does stiffness matter (w/numbers)

Discuss light weight issues concerning road bikes & parts.
wwnick
Posts: 106
Joined: Wed May 10, 2017 8:49 pm

by wwnick

anyone want to talk about stiffness? I got these numbers from roadbike.de. I understand they use the same test protocol for all bikes.
these are all models from 2015+. Lightweight bikes, ie, not aero, and even an aluminium for contrast.

Image

so what gives? it seems for each measurement these frames fall into low, medium and high results. the alu is always in the medium results, ie, always better than worst carbon but never better than best carbon. even on comfort/vertical compliances (shown sorted in image) but these are super high end carbon bikes :?

what should be made of these numbers for stiffness? are they important?
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Marin
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by Marin

No. All current bikes are stiff enough not to shimmy on fast descents, and apart from that it has been proven that no energy is lost to frame flex.

They might feel a bit different, but I wouldn't overestimate the differences.

by Weenie


fromtrektocolnago
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by fromtrektocolnago

Any proof that someone riding on a stiffer bike will complete a ride quicker than someone on a less stiff bike? Never quite understood this one.
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wwnick
Posts: 106
Joined: Wed May 10, 2017 8:49 pm

by wwnick

OK, thank you, got it, difference in stiffness does not matter, they all more or less the same.

now is it the same with lack of stiffness/flexibiiity for the rear, in other words, vertical compliance.

will the cannondale at 320 N/mm be just as comfortable as the Canyon CF SLX at 155 N/mm. more or less.
also will the aluminium bike feel as comfortable as it is in the same range of measurement, though it is a different material

Delorre
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by Delorre

For a small to medium and fit rider (lets say <180 cm and < 70kg), I don't think it matters that much, but for the more athletic, powerfull and tall riders, it can make a huge difference in how well the frame copes with all the forces that are going on. Taller, heavier riders will also have less negative impact from a less vertical compliant frame, those riders weight more, so will overcome the high vertical stiffness more easily as a 60kg climber.

So for me, those numbers can be an important criteria for some of us.

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

by C36

fromtrektocolnago wrote:Any proof that someone riding on a stiffer bike will complete a ride quicker than someone on a less stiff bike? Never quite understood this one.

Yes, read few earlier posts.


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youngs_modulus
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Location: Madison, WI USA

by youngs_modulus

Marin wrote:No. All current bikes are stiff enough not to shimmy on fast descents, and apart from that it has been proven that no energy is lost to frame flex.

They might feel a bit different, but I wouldn't overestimate the differences.


Marin is basically right about this, but since the OP seems to be new to these sorts of questions, I should clarify two things:

- Shimmy still exists; stiffer bikes tend to be less prone to it, but riders of new bikes still sometimes experience shimmy.

- Energy is absolutely lost to frame flex. Perfect springs don't exist; they all dissipate some energy. That said, the amount of energy lost to frame flex is so small that no one (to the best of my knowledge) has been able to measure it. The difference between frames is likely to be lost in the noise of any power meter, including SRM's "lab" meter.

I wrote a sidebar for the Fairwheel Bikes crank stiffness article that attempted to establish an upper bound for energy losses due to crank bending. (In other words, if all the energy that went into bending a crank was lost, how much would those losses be?) The numbers were small for a very flexible crank (roughly analogous to a Campy Super Record crank). The maximum possible losses for a stiff, modern crank would be even lower.

This is one reason why it drives me a little nuts to hear people talk about "power transfer." First of all, the term is meaningless from an engineering perspective. Second of all, all frames "transfer power" so similarly that no one has measured the difference between two frames. I understand and accept, however, that "power transfer" is just shorthand for "it feels stiff and fast."

gewichtweenie
Posts: 148
Joined: Fri Sep 25, 2015 3:12 pm

by gewichtweenie

wwnick wrote:anyone want to talk about stiffness? I got these numbers from roadbike.de. I understand they use the same test protocol for all bikes.
these are all models from 2015+. Lightweight bikes, ie, not aero, and even an aluminium for contrast.

