Stelvio CARC failure in Oz AND South Africa

[oldbike54]

Serious, yes but so long as you check for it before heading out what is the chances of it starting to crack and then coming apart all on the same run?

 Chances are good that once that type of stress crack begins it will expand rapidly . Not sure doing a visual inspection will catch a crack in its earliest stages .

  Dusty

[canuguzzi]

Chances are good that once that type of stress crack begins it will expand rapidly . Not sure doing a visual inspection will catch a crack in its earliest stages .

  Dusty

Is that because it is cast and not forged part? Is it more likely a forged part bends whereas cast breaks or are their too many other variables in the mix?

[oldbike54]

Is that because it is cast and not forged part? Is it more likely a forged part bends whereas cast breaks or are their too many other variables in the mix?

 Yeah , castings tend to be a bit like glass . Certainly some modern techniques have improved the process , but remember , cast parts are somewhat like a brick . One of our smart guys can probably explain that much better . Forgings or extrusions will be more able to bend , something to do with crystalline structures and molecular alignment . Dang , now my head hurts , TJ or one of you geniuses , help me out here 

  Dusty

[oldbike54]

 OK fellas , no politics , as Rocker would say , plenty of other places on the internet for that , and we are all friends here .

 Thanks

  Dusty

[Triple Jim]

TJ or one of you geniuses , help me out here 

TJ or one of you geniuses.  At least you kept me separate from that group.   

[canuguzzi]

Yeah , castings tend to be a bit like glass . Certainly some modern techniques have improved the process , but remember , cast parts are somewhat like a brick . One of our smart guys can probably explain that much better . Forgings or extrusions will be more able to bend , something to do with crystalline structures and molecular alignment . Dang , now my head hurts , TJ or one of you geniuses , help me out here 

  Dusty

I was curious because I had some time back drilled an tapped an aluminum piece being held onto some alum angle. Somehow it got bent and the drilled and tapped piece cracked and came off (well, part of it anyway) while the angle piece bent.  The larger piece looked sandcast. Materials were not too thick, maybe 1/8" for the angle and 1/4 for the cast. Hole was a 1/4-20 through bolted.

[boatdetective]

While aluminum as a rule is considered an extremely ductile material, castings certainly will behave more "brittle" than an equivalent wrought or extruded part. Yes, grain structure has a lot to do with it.

[oldbike54]

TJ or one of you geniuses.  At least you kept me separate from that group.   

 Kinda always thought YOU are one of the geniuses here 

 

While aluminum as a rule is considered an extremely ductile material, castings certainly will behave more "brittle" than an equivalent wrought or extruded part. Yes, grain structure has a lot to do with it.

 Thanks Jonathon , some times the terminology escapes me .

  Dusty

[MotoG5]

Ok, but Guzzi isn't the only one.

[Triple Jim]

The whole subject of a single sided, monoshocked arm is interesting to me.  The first time I heard of one, I thought that it seemed odd to replace a symmetrical assembly that has the support springs near the axle, so that the arm guides the wheel but doesn't have to do much supporting, with a single sided one with a spring near the pivot.  That makes the single arm take a lot of torsion and bending load that the old fashioned type doesn't have.  Statistics aside (only 3 out of a zillion, etc.), seeing these photos doesn't help me like the concept.

[Aaron D.]

I thnk it's heavier, too-but it looks great and would certainly make wheel removal easier.

Still..I wonder how VFRs do.

[oldbike54]

I thnk it's heavier, too-but it looks great and would certainly make wheel removal easier.

Still..I wonder how VFRs do.

 Not many Viffers traveling on dirt roads  Actually , single sided swingarrms with shocks mounted near the front are fairly common on sport bikes and ...

 Beemers . Ouch ! Yes , Guzzi ain't the only one with a problem here 
 
 

Ok, but Guzzi isn't the only one.

