Cycle Frame Weight And Materials

The Cycle Frame Weight Wars by Don Ferris

Bicycle frame longevity essentially lies within the realm of cyclic stresses, the material selection to meet those stresses, and fatigue. In the real world, frames experience fluctuating loads with use. Those applied loads are as varied as we are and two different riders will not apply the same loads to a bike even if riding the exact same bike.

Some materials have what is called a fatigue limit. A fatigue limit is a stress value denoting a point below which a material no longer exhibits cyclic stress failure with time (theoretically meaning they can withstand a cyclic stress below its fatigue limit forever). Again, some metals have this "fatigue limit," e.g., steel & titanium, but most non-ferrous metals do not, e.g., aluminum.

Aluminium will eventually fail under cyclic stress no matter how low the applied stress is, it's really just a matter of time (or number of stress cycles). The key point to remember here is that is may take a very long time or a very short time depending on the applied stress relative to the frame's design strength. On a heavily built aluminium frame it may take millions of cycles so say years, perhaps even decades, but on a super light frame, it may last only a few thousand or tens of thousands, meaning it could fail in months. Of course, this is assuming that corrosion or stress concentrations are not present for if they are, then the life cycle can be reduced significantly (this applies to all materials, not just aluminium).

The pro of aluminium is its density per unit of volume; its con is fatigue failure. As you decrease an aluminium frame's weight, and in turn, its strength, you reduce its ‘overhead’ ability to withstand cyclic stresses resulting in a decreased life cycle. This is why it may not be a good idea for the average rider to bet his bike money on an ultra-lightweight aluminium frame. This same trait is why some aluminium frames are stiff as hell and can last seemingly forever; they're overbuilt so that the applied stresses are significantly below the strength of the structure allowing them to last millions of cycles. It should be noted that overbuilt aluminium frames really lose their weight advantage over Ti and steel. Is that my bias showing?

When discussing aluminium, someone always brings up airplanes.
Airplane design showcases what aluminium does best: acceptable strength and a low relative weight. But, aluminium's lack of a fatigue limit is one very good reason why there is stringent monitoring of dynamically or cyclically stressed aluminium structures. Airplanes are also designed to allow sub-assembly replacement as they approach the end of their life cycle, which is an option you really don't have on a bicycle frame. The inverse example along these lines may be why springs and paperclips aren't made of aluminium.

The bottom line is that there is nothing wrong with buying an aluminium frame if it meets your life cycle and ride requirements. You just need to keep in mind that if your lightweight aluminium frame really does ride smooth (i.e., absorbs road shock) as a direct result of frame design (and not tire, saddle, fork, seatpost, stem, handlebar, and crankset choices), it may not do so for long.

The difference between Columbus Altec 2 and regular Columbus Foco steel for the same size frame, designed for the same rider, is about 60 grams. Let me repeat - 60 grams. Want to lay money on which one lasts longer? I could give examples that sway the difference either way, just like the boys in marketing do, but it's only loosely based on reality for the average rider. I can build a Foco frame that cuts that difference, but then I could build an Altec2 Plus that gains it back, either way, you're compromising something to get there. There is no free lunch.

The super light aluminium tubes are not what I would call long-term investments for aggressive recreational riders (that includes Pros that don't get free bikes), nor are the super light steel or ti tubes. Further, none of these tube sets are proven anywhere except in the eyes of the marketing boys and their ad copy. You should see the list of warnings, cautions, and advisement's that go with building with these tubes. They simply haven't been around long enough to be proven. Frames can be a lot like wheel sets. Someone mentions the lightweight ADA/Lew/Zipp/Zero-Drag-Teflon-Coated wheels and most folks think "race wheels"; don't train on them, fragile, crash and they're history. I offer that many, if not most, sub 3-pound 55cm frames fall into the same category, regardless of the material used to build them. That's not marketing, it's just fact.

Steel and Ti, fatigue limit be damned, can still break through fatigue. Sure, design the frame to keep the stresses comfortably below the limit, use a careful build to eliminate or minimize stress risers/concentrations, and it can last forever. But, many of today's super lightweight steel tube products have a yield strength that closely approaches their ultimate strength. Overstress it just a few times and you'll have problems. Aluminium has no fatigue limit. Each stress is one less stress that it will tolerate in the future. Aluminium frames are built to survive a given number of cycles, and that is determined by the ratio of expected stress to design strength. Aluminium frames can be exceptionally long lived, but those that are don't weigh in at a kilo either. There are reasons why builders and manufacturers recommend weight limits based on the material and the design and there are reasons why many of these frames fail.

