of it's stored
energy, and though it helps you get height in one
scenario, it brings you to a dead stop in the other.

If you apply this analogy to the bicycle you will
see that the second scenario is actually the proper
analogy for a bike. You are extending your leg as
the bed goes down (BB deflects) then retracting your
leg as is springs back. This is EXACTLY the action you would use on a trampoline to COME TO A STOP.
Sorry for all the CAPS, they look obnoxious.... (nm)speed-chump
Feb 25, 2003 1:57 PM
All CAPS is yelling/obnoxious, cap word is just 'emphasis' of a point ,IMO... (nm)wasabekid
Feb 25, 2003 2:12 PM
not sure what you are mr.dan are sayingishmael
Feb 25, 2003 3:13 PM
but, tell me if you follow/agree with what I'm saying. a tool can direct energy. A car, bicycle, whatever, are all examples of tools that can direct energy towards a purpose. The body also is a tool. the combination of the body and another tool is often not the most efficent fit. For example- the bicycle is the fastest human powered land machine but it only really utilizes the legs. The bicycle, although the best thing we've got, is far from a machine that is able to fully utilize the human body's energy. Funny that it's better than running even though running is what we were breed for since the dawning of time. But back to the point. I think the problem of the human to bicycle transfer of energy is largely in the ergonomics of the bicycle. Cranks spinning in a circle arent the best, there's the dead spot, besides that it only utilizes a few muscles and requires others to waste energy sitting in a certain positon. I dont know what would be a better fit for the body than the bike but I'm sure there must be one. So, I'm tring to have a point. This is it- think of how awkward it is riding a bike up a hill out of the saddle(bike swinging side to side, arms, neck, and everything else used while not directly helping the forward movement. The main problem of the bicycle isn't stiffness of the design, it's the design itself, it's not ergonomic. So when someone tells me they feel more natural with flex in the bb, that "more natural" I equate to a more ergonomic fit. And that's, in my estimation, more of an issue than flex. That's my argument anyway, and it makes sense to me.
Feb 25, 2003 8:33 PM
See my replies and "carcass"'s replies. I see that agree that there is a push back in the pedals, but you believe that it is not happening at a time where that energy can be directed back into the chain. Remember that the frame is not some slack thing with a delayed reaction - it pushes back at the same time as you push against it, Newton's third law. As you go through a pedal stroke and your force on the pedals increases, say from 1 'o clock to 2 'o clock (looking from the right), the frame deflects at the same time to the other side. As soon as you reach your maximum force on the pedals, say at 2.5 or 3 'o clock, the frame is at maximum deflection and ready to start pushing back. At that same moment, say 3 'o clock, it is pushing back with it's maximum force, as is with the nature of springs... the greatest force is at the greatest deflection. As you get to 4 'o clock, it is pushing back with lesser force because it has already "sprung" back partially. By the time you get to 6 'o clock, the frame is already in a neutral position - after all, it is going to be flexed in the other direction very soon by the left power pulse. You say:
> You are extending your leg as
the bed goes down (BB deflects) then retracting your
leg as is springs back.

This is clearly not the case.
what about this example .. breaking my vow of silence...MrDan
Feb 25, 2003 1:52 PM
take the trampoline and stand it on end... now stand with your shoulders at a right angle to the trampoline canvas ... then extend the arm adjacent to the trampoline canvas ... lean into the canvas and push yourself back to your upright standing position... how far forward did your body travel? Did you feel any push forward or backward even? This is a fairly good model of what happens...
what about this example .. breaking my vow of silence...xcandrew
Feb 25, 2003 8:15 PM
Your example has no relation to the situation at hand and you know it. Where in this example are you even trying to move forward?

I think you are hung up on how the energy stored in the flexed bicycle frame can be returned to propel the bike. I think that it is very clear from my example (in earlier replies) that it manifests itself as a push upwards in the pedal as soon as the peak load is past. "carcass" has the same idea below.