  Dusty

[canuguzzi]

The whole subject of a single sided, monoshocked arm is interesting to me.  The first time I heard of one, I thought that it seemed odd to replace a symmetrical assembly that has the support springs near the axle, so that the arm guides the wheel but doesn't have to do much supporting, with a single sided one with a spring near the pivot.  That makes the single arm take a lot of torsion and bending load that the old fashioned type doesn't have.  Statistics aside (only 3 out of a zillion, etc.), seeing these photos doesn't help me like the concept.

Maybe one company did it to be different and look cool, others followed suit.

It could be a weight saving thing but really,  on the bigger bikes is it saving all that much vs the older design? Once you get near 600 and up, does the other 10-15 pounds really make a difference? The thing is heavy at that point. Why you'd take a nearly 600 pound bike off road is a whole new question.

Seems like the trade was from lesser stressed and balanced loads to high stressed, twisting loads and more super critical parts with less room for margin.

[canuguzzi]

Not many Viffers traveling on dirt roads  Actually , single sided swingarrms with shocks mounted near the front are fairly common on sport bikes and ...

 Beemers . Ouch ! Yes , Guzzi ain't the only one with a problem here 
 
 


  Dusty

I don't think the VFRs are using cast swingarms, they all look extruded to me. Appears the same for most sport bikes, extrusions for the swingarms.

When you get down to it, why cast them anyway?

See many cast frames?

[oldbike54]

I don't think the VFRs are using cast swingarms, they all look extruded to me.

 Probably correct 

  Dusty

[motrhead]

 Castings are far better now days than what they once were. Vacuum and pressure casting processes have significantly increased strengths of components. But there is always the possibility of an inclusion, or pouring too cold...stuff happens. I remember the pics of the 2001 BMW GS forks and the grain structure looked like foam with voids and inclusions.  Thankfully bad stuff doesn't happen very often.

Here is another one that popped up on my facebook last night, a GS800:


not my pic.

[Wayne Orwig]

The monoshock connection point on my Honda XL600 collapsed. Who do I sue?

[canuguzzi]

The monoshock connection point on my Honda XL600 collapsed. Who do I sue?

Yamaha?


Sorry, had to.

[Randown]

The monoshock connection point on my Honda XL600 collapsed. Who do I sue?

Weight watchers?

[pauldaytona]

Guzzi had a single sided swingarm proto around 1990, before even Ducati thought about it.

[MotoZA]

Is that because it is cast and not forged part? Is it more likely a forged part bends whereas cast breaks or are their too many other variables in the mix?

Castings, in general, behave in a brittle manner. A ductile material can however become brittle if, for example, it's strain-hardened into the material's plastic region (beyond its elastic limit). It may then behave like a brittle material if loaded repeatedly close to and/or beyond the point that corresponds to the plastic set. A ductile material can also behave like a brittle material if working in low temperatures (much like a toffee that was left in the sun vs one that was left in the fridge -- which would you rather chew? Odd analogy, but you get my point). There are other mechanisms too that can cause a material to behave in a brittle manner.

As an example that you can try at home: Take a soft steel wire that you bend permanently by hand, initially it will behave like a ductile material, but if you bend it back and forth enough times you can feel how the wire becomes stiffer (less compliant) and it will eventually snap because it becomes brittle. This process, in essence, is fatigue (albeit very low cycle fatigue).

[Andrew_C]

Actually, yes, Dear Leader Mod, I do see the statistical evidence of the successful many as outweighing the failure of a few, especially as the only persuasive failure to me seems to be the OP's.

TL;DR

Isn't that exactly the thinking that resulted in the Challenger explosion?  "Lots of safe launches, so this one in the cold will be fine" when the question should have been, "We've only launched a few times in the cold, is this safe?"

[MotoZA]

I see on one other forum that some people are mentioning abuse...

There were 6 other riders with me (two of whom were going much faster than me) that can attest to the fact that I wasn't abusing the bike. I weigh 81 kg, the bike wasn't loaded, no pillion (fortunately), so not an issue here. I was going 40 to 50 kph. The bike was recovered with a 2-wheel drive pick-up, so it's hardly tough going. The rim and tyre is still perfect, no bends, dents or broken spokes. The front suspension is still perfect (it wasn't close to knocking through and it's on the factory setting). I had to estimate it, but I've done between 4 or 7 thousand kilometers of gravel road riding (never off-road, that's what enduro bikes are for) and have only ever fallen in thick sand, but not with Stelvio. NTX = "Nuovo Tipo Cross", to me that says gravel riding is OK. ABS can be switched off, to me that says gravel road riding is OK. The bike is marketed against and compared to the GS, Super Tenere, etc.,
Spoked wheels...

