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DaveJ

Shattered hub and sprocket

20 posts in this topic

Bad news guys,

Today my sprocket and rear hub shattered, twisting the chain and destroying the swing arm chain guard as well. This after seeing the very same failure on another 426 just last week.

After reviewing the parts from my bike, it appears the same thing occurred on mine that occurred on the other 426. That is, three of the rear sprocket bolts broke which put too much pressure on the remaining bolts, thereby causing the hub to be overloaded and fail.

It appears that the bolts are getting sheared by the counter sunk tapered edge of the sprocket. This seems to only be a problem on MX tracks where the bike is repeatedly jumped, which I assume is causing the sprocket to rotate back and fourth against the hub, (something that would only be inherit to these bikes with the their torgue and engine braking ).

I thought it might have something to do with the fact that my sprocket was aluminum, but I now recall his was the factory steel unit.

So....have any of you guys seen this on these or other bikes in the past? And if so, are there any remedies to better secure the sprocket and hub?

Dave

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That sounds pretty hairy...

What I don't get DaveJ is how the sprocket was able to move enough to allow it to shear the bolts in the first place. My gut feeling is that it was coming loose first, and then it sheared the bolts. If it had sheared all six at the same time that would be a different story.

Also, if the bolts are backing out they may have been sheared off by the chain guide cover, then the remaining bolts were finally broken by the torque of the sprocket.

Loctite is your friend :)

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Hick,

During the first months of ownership the rear sprocket bolts always came lose. I tried blue loctite , which failed, then switched to red.

After several rides it was obvious that the red was doing it's job. They never came loose. Then after my second to last ride, a day filled with nothing but jumping, I noticed that I lost one bolt. This was the same day that the other rider broke his hub.

I just assumed it was a fluke, replaced it and returned the following week.

I would say within about 30 minutes of jumping I lost all three bolts, which was quickly followed by the above event.

Based on the level of attention I give this bike after each ride, I know that all bolts were secure and in their place.

When I took a closer look at the hub and sprocket this morning, I noticed that the last three bolts had no sign of wear except the top edge, (contrary to my earlier thoughts). These edges are no longer smooth, and are partially mushroomed, as if the bolts were tipping causing that edge to smash.

However, this may have been the result of the previous events in which the bolts came loose.

The sprocket holes are now ovaled, and the area where it meets the hub appears as though the metal has been melted, then packed back onto the surface. Much like a scored piston would look. Evidence of friction.

However, It's not obvious from the area around the hub that the sprocket was moving - the chain oil residue indicates no movement.

What I really need to find are the broken bolts, but it's becoming obvious that they are either getting sheared or just snapping under the extreme stress of this style of riding.

The track that both of us broke our hubs on is very hard packed causing the tire to stick like it's on asphalt. Often you launch and land in a low gear which puts a lot of stress on the rear wheel , chain and such when landing, (unless you're smooth enough to tap the throttle). In a couble of spots you land faster than your take-off, which beats the bike with the same effect as an accidental down-shift.

The two strokes sound like they hit their their redlines on landing, but they rev so freely they don't seem to be bothered by this.

So I don't think this will be a problem for most, but those of us that really work these bikes like this it's an issue.

I'll try to post some pictures and let you know what I find from Yamaha.

Anything else you can think of on this let us know.

Dave

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DaveJ,

Didn't mean to imply you didn't know how to take care of your bike...

And I agree, blue loctite isn't infallible. I've lost some fasteners even though I loctited them and probably over-torqued them. But the sprocket bolts on my 426 have so far behaved (w/ blue). But I ride offroad, dez, no hardpacked moto 'round here.

If the extra weight and price doesn't represent heresy to a motocrosser like you :0) then maybe the harder Sidewinder stainless sprockets will better resist having the mount holes oval out. Obviously once this happens it is much more likely that you will start breaking bolts.

I've got a few Sidewinders in my sprocket collection and they do feel almost twice as heavy as a Renthal but they last forever. My 49 tooth has outlasted one Regina o-ring and looks good enough to outlast a second.

