Planing

[JohnnyChromatic]

Possibly bogus theory from a long(ish) time lurker:

This is based on widespread, but possibly imagination induced, reports of correlation between different frame materials and rider satisfaction. If frame flex is improving rider performance, a possible explanation is that a more flexible frame will be absorbing vibrations and smoothing out the small shocks.

To illustrate, you're pedalling along and you hit a bump. There is a momentary increase in resistance transmitted from the drive wheel to the pedals. The momentum of your massive pedal mashing leg, when it meets that resistance increases the flexure of the frame. That spring action is absorbing the shock of the impact at the BB/pedal/foot/leg mechanism. The energy absorbed by flexing the frame is returned as the resistance of the pedal diminishes. This has the effect smoothing out the amount of resistance applied to the foot/leg by the bike. This is all happening in the sub 1 second time frame for almost all bumps, in milliseconds perhaps for the majority. This is prabably happening almost impercipibly of any kind of road vibration. Anything that increases rolling resitance in an unsmooth manner would see the smoothing effect.

This effect is very small and compared to other vibration damping parts of the bike, e.g. tires, wheels, saddle and rider, in terms of magnitude of displacement, is probably negligible. However, frame flex damping might be particularly effective in smoothing out higher frequency vibration, which can be very fatiguing to the rider.

At this point, if you're still reading, you're may be thinking, "Everybody knows that a a flexible frame absorbs road shock and vibration." Yes, but I think a lot of people posting in this thread are ignoring that and may be unaware of this (possibly bogus) theory.

I hope I have made this all clear.

[dinucci]

Oh come on you guys, you are not going to leave this interesting subject just die here, at this point, are you?
Even those of you who are disbelievers must admit that some bikes 'roll' better than others. Jan does not mention/equate this rolling resistance to planing, which I believe is an important part of the equation. Too much good work has been done by the OP and others in this thread to just leave it here.

DiNucci

Possibly bogus theory from a long(ish) time lurker:

This is based on widespread, but possibly imagination induced, reports of correlation between different frame materials and rider satisfaction. If frame flex is improving rider performance, a possible explanation is that a more flexible frame will be absorbing vibrations and smoothing out the small shocks.

To illustrate, you're pedalling along and you hit a bump. There is a momentary increase in resistance transmitted from the drive wheel to the pedals. The momentum of your massive pedal mashing leg, when it meets that resistance increases the flexure of the frame. That spring action is absorbing the shock of the impact at the BB/pedal/foot/leg mechanism. The energy absorbed by flexing the frame is returned as the resistance of the pedal diminishes. This has the effect smoothing out the amount of resistance applied to the foot/leg by the bike. This is all happening in the sub 1 second time frame for almost all bumps, in milliseconds perhaps for the majority. This is prabably happening almost impercipibly of any kind of road vibration. Anything that increases rolling resitance in an unsmooth manner would see the smoothing effect.

This effect is very small and compared to other vibration damping parts of the bike, e.g. tires, wheels, saddle and rider, in terms of magnitude of displacement, is probably negligible. However, frame flex damping might be particularly effective in smoothing out higher frequency vibration, which can be very fatiguing to the rider.

At this point, if you're still reading, you're may be thinking, "Everybody knows that a a flexible frame absorbs road shock and vibration." Yes, but I think a lot of people posting in this thread are ignoring that and may be unaware of this (possibly bogus) theory.

I hope I have made this all clear.

[Dallas Tex]

A couple weeks ago, I sent a bike abroad. My first out of country sale. I could track the package via the freight carriers website.

At 8:54am on November 20th 2014 my bike boarded a plane.

At the exact same time, 8:54am, I was in my workshop planing a board.

Coincidence??? I think not.....

[Mark Kelly]

Oh come on you guys, you are not going to leave this interesting subject just die here, at this point, are you?
Even those of you who are disbelievers must admit that some bikes 'roll' better than others. Jan does not mention/equate this rolling resistance to planing, which I believe is an important part of the equation. Too much good work has been done by the OP and others in this thread to just leave it here.

DiNucci

Well only two data points so far, but here's an observation: the only bikes I know of designed explicitly on acoustic principles roll downhill exceptionally fast (and it isn't be the tyres).

[Too Tall]

Scoobie what??? Mark, go for it. Now I want to hear, no pun intended, about this.
As for Dinucci, you are trouble maaaaan nothing but trouble ;)

Well only two data points so far, but here's an observation: the only bikes I know of designed explicitly on acoustic principles roll downhill exceptionally fast (and it isn't be the tyres).

