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Aluminum frame purpose?
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Other things ktm does to get thier bikes lighter is using aluminum bolts instead of steel.
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Personally, I think there was a perceived weight savings on the part of us (the customers) but in practice that hasn't really panned out for most of the bikes. Maybe due to concerns for durability by engineers, testers and management they added material to the aluminum frame designs. I believe we can only compare the weight of a YZ125 or YZ250 directly between steel and aluminum because the rest of the bike and the format of the frame was not really changed on those bikes. The Aluminum YZ frames are substantially lighter (several pounds) than their steel versions.
The reality is that aluminum frames have certain manufacturing benefits when you consider a large scale operation. Most of the parts used on aluminum frames are forgings, castings and extrusion which get CNC machined and robot welded in a jig. These processes eliminate a lot of manual operations (skilled labor is expensive) from the production process. On the other hand, a fabricated steel frame requires a lot of bent tubing sections and stampings which take a bit more hands-on in the fit-up and welding process.
Personally, I'm in the aluminum frame camp because they seem to have a longer fatigue life in practice.
Steel frames eventually become brittle and crack while aluminum frames seem to take abuse for a much longer life span without cracks and failures.
I don't consider early failures in aluminum welds (Yamaha) to be a fatigue life issue. That's clearly a manufacturing defect due to some kind of welding process or preparation issue.
Most of us have heard how the factory teams used to swap out steel frames every few races and how they get a lot more time out of aluminum. Some of that has to be attributed to the improvements in FEA software and the engineering applied to these bikes, but some of that can be attributed to the choice in materials. Personally, I believe that aluminum can be a tricky material to design with, so it forced some of that engineering improvement, and the rest is simply due to the rise in computing power and availability of good software combined with engineers trained to use it. Aluminum motocross frames and ubiquitous FEA software both became a reality over about the last 20 years.
I would love to see what a current KTM frame weighs stripped down with nothing on it but the steering bearing races.
I doubt it's dramatically lighter than a 2005 - current YZ250 aluminum frame, but probably pounds lighter than the old YZ steel frames were. I have a 2002 YZ steel frame sitting bare (it's cracked) and will get a weight on it sometime.
KTM may have other incentives to stay with steel frames too:
1) Cost of converting an entire manufacturing line includes a lot of equipment ( forging, forming, welding, etc.)
2) Cost of changing a lot of other parts of the bike to work with the frame layouts beneficial to aluminum frame design.
3) There may be no real weight savings to be had by the time you beef up an aluminum frame.
4) Steel frames are more compact (This is related in some way to item #2).
5) Customers have not demanded it and it doesn't seem to be limiting performance of their bikes.
On #5 above, KTM changed their PDS to a rising rate linkage to satisfy a real or perceived performance improvement, I believe if aluminum frames are a performance enhancement, they would have or will eventually change to aluminum. Evidence that they don't see a performance gain is in the fact that every KTM still has a steel frame. I've heard that KTM did experiment with prototype aluminum frames. They seem to be willing to at least experiment with everything and keep what works best.
Something to consider... Aluminum is about a third the density (mass/volume) relative to steel. On the other hand, aluminum has around half the strength. So theoretically you can use roughly double the quantity of aluminum and have equivalent strength at about two-thirds the weight... In reality, aluminum gets beefed up because when it does fail, it fails catastrophically.
Pit Row
On Thumpertalk someone weighed bare steel YZ250 and aluminum YZ250 frames and the steel one was 5.8lbs heavier.
Obviously you can't just drop an aluminum frame on a KTM and have a 5 pound lighter bike, as they've had quite some time to optimize their steel frames, and they use chromemoly instead of the milder steel the Japanese were using. Even in theory they should be pretty close, as steel is three times stronger than aluminum but is three times heavier too.
At this point it barely matters, people are too entrenched in their camp to consider rational thought.
Are the bikes so much better now? Does that even matter, to the manufacturer, as long as profits rise?
The first gen aluminium frames for dirt bikes they made them out of tubing. Very rigid with not much flex and this held up dirt bikes getting aluminium frames for many years.
Steel frames stretch over time and go out of true.
If you buy a fairly new bike regularly it doesnt matter that much, if not its something to consider.
The truck industry must be like this. Why else would Ford go to all the work just to make an aluminium box? I can only guess because they are running out of things to change or make new and improved. If the new model is not "new and improved" in some way. why buy a really expensive new item when the "old" one is perfectly fine.
My bigger point. It is a marketing scam. Otherwise why would we not all have at least three 2 thousand dollar 2 strokes sitting in our garage. A 125, 250 and 500 that we have fun with and even race. Or at least one bike for the average or even almost poor guy. I realize those are early 80's prices but it is to my point. Imagine technological growth, as it is, stopped with the last steal framed YZ 125 and 250. Would have racing and riding MX suddenly become unappealing and boring? In reality, local racing gets boring, or non existent, because no one is even there due to prohibited costs of keeping up with The Jones.
Marketing is a bitch.
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