Can Alu frames ever feel like Steel frames?

The bottom line Try for yourself. Our hobby is cheap and cheap enough to experiment - there is so much out there and all the empirical knowledge out there can very much help design a bike fit for a purpose - for a one off event or even that crazy notion of one bike for life and do it all.

The "best" is simply imaginary and 3rd hand marketing bull mostly. It's just got to work for you ultimately with design. IMHO.

pileyk said:

I'm talking regular fully rigid normal/hybrid bikes, not the extremes of MTB or road bikes. I've read people claiming that frame material itself does not affect stiffness rather how the tubing is selected and put together, etc. By their logic the saying is that ALU is lighter than STEEL for a given stiffness, i.e if you wanted a steel bike as stiff as alu then it would be much heavier.

But the few alu bikes I've ridden (with no susp) all feel horrible and harsh with vibrations on rough terrain vs steel. So if it's not the frame material causing it, why can't manufacturer build an aluminum frame that way less stiff, similar to steel (even if it's more "noodley") that has the same comfort but it just lighter??! Most people who aren't riding serious would prefer an absorbent comfier ride at the expense of some flex surely...

Do such frames exist? I'm 200lbs so it can't be too delicately built but it annoys me when folks say "it's not the frame material" or "alu can be just as smooth riding as steel" because in my (limited) experience all alu frames so far feel quite harsh in comparison! If it was possible to make them less stiff with same flex as steel but just lighter, then surely that would be a very popular thing to sell.... Thoughts?

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My secret sauce back in the day was a quality aluminium frame with the fattest tyres one could squeeze on.

Just my own 2 penneth worth: I've owned just one aluminium rigid bicycle and I hated it- transmitted every pebble on the trail to my spine. My Al Mokomoko's fine- but that's full sus' and it still feels a little lifeless at times. Every decent steel frame I've ridden has felt oodles(!) better. Titanium though, in my experience, is something else: I've ridden the same trails on all three examples and the Hei Hei seemed to soak up the vibration while still being super rigid and nippy...

highlandsflyer said:

My secret sauce back in the day was a quality aluminium frame with the fattest tyres one could squeeze on.

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Yup. Basically that. Then get into very stiff wheels with no flex, then work on tyre volume and pressure it becomes easy to compromise and with almost immediate effect what is exactly the major factor at play.

Going forward quicker with positive immediate feedback vs. traction in sloppy mud vs. that long haul needed comfort mostly is indeed solved on the contact with the surface of the road / off-road.

The human contact points on a bike are only three, and in cases just only two. All these bullshit flawed reviews and comparisons when the very human interaction and physical contact with the bike is more than often not consistent amongst the battery of tests would get you kicked out of any college 2nd year for being just stupid to make such a false assumption.

In the car industry, it is known that the majority of the decision to purchase with a potential buyer is at the first physical contact with the car - the door opening.

In the perfume industry, it is known that the majority of the decision to purchase with a potential buyer is at the first physical contact with the jar and the top cap opening.

We are so flawed and so swayed as humans. And thanks why we have space here to discuss just what the bike industry is up to.

highlandsflyer said:

@fennec has an interesting example of a frame designed to flex at the stays. Aluminium.

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I was thinking about his Castellano. I also wondered how the aluminium plate chainstays stand up to flexing. Is it just accepted that they will have a finite lifespan then crack?
I have ridden a lot of aluminium frames. It's my favourite frame material by far. I think a lot of its "comfort" and durability can come down to who makes it. My Spooky Supertouch cross bike has endured extreme levels of use/abuse over 20 years and is still going strong. It is a comfortable bike (by my standards) and it has been my pick for long rough rides on and off road (full Tour of Flanders, South Downs Way in a day etc). It has worked tube profiles that seem to dampen things, but it goes like the clappers when you stamp on the pedals. On the other hand another bike i loved, my Stoemper Ronny, always felt harsher and cracked at several welds.

