KCNC Scandium BB - Sheared off!

[chicane]

Someone please explain to me how this happens?

1. Yes it was tight.
2. The crank bolt does not appear to be damaged, AND it extends further into the axel than where the break occured.

I got back from a ride and changed training wheels to my new Nimble Crosswinds to take them out for a spin. 1 block from my house I stand up to accelerate down the road and BLAM, my right crankarm is now no longer 180 degrees from my left. Very weird sensation. The crankarm stayed attached through all of this. Rode back home carefully and disassembled to find it in this configuration.

I have emailed KCNC to see if they want to warranty the item. It has always been tightened to spec, so I would assume this is a defect in the axel material itself. I only weigh 135lbs.

Chris

[ergott]

From what I've read, that is the weak point of ISIS. The BB design leads to a stress riser right where it looks like it broke. Maybe someone elso can verify this.

-Eric

[madcow]

Not too uncommon with the scandium axels. I think the Ti is a much better way to go.

Did you have the stock bolts in the BB?

[jeremyb]

I wouldnt ride any scandium axled BB. Save weight someplace else.

[ScienceIsCool]

I agree with everyone else. The axle is cantilevered at the point it broke, and it undergoes a huge number of stress cycles (90 per minute on average... ). Without clearer photos, I'd say this was a case of fatigue failure. There may be ways to reduce the stress in the part through better design (i.e., not having an isis shaped spline), but aluminum - scandium doped or not - is not the greatest material for this application.

John Swanson

[onyourleft]

I just bought the Token Scandium unit and installed it today, but didn't have time to ride. Hope I'm not posting the same story as chicane tomorrow.

[Amadeus]

Scandium is like Aluminium a metal that breaks immediately (if it breaks). Titanium is different and has the advantage that - before it will break - the metal will bend so you know when to replace it in time. Titanium is rarely to break at once.

For riders over 75-80kg I would not recommend Titanium BB and Scandium or Aluminium BB I would never recommend. The weight saving is not worth the risk .......

[divve]

The aluminum BB spindle from the Cannondale SI Hollowgram is very reliable and strong. Personally, I've been using it in my MTB for the past 4 years.

[chicane]

Not too uncommon with the scandium axels. I think the Ti is a much better way to go.

Did you have the stock bolts in the BB?

I was using the standard bolts, which is what makes this failure weird. They extend very far into the axel (past the break), and were not damaged during the failure.

I agree about the Scandium BB in general. It always felt like a toy, so I have already ordered myself a replacement Token BB w/ TI axel (While this was sold as a KCNC, it looks identical to the Token, so I assume it was made by the same folks)

[onyourleft]

I've got the OEM bolts installed and properly torqued also chicane, so I'm crossing my fingers at this point. I'm leaving in 20 minutes to take it on a 23-mile rolling "time-trial" ride, on which there are several quick stand-up climbs. I'm going to pour the beans to it and not baby it, so we'll see if it's going to hold up initially.

If it eventually borks, I'll probably give the Ti/CF version a try too. If that one dies, then I guess it's back to boat anchors for me.

Damned WW parts!!

[DocRay]

ride the thin edge of weeniedom, you'll get cut.

These aluminum axles are like aluminum cassettes: disposable for race-only bikes.

[Provence]

I've got the OEM bolts installed and properly torqued also chicane, so I'm crossing my fingers at this point. I'm leaving in 20 minutes to take it on a 23-mile rolling "time-trial" ride, on which there are several quick stand-up climbs. I'm going to pour the beans to it and not baby it, so we'll see if it's going to hold up initially.

If it eventually borks, I'll probably give the Ti/CF version a try too. If that one dies, then I guess it's back to boat anchors for me.

Damned WW parts!!

Boat anchors haha

[520 Dan]

The aluminum BB spindle from the Cannondale SI Hollowgram is very reliable and strong. Personally, I've been using it in my MTB for the past 4 years.

The SI BB spindle is almost twice the diameter of an isis.

[divve]

About 30mm versus 22mm. My point was that aluminum can be used for a BB spindle provided it's designed properly.

[ScienceIsCool]

Yes, I agree with most of the posts above. But it really doesn't look like it was a shear failure. To shear a piece of aluminum that size would take a horrendous impact. Like dropping an anvil on it from three stories up. I think what we're seeing here is a fatigue failure at the point of maximum stress.

I think an increase in diameter and a change in spline pattern could help in increasing fatigue life, but aluminum really isn't up to the task. Titanium would be much better since its fatigue characteristics are that much better.

John Swanson