YZ450F Sheared hub!

[BSWIFT]

First off, it was not my bike.  A local guy was running in the Team event of the Goldrush EnduroMXRoadrace at Hallet Motor Speedway today and sheared off the hub at the rear sproket.  I heard about this right before I went riding.  I checked the bolts holding the sproket on and they were all tight.  Not sure of all of the circumstances but you might want to double check yours before your next ride. 
Sounds like he sheared the cast areas on the hub.  I will try to get more info tomorrow.

JA

 

 

[KWJams]

Ouch! :flame: wonder if the chain tension may have been to tight or wheel alignment may have been off?

Let us know what you find out :thumb:

 

[BSWIFT]

I got the info third hand so maybe tomorrow I will get a first hand account.

 

[BSWIFT]

Well I found out what happened to the 450F hub.  The course at Hallet was part MX, part H/S, and part Road course.  The guy is an A rider and a flat tracker at that.  He was transitioning onto the pavement under hard acceleration and when the tire grabed the asphalt, sheared the sprocket from the hub.  Saw a KDX do the same thing today.  I don't think this is really too unusual.  Automotive rear ends will do the same thing.  :debil:

 

[KWJams]

That will do it :(

Did the wheel lock up and spit rider over the bars?

 

[BSWIFT]

Locked up the wheel for sure, the guy kept it up right some how.

 

[Earthpilot]

There have been numerous reports of broken 450F hubs on other bulletin boards for the past month or so.. many therioes but nothing
that targets the problem

 

[PeteN]

I race Supermoto, currently with a CRF, but also with a WR and DRZ, and have never seen this happen. We transition from dirt to pavement at least once a lap, usually with a lot of wheel spin. This seems bad. IMO

 

[XRpredator]

Too tight of chain will do it. My brother has grenaded two (maybe even three!) hubs on his 426, and that's what we've attributed it to.

Better too loose than too tight, that's fodangsho!

 

[BSWIFT]

I suspect that XRP is right.  It happened on the third lap, first transition onto the pavement.  Likely he adjusted the chain right before the race and tweaked it a bit too much.  I'll check on it this week.

UPDATE:  The guy had just put a new tire on before the race.  This happened right in front of the tower and reports say he was sideways on the gas hard.  Decide for yourself what the cause was but I think it was a combination of hard/abrupt change in traction and too tight of a chain.

 

[tiddler]

sheared hubs are not so uncommon on torquey four stroke mx bikes. I sheared my 426 hub two months ago. A vet pro buddy of mine did the same a month ago. Both chains were adjusted to proper specs. I talked to some race bike tuners in my area and they said the big 4-stroke mx bikes make enough torque that the hub will actually flex. They also further stated that if just one bolt starts to come loose that the rest of the bolts will shear with nasty results. My suggestion (one that I follow religiously now) is to check your sproket bolts between motos at the track.

 

[nikki]

Yep... Red shattered one on his 426, too... this Spring. Then we started reading more into it and found out that it was kinda common in 426's for the exact reason that tiddler said - the torque and the sprocket bolts. I wonder if the YZ 450F has the same rear hub as the 426's did. I would have expected Yamaha to beef up the hub a little bit where the sprocket bolts on to help prevent this?

 

[SFO]

Pro dirt trackers were known to shear the flanges off of the hub that the spokes affix to.
Kosman had a box of grenaded ones and they ended up making a thicker flange with a larger radius in the base of the fillet.
I would think if there was a positive drive dog on the hub/sprocket interface we wouldn't put the bolts into shear and could bypass making a heavier hub to fix the symptom of hub breakage due to bolts doing double duty.
just a thought...