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Flame flex: John Rasmussen responds

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Flame flex: John Rasmussen responds

Old 02-11-07, 03:17 PM
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Flame flex: John Rasmussen responds

Hello Joe,

Interesting business you are in. I love good beer.

Anyway, at the time that paper was written, we worked together with a small
Danish bicycle manufacturer, Principia, and were helping them design bicycle
frames. So the claim initially came from their market research feedback.

However, there also is a more scientific explanation: Muscle work is not
conservative. This means that it makes a difference to the energy
consumption exactly how the work is done and the process is irreversible. To
give an example, consider walking up a staircase. It will make you tired
because you consume energy. If you them run down the staircase it will be
easier for you but you are not getting the energy you initially spent back.
In a conservative system you would get it back.

An elastic bicycle frame is a conservative system. The elastic energy you
put into its deformation comes back when the frame regains its undeflected
shape. However, the muscle system that causes the deflection is not
conservative, and it actually ends up spending energy both on the
deformation and on the restoration of the shape, just like walking up and
down stairs.

I hope this explains the matter. Otherwise, please do not hesitate to ask
again.

...

Best regards,
John
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Old 02-11-07, 03:23 PM
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That's an interesting point regarding the staircase example. Makes perfect sense for a pedal stroke. I wonder how that compares to the efficiency gains/losses related to instantaneous acceleration and deceleration during the pedal stroke.

I guess everyone will disregard the part where he says the elastic bicycle frame is a conservative system

Thanks for posting back so fast, and thanks to John R. for being so responsive!
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Old 02-11-07, 03:26 PM
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I missed something--who is he?

The argument makes a lot of sense, though. It's not like a flexed frame is somehow transferring energy back into the muscles for pedaling power.
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Old 02-11-07, 04:29 PM
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This whole topic is SO overplayed...

How much extra "energy" do the muscles expend as a result of frame flex? Not enough to matter is the answer.

I do agree with the part about the frame being a conservative system though - no energy loss (or again, not enough to matter).
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Old 02-11-07, 04:57 PM
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Real-world testing has shown frame-flex to be irrelevant; the most flexible bikes posts the same TT times as the stiffest bikes. So what's the big deal? If you want a stiff frame, build it out of concrete.
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Old 02-11-07, 05:39 PM
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here's the thing and I think the main point of his explanation.

You expend energy in the deformation of the frame that is not given back to you. With a less flexy frame you lose less non-conservative energy even though the frame gives back; the muscle expenditure does not come back.
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Old 02-11-07, 06:02 PM
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Originally Posted by DannoXYZ
If you want a stiff frame, build it out of concrete.
Stealing my 1-minute pshop from yesterday: no stiffer frame than this. That front hole is the head tube, and rear is the seat tube. It doesn't have cranks because they flex too much
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Old 02-11-07, 06:32 PM
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There is a crack in that head tube
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Old 02-11-07, 06:38 PM
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All I know is when I push hard on the pedals of my Giant TCR Team Advanced, it feels like thereis an instantaneous response with most of the power going directly to forward motin; when I do the same on my MerlinExtralight, the resopnse is not as crisp or as quick. I'll readily admit that there may be a huge dose of plecebo here. But at least I'm enjoying the pill.
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Old 02-11-07, 07:36 PM
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Originally Posted by merlinextraligh
I'll readily admit that there may be a huge dose of plecebo here. But at least I'm enjoying the pill.

great quote!!
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Old 02-11-07, 07:44 PM
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Originally Posted by merlinextraligh
All I know is when I push hard on the pedals of my Giant TCR Team Advanced, it feels like thereis an instantaneous response with most of the power going directly to forward motin; when I do the same on my MerlinExtralight, the resopnse is not as crisp or as quick. I'll readily admit that there may be a huge dose of plecebo here. But at least I'm enjoying the pill.
The laws of Kinetic Energy say you're right. But apparently if it doesn't come in Homer Simpsonesque pictures and language its too difficult to understand for some.

Thats why people like that work for people like me.
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Old 02-11-07, 08:20 PM
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I'm not a engineer... but, isn't there one small caveat to the frame being a conservative system: when the frame flexes, it creates heat, and that heat is energy lost. Whether it is a significant amount or not is another question, but it is not a perfectly conservative situation, right?

