Tank Slap - Motorcycle Forum

I bought some aluminium sprocket nuts at the end of November when they were having a sale on.

Half have split. They've only recently split as they were fine last week when I cleaned the bike. I've no idea what's caused them to split, they were all fitted at the same time and using the same method and torque.

Blimey - i'd put the originals back on and drop in a complaint...

send them those photo's in an e-mail and I'm sure they will send you some replacements although you may be better off going for stainless steel rather than aluminium .

Yep, I've messaged them.

I've not had a problem with aluminium sprocket nuts in the past. Steel next time.

Wow, one of them hasn't even split at the thinnest part so I wonder what has happened there, glad you found it before bits fell off!

Fwiw I've never been a fan of alloy fastenings for anything stressed. Probably one of our engineering guru types will come on shortly and explain that I should have no real grounds for my reservations.???
That image is pretty shocking though, my complaint to them may have covered the subject of how lucky I felt to still be alive.
Be very interested to hear what they have to say.

Stonesie wrote:glad you found it before bits fell off!

^^ this ^^

Very poor IMO, for such a critical part you would have thought each and every batch was well tested.

That’s terrible Kwacky, but it’s lucky you caught before any major incident!
Fwiw I’m not a fan of alloy nuts on anything, let alone on plated steel studs, did you take one off to check on the condition of the stud?

StMarks wrote:Fwiw I've never been a fan of alloy fastenings for anything stressed. Probably one of our engineering guru types will come on shortly and explain that I should have no real grounds for my reservations.???
That image is pretty shocking though, my complaint to them may have covered the subject of how lucky I felt to still be alive.
Be very interested to hear what they have to say.

How stressed is a sprocket nut?

My guess is that rightly or wrongly, they might say its been over tightened but strange it has taken so long to been an issue.

have you removed one yet?

edit:

I notice Probolt sell some specific alu sprocket nuts with a steel helicoil centre. That might explain its not an unknown issue to them.

https://www.pro-bolt.com/aluminium/alum ... -nuts.html" onclick="window.open(this.href);return false;

Not yet. A family meal out took precedence.

Kwacky wrote:..How stressed is a sprocket nut?

I guess that the torque from your engine should be transferred to your rear wheel with all the stress being taken by the stud shafts. However the nuts must be necessary to prevent the rear cog from being thrown from those studs.??
Just to be clear Kwacky, when I said not stressed I was meaning that I would only really use them to hold a fuel filler cap or fairing screen. Perhaps even casings.
I'm not saying my preference is valid mate, just my choice.

edited my post above.

I see Pro bolt do sell specific aluminium sprocket nuts with a steel helicoil centre. That suggests to me they have had this issue before.

I imagine a lot of bolts and bits on a moden bike are alloy.

If aluminium isn't fit for purpose as a sprocket nut then they shouldn't sell them.

Kwacky wrote:

StMarks wrote:Fwiw I've never been a fan of alloy fastenings for anything stressed. Probably one of our engineering guru types will come on shortly and explain that I should have no real grounds for my reservations.???
That image is pretty shocking though, my complaint to them may have covered the subject of how lucky I felt to still be alive.
Be very interested to hear what they have to say.

How stressed is a sprocket nut?

It’s under whatever initial torque it was tightened to, plus gyro force as the bike changes direction which adds to the initial torque (including any side whip of the chain).
The drive wouldn’t normally be borne by the studs in a shear loading, the drive is transmitted via the sprocket to wheel hub contact which is clamped in place by those nuts. The studs (theoretically) only have a shear loading if the sprocket nuts are loose. HTH.

Edit - best guess from pics is approx 2.5-3 linear tonnes of clamping force per nut, but tbh there are some huge variables in that figure so please don’t take that as gospel.

Tbh I'm surprised they were sold as sprocket nuts.

I was under the impression you can't use aluminium in load bearing fasteners and thought that was a given, which explains the reason the ones Duke mentions have steel helicoils. If that's wrong it's news to me as I'm old school and things may have changed.

Going a little further in studs the steel needs to be high tensile steel (Gr12.9) and not just mild or plain carbon steel.

Also what torque were they tightened too and were the threads dry or a thread lubricant used ?
If you can use alloy in this application I'm pretty sure it will have a different torque spec to steel.

To get the same bolt stress you can need double the torque between different thread lubricants dependent on their coefficient of friction.

To be safe. On anything load bearing I would only use oem spec materials tightened to manufacturers recommended torque settings using the specific thread lubricant if specified in the maintenance manual.

Probolt have come back to me.

In brief:

Aluminium is fine to use.
They're happy to replace the set
I've got a choice of stainless steel or aluminium
They want the failed set back for testing.
They sent me a returns form.

That seems fair enough , decent service from a reputable company , just as it should be .

Looks like torque settings are the likely culprit. I used the OEM of 44 ft/lb, they recommend 33ft/lb for the aluminium ones.

They're still going to examine the failed bolts and give me a free set. I did offer to pay for the replacement set if it's my fault.

Kwacky wrote:Looks like torque settings are the likely culprit. I used the OEM of 44 ft/lb, they recommend 33ft/lb for the aluminium ones.

They're still going to examine the failed bolts and give me a free set. I did offer to pay for the replacement set if it's my fault.

that's great customer service