What it all comes down to is simply strength to weight ratio with durability/fatigue being the wild card. Aluminium is three times lighter than steel and its best examples are also three times weaker than steel's best (410-470N/mm2 for Starship/U2 vs. 1400-1450 for Foco/EOM). Aluminium proponents like to profess that Al is three times lighter and only two times weaker, such is the case if you compare it to your garden variety 4130. Anyway, Al can build the lighter bike because it can be used in the larger tube sizes needed to provide the necessary rigidity (and this is why Al bikes aren't three times lighter than steel or ti frames). But, as its weight is pared away, so is its life cycle. It becomes more and more reliant on exceptional manufacturing processes, and much less tolerant of errors, and its failure mode is not pretty. Maintain your dental insurance. Ti & steel at the limit become too flexible and susceptible to crippling (beer canning) when formed to diameters large enough to build a rigid structure at such a low weight, unless the tube is highly shaped and worked, and even then, it can still fail suddenly, fatigue quickly due to a manufacturing fault, and it is very difficult to produce and work with.

Steel is heavy, there's no getting around it. Steel bike frames on the other hand don't have to be heavy. Steel is 2 to 3 times denser than Al or Ti, but it is also 2-3 times stronger. That strength allows thinner walls in larger tubes to provide adequate stiffness but steel can never catch up to aluminium when it comes to light frames and it has a hard time hanging with ti. As it is, the Al guys can build a 2-pound frame with a weight limit but it's going to be flexy and it ain't going to last. I just finished up an 1168gram 55cm frame (it's not painted yet so that'll add a few ounces) using an odd mix of tubes and it's the lower limit of any frame that size I'm going to build. Maybe if I built a compact 52 and got jiggy with the mill on some parts I could get down to 2lbs, but why?

By relying on steel's superior strength, you can build a super-light steel frame but it rapidly becomes an issue of workability. The thinner the tube, the less tolerant it will be of faults. We're seeing this with the super light Al tube sets all the time. Add the fact that lightweight steel is one of the hardest frame materials to work with. Due to its high strength and surface hardness it is exceptionally hard to mitre and to weld, let alone form and butt even in an annealed state. If the Carpenter Tech guys had found a way to economically butt and form Aeromet, guess what we'd all be riding? Yep, Aeromet with 0.5/0.3/0.5 butt profiles. Another downside was that Aeromet was so hard you couldn't mitre it, you had to grind the mitres in. No big deal, but indicative of the material's properties.

What about denting? Dent resistance is directly related to yield strength and surface hardness. The higher the value, the harder it is to dent. So yes, high strength thin steels can be dented easier than lower strength thick steels if the thicker product's cross-section allows a higher yield. This applies across materials also. Steel can be as much as 3 times (or more) stronger than aluminium, which means that it can be thinner while maintaining equal dent resistance. Think of the difference between aluminium foil and steel shim stock of the same gauge. Big difference. Tube denting can be a problem with any tube once it gets down to a certain wall thickness regardless of material.

Me, I can have anything I choose. I like Ti, it's a neat material, but my pragmatic side bristles at the costs of not needing paint. I chew through thousands of pounds of aluminium a year and pay the mortgage because of it, but have no appreciation for it as a frame material outside of dual suspension bikes. That leaves steel...until someone prods a tube mfr to produce a superior heat treatable stainless steel tube set in a WIDE variety of sizes and shapes. Did you know that some stainless alloys can be cold worked and heat-treated to tensiles approaching 400KSI?

If you'd like further references on this subject, try: ‘Materials, Bicycles, and Design’ a design and materials selection paper by Prof. M.F. Ashby

‘Fracture and Fatigue Handbook’ available through ASM International (depending on your age you may know it better as American Society for Metals) Press

‘Handbook of Materials Selection for Engineering Applications’ also available through ASM Int.

‘Strength of Materials’ by J.P Den Hartog -this is more of an engineering textbook for determining a member's ability to withstand or response to a static load and not a ‘properties of materials’ reference.

…or, you can just enjoy your bicycle and forget about the technical gee-whiz stuff.

Cheers! Copyright2005 Don Ferris, Anvil Bikeworks Inc.,

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