For all this pondering, the right set of wheels and tyre pressure probably makes more difference. You could argue that a nice shallow set of rims will bring you more comfort on any frame.

Woz said:

The human contact points on a bike are only three, and in cases just only two. All these bullshit flawed reviews and comparisons when the very human interaction and physical contact with the bike is more than often not consistent amongst the battery of tests would get you kicked out of any college 2nd year for being just stupid to make such a false assumption.

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Quite. Unless you test with the same saddle, grips and tyres you won't have a clue what you are comparing. Ideally of course also the whole finishing kit!

My singlespeed (Kona Kaboom) rides totally differently with different forks - swapping from triple butted P2 to Exotic carbon removed loads of trial buzz and made it far less brutal.

Woz said:

The "best" is simply imaginary and 3rd hand marketing bull mostly. It's just got to work for you ultimately with design. IMHO.

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Cycle journalists (i realise the term is a stretch) love a good frame cliche. "I noticed no shortage of lateral stiffness and at the same time there was ample vertical compliance". Of course there was.

maybe we aren't asking the right question.
maybe the question is "Should an aluminium frame feel like a steel frame?"

hamster said:

Quite. Unless you test with the same saddle, grips and tyres you won't have a clue what you are comparing. Ideally of course also the whole finishing kit!

My singlespeed (Kona Kaboom) rides totally differently with different forks - swapping from triple butted P2 to Exotic carbon removed loads of trial buzz and made it far less brutal.

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And wearing known shoes compatible to whatever the test bed as got. Which is usually some crap with reflectors necessary by law at the point of sale. Insisting "please put these on" for a test ride was typically greeted with a "OK" in my limited experience - XT SPuDs.

wynne said:

I was thinking about his Castellano. I also wondered how the aluminium plate chainstays stand up to flexing. Is it just accepted that they will have a finite lifespan then crack?

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The Castellano flex plate was designed so that it flexed over a fairly long distance so the stress per mm³ (ksi/MPa/psi etc) was kept very low and well away from any welded areas or stress raisers.




This whole thing about aluminium not having a fatigue limit is often mis-represented by bike journalists, keyboard warriors and such.
This comparative Stress Life curve (off Wikipedia, of course) that's always trotted out in these arguments does NOT show the cumulative reduction of strength (Y-axis) over repeated stress cycles (X-axis)...The "gradual inevitable fatigue failure of aluminium vs the infinite life of steel" as it's portrayed.





Any point on the line actually indicates the likely number of cycles to failure (X-axis) of material at a given stress level (Y-axis). ie: how many cycles at a set stress until a sample typically fails. It does indeed show that steel has a clear 'endurance limit' below which it can handle an infinite number of small load cycles without any reduction in physical properties. This point is around 1,000,000 cycles at 30ksi for 4130 steel.
Aluminium does not have this inflection point, so will still experience fatigue from loads down to lower levels. However, 6061 is still found to endure 1,000,000 cycles at 25ksi....and infinite numbers of cycles below 15ksi.

Unlike many engineered structures, bike frames are subject to a wide and unpredictable range and frequency of loads, so the S/N Curves are interesting, but not particularly useful indicator of likely fatigue life. The question of course is will a bike frame of given material ever actually experience cumulative stress cycles at a sufficient frequency to fail in normal use. This is inevitably a trial and error/ finger in the wind assessment. Most likely it will be a low number of high stress cycles that will kill a frame. Crashes, bad landings and heavy pedalling cycles, up and over to the top left of the chart, well above the low load and high cycles associated with the endurance limit of steel.

Good frame design and construction should aim to reduce localised stress (in any material), which will always extend the life of a frame. Unfortunately, the easiest and cheapest (so most common) ways to do this with an aluminium frame is to stiffen it up (which reduces stress) using fatter tubes, or thicker walls, which usually for a lousy ride on a non-suspension frame...

All the best,