You wonder how a lateral return of the BB can affect anything. Well, its obvious that the BB movement is not strictly lateral. The pedals are outside the centerline of the frame and it stresses the down tube about 2/3 in torsion, the rest in bending. It stress the seat tube about 80% in bending. This is from FEA, and makes perfect sense to me - it depends on the tube's orientation: if the seat tube were more vertical, it would be stressed a higher percentage in bending, if the down tube had a lower angle, it would be stressed more in torsion. The BB has more of a rocking motion when a force is applied from the pedals. If the force weren't out of plane, the frame would not flex laterally, period. If you can flex the frame laterally by applying a force downwards into the pedals, why can't the exact opposite occur, the equal and opposite reaction of the frame springing back and applying force to your foot (assuming that you have chain tension)? Have you ever heard of torsion bar springs (as opposed to coil springs)? I had them in my old car. They are a good model for this.

Another thing. How if ALL the energy taken to flex a frame is lost like you claim, how is it that bike racers in the '80s raced on frames like the vitus/bador 979, and used them and even flexier small tubed carbon frames on their climbing bikes? I scanned the first page of Bontrager's article and he mentions it too, with the example of Sean Kelly winning a zillion sprints on his bike. Think about how must energy is necessary to flex a frame out of alignment... go push on your frame now (I realize that much if the flex will be in the wheels and tire too, but even so). It is MORE than a FEW watts that would be lost considering this happens 2 times per crank revolution. If ALL this energy is lost as you claim, you would just plain not be able to compete on a flexier bike. Now consider that a stiffer frame that deflects a lesser amount, say half. Just as much energy was put into the stiffer frame to deflect it half as much as the more flexible frame. The "spring rate" of the frame is twice as high, but the energy put into it is similar, and the loss would be similar IF it were all lost as you claim. I would say the flexier frame would lose slightly (maybe not measureably) more energy because of greater elastic deformation of the frame. The stiffer frame may be more stable/controllable, but it's probably more a matter of "feel" preference as Sean Kelly's example shows.
A thought experiment....carcass
Feb 25, 2003 4:53 PM
Personally, I have several steel and several aluminum bikes. I tend to prefer a stiff frame to a more flexible frame, but have not experience what I would consider a large difference in efficiency between frames based on flex. I think that much of the argument is psychological in nature, and people perceive a stiff frame as more efficient. Furthermore, I suggest that the real issue is that the flex may hinder your pedaling flow if it flexes in an out-of-touch manner with the rider. If you can dance on the pedals with a flexy frame, you are fine. If you and your frame are not good dance partners, you will probably chalk up the awkward feel to frame flex. The real issue is that maybe the bike and you are not cut out for each other or it could be a lack of a fluid and efficient pedal stroke among other things.

One thing that people leave out with their thought experiments on this subject is conservation of energy. If you state to a physicist that you are loosing energy in the frame, the first thing they will ask is where does the energy go? I think that most of us are in agreement that the frame itself does not absorb the energy. It acts like a spring, which is very efficient, storing nearly 100% of the energy as potential energy. People state that the when a frame flexes, there is lost energy. The big question is: Where does all of this lost energy go?

If a spring is compressed by a hand, and then released, the energy is stored in the spring as potential energy, and then released into the hand when the grip is relaxed. Since your hand does not store energy like a spring, it is dissipated mainly as heat in your hand muscles. Energy is conserved.

In the handheld spring-exerciser example, it goes into the muscle. This is impossible on a freewheel bike. The spring of the frame cannot push your leg up on the backstroke, the freewheel will spin before that happens.

On a bike, you flex the spring (frame) with your leg, which stores energy in the frame. The important difference here is that all of the flex of the frame has sprung back by the time you reach the bottom of your pedal stroke (full leg extension); the muscle is never compressed by the rebounding frame (again, impossible on a freewheel bike).

Spring Dynamics:
Steel, aluminum and carbon bicycle frames can act as very efficient springs until they approach their plastic deformation point, at which point they will start absorbing significant amounts of energy (and/or possibly break). Since bicycle frames never approach this point in day-to-day operation, they are operating as nearly perfect springs, and do not absorb more than an insignificant amount of energy.

