Bike Gremlin

Hi, this subject has been talked over and over and over in this forum. Don't want to start a debate, just curious about measurable, scientific approach to the frame material subject. Is there a test showing how much a steel frame flexes, compared to an aluminium one? mm travel per N of force, etc, on two similarly made frames, one alu, one steel.

Kimmo 

Your question is moot because the ideal frame design for each material varies.

Further, the parameters are affected by other choices such as method of construction (welded, lugged and brazed, fillet brazed, bonded) and specific tubing choices eg normal cromo or 853, or hyrdoformed ally versus butted and ovalised.

In short, there's no point considering identical frames made of different materials, because at least one frame will be making poor use of its material.

The closest you can get to a fair comparison is the output of a high-end builder equally exerienced and talented with each metal.

And, as anyone who's looked into the subject can tell you, the frame's design is far more important than what it's made from, in the same way that buid quality is the most important component of a wheel.

acidfast7

this is a good place to start reading:

https://sheldonbrown.com/rinard/EFBe/...tigue_test.htm

Sheldon does a reasonable job with translation and describing the issues.

here's a more recent version of the results (in English)

https://www.efbe.de/testergebnisse/fu...?typ=1&sort=15

the actual testing methods are here:

https://www.efbe.de/pruefservicenew/enindex.php

also, it is Germany, so I can't imagine anywhere else doing more rigorous tests (as far as I know there are no specific minimal requirements in the US).

acidfast7

also, it's quite a reasonable cost, €2k for a full frame test.

Airburst

Yes there is. It's called maths. You'll need to know the Young's modulus ("stiffness") of the two materials you want to compare, the geometry of the frame and the dimensions of all the tubes, as well as the point you intend to apply the force and the direction you intend to apply it in (i.e. is it flex under pedalling, or under the weight of the rider that you're wondering about?).

Since a lot of frames these days are made of oddly-shaped tubing, it's likely to be really quite complicated to work all of this out, but it's a fairly simple principle. It'll give you the deflection in the frame for a given force.

acidfast7

actually there's something called empirical testing and that what the links I provided did. the inherent attributes of the material are irrelevant when they're used in a bicycle frame ... the entire frame must be tested ... as per above.

Airburst

Oh dear....

acidfast7

of course, as the points are failure to to be elsewhere, as my links clearly state. most failures are within a few cm of a modification to the tube (i.e. a weld, a boss, a hole) etc...

for all practical purposes, Young's modulus doesn't matter, the modifications to the frame material matter much more. that is, of course, unless you just want to intellectually wank it to arguing which material has "more desirable" properties ... which is a useless metric as it pertains to a manufactured/assembled frame.

if you can't see that, then feel free to intellectually masturbate.

Bike Gremlin

I took a 20 year old MTB for a ride. Salvaged old friend's bike. Intend to use it in the winter. It is a steel frame. It felt so much more comfortable on bumps, than my aluminim treking bike. I was sure it was down to a lot fatter tyres, but it got me thinking. How good a frame can be in absorbing shocks, making ride comfortbale? I heard lots of talk about steel being more comfy, then some people swear in carbon fiber, but my personal feeling is that it's 99% down to fit and tyre thickness/pressure.

This test, however, shows that carbon fiber and steel are less stiff. Still not sure how much it influences ride comfort, but it must make some difference.

It's also comforting to know that alu bikes don't have to break apart. Not sooner than steel bikes.

I'm not sure about alu rusting? I thought it doesn't rust besides surface oxidation. Here it says: "...some aluminum alloys typically used in bicycle frames are comparatively susceptible to so-called "grain boundary corrosion", which can operate very briskly in the material and so cause a break..."

acidfast7

I agree with your assessments. Most of the forces transferred from road to rider have more to do with the non-frame components of a bicycle.

