My Output Shaft Snapped.

[TCMB371]

So my output shaft snapped today just behind the front sprocket. Luckily the rear tire didn't lock up but rather the bike freewheeled. Chain slack was normal. I was going through a fast roller section.



Heres a pic with the boot guard removed. Collar still over the end of the the shaft on the sprocket side and the sprocket still on.

Heres after i got the sprocket off. Looks like a clean break to me.

 

[C5tor]

This makes me very, very sad. Too bad you don't have a backup bike anymore.

 

[TCMB371]

Looks like it broke here:

 

[TCMB371]

This makes me very, very sad. Too bad you don't have a backup bike anymore.

Luckily it looks like Liquid has the replacement parts, but i may need some guidance replacing it. For now i'll have to shred my 2004 KX 125.

Anyone ever replace the shaft before?

 

[Rashid510]

Yep. Its not fun. Requires removing the motor and then un-pressing the shaft out (as seen with an old MI we used)

 

[VINSANITY]

Wow, a little scary on what could have happened.

I am guessing you ride a little harder than most people, but still you wouldn’t expect this to happen. Alta’s have a lot of torque especially in map 4 so maybe it’s just a matter of time for this to happen on other bikes.

That groove looks pretty square edged so maybe it is a design flaw?

 

[Rashid510]

Your probably gonna want these as well since the O-rings were "custom"

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

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Bearings are all press fit..so if you got a hydraulic or arbor press - your set.

 

[TCMB371]

Your probably gonna want these as well since the O-rings were "custom"

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

Bearings are all press fit..so if you got a hydraulic or arbor press - your set.

On that topic, I'm probably going to want to replace all of the orings in there. Do we have a source for the large Oring that goes around the motor?

 

[Rashid510]

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

 

[VINSANITY]

TCMB371 should also get new bearings from Liquid, would you have the part numbers as they are likely not listed on ebay.

 

[UKLee]

So my output shaft snapped today just behind the front sprocket. Luckily the rear tire didn't lock up but rather the bike freewheeled. Chain slack was normal. I was going through a fast roller section.

View attachment 7517

Heres a pic with the boot guard removed. Collar still over the end of the the shaft on the sprocket side and the sprocket still on.
View attachment 7519
Heres after i got the sprocket off. Looks like a clean break to me.
View attachment 7520

View attachment 7521

In the last picture can I also see the spline cracking right next to your thumb? That would be the back side of where the sprocket sits.

 

[enjoythesilenc]

It looks like it had a fissure forming around the circumference of the shaft given the look of the oil contamination. Finally weakened to the point of the remaining material shearing off "cleanly". Point is, maybe it was visibly cracked for a while and that area might need watching as our bikes wear in?

Also, maybe a longer threaded sprocket bolt would help strengthen the shaft as it would span across the fracture zone?

 

[Mark911]

Wow! Cracked at the weakest point, the o-ring slot. That square slot is a perfect stress riser. Probably should have machined a slot with much bigger corner radius to reduce the stress concentration. When you get a new shaft you'd be advised to carefully radius those corners with a dremel and then polish the slot to remove any tool marks. Maybe even shot peen as well. Wonder if chain tension was too tight?

 

[rayivers]

Based on my motor-replacement experience in my non-Alta bike, if this happened to me I'd change out the entire motor if feasible (I was surprised at the difference fresh magnets made!) and/or send the old one out to Accu Electric or similar for shaft/bearing/seal etc. replacement - remagnetizing - rebalancing if needed.

 

[TCMB371]

In the last picture can I also see the spline cracking right next to your thumb? That would be the back side of where the sprocket sits.

Nah thats just the teeth marks from the vice grips i had to use to hold the broken piece while taking the sprocket off. Before doing that the piece was smooth. It was a clean break right at the oring seat.

 

[Flynryan]

How many hours on the bike? Ive snugged up the chain a little too tight before and could feel it in the suspension. Wonder if anyone else has experienced something similar

 

[rayivers]

Ive snugged up the chain a little too tight before and could feel it in the suspension. Wonder if anyone else has experienced something similar

Yes, I felt that too last year, so I started running my chain at 40-45mm of slack (Alta manual calls for 30-35mm). When I changed out the shock recently I positioned the swingarm at minimum slack, and found it was almost binding-tight at one point in the wheel rotation. Now I run the equivalent of 45-50mm on a stock bike (mine's been lowered) and find it needs very little adjustment compared to before.

 

[TCMB371]

How many hours on the bike? Ive snugged up the chain a little too tight before and could feel it in the suspension. Wonder if anyone else has experienced something similar

My MXR is right at about 180hrs. The chain slack wasn't tight. In fact, it was a bit on the looser side.

 

[Elpeladodelsam]

Thankfully someone has the spare so change it and continue enjoying it!

 

[Motophyllic]

Glad it didn’t happen on the face of a downhill double.