[lumpy]

Well only two data points so far, but here's an observation: the only bikes I know of designed explicitly on acoustic principles roll downhill exceptionally fast (and it isn't be the tyres).

I too would like to hear more; Mark has referred to the theory that ride quality is an acoustic phenomenon and let's just say it resonates with me.

My name is Edward Carman.

[Mike Mcdermid]

Well only two data points so far, but here's an observation: the only bikes I know of designed explicitly on acoustic principles roll downhill exceptionally fast (and it isn't be the tyres).

Acoustic or constrained layer?

[Mark Kelly]

First, I have to say that the rolling thing was serendipitous and the idea that it is a result of the acoustic design is post facto, so it might be a statistical blip and all that follows hot air.

The best explanation I can give is that I'm building the frames as though they were musical instruments. The best website on the acoustics of instruments I know of is from my alma mater here: UNSW physics acoustics. Please note I did not study acoustical design there: I studied aeronautical engineering then switched to biotechnology, biochemistry, microbiology and philosophy then went into winemaking and brewing. Later on I got interested in audio design and acoustics. But enough about me, onto the bikes:

I'm applying some ideas gleaned from luthiers, including my father in law: after all a guitar is a highly stressed lightweight hollow structure. The observation that started this was that "tonewoods" are distinguished by the spectral distribution of their loss tangents. Like anything in this field, theory will only get you so far, beyond that it's trial and error and an ear for what's happening.

Mike, as it happens a structure I designed in for other reasons is also acting as a constrained layer but importantly the viscoelastic element is not very lossy at low frequencies, see comment above re spectral distribution of loss tangent.

[Too Tall]

Mark, I'm digging this. So many exquisite things are created because of intuition, hard work and some luck than explained with science. Why should this be any different?
Seems like you have more to say. Thanks for sharing this.

[afwalker]

If some of the energy from flexing is returned to the bicycle, how to we know it is a positive force? What if it returns out of phase and is negative or only partly positive? I'm no engineer, but find this discussion theoretically interesting. We are assuming the stored energy is returned during a pedal stroke phase that is additive, but is it the correct harmonic? Is this what makes the "feel", just a less negative force, or a partial positive force?
cheers
andy

[jclay]

Mark, I'm digging this. So many exquisite things are created because of intuition, hard work and some luck than explained with science. Why should this be any different?
Seems like you have more to say. Thanks for sharing this.

Scientific/technical proofs, explanations and such usually (always?) follow intuition and the luck of wrong turns.

[GrantM]

Some interesting and perhaps relevent thoughts on stiffness here in the latest crank test by the fine folks at Fairwheel.

https://fairwheelbikes.com/c/reviews...crank-testing/

"Notes about stiffness: According to conventional wisdom, the more pedaling stiffness the better. Stiffness implies efficiency, along with the notion of a stiffer bike inspiring confident and responsive handling. As desirable as those traits are, and even after a crank’s stiffness is proven empirically, the outcome still seems subjective—we are simply going by how many people feel more efficient on a bike with a stiff drivetrain. But it’s far from clear whether that tactile, from-my-quads-directly-to-the-road sensation is actually faster. This is the question: is a stiffer drivetrain actually more efficient, or does it just feel that way? Looking at the numbers, we can see that average deflections range from roughly 0.20 inches to 0.30 inches. From this, we can generalize and say that the most flexible crank is about 50% more flexible than the stiffest crank. It’s easy to imagine that the stiffer cranks feel better, or have better “power transfer,” which is a particularly vague and ill-defined concept.

[sk_tle]

One thing is sure, any test shouldn't rely on "feel" to be considered rigorous.

Our local television did a test a few years ago. They filled several bottles with syrup using the very same flavor (mint or grenadine, I don't remember) but colored differently. People in the street where asked to drink a glass of different ones and tell which flavor it was. The unanimous answer was that the yellow liquid was citrus and the green one was mint. The brain is easy to fool, especially if you have your own idea in mind prior to the test.

[EricKeller]

If some of the energy from flexing is returned to the bicycle, how to we know it is a positive force? What if it returns out of phase and is negative or only partly positive?