A spring has a rate. The rate is expressed as the amount of force that it takes to flex it a given amount. This could be expressed as 250lb/inch. If a force of 250 lbs is applied, it will flex 1 inch. Also remember that if you applied only 25 lbs to it, it would compress 1/10 inch. It is a linear relationship, as more force is applied, the more it flexes, and as force is removed, it returns to its original state in a linear fashion.

What everyone seems to forget is that the force exerted on the frame is sinusoidal in nature. The force is very weak at the top, greatest in the middle, and very weak at the bottom. The frame gradually deforms as the force increases, and then gradually spring back as the force is removed. It does not just all of the sudden snap back at the bottom of the stroke. This gradual change in force allows the frame to start rebounding just after maximum pressure is applied (middle of stroke), and is fully rebound by the bottom of the stroke. In other words, the energy is being released gradually into the downward pedal stroke as soon as pressure starts to decrease after the middle of the pedal stroke. If you doubt that this bicycle spring could keep up with your 200RPM cadence, I suggest you consider that the steel springs that contr
Feb 25, 2003 4:58 PM
What everyone seems to forget is that the force exerted on the frame is sinusoidal in nature. The force is very weak at the top, greatest in the middle, and very weak at the bottom. The frame gradually deforms as the force increases, and then gradually spring back as the force is removed. It does not just all of the sudden snap back at the bottom of the stroke. This gradual change in force allows the frame to start rebounding just after maximum pressure is applied (middle of stroke), and is fully rebound by the bottom of the stroke. In other words, the energy is being released gradually into the downward pedal stroke as soon as pressure starts to decrease after the middle of the pedal stroke. If you doubt that this bicycle spring could keep up with your 200RPM cadence, I suggest you consider that the steel springs that control the valves on your car engine have no problem handling four thousand RPMs.

In addition, it does not matter in what direction a frame is flexed. It can twist the bottom bracket, or move it side-to-side, it is all attributed to pedal pressure, and the windup of the spring and the stored energy is released gradually as the force is released.

What this shows is that the muscle does not absorb the potential energy of the spring as in the hand-exerciser example, the muscle does not even have a chance to get pushed back by the spring.

The main point is that energy must be conserved. If energy is not absorbed by the frame, and it is not absorbed by the muscle, then it must find its way into propelling the bike forward. This is only a hypothesis at this point because I have not performed any experiments to prove my hypothesis. It is not a sufficient argument to state that a flexible frame wastes energy without stating where you conclude that the energy goes. I have not heard a reasonable argument that explains this lost energy (if it truly exists).

In the end, I do not care either way, as long as people do not suggest that it is the frame that is absorbing a significant amount of energy.
I see it as you see it. nmishmael
Feb 25, 2003 5:48 PM
Feb 25, 2003 7:57 PM
I particularly appreciate that you used my own example
against me in a particularly clever way. The spring
begins to rebound as soon as force is DECREASED, not
REVERSED. I have to admit that you are about to make me
change my position. As far as my drink-addled mind can
figure, you have succesfully repudiated my point. Touche.

Since this may be quite a monumental event in the world of
internet arguements, (someone admitting they are wrong) I
will go ahead and disclose exactly where you shot me down -
"In other words, the energy is being released gradually
into the downward pedal stroke as soon as pressure starts
to decrease after the middle of the pedal stroke".

As much as I want to, I can't argue with that. It does
repudiate my point.

Here's my only caveat - it repudiates my point ONLY if ALL
the stored energy is released bfore bottom dead center. Is
it? I don't know. Maybe, maybe not. Argueing about it
sure is fun, though. And you do (in my mind) show that the
effect is much less than what I was thinking.

Thanks for insightful arguement without a personal attack.

I'm going to go back into lurker mode, now that I've gotten
righteously smacked down.

Ride on - stiff or flaccid. Makes no difference to me.