Since you didn't wreck your chain guide cover I guess it wasn't what was shearing off the bolts...

I think I'll go home and retorque my rear sprocket bolts...

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Good news! My dealer got Yamaha to warranty the cracked tank. I hope to get a new one by the weekend. They refused the hub, but I really think they have a hub problem and should warranty them also, along with all the damage they cause. I hope no one gets hurt from exploding their hub while landing the big one.

Scott F

>Today my sprocket and rear hub shattered, twisting the chain and destroying the swing arm chain guard as well. This after seeing the very same failure on another 426 just last week.

My failure was the same, plus the sprocket got jammed into the airbox and cracked it, so I had to replace it. I had just recently mounted a new sprocket and chain, so it was an expensive repair.

>After reviewing the parts from my bike, it appears the same thing occurred on mine that occurred on the other 426. That is, three of the rear sprocket bolts broke which put too much pressure on the remaining bolts, thereby causing the hub to be overloaded and fail.

I don't think the bolts are breaking. Do you have the broken bolts? I think they are either falling out, or the casting is failing. None of my bolts broke.

>It appears that the bolts are getting sheared by the counter sunk tapered edge of the sprocket. This seems to only be a problem on MX tracks where the bike is repeatedly jumped, which I assume is causing the sprocket to rotate back and fourth against the hub, (something that would only be inherit to these bikes with the their torgue and engine braking ).

No way an aluminum sprocket is going to shear those bolts. And the only way a steel sprocket would do it is if the bolts came loose.

>I thought it might have something to do with the fact that my sprocket was aluminum, but I now recall his was the factory steel unit.

The YZ stock sprocket is aluminum.

>So....have any of you guys seen this on these or other bikes in the past? And if so, are there any remedies to better secure the sprocket and hub?

Be sure the locking plates in the nuts are fully engaging the bolt threads, and use lots of blue or red loctite.

Scott F

[This message has been edited by Scott F (edited 08-29-2000).]

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Scott,

Good call on the aluminum sprocket verses steel bolt issue. Second thought (and after closer examination) and it now makes sense.

But somehow I, and perhaps the other guy, went from six well loctited and snug bolts to three. All in less than an hour. Perhaps too tight...huh?

Wish now that I had gone back looking for those broken bolts for a closer look.

Perhaps all the thrashing from the earlier days of loose bolts caused some damage to now cause this.

Dave

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I found all but one of my bolts. The other five were still tightly attached to the broken hub and sprocket pieces. I looked hard for the other bolt, with the missing pieces presumably still attached, so I could try to figure out the cause. I still cannot prove the cause of this failure.

I tighten the nuts as tight as I can by doubling up on the box wrench. I don't think it is possible to overtighten them with this method.

BTW, do not tighten with the bolt head. It will not be sufficient. Once snugged up, the bolt head will not move, and the nut can be tightened further.

Scott F

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Originally posted by FITA:

Sounds to me like you just had a bad hub and/or some loose bolts. Spokes almost always break before a hub will shatter (when coming up short off a jump).

P.S. Keep an eye on that clutch basket. Mine just selfdestructed! YAMAHA sent me a new, redesigned 2001 clutch basket/gear/fiber & metal plates and springs! Overnight! It's good to see they stand behind their product.

Good luck with that hub prob.

FITA, how did you go about getting your replacement clutch basket? do you think 426 owners need to wait for theirs to fail before yamaha offers up a replacement, or is being a 426 owner enough to warrant a replacement? maddog in o.c.

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Sounds to me like you just had a bad hub and/or some loose bolts. Spokes almost always break before a hub will shatter (when coming up short off a jump).

P.S. Keep an eye on that clutch basket. Mine just selfdestructed! YAMAHA sent me a new, redesigned 2001 clutch basket/gear/fiber & metal plates and springs! Overnight! It's good to see they stand behind their product.

