Investigating cause of a sprocket hub failure

Hi everyone. I was running a practice day at the track with my LO206 kart (senior), 2015 OTK chassis, when three mounting points on my rear sprocket hub broke. Since I haven’t encountered this, I’m hoping some folks here might have some insight as to what might have happened and how to prevent it in the future. It failed on acceleration through a high speed left-hand corner after running about 8-10 laps; where I would normally go flat if I was pushing I was letting up and then accelerated because I was going to come back to the pits. Prior to the session, I installed a different sprocket (58 from a 61) and sprocket guard, including new hardware that included flanged nuts (no flanges on previous nuts). Three points failed on the sprocket hub, but I’m wondering if it started with one and the other two followed, after which the sprocket deformed and took away my drive and I slowed down. Could it be possible I over torqued the nuts which led to a fracture in the hub? Could it be something else?

This was a used kart, and I’ve run it three other times since purchasing. The photo included hopefully shows some of what failed.

What was your process for installing the new sprocket?
It should be:

  1. Line up the line up the hub and drive sprockets first and tighten the locking bolt slightly.
  2. Attach new sprocket to hub and snug the bolts. Don’t torque yet.
  3. Verify the two sprockets are correctly aligned. Adjust as needed. If you don’t have a laser like me, I just use a metal ruler and it works fine.
  4. Tighten the hub to the axle and rotate the axle to see if there are any high spots in the rear sprocket. The chain with go tight or slack if there are.
  5. Loosen each sprocket bolt one at a time and then torque it down.
  6. Recheck for high spots and adjust the chain tension as needed.

That is an unusual failure in my experience. It has me thinking that if you tightened the sprocket bolts first then the hub bolt, you may have put uneven tension on the mounting points of the sprocket. The distance between the mounting holes will change slightly when you tighten the hub to the axle due to the gap the bolt closes to tiighten. That could stress the metal causing cracks or outright failure. And yes, once one goes, the others will domino on you. Those parts are magnesium and more brittle than the steel parts that bend.

As this kart is not new, but new to you the previous owner could have dropped a wheel one to many times smacking the sprocket and started the process before you got it.

Hard to tell because its a photo subject to lighting but it looks like there was a existing crack in the mounting tab visible in the photo. The existing crack area looks dull while the shiny part is where it ultimately failed. Maybe a crack developed in the tab from being straighten out after an incident.

I’ve never head of a sprocket hub failure.

Apologize for the dumb question, but the sprocket was on the left side of the hub correct?

Ultimately carriers are a wear item and I’d suggest always having a spare. They take a ton of load when you hit curbs or flex the chassis heavily and in the even that you ever throw a chain can be subject to tons of forces that could ultimately cause fatigue leading to failure. Magnesium is also a material that gets brittle with age so all of those factors together could have been a contributing factor to the failure.

The OTK sprocket carriers are probably the lightest but most fragile around, I’ve had situations where I mounted a brand new OTK carrier and ran a few sessions without ever going off track or having any “incidents” and they still were slightly bent when nearly new. I’ve had better luck with Kart Rebublic carriers, they seem to be the most robust of the mag units I’ve tried. If you don’t’ care about rotating mass comet has a forged aluminum one that’s pretty heavy duty and holds up to wear and abuse better.

I don’t have much experience changing the sprockets, and my process so far has been nothing that polished. Your point about the sprocket bolt tightening before the hub bolt is interesting, because I remember that’s exactly what I did this time; I tightened the hub bolt last.

Between wear and tear from the past and present, not great approach to sprocket changes, it could all combine to result in the failure I experienced.

The sprocket was mounted on the driver’s right (shown in the picture) of the hub, not on the left

That’s helpful advice, thank you. From the technical side of things, I certainly care about rotating mass; however, between my skill, other greater factors at play, and the fields I’d generally be competing in, I’d probably say it’s safe to worry a little less about it for this particular case.

That’s probably why it failed. The sprocket should be on the left side of the carrier on the shoulder. This distributes more load to the center of the hub and less to the bolts. I usually only run 4 bolts on my carriers with split sprockets, one on each side of the split.

Is that what the edge is for on the shoulder of the carrier? The previous owner had it mounted on the right side, with the guard on the edge on the left, and I mistakingly thought that was the correct placement.

Yes that is where the sprocket should reside. Unfortunately it sounds like you were kind of setup by the previous owner. Some sprocket carriers have a shoulder on both sides so offer the flexibility to mount the sprocket on either side; however, my preference is always on the left as it’s more open and easier to take the sprocket on and off.

Well, thank you! And while I was changing the sprocket I was a bit frustrated by the position as well, so any serviceability hack for it is also helpful. Do you recommend a side for the nuts as well?

My preference is nuts on the left (sprocket side) with socket head allen bolts. This allows you to use an allen key (preferably a T-handle) to hold the bolt and use an impact with a 10mm socket to tighten the nuts. Always tighten the bolts around the splits first, ensuring the chain is fully seated on either side of the split before tightening to make sure the tooth spacing is correct.


Thanks! By “splits” do you mean a split sprocket and the point where it is split?

That’s correct, the points where the two halves come together.

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