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Aluminium vs steel frames

Discussion in 'Technical and Troubleshooting Torque' started by kols_kebabs, Oct 17, 2009.

  1. Why is aluminium the modern standard for motorcycle frames?

    Why don't ducati use aluminium for their trellis frames?

    Why do KTM still build their dirtbikes with steel frames?

    Aluminium is far more vulnerable to fatigue, so I'm told. Aluminium also has to be thicker for a given strength. These are things I have heard.
  2. I'm no materials engineer, but to start the thread off:

    Because it is easy to cast into nice sports-bike shapes. You could do the same with steel, but it is more costly to cast and would be much too heavy.

    To answer the other questions, you need to know what you mean by 'strength' before we begin. I'm not going to give you a mini lecture on materials science, because it'll take too long and I'm sure to make mistakes. Maybe someone else will though...

    Go to the library and read some books - J.E Gordon is thoroughly outdated but still relevant, and easy to follow for the non-engineer. Or do a quick google :rolleyes:

    Once you understand your question, you will understand the answers - or be able to answer them yourself.
  3. some more reasons.

    It don't rust so you don't need to paint it, it's lighter, it's easier to work with, noobs strip thread in it easier so they make money off you when you take it in for repairs, its cheaper then magnesium, sounds cooler to say and Americans cant say or spell it.
  4. There's the reason, right there.

    The frame of my 1987 FZR1000 is 12kilos (cast aluminium) that
    might have something to do with it.
  5. don't forget that it also mimics race bikes, so that gets the boys in ....
  6. For all practical purposes, you can make a frame light and stiff enough for any road bike out of either aluminium or steel.

    However, bike buyers are one of the most fashion driven markets there is and the current fashion for sports bikes (possibly the most fashion concious subsegment of the market) is for ally frames.

    Who cares that ally frames are fragile in impacts, difficult to repair after crash damage, have a finite lifespan due to the material's fatigue characteristics (although I'll admit there are still quite a few early GSXRs running around which don't seem to have cracked yet) and are generally made from bloody great sections that limit access to pretty much everything? If it ain't ally, the Stoner and Rossi wannabes won't buy it.

    Of course, they might be slightly less enthusiastic after repeated hard landings from wheelies flog out the rather soft head bearing seats.

    Flame suit on :bolt:.
  7. No PatB you are right
    My 1987 FZR1000 has a cracked frame at the moment after only 22years and
    295000 kays together, they don't build 'em like they used too.
  8. I'll jump in.

    You have more control over the amount of 'flex' you can introduce into a frame using steel than ally. You only have to read about the problems they had when first using carbon fibre in frames (and still do). People think that flex is a bad thing for one reason or another.
  9. From what I've heard, steel is both cheaper and easier to work with. Apparently aluminium is quite affected by heat (e.g. from welding) and needs special treatment.
    But from what I've heard, for a given density, aluminium is stronger.

    This is an article worth reading: http://www.kleinjapan.com/at_klein/garys_speech.pdf
    Admittedly it is related to bicycle frames, but it does delve into this alleged fatigued strength. The author claims that under normal use, aluminium will have a better fatigue behavior than steel. Furthermore, he(?) believes that the weaknesses in aluminium's application are due to its engineering and construction.

    Its probably also worth pointing out that most motorcycles rely on aluminium for the swingarm.

    I suspect one reason why Ducati doesn't use aluminium for the frame is because the engine is narrower, meaning that they don't have to cast a complex meandering structure to get a straight beam running from the steering head to the swingarm pivot.
  10. Why do they use aluminium to build most civilian aircraft?

    Because it is light and strong. It doesn't corrode easily (in pure form) and it's much easier to work with during the manufacturing stage when they will be bending and cutting and casting the ally.

    Depending on the type of heat treatment and what other metals it is alloyed with gives it different properties.

    For example, if you mix magnesium and aluminium you get a VERY strong yet flexible material (the main spars in the wings of aircraft are made of this very material).

    EDIT: To make my statement about corrosion more clear. It does corrode almost immediately once exposed to oxygen but this very thin layer of corrosion protects the core of the material.
  11. Aluminium has a better stiffness to weight and strength to weight ratio.