Good luck with that hub prob.

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Originally posted by MADDOG:

[b...is being a 426 owner enough to warrant a replacement? maddog in o.c.[/b]

I doubt it.

Yamaha 's official line is that only a small number of bikes had the defective clutch, those that are defective will be replaced.

There was an article in MXA on this, they showed a picture of a "bad" clutch basket. Defective units did not have reinforcing ribs on the inside of the tangs (inside clutch basket).

So if your basket does not have extra material on each edge of the tangs added to the inside of the basket then Yamaha will give you a new one.

But clutch problems have not been relegated only to these units. A few other people (me included) have had clearance problems with rivets on back of basket and case or main drive gear and others have had oiling problems and grabby, chattering cluthces. Sometimes a Hinson cures the oiling problem, sometimes enlarging the grooves in the actuation rod allows enough oil to reach the clutch.

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Dear DaveJ.

The same thing happened to me.

My hub has now got back from serious material tests wich found that there was some casting problem.

I got all new parts from Yamaha (warranty)and mounted a JT steel 50tooth sprocket since they found that the stock Yamaha one is really bad.

So to all You out there throw Your stock chain and rear sprocket in the litter can.....

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Numpsy,

Your'll have to share the details of what happen to your bike..terrain, riding style...etc. And hope you saw my later post as to what caused my failure (aka, my hub tested okay).

Dave J.

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Dave,

I was riding a normal sand/dirt track with no "super-jumps".

I went into a 2:nd gear turn and when I got out and shifted to third the rear wheel completely stopped.

My sprocket was twisted 90 degrees over the swingarm, the chain and chainguard was shattered and my swingarm was badly cut.

And my bike was only two hours old, I was devastated !!!

I called Yamaha and they told me to send the broken parts to them and I asked them to tell me the result and here we are !

By the way, You asked about my riding style ! I am a normal old school mx:er. No fancy jumper or anything like that.

Regards Numpsy

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Numpsy and Obrian,

I eventually discovered that the cause of my failure was the chain being too tight. The 40mm spec that Yamaha has in there manual is too tight.

Any chance this was the case with yours?

DaveJ

ps - As things are right now, it looks like there are going to warranty it.

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Davj:

I've also found that the minimum chain slack listed in the manual is too tight. When adjusted to the min. of 1.6", the rear wheel hops and skips over acceleration bumps as the chain tightens and loosens. I found the problem when I cinched the bike to a trailer with tie-downs from the foot pegs, and saw that the chain was very tight with the rear suspension compressed.

I imagine running it this way is very hard on the drive line.

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Make sure your chains are not too tight!!! All those failures sound alot like an overly tight chain. Given the amount of power the 426 produces, this is not surprising. I've been riding my 426 since Dec. 99, and have never had any of these problems that I am reading about here. No clutch trouble, no sprockets and hubs exploding, nothing at all. It seems to be very reliable to me, and it's terrible that so many people are having so many problems with this bike. It doesn't even shed fasteners like my old RM used to do.

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Dave,

My chain was not too tight.

Yamaha thinks that this was caused by the procket who shattered and that the hub was not perfectly casted.

I am not sure if they´re really telling the truth but I have now recieved two explanations of my problem and one is "air bubbles" in the casted hub !?!?

To be frank, I think that´s bulls**t.

Hope You have as much fun with Your 426 as I have and will have with mine.....

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All,

You guys are correct. The minimum chain spec that Yamaha has listed for this bike is too tight. And even the smallest variance from it is too tight, so be extra EXTRA careful about this. AND TELL YOUR MECHANICS AS WELL!!

This is clearly what caused my hub to fail.

As for other hub failures, when I had my dad look at mine (he's the engineer in the family) he told me that "usually" when castings fail, there are shining spots along the breaks, indicating that friction between the gaps was present before the failure. If it's a clean rip, then most likely the casting was not at fault.

I didn't think to ask him if air bubbles would show up the same way.

Thoughts?

DaveJ

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