does stiffness matter (w/numbers)

Discuss light weight issues concerning road bikes & parts.
wingguy
Posts: 3830
Joined: Thu Mar 08, 2012 11:43 pm

by wingguy

wwnick wrote:"A material does not have a resonant frequency. A structure made of that material will have resonant frequencies, and they will depend on the shape and size of the structure as well as the mechanical properties of the materials used to build it."
Kim Aaron, Has PhD in fluid dynamics from Caltech

But the material used dictates the best shape, size and wall thickness of the structure built from it. As in, for a given purpose the best steel bike won't have the same tubes as the best aluminium bike or the best carbon bike.

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

wwnick wrote:"A material does not have a resonant frequency. A structure made of that material will have resonant frequencies, and they will depend on the shape and size of the structure as well as the mechanical properties of the materials used to build it."
Kim Aaron, Has PhD in fluid dynamics from Caltech


Aaron has a Ph.D. In aeronautics, not "fluid dynamics." I put the latter in quotes because American universities don't typically award degrees in fluid dynamics. Fluid dynamics, which Aaron's dissertation incorporated, is a subfield of mechanical/aerospace engineering. I'm nitpicking here, but the quote is accurate.


wwnick wrote:so you would need to test for "resonance frequency" to understand a little more about comfort properties of a frame.


Not really. Resonant modes are a fairly simple function of stiffness and mass. For metal frames, you could get the mode shapes/frequencies pretty accurately with a hand calculation. You'd need an accurate FEA model to do the same with a carbon frame, and even then frame-to-frame variations in construction will mean the model only gets you in the ballpark.

You can always pluck the frame and and measure the frequencies that way. You can even use an audio FFT app on your phone to measure the ringing frequency of a plucked frame. The ring-down time would be a pretty interesting...the shorter it is, the more damping is built into the frame.

wwnick wrote:as you say... measurebating...

I'm not picking on you, wwnick, but I most often hear this expression as shorthand for "This discussion has become too technical for me and I have lost interest." I get that I care about this stuff way more than most people, and I don't think everyone needs to care as much as I do. But it's funny how no one uses this expression when their pharmacist is titrating chemo drugs or when aerospace engineers are discussing how to make sure the 787 doesn't fall out of the sky due to wing delamination...


wwnick wrote:would you agree, that, in the end all frames are more or less the same, and the only appreciable difference in a well designed frame is weight. carbon cerca 750g, alu, 1100g, steel 1500g.


No, but the right answer is widely misunderstood. Carbon frames have more inherent damping than metallic frames because of interlaminar shear strain, the same characteristic that causes much of the rolling resistance in bicycle tires. Magnesium is the sole exception...it's really dampy (so to speak) for a metal.

People talk about how aluminum frames beat them up because they're so stiff. That's not quite right. First of all, if you made equally stiff frames from carbon, steel, aluminum and titanium (totally doable, by the way) the initial jolt from an impact would be the same in all of them. The difference comes in how quickly the stop vibrating, and carbon has an advantage here.

I recall that Merlin/Spectrum built a series of titanium match sprint bikes for Ken Carpenter, but abandoned the project because either they couldn't build one stiff enough, or they could but it was heavier than his steel bike. This is silly. Because titanium is less dense than steel with similar strength, you can absolutely build a Ti bike that's stiffer than a given steel bike. And it'll be lighter, too. Any competent sophomore mechanical engineering student could prove it, but there's so much voodoo and hand-waving in the bike industry that silly myths just don't die.

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

euka wrote:How much do the stiffness readings decline after few years of riding?

Stiffness doesn't decline at all unless the frame has cracked/delaminated.

One of those silly myths I referred to above was common into the 1980s: that frames "went soft" after a year or two of use, especially hard use. There's no basis for this at all.

euka wrote:I have raced with steel, aluminum, titanium and carbon frames. The worst of all materials was mid 90's carbon VTT with aluminum lugs, that one was barely ridable after two seasons. But it didn't crack like some of my aluminum and titanium frames.

If your TVT wasn't delaminated/debonded (you say it wasn't cracked) then it was only unrideable because you decided you didn't want to ride it any more. I mean, unless you're leaving out some important detail. Those frames were very flexible...Andy Hampsten once mentioned seeing his front tire nearly contact the down tube under braking, and he's not a heavy guy.

Stiffness is a nearly invariant material property. The valve springs in car engines don't "go soft" even after millions of cycles. Neither do the fiberglass leaf springs in Corvettes. if materials "went soft" as they fatigued, aircraft and wind turbine companies wouldn't spend millions-billions of dollars trying to detect fatigued parts before they failed. They could just measure their change in stiffness and be done with it.
Last edited by youngs_modulus on Wed Jul 05, 2017 11:22 pm, edited 1 time in total.

wwnick
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Joined: Wed May 10, 2017 8:49 pm

by wwnick

@youngs_modulus thank you for detailed response, all very interesting and makes sense.
regards "I'm not picking on you". No, not at all, my ears are always open to learning. In fact I only put the phrase in as I thought I maybe getting on the nerves of people who were starting to think that this is all starting to come a wee bit "academic" (another euphemism).

