Hey Guys,
Over on the Vital MX forum where John and Dr. Sweden are discussing some things, I saw his most recent post where he seemed to feel that KTM may be remiss in not using aluminum for it's frames. I wanted to write to him when I saw that because I've been thanking god that not everyone uses aluminum yet. It has bad fatigue traits that are well known to engineers and really quite alarming. I tried to do my best to explain my point of view about it to Dr Sweden in the post below.
I'm curious about what you guys think of aluminum and steel frames.
Thanks,
Jim
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Hello Dr. Sweden,
Don't be uneasy about a steel/chromoly frame. KTM is the last manufacturer offering you a frame that you will be able to count on for the long haul as long as you take care of it... There has been a lot of discussion about how the modern 4 stroke engines, which must run quite close to their failure thresholds in order to perform acceptably, are like time bombs. How much time is on the clock before the engine goes to pot depends on many variables, but inevitably in the end, damage and wear resulting from multiple repeated stresses will result in a sudden structural failure of one or more parts and quite often the engine fails in cascading fashion, just like a card house.
Aluminum is just one more pie in the sky for the manufacturers, kind of like an overly inflated and stressed 4 stoke engine. It is like a stopwatch ticking every time you ride or even sit on your bike, or do ANYTHING to apply a stress to the frame. Aluminum's lack of an "Infinite fatigue life" guarantees that after a certain number of stress cycles, your frame will fail.
The reason to be glad if you've got steel results directly from the characteristics of Steel vs Aluminum as they endure repeated loading below the point at which either will fail initially. "Fatigue Life".
Any bar of material has a "Moment of Failure" threshold, a limit load which will break the material in question immediately when applied. All of our motocross frames, no matter what type, are designed well beyond that point. Even Travis Pastrana shouldn't have to worry about that type of failure. He can rest assured that his head tube will stay where it should be, at least while he is doing anything even remotely concievable by the people who designed his bike. But if he's got an aluminum frame, that confidence would only be warranted while his bike was fairly new .....
I'm going on real long about this I'm sorry, and it's complicated, but let me try to explain it this way:
Say we have 2 brand new, equal, fully charged car batteries. One battery, we'll call "Steel" and one battery we'll call "Aluminum". A "Dead Battery" equalls a broken frame.
A "Dead Short" would be the equivelant of a single huge jump with Fat Albert riding, where the frame broke as soon as he landed.
For all of us smart enough not to "Dead Short" our battery with one huge application of force, our normal usage of our bikes could be likened to drawing different amounts of current from the batteries, and making different demands on them, to do different jobs.
Riding down a smooth straight for example, might be like... running the map and dash lights in the car. Blitz the woops = maybe, headlights or wipers or both. Make the doubles, = maybe same or maybe same plus you turned on the radio. The big stresses that are easy to see are like when you come up short and land on the upside of something you were trying to clear, or go too far and flatland, or things like that. Their draw on the "Battery" might be like if you idled the car and played a car stereo system like you see on "NJ Shore" as loud as it would go, with big amps drawing a lot of power..
At some point, as the demand for current increses a point of balance is reached where the demands placed on the battery and the recharging power from the generator are equal and the battery's potential remains static and constant as a buffer, further demand can surpass the ability of the generator to replenish the charge. After this point is surpassed, the battery will be drawn down by the excess demand until it's dead as a doornail.
Steel is great because it behaves kind of like a car battery with a generator recharging it. There is a level of stress at which it will break immediately, and you do your best to avoid that, (Don't ride off a cliff) but the GOOD news is that steel ALSO has a level of stress under which it aquires NO damage from cyclical stresses. A level of demand under which it does not lose any charge. This spectrum of demand is where steel has what's known as "Infinite Fatigue Life".
The reason why Aluminum is a little scary to me as a motorcycle frame material is that it has NO level of stress below which you are not damaging it. It's like using a car battery without the generator. If it takes 100lbs in the middle of a bar to break it, and you apply a relatively small fraction of that load numerous times, it will break after some number of load cycles.
If you had a very beefy steel frame, it would be quite possible that nothing you ever did on your bike would ever cause any stress cracking, and your frame could last you as long as you wanted to keep it. The break bomb theoretically never has to go off...
On the filpside, beefing up an aluminum frame would only add some length to the fuse.
And they're making it worse by trying to add some flex into those aluminum frames. By adding those highly touted points of flex to the frames, you concentrate the stresses and expedite the failure.
Maybe it's just me, but it just don't seem right.
Thanks,
Jim