I'm pretty sure we can rest assured that this isn't true. There is one force diagram I would draw if I were an artist. Any force that propels a bicycle is translated through the chain. Any motion of the frame that somehow ended up as tension in the chain would have to be opposed by the muscles in the leg. You might imagine that the forces opposing the leg at one point in the pedal stroke are reduced, and correspondingly increased somewhere else in the pedal stroke by a flexible bicycle. I assert that flexiing, at best, produces what's called "wasted motion." But I don't see how it's going to end up causing more force to be expended than a rigid frame

[afwalker]

Ok, sounds good but do we know it's true? The return of the flex may or may not come at a time when it positively or negatively contributes to a positive harmonic in time with the chain force. All I'm saying is there is probably a harmonic that when it's in tune will be a nicer riding bike, just how do you know when you're building it?
Flexing may just be "wasted motion" but sometimes it may be additive, or less subtractive but it still may "feel" better.
cheers
andy

[EricKeller]

I think that it's a static force balance under most riding conditions. The notion that forces will be transmitted down the chain without a balancing force from the leg and without a noticable inefficiency in the drive train is hard to believe.

[Mark Kelly]

Well here I am compounding the sin of resurrecting a zombie thread by choosing the one about planing:

A propos of an article about stem length, I had cause to review the very good if somewhat mathematically dense paper on bicycle dynamics published a few years ago.

When an uncontrolled bicycle is within its stable speed range, lean and steer perturbations die away in a seemingly damped fashion. However, the system has no true damping and conserves energy. The energy in the lean and steer oscillations is transferred to the forward speed rather than being dissipated. As the forward speed is affected only to second order, linearized equations do not capture this energy conservation. Therefore, a nonlinear dynamic analysis with Spacar was performed on the benchmark bicycle model to demonstrate the flow of energy from lateral perturbations into forward speed. The initial conditions at t=0 are the upright reference position (ϕ, δ, θR)=(0, 0, 0) at a forward speed of v=4.6 m s−1, which is within the stable speed range of the linearized analysis, and a lean rate of Embedded Image. In the full nonlinear equations, the final upright forward speed is augmented from the initial speed by an amount determined by the energy in the lateral perturbation. In this case, the speed-up was approximately 0.022 m s−1. Figure 4 shows this small increase in the forward speed v, while the lateral motions die out, as expected. Figure 4 also shows that the period for the lean and steer oscillations is approximately T0=1.60 s, which compares well with the 1.622 s from the linearized stability analysis. The lack of agreement in the second decimal place is from finite amplitude effects, not numerical accuracy issues. When the initial lateral velocity is decreased by a factor of 10, the period of motion matches the linear prediction to four digits. The steering motion Embedded Image has a small phase lag relative to the lean motion Embedded Image, visible in the solution in figure 4.

This suggests a possible mechanism whereby planing can return energy to forward motion. As is obvious from footage of Sean Kelly sprinting on a Vitus 979, some of the strain in the frame serves to twist the frame and thus move the wheels away from co-planarity, a lateral perturbation in the model referenced above. According to this model, these lateral perturbations are energy conservative and the energy is returned as forward motion. Interestingly there is a phase lag here, just as posited by me somewhere above.

In layman's terms, some of the strain energy is transferred to precession of the front wheel, then returned to the frame as forward motion a short time later.

Note that I'm not saying that this as "the answer" regarding planing: as someone said recently, amongst other things the paper cited makes it obvious that only a handful of people understand how bicycles actually work and I'm not one of them.

[swt]

As far as I can tell, that paper doesn't model mechanical deflections of the frame itself.
F2.medium.gif
The oscillations they are talking about are the sideways side-to-side movement of the front wheel and the lateral side-to-side movement of the frame. This model is all about coupling the various angular momenta to the linear momentum of the machine. Not saying your proposed mechanism is wrong, but I don't think you and the paper are on the same page.

I'm not sure about the statement that no one knows how bicycles work. I think this covered it pretty well a while ago.
Bicycling_Science.jpg

[Mark Kelly]

I know they're using a rigid model, they discuss it at length.

Yes, they're talking about energy being recovered from lateral wheel movement, my point was that some lateral movement is caused by frame flex and this energy could also be recovered.

I didn't say no-one understood how bikes worked, I referred to someone who said only a handful of people do. As I said, I'm not one of them.

[jclay]

Pretty neat demonstration of frame strain energy being returned to the rear wheel that Jan sent to folks who subscribe to his emai newsletter. I wish I'd thought of it!