rear caliper bolt sheared off help!!

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myglaren
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Post by myglaren »

The modification of heat transfer characteristics is something I had not considered. A valid point and may be the reason that Citroen have not introduced something like this themselves.

One thing that did occur to me recently was that there would be the same EMF and therefore the potential for corrosion on the retaining bolts.

Although they are very tight it is presumably down to all the threadlock they are smothered in.
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Post by admiral51 »

I should really update this particular thread but as of yet it has not been looked at :oops: :oops:

It has made 5 round trips of 30 miles to and from work albiet very early in the morning and returning quite late so i have been very lucky so far :) :) although i like to think im quite good at defensive driving ie expect everyone else to do something stupid and i should be ok :lol: :lol:

The plan is for someone to have a go at removing the bolt on tuesday as i dont need to move the car from now till wednesday :D :D
I dont have a welder or tap so they will be coming to me just have to help dig out the new fish pond area as payment :( :(
This weekend is being devoted to my VSX :D :D

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Post by xmexclusive »

Hi All

I suspect that the bolt failure is due to fatigue cracking. Any movement on the bolt will be well within the elastic limit of the steel so it is unlikely to be a tensile failure of the bolt until the fatigue crack covers a large part of the crossection of the bolt. The most common place for this fatigue cracking on high tensile bolts is at the end of the thread where it finishes and the main shank of the bolt starts. The thread cutting tool can leave micro cracks here and the higher the tension on the bolt the greater the chance of fatigue starting one of the micro cracks growing. The eccentric loading on the bolts due to braking force will greatly reduce the fatigue strength of the bolts. So will any uneveness between the caliper and the rear arm.
Using harder steel to try to solve a tensile failure problem has a high chance of success. Unfortunately in making steel harder it becomes less ductile and far more suceptible to fatigue failure.
With those limitations in mind I have always cleaned up the faces between the caliper and arm every few years and fitted things back snuggly so the problem never gets out of hand.

John
Last edited by xmexclusive on 01 May 2009, 22:04, edited 1 time in total.
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Post by admiral51 »

The most common place for this fatigue cracking on high tensile bolts is at the end of the thread where it finishes and the main shank of the bolt starts
This is exactly the place where the bolt has broken in 2 :cry: :cry:

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Post by xmexclusive »

Hi Colin

If the broken end is still clean and examinable then you might be able to see the separate fracture surfaces. Fatigue cracking is a long slow process and has a smooth surface often black in colour and will spread out in a circular shape with the circle centre the point of crack initiation. The final tensile failure will be crystaline in appearance, bright silver in colour initially but very light brown after a few days as the early stages of rusting start to appear. My guess is that you put the correct tension on a bolt that already had a fatigue crack growing in it. Just luck of the draw. In its previous state it has stress relived its self a bit. So now you come along and torque it back up increasing the fatigue stress, the crack grows faster until the remaining metal cannot take the tension and snaps like a carrot. Pity the remains do not drill out like a carrot.

John
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Post by myglaren »

An excellent recommendation for using new bolts.

The guy who did my rear callipers mentioned that the bolts often shear off but put this down to them being stressed as they are pushed out of line by the tilting of the calliper body as the corrosion pushes it away from the suspension arm.
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Post by cachaciero »

myglaren wrote:The modification of heat transfer characteristics is something I had not considered. A valid point and may be the reason that Citroen have not introduced something like this themselves.

One thing that did occur to me recently was that there would be the same EMF and therefore the potential for corrosion on the retaining bolts.

Although they are very tight it is presumably down to all the threadlock they are smothered in.
Hi Steve
There is as you point out more than one path for current to flow through and more than one generator, my philosphy is wherever possible to try and reduce the amount / capacity of the generators.
Gen1 caliper to arm:- gasket of some kind paper / silicon / ptfe for choice.
Gen2 Steel Bolts in caliper bolt coated in Duralac or silicon grease.
Gen 3 Steel bolts into Steel arm - very low potential as for Gen 2

Stainless alters the equation for both 2 and 3 makes 2 worse and 3 possibly slightly better.

Citroen don't use gaskets between arm and caliper for cost saving reasons almost certainly.

Threadlock on the bolts will go a fair way to reducing corrosion potential between the bolt and the alloy caliper while at the same time offering cost reduction on assembly (no locking washers etc.)


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Post by fred1 »

Hi,

I did think about the thermal aspects but the rear brakes do very little work compared to the front and stay relatively cool. The bolts also conduct the heat.

I have had the polythene in place for some 60K and it was still pristine and re-usable by the garage who replaced my radius arm bearings recently.

They put the same gaskets back.

I think this is quite good evidence that in normal use there is not a thermal problem. Clearly PTFE would provide more margin.

One further point I would not recommend putting locktite along the shank of the new bolts (as citroen do on the C5). This just prevent the easy removal of the callipers as the bolts will no longer turn easily and are very, very difficult to remove from the calliper. Use a good coating of grease.

However, do put locktite on the threads.

New bolts are readily and cheaply available from Citroen and come I think, with a little loctite already on the thread.

Regards

John
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Post by cachaciero »

fred1 wrote:Hi,

I did think about the thermal aspects but the rear brakes do very little work compared to the front and stay relatively cool. The bolts also conduct the heat.

I have had the polythene in place for some 60K and it was still pristine and re-usable by the garage who replaced my radius arm bearings recently.

They put the same gaskets back.


John
As I said I had no thermal data it was just a consideration, 60K does it for me polythene it is ! :-)

Cachaciero.
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Post by MikeT »

Hi Colin, if you think you need a hand with this one, give me a shout and I'll pop over or have you sorted it now as you always seem to manage in the end?
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Post by admiral51 »

MikeT wrote:Hi Colin, if you think you need a hand with this one, give me a shout and I'll pop over or have you sorted it now as you always seem to manage in the end?
Thanks for offer Mike and the confidence you seem to have in me :D :D

The lad i used to help/do the welding on the seat mount is coming over to sort the stubborn bolt out on tuesday as he has the mobile welding equipment that i dont have :oops: :oops:
Glad to see you have been exploring the infamous wiring loom :wink: :wink:

Colin
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