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Old 02-11-07, 09:11 PM
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Originally Posted by ratebeer
An elastic bicycle frame is a conservative system. The elastic energy you
put into its deformation comes back when the frame regains its undeflected
shape. However, the muscle system that causes the deflection is not
conservative, and it actually ends up spending energy both on the
deformation and on the restoration of the shape, just like walking up and
down stairs.
Not sure about this. I thought the main problem with frame flex is that in every cycle some of the energy that should have gone into rotating the crank is spent on deflecting the frame laterally (which doesn't help to propel the bike forward). Eventually, the energy in the lateral flex is *dissipated* as internal energy or heat energy. All mechanical strurctures have inherent damping and are not conservative systems.
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Old 02-11-07, 09:14 PM
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Similar to the reason cycling shoes don't have a lot of sole, to waste energy compressing the rubber on each pedal stroke. The wasted energy is not given back to you. Frame flex loses energy because unlike in a perfectly flex free frame, some of the energy actually propels the bike backwards when it regains it's form. You can do this too as a rider with poor form.

It may be a just a tiny quantity, but often significant at the highest level of competition. Not unlike the tiny savings you realize from the use of lighter components...or negatively, by installing an aero wheel backwards as one Jan more recently found out, or by unfortunately using an front aero wheel as one Laurent discovered once upon a time...
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Old 02-11-07, 09:33 PM
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The funny thing is, that it *is* a conservative system. In this house, we obey the laws of thermodynamics.

Frame flex does give off heat; the amount of heat given off is how much work is 'wasted'; but the work is still conserved, i.e.:

Work by muscles on frame - work converted to heat by drivetrain friction - work of wind on bike and rider - additional energy due to potential changes (going up a hill) - heat of frame flex hysteresis = 0.

The frame hysteresis is negligible. The amount and frequency of frame flex affects your cadence negligibly. Ergo, frame flex is negligible.

Ahh. I feel better now. A month from now I'll repeat myself.
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Old 02-11-07, 09:35 PM
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Originally Posted by rufvelo
to waste energy compressing the rubber on each pedal stroke.
Rubber soles that can support the same loads have much more mass than CF soles with the same mass.

Rubber does have hysteresis, aluminum/CF/steel have only negligible hysteresis.
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Old 02-11-07, 09:35 PM
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Originally Posted by catfour
Eventually, the energy in the lateral flex is *dissipated* as internal energy or heat energy. All mechanical structures have inherent damping and are not conservative systems.
Everyone is saying this energy is negligible. Rasmussen doesn't claim this. He claims that the leg's reabsorbing the the shock is a primary energy expense.

A related argument is being put forth by SoftRide company. It's related in that their frame is meant to absorb energy that the body (muscles) would have to spend absorbing the shock -- essentially what Rasmussen states is one of the primary costs of a flexing frame. SoftRide has done very well in competition, so I'd be interested in those 40kTT times, Danno.

DannoXYZ: Would love to see some citations!

Dr Pete: John Rasmussen has his name on numerous papers mentioning the claim that the vast majority of mechanical scientists believe stiffer frames are certainly better than more pliant frames. I contacted Doctor Rasmussen in an effort to better understand these claims.
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Old 02-11-07, 09:45 PM
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Originally Posted by ratebeer
Everyone is saying this energy is negligible.
I also mention that the way this affects your cadence is negligible. My assertion is that the actual amount of frame flex doesn't actually change the motion of your legs in the range of what is noticeable.
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Old 02-11-07, 10:09 PM
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Originally Posted by ElJamoquio
Frame flex does give off heat; the amount of heat given off is how much work is 'wasted'; but the work is still conserved, i.e.:

