Posts
76
Joined
4/1/2017
Location
IE
Edited Date/Time
10/9/2020 12:04pm
Recently I had a freak mechanical problem which caused a large accident and numerous injuries. Just before a jump a rock caught the chain block at the rear sprocket and ripped it off at the welds on swingarm (showed no prior marks of stress) . As it wrapped up around the back wheel I went straight over the handlebars. Now that this has happened it seems like a very bad design compared to other brands I’ve ridden e.g KTM. I feel now that I have lost trust in the otherwise great bike that the yz450f is 
Not common but it happens. Had one guy on a 18 ktm450 get a rock stuck and do the same thing also his350 got a rock stuck in his linkage and stopped his suspension. These are anomalies not in the norm.
The Shop
I’ve gone on a tirade about this before, the way the Japanese companies mount their chain guides is absolutely pathetic and embarrassing. It’s not a freak accident when those things pop off, it happens way more than people think and when it does the results can be ugly since it has a tendency to bring your back wheel to a sudden stop. The fact that they’re still just bird shit welding these things on (and missing with the automated welder half the time) is mind blowing to me.
It's been a reasonably common thing since they introduced the cast swingarms. They tend to rip the bolts, most
notably, the forward one, right through the casting .
There's a whole bunch of companies that make strengtheners/ guards for the KTM, KTHuskies, and the coming KTGeeGees. They sure aren't producing them for no reason.
A good hit, a rock etc into the guide like your problem, will have a good chance at doing damage like yours on any bike. Probably the worst for this are CRFs over recent years, with many have only partial welds on the mounts, though I've seen, and (re) welded mounts on pretty much all brands, over the decades.
If it's a good, full length weld, there's not much point in adding to that weld.
A crap, partial weld, which is quite common, well, of course.
It's a murky area, with aluminium frames and swingarms, as to what actual Grande of aluminium is used. Many manufacturers, have their own variations on the metallurgy they use - proprietary, so to speak.
But, they mainly are 'blends' specifically developed to Not require artificial ageing / solution heat treatment after weldment. Mainly in 7000 series /H20 ( I may have that H number incorrect), that age back to specification in a short time. But then, there can be a wildly varying amount of grades of aluminium used on one single frame.
I've nearly always added reinforcing / addition tabs / brackets to my own swingarms, and to repairs I do for people, to ad in some geometrical reinforcement to the guide mount. And, to make up for HAZ zone 'damage' to the swingarm / repaired mounts / added bracing. I try to persuade owners to leave the Swingarm aside / unused for as long as they can ( which is usually the next bloody day, to allow for as much 'aging' as the materiaml allows.
And, I use / recomend the use of a full plastic guide / guard such as a TM, BRP, Acerbis, UFO, Polysport etc for some flexability to it, to both give the mounts an easier time, and to allow the guide to move back in line - bend the alloy cage, it stays bent, and can derail the chain, even if lightly tweaked. Many riders will just jettison the alloy part of the guide, if the design allows it, and the internal OEM plastic portion of the original set up is substantial enough - but, you are generally far better off to use aftermarket items, such as I listed, that are made
specifically to be used sans ally cage.
Pit Row
Think it is just one of those things...
Also, you might want to go look at the current mounting brackets on the other brands before being convinced that Yamaha is any different. I've had a few, on Kawasaki and Honda, that had welds that were incomplete.
There's even a few places that offer a servise to thoroughly re-weld and gussett the mounts on your swing-arm ...most notably on Hondas ...as they've done FOR YEARS...like JCR...
So, there's always that.
But if you get a crack. and all welds have cracks, its how that propagate that causes a failure like this,
I have done post mortems on failed welds, and sectioned ones that havent failed yet, and when you see what they are like, these types of problems are not a surprise, but by the same token, you cant expect they will last forever.
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