For example, if one compares the vibrations transmitted from the handlebars to the riders hands/arms via the hand grips ... you've have two different intensities of transmission via two similarly shaped grips:

GP5 BioKork: https://www.ergon-bike.com/us/en/product/gp5-biokork

GP5 Standard: https://www.ergon-bike.com/us/en/product/gp5

same with the tires, wheels, pedals, seat posts, handlebars and hand grips. although I haven't seen an empirical test, i would bet the bank that the items are responsible for a greater percentage of total variability in ground-rider vibration transmission that the frame itself.

i do find the German testing quite interesting. not only the testing methods, but also the consumer's desire to see the test results, and the average German's compulsion for owner the "highest quality" product at a given price point.

Bike Gremlin

)) Yes. Typically German. Funny and admirable at the same time.

Burton

Those test results are interesting. I'm just not sure they're easily applicable accross the board to all models made of those materials regardless of price point. I checked a couple of those frames - they were $1,500 mtb bikes. Since the averge consumer in NA typically buys a $400 to $600 hybrid - it might be of more practical value to see tests done on items in that price range.

garage sale GT

You can't use "maths."

A frame is too complex to model. It's not like predicting how far a tensile test specimen will stretch. You could say we don't yet have the maths.

A computer could break it down into a bunch of simplifications and compute a result using finite element analysis. So could a person but it would take forever.

Sorry old chum, couldn't resist.

garage sale GT

An inexpensive steel bike can be fairly stiff.

A bike made from 28mm tubing with 0.6mm thick walls can feel quite flexy especially if you exceed the rated weight by 70lbs.

A good aluminum frame feels stiff but it's in the direction you want. The steering seems to respond quicker and even standing up and hammering produces no feeling of flex. However, if you have a carbon fork and stays, it can ride better than the one made from the 0.6 mm tubing by far. I hate that they're going away from carbon stays.

My 2 cents.

contango 

Curious, so presumably I could make a frame out of leaves or something, if I did a really good job of it?

Retro Grouch 

Don't know about leaves, but certainly bamboo.

acidfast7

essentially one can make a bicycle frame out of anything solid with reasonable properties ... several companies make frames out of "plastics."

although common sense for most, it should be stated, that the individual characteristics of the original materials pales in importance to how those materials are "joined/connected" to make a complete frame.

and, leaves would be fine for a bicycle frame as long as they were joined/held together by some type of resin.

acidfast7

plastic bike.



how do you think the characteristics of those plastics compare to steel/aluminum alloys? probably not so well, eh? it's about the design/connection

full story:

https://www.gizmag.com/frii-recycled-plastic-bike/19337/

acidfast7

it would be great to have two of those plastic bikes, snap them apart and have a picnic bench/table for a brunch next the river, then snap them back into bikes and ride home.

that's more important to me than Young's modulus.

oh, and, the table better have two champagne bottle holders.

MichaelW

If you were to take two bike of the same material, brand and model but in different sizes (eg S and L), then a deflection test would give different results. It almost always true that bigger frames have more deflection for any given force, compared to smaller frames. This is the inverse of the requirement, where big frames are ridden by stronger, heavier riders.
You could build big frames with identical deflection or even better, with less (ie a stiffer frame made with fatter tubes) but it is expensive because you need to hold more stock in tube types. I know that Principia used to build like this.

JohnDThompson 

Steel and aluminum frames can be designed to have nearly identical stiffness, but since steel is much denser than aluminum, the steel frame will tend to be heavier and have thinner tubes. But your query presupposes that "stiffness" should be the over-riding design criterion in a bicycle frame. Is it really?

linus

OP, see what you have done?

This is the internet, where people talk **** about stuff that they have no clue about.

garage sale GT

I'm depressed now.

I am sure you would experience a substantial savings on your train fare if you took enough picnics.

acidfast7

I live right next to the river, no train fare required

However there's some nice vineyards upstream and downstream that make great spots for picnics

FBinNY 

It's impossible to generalize, because while the materials have different properties, the manufacturers can alter dimensions such as tube diameter and wall thickness to compensate and produce a frame with the ride characteristics of their choice.

It's not like there are ideal characteristics where more is good and most is best, except maybe for weight, where least may be best. Good frame design is a balance of characteristics, some of which conflict, to give the best overall ride. This can be done effectively with any material.