Sam2 rear axle working loose while riding..

[millemille]

2020 Focus Sam2, completely standard, and am having problems with the rear axle repeatedly working loose while riding.

Anyone had any experience of this or any pointers as to what could be causing it?

TIA

 

[Mikerb]

do you mean the thru axle?
If so ensure the shaft and thread of the thru axle are clean and lightly greased and tighten to the recommended torque using a torque wrench.
If it still continues to come loose then it is likely the thread has been damaged ( by being loose) and it will need to be renewed.

 

[steve_sordy]

I had this on my Focus, it will be the same design as yours. I cleaned the whole thing and retorqued. Still came loose. So I redid the cleaning and retorquing, but this time I added one more Nm. Still came loose. Next time I redid the cleaning and torquing to the correct amount, but this time I added some blue Loctite to the thread. It never came loose again. Even when I removed the wheel later and had to remove the axle, I just reassembled and retorqued without going through the cleaning and Loctiting and it still didn't come loose. The residual Loctite was still doing its work.

 

[millemille]

I had this on my Focus, it will be the same design as yours. I cleaned the whole thing and retorqued. Still came loose. So I redid the cleaning and retorquing, but this time I added one more Nm. Still came loose. Next time I redid the cleaning and torquing to the correct amount, but this time I added some blue Loctite to the thread. It never came loose again. Even when I removed the wheel later and had to remove the axle, I just reassembled and retorqued without going through the cleaning and Loctiting and it still didn't come loose. The residual Loctite was still doing its work.

Thanks, glad to know I'm not the only one.

But you weren't able to get to the bottom of what was causing it?

Sticking some threadlock in there is an option, but I have the wheel in and out 2 or 3 times a week for cleaning/maintenance so not sure how long it will last.

My thoughts at the moment are it's either something to do with the rear hub (if the distance across the hub is closing up when the bike is being ridden) or the rear triangle is flexing excessively....

 

[Mikerb]

the loctite worked because it helped to remedy the wear/damage in the thread caused by movement having come loose. If the thru axle comes loose when riding and is not immediately rectified the thread will be damaged. So loctite can be a remedy as long as the damage/wear to the thread is not too severe, but a replacement thru axle is the full remedy. Loose thru axles have been reported on this forum for other bikes besides the Jam. The torque setting for thru axles is very specific but it will not be achieved accurately if there is too much friction. That friction can occur on the threads of the thu axle, the threads of the chain stay and between the head of the thru axle and the chain stay. Significant forces act on the thru axle of the back wheel in addition to vibration.........the rear wheel also gets the worst of any dirt from the trail......best not to disturb it other than when cleaning and servicing.

 

[steve_sordy]

Thanks, glad to know I'm not the only one.

But you weren't able to get to the bottom of what was causing it?
..........

I am convinced that it is poor detail design. A fastening properly designed with the correct torque should not come undone, otherwise bridges would fall down and engines would fall out of cars. If the constraints of the materials and the dimensions prohibit a good secure fastening then other measures should be specified at manufacture, such as anti-rotation washers (not suitable for a frequently used fastening), pins (ditto) or an anti-shake adhesive such as Loctite.

 

[millemille]

Having had it all apart this morning I can say, as a chartered mechanical engineer, that the rear axle design is ripe for losing prevailing torque.

Any 3rd body contamination - dirt - between the axle head shoulder and flip chip, flip chip and outside face of drop out, inside face of drop out and hub on either side and the integrity is compromised.

Lesson learnt; get everything - flip chips, drop outs, axle, hub etc. - super clean before assembly.

I also found the rear hub disc side outboard spacer, the top hat through which the axle runs, finger tight. Again, this could lead to a loss of prevailing torque if it tightens even slightly and affectively narrows the hub width.

No damage to the male thread on the axle or the female in the drop out/mech hanger.

 

[Mikerb]

Having had it all apart this morning I can say, as a chartered mechanical engineer, that the rear axle design is ripe for losing prevailing torque.

Any 3rd body contamination - dirt - between the axle head shoulder and flip chip, flip chip and outside face of drop out, inside face of drop out and hub on either side and the integrity is compromised.

Lesson learnt; get everything - flip chips, drop outs, axle, hub etc. - super clean before assembly.

I also found the rear hub disc side outboard spacer, the top hat through which the axle runs, finger tight. Again, this could lead to a loss of prevailing torque if it tightens even slightly and affectively narrows the hub width.

No damage to the male thread on the axle or the female in the drop out/mech hanger.

I agree dirt which causes friction and prevents correct torque is the problem..........but disagree on one aspect of your investigation. If everything is clean when assembled, the inevitable dirt accumulation on the back wheel during a ride will not impact on the thru axle. It is assembling the thru axle with that dirt already present that causes the problem, and is then compounded by the thread becoming damaged if the bike is used with the thru axle not at the correct torque and coming loose during a ride as a result.
It is a basic principle that applies to all fastenings that the recommended torque is calculated to ensure maximum friction between the male and female thread sections whilst being below the shear force .....all set out in manuals that enable engineers to specify in line with material, thread pitch, and diameter.

 

[Jackware]

So in summary;

Mille - My rear axle keeps falling out, the bike may have a design flaw - did I mention I'm an engineer?
Mike - It's called a thru-axle - let me explain about threads and recommended torque
Steve - Just glue it in
Mille - I'll just clean it before I tighten it all up
Steve - Just glue it in

 

[steve_sordy]

Not quite "glue it in", just an anti-shake product designed for the job!

 

[millemille]

Couple of hours riding this morning, which would previously have seen the axle loose, and it was resolutely involving.

So it appears the secret is to make sure that all parts of the rear axle assembly ( drop outs, flip chips, insert, axle and hub faces) are spotlessly clean and that the rear hub is solid with no looseness of the disc side outboard spacer.

 

[steve_sordy]

@millemille
"resolutely involving" ?

 

[millemille]

@millemille
"resolutely involving" ?

Resolutely unmoving...

Typing on a phone and the oh so helpful predictive text.