No abuse people, just normal adventure riding, exactly what it was designed for.

[Vasco DG]

I doubt very much you were abusing it. The first case I heard of here in Oz that was my first thought but looking at the damage to that and then hearing of your very similar experience makes me doubt abuse. Firstly you're lucid and obviously not a knuckle dragger and secondly the circumstances of the failures seem quite similar, (I know nothing of the Canadian event.). One failure could be abuse or bad luck. Three, all seemingly on bikes of a similar vintage, would seem to indicate a systemic problem.

Pete

[Rough Edge racing]

  I didn't read all the posts......... Millions of cars have aluminum suspension control arms (forged) so it's not something experimental...Alum inum has no limit of fatigue  ,other words an aluminum part can fail even if not stressed to the design limit unlike steel that can last forever if not stressed beyond the limit. This is why aluminum connecting rods are generally only used in drag racing engines....And some vintage British motorcycles 
 If those swing arms aren't forged aluminum ..they should be...if they are forged then they need to be made of a different material or a totally different design...

[molly]

A cast aluminium swing arm originally designed for the 1100 Breva road bike is being used in the Stelvio a heavy weight 'adventure' bike used by owners in difficult terrain and a few failures have occurred, hmmm..
Possibly the casting is being pushed to it's limits off road and any slight structural flaw during manufacture can result in cracking. I would suspect the inspection of the castings will be on a sample basis so the odd rogue unit could easily creep through to assembly.
Surely the problem is safety related and the factory are obliged to investigate and recall effected machines immediately (unlike cam wear) if there is a problem.

[molly]

  I didn't read all the posts......... Millions of cars have aluminum suspension control arms (forged) so it's not something experimental...Alum inum has no limit of fatigue  ,other words an aluminum part can fail even if not stressed to the design limit unlike steel that can last forever if not stressed beyond the limit. This is why aluminum connecting rods are generally only used in drag racing engines....And some vintage British motorcycles 
 If those swing arms aren't forged aluminum ..they should be...if they are forged then they need to be made of a different material or a totally different design...

I would suspect a forging would difficult/costly to produce and would be heavier.

[Aaron D.]

A forging would be more expensive.
A forging requires different and more equipment to make.
A forging would not have broken.

[WitchCityGuzzi]

I still don't believe it's a design issue.

Look through some of these pictures. This Stelvio did a 62,000 mile trip around the world in far worse conditions than the OP posted pictures of, and no CARC failure.

http://www.stelvio2stelvio.it/images

I would suspect a bad run of castings. My other theory is perhaps galvanic corrosion with a dissimilar bolt material into the aluminum casting. I can't imagine why that would happen on a particular model year more than another unless perhaps they changed the material of the bolt.

It'll be interesting to see what Piaggio/Guzzi come up with on that. It's clearly going to require a metallurgist looking at it to determine cause. The part is about 500 bucks I think and in the long run, a recall of them (if it comes to it) sounds like it'll be cheaper than the roller kits.

[boatdetective]

This part would be impossible to forge in one piece. The Aprilia swing arms appear to be extruded in several pieces, then welded.

Galvanic corrosion- I don't think so. You would see the threads packed with aluminum oxide.

Honestly, I think that the hole was an add on that was drilled and tapped after designing the plug for the mold. The hole is in the bold face of the casting- so it really is a weak spot- or at least an anomaly- in the rest of the surface. Normally, you'd have a thickened section, or boss, which then would be drilled and tapped.

It's very hard to predict fatigue and it doesn't mean all units must fail in order to be a design issue. The onset of fracture initiation can be a real coin toss. 

So Doug- tell me you didn't rush up to the garage and shimmy under your bike to check your swing arm??? :)