Work by muscles on frame - work converted to heat by drivetrain friction - work of wind on bike and rider - additional energy due to potential changes (going up a hill) - heat of frame flex hysteresis = 0.
I would tend to disagree, as I think there is an important semantic distinction here. You are accounting for all the energy, but that doesn't make it a conservative system. The relevant "system" in this specific quesiton is how much of the work is contributing to forward motion... and heat dissipation is, indeed, wasted, as it is not contributing to forward motion. Yes, it is conserved if you are taking a universal view (of course, it has to be conserved in that sense).
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Old 02-11-07, 10:29 PM
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Originally Posted by ElJamoquio
...
Frame flex does give off heat; the amount of heat given off is how much work is 'wasted'; but the work is still conserved,...
Only if you consider the conservation within the universe as a whole, relative to your bike though...you lose.
Same problem when you brake. Else a 'rubbing brake' would be a non issue, hysteresis or hysterics.
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Old 02-11-07, 10:40 PM
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Here is a good technical discussion.

https://www.bikethink.com/Frameflex.htm
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Old 02-11-07, 11:47 PM
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Originally Posted by ElJamoquio
I also mention that the way this affects your cadence is negligible. My assertion is that the actual amount of frame flex doesn't actually change the motion of your legs in the range of what is noticeable.
FWIW, Rasmussen doesn't claim this either.

Again, the claim is that it's the muscular energy applied to distort the frame and also energy lost to reabsorbing the distortion, not the frame behavior independent of the rider, nor cadence, that is the alleged cost of a flexing frame.

There is research to show the legs poorly store elastic energy. However, I still haven't found any researchers claiming calculated speed improvements as a result of a rigid frame vs. a noodle.
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Old 02-12-07, 06:03 AM
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Originally Posted by ratebeer
Everyone is saying this energy is negligible. Rasmussen doesn't claim this. He claims that the leg's reabsorbing the the shock is a primary energy expense.
Still trying to figure out what that means. Is this another way of saying that most of the flex energy in the frame is actually dissipated in the legs because of, I don't know, more damping/hysterisis/friction in the joints and muscles than present in the frame itself? This sounds reasonable to me and it can cause the flex energy to be wasted at a greater rate (compared to structural hysteresis in the frame alone). But still, you have to assume that the flex energy is not negligible in the first place, for otherwise whether it gets dissipated in the frame or in the muscles doesn't matter.
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Old 02-12-07, 07:44 AM
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Originally Posted by MacMan
The laws of Kinetic Energy say you're right. But apparently if it doesn't come in Homer Simpsonesque pictures and language its too difficult to understand for some.
Ok, ok, we get it. You're not smart enough to explain what's so obvious. Can you just shut up then?

BFers will go to great lengths to discuss the finer details of HR zone training, power training, bottle cage bolt color, but asking for detail on something as important as power delivery is just too far out there.

Originally Posted by MacMan
Thats why people like that work for people like me.
What are you, 12?
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Old 02-12-07, 08:01 AM
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Originally Posted by ratebeer
FWIW, Rasmussen doesn't claim this either.

Again, the claim is that it's the muscular energy applied to distort the frame and also energy lost to reabsorbing the distortion, not the frame behavior independent of the rider, nor cadence, that is the alleged cost of a flexing frame.

There is research to show the legs poorly store elastic energy. However, I still haven't found any researchers claiming calculated speed improvements as a result of a rigid frame vs. a noodle.
I've been thinking about the stair climbing and descending explanation a little bit. What do you think of this? :

Stairs may not be a perfect analogy for what's happening since the process is discontinuous (the muscles relax before you have to absorb the return energy). If you ran up the stairs, then jumped off a balcony, your legs would still have to absorb all that energy at the bottom, so there's no way to get back down stairs on foot without using energy on the descent.

In a bicycle stroke, there is a point where the force from pedaling will become lower than what's necessary to maintain the maximum amount of flex. This isn't "bottom"-dead center, but probably somewhere close to 4 or 5 o-clock. At this point, the frame will unwind and push back into the downstroke foot gradually as the stroke becomes less powerful. What this does is allow the rider to continue pushing a little harder at that point in the stroke, even though leverage has decreased.

This is a continuous process, as the muscles had not relaxed when the unwinding began. It's effectively making the pedal stroke a little longer, which will make up for the extra push at the beginning of the stroke that wound the frame up instead of propelling the bike.

On a stair descent, the returned energy results in the leg absorbing the energy and moving in the opposite direction as when climbing. In the pedal stroke, your leg is continuing to push and move in the same direction when receiving the energy return.
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