Image

so what gives? it seems for each measurement these frames fall into low, medium and high results. the alu is always in the medium results, ie, always better than worst carbon but never better than best carbon. even on comfort/vertical compliances (shown sorted in image) but these are super high end carbon bikes :?

what should be made of these numbers for stiffness? are they important?


just think how much 1 Newton is and how much a * or mm makes a difference

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wheelsONfire
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by wheelsONfire

If you would like to test this so you really notice what happens, you should try a suspension fork on your bike over a stiff fork.
This is a huge difference from looking at a frames stiffness, but you would notice what alot of flex do in terms of speed and the bike feeling snappy, slow or even mushy.
Sure,this is overdramatic and perhaps even off topic, but this way you see what flex do.
Next thing is, as a normal biker, how much force can we induce?
Do we really possess such watt output that we really flex/ bend a bike to a degree where speed is affected?
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gewichtweenie
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by gewichtweenie

wheelsONfire wrote:If you would like to test this so you really notice what happens, you should try a suspension fork on your bike over a stiff fork.
This is a huge difference from looking at a frames stiffness, but you would notice what alot of flex do in terms of speed and the bike feeling snappy, slow or even mushy.
Sure,this is overdramatic and perhaps even off topic, but this way you see what flex do.
Next thing is, as a normal biker, how much force can we induce?
Do we really possess such watt output that we really flex/ bend a bike to a degree where speed is affected?

not overdramatic..... extremely overdramatic. like...5N/mm .... and the flex is in a different direction.

wwnick
Posts: 106
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by wwnick

have been watching some Mike Burrows content in youtube regards materials, particularly this: https://www.youtube.com/watch?v=JjYNIaI26EQ
as many know he is the inventor of modern racing bike design. in his hat and slippers he looks a little like an old wizard which also adds to his credibility.

so am I to believe if a frame has vertical compliance of 238 N/mm and is made in aluminium, it will feel just as comfortable as a frame of 238 N/mm and is made in carbon.
closest examples in spreadsheet image above is Canyon AL SLX (238 N/mm) and S-Works Tarmac S3 SLight (241 N/mm) . so these would possess a similar comfort feel?
much like a kg of lead weighs the same as a kg of feathers?

or is that wizard is just a crazy old man?

gewichtweenie
Posts: 148
Joined: Fri Sep 25, 2015 3:12 pm

by gewichtweenie

wwnick wrote:have been watching some Mike Burrows content in youtube regards materials, particularly this: https://www.youtube.com/watch?v=JjYNIaI26EQ
as many know he is the inventor of modern racing bike design. in his hat and slippers he looks a little like an old wizard which also adds to his credibility.

so am I to believe if a frame has vertical compliance of 238 N/mm and is made in aluminium, it will feel just as comfortable as a frame of 238 N/mm and is made in carbon.
closest examples in spreadsheet image above is Canyon AL SLX (238 N/mm) and S-Works Tarmac S3 SLight (241 N/mm) . so these would possess a similar comfort feel?
much like a kg of lead weighs the same as a kg of feathers?

or is that wizard is just a crazy old man?


yes and no

this measurement just says the deformation is the same when subjected to the same force. but says nothing of resonance or buzz. things like this gets lost when people get mired into numbers without context. and also lost is the fact that fit is first order determinant of comfort and i bet nobody cant detect 5-10-15% differences in these measurements anyway.

for me its more akin to arguing whether a 450hp 1400kg car will be faster in a straight line vs a 460hp 1400kg car even with same tires and transmission and body shape when it still neglects power curve and driveability and moreover both are still damn fast for all intents and purposes.

its measurebating for the sake of measurebating

wwnick
Posts: 106
Joined: Wed May 10, 2017 8:49 pm

by wwnick

"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

so you would need to test for "resonance frequency" to understand a little more about comfort properties of a frame. as you say... measurebating...

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.

euka
Posts: 15
Joined: Mon Sep 19, 2005 10:35 pm

by euka

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

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.

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kgt
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Location: Athens, Greece

by kgt

Time's AKTIV technology manages to dampen the vibrations without sacrificing the stiffness of the frame or the fork. That's the kind of technology more manufacturers should invest into IMHO.

by Weenie


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