    However, I suspect the major reason behind it is ease of mass production. To achieve the same frame stiffness and strength using steel you have to use a space frame like Ducatis. Which means, lots of individual tubes that all have to be cut and welded together. Its probably the reason why duc's and the HP bmws cost so much

    Steel has a fatigue limit of about 50% of its ultimate tensile stress, that means that if the cyclic loading is less than that stress it will not fail from fatigue. Aluminiums fatigue limit is less well defined but is around 25% of UTS. However, designed around achieving high lateral stiffness. That suggests that stiffness not stress is the limiting design factor.
  12. Not bad, but I can point to a few steel framed bikes that have managed that many miles of thrashing without failure. And, of course, the steel framed BMW F650 that broke its frame after less than a tenth of that :eek:.

    Aluminium is an excellent material for many things, including aircraft and (notwithstanding my earlier comments) motorcycle frames.

    However, I would contend that aircraft are a rather different case from motorcycles, if only because an aircraft will be subject to a much more rigorous regime of inspection and of replacement of time expired components.

    I would also contend that, regardless of any theoretical advantages of ally over steel for bike frames, the manufacturers give them to us because of (a) cheaper/easier mass production as alluded to upthread (b) because they're what sell, even to the 99% of riders who will never push their bikes hard enough to be able to tell any difference and (c) for homologation purposes for stuff like WSB where any advantage, however marginal, is worthwhile.
  13. That's it- really; aged aluminium allow shows better fatigue characteristics, but you must remember - you could do a lot, for less cost, with steel, but aerospace is heavier-than-air flight; The maintanance schedule only comes from necessity due to the materials

    Back to bikes, Aluminium is softer, more easily machined, but costlier to produce then steel; but, it's lighter. Steel produces stiffer frames and corrodes more readily (sic) then aluminium.

    As for production cost - Aluminium - 222.5 MJ/kg (fresh material, likely overquoted; hippie source) - Steel - 34.4 MJ/kg (same source, likely similar bias)

    Though with bikes, either will preform as well as the other under the same design limits - Aluminium is the "exotic" material, and steel/cast steel is the "cheap" material; Aluminium will require a greater volume; Steel less but weighs more. It depends on whatever the engineers/product designers have decided...

    Overall; I'd be inclined to steel for ease-of-reworking, but it depends what's important to you, in the end; AFAIK, alumimium tends to fracture as it bends; steel is easier to rework.
  14. Strength to weight yes, but total stiffness no. The Young's modulus for Aluminium is around 70 GPa as opposed to 200 GPa for Steel. this means that whilst aluminium has about a density about 1/3 of that of steel, you have to throw about 3 times the material at it if your section modulus is the same.

    This is why they make aluminium chassis' are so chunky.

    So you don't gain as much mass as the 3:1 would suggest.

    Then if you factory in you can make a tressle steel frame, which doesn't work in aluminium due to fatigue at the joint, suddenly there is nothing in it.

    Yep that would be my take too. This would factor second after marketing.
  15. +1 Everything Ibast said; stress cincentration is a major consideration, I remember a couple of years ago there was a bike (thinking one of the supersports) that was being recalled to have a support subframe glued on at the head stock, due to too many fracture failures from shock loading on the front end (too many wheelies); it's light, but they have to be very careful with their designs
  16. The aluminium vs steel debate is a big one in cycling, and there's lots of rubbish written on it. Probably the best article is from the late cycling mechanic Sheldon Brown http://www.sheldonbrown.com/frame-materials.html
    I imagine the concepts are similar for motorbikes.
  17. Aluminium does have a higher stiffness to weight ratio. The material selection index for maximum bending stiffness of a beam with minimum weight is the square root of youngs modulus divided by the density. For aluminium the index is 0.0031 and steel is 0.0018. So for a given weight of aluminium you can make a beam 70% stiffer than a beam the same weight of steel.
  18. Mine is half aluminium and half steel. Win-Win? But then the two pieces are glued together, so maybe that's not so great.
  19. OK I'm missing something from my engineering knowledge here. E is linear in every deflection formula I can think of, so why would you square root it for any comparison?
  20. Google " material selection indices for a stiff beam" there are some lecture slides that go through the algebra.

    Its basically because the stiffness of a beam goes up with the square of its area. A lower density means are larger area, means a proportionally stiffer beam. However, to add a qualifier, everything I've only been talking about is in the case of an ideal simply supported beam with a point load, which is a bit of a step back from reality.