O/T bolt strength innhigh stress situation
Turbo Mini Forums

this is a bit off topic but i guess could still be applied to minis,
my mountain bike has a four bar linkage rear suspension with the shock mounted vertically in the frame using two bolts, an M8 allen key head bolt at the top and an M6 allen headed bolt and nut at the bottom.
the original bolt has 12.9 in the head which is related to the strength i think?? and the lower bolt has no markings on it they both take a lot of load and i've snapped the top bbolt before,
i snapped the lower bolt the other day and on inspecting th etop bole it is bent, i was told by thge bike shop that stainless bolts would be better, but isn't that more brittle??
i need some advice from someone who knows about the loads these bolts can take and some advice about what i should use (ideally the strongest bolt possible) should i buy some titanium bolts or will stainless be be ok, i think the 12.9 isnt really up to the job as i've snapped one and this one's bent.
hopefull y that makes some sense
thanks in advance
DAN :)

just so you can see waht i mean this is pic of the way the rear is layed out, the bolts in question are the ones at the top and bottom of the rear shock, so as you can see take a lot of load especcially when ihave my fat arse on the bike

DAN :)

The 12.9 relates to the tensile strength of the bolt, e.g. when you are trying to stretch it.
In your application, the bolt is in shear, which is not (by far) its ideal loading condition, hence you have broken a couple.
Stainless will be no better, if anything worse, it is more brittle as you say due to the grain structure, and also it generally has a lower UTS (ultimate tensile strength).

Basically it is bad design by your bike manufacturer! But a 12.9 bolt should be adequate, and you will struggle to get anything any better.
My suggestion would be to either change the bolt regularly, or if possible drill the mountings and shock out and use a 10mm bolt instead.

cheers tom, also one other thing, i noticed that the bolt that snapped did so at the start of the threaded section and the bent bolt is bent at the start of the thread, neither of them need as much thread as they have so would getting bolts with only the minimum amount of thread needed improved matters or not??

As Tom says, 12.9 is pretty good. You can get higher, but usually strengthening one area leads to a failure in another...

You mention a 'bending' of a not-yet failed bolt...

Is it a double shear mounting? It might well be in principle, but a true double shear is only achievable with no gaps between the two shear pieces - if you have a gap, bending becomes the mode that will promote a failure long before the shear itself.

Detailled pics of the area in question needed!

D

Thought of something else - Are you replacing them with bolts from the manufactuer of the frame?

There are dubious things happening in industry - it could be that the 12,9 bolts aren't up to the spec they should be.

Find a profesional nut/bolt supplier and source some unbrako cap heads (allen key head bolts) and replace with these. Also check out the ARP website and see if they list any bolts of the correct length.

D

Unbrako are the ones you want

also as you say, keep the thread as short as possible, it is however difficult most of the time to do this on the smaller cap screws.

Stainless steel brittle ? Which stainless steel ? There are 4 or five different stainless steels depending on the grain structure etc. austenitic, austenitic-ferritic, martensitic, ferritic, etc. They all have different features, with different applications. It is similar to "billet" aluminiun or "aerospace" materials.

Austenitic stainless steel is very ductile material compared to e.g. most ferritic steels.