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20th Jun 2017, 09:16 PM #16I break stuff...
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I believe you made the right call - chances of inducing a failure aside, keep in mind that the pinch bolt torque around the fork legs on the lower triple, particularly on 'billet' triples like yours (6 Day?), is considered to be crucial. Something like 10nm on those clamps from memory, anything more is generally considered to cause a tight spot in the forks movement as the internal bushes etc pass that point. Whether the guys that claim that can actually notice this, or are just talking out their ass, I don't know, I tend to suspect the latter. However, I would suggest it would be very very difficult for you to get it back dead straight (especially with respect to being in plane to the upper triple) without some investment in special tools, thus possibly causing accelerated fork wear/stiction, and also potentially throwing the front axle out of alignment somewhat.
As Jhovel mentioned - how do the fork legs look? To be able get a bend in KTM triples suggests the fork has moved a fair way - I wouldn't exactly consider KTM triples weak... Must have been an impressive stack, I suspect the involvement of a tree or a large cliff?
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20th Jun 2017, 09:38 PM #17Most Valued Member
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- Oct 2007
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Don't feel bad about asking, as it is completely understandable going by the spiel out in there. Many, MANY after-market components will be advertised something like "material type: aluminium billet" [or better yet, "aircraft billet" if they're really towelling the BS on!], because that sounds impressive. What they imply from that is the component was machined from one block of material, yet I've seen examples that clearly weren't even that, so they can't even get their own BS consistent
As a sparky you're no doubt aware there are many different types of "plastic", so substitute aluminium for "plastic" and you'd no doubt appreciate it would be impossible to answer the question and might help un-muddy the water a tad. Just like plastic, there are literally hundreds of different grades of aluminium, and even then, how they behave will depend on how they are worked/machined and treated in manufacturing. However it also goes further.
Pure aluminium is pretty useless stuff in engineering terms, so it's alloyed with various components, and they will radically change the strength and how the material behaves. Some aluminium alloys will show a very gradual and progressive failure (grab a window seat next time you go on a flight and watch the wings!) even past their yield point, which is what is done in ultimate failure testing of aircraft. Other alloys will fail suddenly and catastrophically with little or no warning.
One characteristic of aluminium (as far as I'm aware, all the alloys) is they have a finite fatigue life. What the life is will depend on the alloy and how much it's deformed. So yes, every aircraft wing will eventually fall off In that case the aircraft manufacturers make the fatigue cycles so great that the aircraft will be well and truly retired by the time that happens! Another example, if you like riding an aluminium bicycle, it WILL eventually fail even if you never crash it, just from the road buzz. That's just the way aluminium behaves. Generally the cycle amplitude is so small that the number of cycles is hundreds of millions, so it's not normally a problem. However in your case the amplitude of that cycle that caused the bending could be great enough that the cycle life is now very low, so even if it straightened ok, it may subsequently fail in use just from normal cycles. An engineer may correct me on all that, but I think I've got it pretty much covered.
Hope that somehow helps.
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21st Jun 2017, 07:06 AM #18Novice
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- Mar 2012
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- melbourne
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- 14
Thanks again for all the great advice and guidance!
In answer to your question Joe and Jekyll, yes, it was a pretty impressive stack. I hit the base of a long, steep hill at around 50kph, dust everywhere from riders in front, and made it about 100 meters. I was staying very close to the left side of the track to try and stay out of the worst dust. Someone had cut off a previously fallen log to clear the track, but it was still sitting about 50cm across the track. I hit the stump doing about 40kph square on the left fork leg, then slammed down hard on my right hand side. I picked up myself and the bike, a couple of broken fingers for me, but the bike, for the most part, was fine, a marvel of engineering!
I had a couple of months off the bike with my busted fingers so I completely rebuilt the forks, both the outer tubes and inner tubes are straight as a die. They really do make these things incredibly well.
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21st Jun 2017, 09:32 AM #19Most Valued Member
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24th Jun 2017, 07:49 AM #20Member
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- Dec 2014
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- melbourne
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Any material that has been permanently deformed has passed its yield point and is on its way to failure.
Straightening it brings that failure closer.
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24th Jun 2017, 11:12 AM #21Mechanical Butcher
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- Oct 2004
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