m12 bolt in single shear 8.8 or 12.9?

[Sean]

Ive been asked if replacing a 8.8 M12 bolt with a 12.9 cap head would be stronger - usually I'd say yes without a doubt but.... The bolt holds the eye of a shock absorber on to a chassis - Its in single shear and in this case the bolt fails regularly as the shocker eye has a rubber bushing there is an degree of torsion too. what worries me, does the 12.9 resist this better or will it be more prone to shatter?

there is no option to change the mount its a rubbish replacement chassis and is poorly designed.

 

[Old Dog]

The 12.9 bolt will take about 30% more load so it will be better, just lube the bolt before you refit because if it does break and the thread of the bolt is seized in the hole it will be way more difficult to remove.

 

[Spark plug]

I would say a 12.9 cap head world be better. But if it still causes problems could the M12 be replaced with a bigger bolt.

It sounds like there is something going on causing too mutch stress in the shock absorber, what is it on a lorry?

 

[Sean]

I would say a 12.9 cap head world be better. But if it still causes problems could the M12 be replaced with a bigger bolt.

It sounds like there is something going on causing too mutch stress in the shock absorber, what is it on a lorry?

nope other end of the scale ....a 2cv main problem is the design of the chassis , not much can be done about it now.

 

[AndyStobbs]

That's a big pin to be shearing off!

 

[lshigham]

nope other end of the scale ....a 2cv main problem is the design of the chassis , not much can be done about it now.

You may be able to make a bracket to take some of the strain off the bolt, something along the lines of this as used on the engine block of a mini

 

[Spark plug]

If the problem persists you could have a bolt made out of something like chromealloy steel or something with a Much higher shear strength.

 

[Sean]

yup the original fitmet was an M14 with a wide land and then the 12mm pin on which the shock mounted this couteracted the bending moment on the stud, the crap replacement chassis just uses a plain bolt into a bushing on the chassis and the bending moment of the shock is right at the start of the thread cut so its a stress raiser right where its not needed. Also being the shock mount its subject to high frequency and larg shock loading

Suppose what im asking is will the 12.9 be more brittle as its more than just tensile strength?

 

[lshigham]

Just a thought but are you sure the damper isn't bottoming out or seized? They aren't exactly the stiffest on a small car like that so I'm really struggling to see how it's snapping M12's even with a poor design

 

[ben8vgte]

12.9 grade bolts need to be torqued to the correct tension for them to be effective really, I think it's 90% of the bolts designed load but you'd have to check that amount from the supplier..

 

[PhillipM]

Have you got a picture at all?

I use M12 bolts in single shear to mount a pair of the biggest dampers you'll probably every see on a car, I never even so much as tweak one!

 

[Sean]

Ok had to go to bed ...it was a long and "Intresting" day yesterday

on the 2cv the way the levers of thre suspension work there is a relativley small amount of travel but the forces are multiplied the shocks also work the wrong way round to a normal car - bounce compresses the shock rebound extends it , not that should make any difference to the bolt. The shocks are about the same as the rears on a 110 defender!
photos some nicked off Ken (Dyanut on here) who is a friend...its not his chassis but an inferior copy

typical failure

Changing the dampers? by slcchassis, on Flickr


No grease used on assembly, by slcchassis, on Flickr

the original pin

Damper washer, which way round? by slcchassis, on Flickr

in its original fitment

DSC00249 by Sean602, on Flickr

 

[AndyStobbs]

It wants bracketry on the front face, a dog leg back to the chassis. I would say its work hardening then snapping rather than shearing off. Too much bending taking place at the inside of the bush.

By the way most shocks shorten in compression and lengthen in rebound. Either way that set up is fundamentally flawed IMHO.

 

[Sean]

the overall design of the 2cv chassis is superb with all the mountings and dampers in a neat and compact form. They make very capable and lightweight off roaders. the problem we have here is that the manufacturer of the replacement chassis ignored all the solutions Citroen found to the problems and fitted cheap and nasty sub standard fixings.

the last rusty one in the pics is mine and it had been raced over Morrocco and nothing failed using Kens chassis with all the original fixings. Its been places you'd be had up for animal cruelty if you tried to take a donkley

 

[dan.taylor.1]

Can you not drill out the threads and fit the oem pin after rethreading to m14?

Dan

 

[Rocketman]

If you could drill the hole out to 16mm on the bracket the existing m12 goes through you could instead use a m12 shoulder bolt properly tightened up to torque and it will have a 16mm shaft then.