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

Wwnick: I'm glad you found my post helpful. I follow why you threw that expression in. I didn't mean to imply that you were using the expression in the way I was describing. I know some people have limited patience for posts like yours (and mine), but I don't particularly care if I get on their nerves.

If they're reading the thread, it's because they decided to read the thread. If they're looking for a bike forum that doesn't revolve around technical details, I'd argue that a site called "weight weenies" is probably the wrong place to do so. :)

pyrahna
Posts: 123
Joined: Wed Aug 14, 2013 9:53 pm

by pyrahna

youngs_modulus wrote:..... The valve springs in car engines don't "go soft" even after millions of cycles. ....


I suspect you know this but valve springs in race engines are replaced at frequent intervals due to the damage caused by work hardening. So, springs on the limit of their design envelope do 'wear out', but it is not because they have gotten softer, it is because they get harder and will eventually crack and break under the stress.

youngs_modulus
Posts: 512
Joined: Wed Sep 20, 2006 1:03 am
Location: Madison, WI USA

by youngs_modulus

I did happen to know that, but it's certainly germane and I'm glad you mentioned it. If you know race engines, then you know (a) that revs=power and that high revs and heavy valves/springs = valve float. Production cars have valve springs that generally last for the life of the engine because their strains are lower.

For a given application, a stiff, light spring has fewer coils and higher strain thank a soft, weak spring. Plus, race engines are rebuilt often enough that the cost of replacing the springs is pretty low in context. But good point!

C36
Posts: 223
Joined: Fri Mar 03, 2017 3:24 am

by C36

"With number..." But before,In summary.
-do frames lose "total energy" you put on them? Not a lot, hysteresis is not large. So doesn't matter? It does!
-the important point is that the energy you spend in deforming an element (any element on the bike) may not be used to move you forward. Even if the element is coming back in position that may not be beneficial for what we are interested in. The easiest example is when you are stepping out of the saddle and move the bike left-right. By doing so you add forces that do not exist as much when pedalling sit on the saddle.
In the test we ran (10 years ago) we measured only stepping out of the saddle. As I previously mentioned, I can't disclose too many details but gains were significant at "constant" power and had a (difficult to quantify) impact on maximum power you could transmit (basically attacking you were "putting more power on the rear wheel" on a stiff equipment than on a soft one). But all things equal 0.25km/h between something soft and something stiff, 0.35 pushing a bit the extremes in terms of stiffness.

Now few side comments
- do material lose stiffness? No, not untill you had physical damages (cracks). The aluminium loosing stiffness is a legend several times proven wrong... Untill you face a structural damage (that can happen quite before total failure.
- where do we see a lot of stiffness variation? Frames... Not as much as 10 years ago, top frames oscillate between 70-90n/mm on the front and 80-100 on the BB? That's enough to be able to measure the gain but... Wheels have a lot more stiffness variation than modern frames from less than 30 to 60n/mm (without going to the extremes and skipping the fact that they are tested differently).



Envoyé de mon iPhone en utilisant Tapatalk
Last edited by C36 on Wed Jul 05, 2017 8:10 pm, edited 2 times in total.

tinozee
Posts: 700
Joined: Wed Jun 01, 2011 7:53 am

by tinozee

Going from years on a s-works sl3 to a evo hi-mod was all I needed to learn that stiffer does not equal faster over the course of a race. In fact the more compliant evo made intense efforts more bearable. For short bursts it may matter some, bb stiffness especially.

Marin
Posts: 2761
Joined: Wed Jan 22, 2014 11:48 am
Location: Vienna Austria

by Marin

wwnick wrote:would you agree, that, in the end all frames are more or less the same, and the only appreciable difference in a well designed frame is weight. carbon cerca 750g, alu, 1100g, steel 1500g.


I would agree. My current stable (about to be reduced) is a 890g carbon frame, 1770g steel, 1250g Aluminum and 1500g Titanium frame. All bikes have more or less the same fit, with the only difference being how low the bars are.

The main differences I feel are 1) exposed seatpost - a frame with a horizontal top tube will kick you more because the post doesn't flex and 2) tires - they change the feel & handling more than geometry numbers or frame material.

In a blind & deaf test, I don't think I could tell my frames apart. But I could surely tell you if I was riding Conti Attack or Grand Bois Cerf tires, probably after 10m of riding.

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