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Just Some Info/to Be Aware Of - Rear Subframe

subframe corrosion

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#41 Arno

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Posted 10 July 2014 - 05:56 PM

On boats this works as they are floating in this nice conductive stuff called water (with dissolved salts/ions..) to close the circuit so the sacrificial metal gets 'eaten' before the other metals so to speak..  ;)

 

On cars the best you can do is as described above.. Clean/remove the corrosion, apply a galvanic corrosion barrier/inhibitor like DuraLac or TefGel, re-assemble and then after it has all hardened/dried, for good measure, spray the exposed joints with a wax sealing compound like waxoyl or tectyl.

 

The ali corrosion is partly galvanic corrosion caused by the dis-similar metal contact, but part of it is also crevice corrosion where water trapped against an ali surface and in an oxygen-deprived environment starts to basically 'etch' the metal and create an ongoing reaction and stinky white oozing residue.

 

The last one (crevice corrosion) does NOT need any contact with other metals, just 'trapped' water on an ali surface. The floor corrosion issue on S1 (and early S2) elises because of water trapped between rubber floor mats and the ali floor is a prime example.

 

Of course once a galvanic corrosion reaction (usually this is sped up by contact with water or brine/road salt) has created small voids in the contact area between the metals, water can get in and the crevice corrosion kicks in as well.

 

The chassis - subframe interface can be prone to both types of corrosion and the double-hit of duralac on the mating surfaces and then, once assembled, a wax coating should stop both types pretty much from occurring though.

 

Luckily, once you clean the corrosion off well and treat the surface, aluminium will just be fine and not react further by itself unlike steel where the rust devil never sleeps and keeps on gnawing/growing from every tiny fleck of rust..

 

Bye, Arno.



#42 Duncan VXR

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Posted 10 July 2014 - 06:30 PM

Thinks that clears it up Arno ;) DG

#43 siztenboots

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Posted 11 July 2014 - 09:48 AM

could a stray electrical path from engine, via exhaust and the hangers pass across the subframe, chassis face to get to the earth strap. the engine mount musts be another possible path

#44 DX220

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Posted 18 November 2014 - 08:52 PM

Ok, just for a laugh; how many hours or £s for a garage to tidy this up? (ball park figures)  :unsure:



#45 robin

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Posted 19 November 2014 - 08:15 AM

Ok, just for a laugh; how many hours or £s for a garage to tidy this up? (ball park figures)  :unsure:

 lots better doing yourself



#46 Pidgeon

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Posted 19 November 2014 - 08:58 AM

A lot.  Clam off, engine out, suspension and brake lines removed.  Couple of thousand?

 

However bad it looks, there haven't been any bolt failues yet and we don't know if there ever would be.



#47 Tony H

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Posted 19 November 2014 - 09:01 AM

Over £1k+vat I reckon. Of course if you time it with some other engine out work the marginal cost would be much less.



#48 FLD

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Posted 19 November 2014 - 09:19 AM

You dont need to take the engine out, you just need to suspend it.  I did mine and left the engine on the crane so didn't need to remove wiring, fuel lines, driveshafts etc.



#49 DX220

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Posted 22 November 2014 - 07:19 PM

£1-2k then  :( , not going to claim I'm surprised!

 

If only I had the time! (never mind the equipment).

Looks like I'll be waiting til next time engine needs something doing



#50 Aerodynamic

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Posted 05 October 2015 - 06:06 PM

 

Is it difficult fitting everything straight again or does it sort itself out?

the subframe bolts to the chassis okay, but the diagonal arms should only be tightened when the car is on it's wheels - not on axle stands.

 

Otherwise the roof doesn't fit any more, but if you loosen the bars and retighten them all is well again.

 

 

Just found this thread ad have sone questions?

I only see shim, nowhere anyone is talking about different thicknesses, whats the purpose of the shim if you don´t adjust

imperfection of the chassi with shim thicknesses?

 

What is the material of the shims?

 

Anyone who have done any replacing guide for this shim change?

 

Diagonal arm?  Have I missed something? Or diagonal arms from roll cage?  Isn´t it just 2 bolts on each side

holdig the subframe?

 

Thank you, Per



#51 Pidgeon

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Posted 05 October 2015 - 06:22 PM

The shim is just a mild steel pressing that fits between the subframe and the chassis.  I guess it has no function other than to separate the two components.

 

The diagonal arm will refer to the bar between the chassis and roll bar that fits under the rear buttresses.



#52 gaffer1986

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Posted 05 October 2015 - 10:48 PM

Damn, why did I have to see this topic. Does that mean I need to worry about my rear subframe falling off?

Edited by gaffer1986, 05 October 2015 - 10:48 PM.


#53 Tibbles Stryker

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Posted 06 October 2015 - 06:39 AM

Damn, why did I have to see this topic. Does that mean I need to worry about my rear subframe falling off?

Ha ha made me smile 👍

#54 Aerodynamic

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Posted 06 October 2015 - 07:56 AM

Thank you Pidgeon. Do you know about what the thinkness is on these? I red some where about they weight over 2kg. What is aligning the subframe to the monocoque? Bolt are not good in aligning and dont see any pigs or holes. Br, Per

The shim is just a mild steel pressing that fits between the subframe and the chassis.  I guess it has no function other than to separate the two components.   The diagonal arm will refer to the bar between the chassis and roll bar that fits under the rear buttresses.



#55 Pidgeon

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Posted 06 October 2015 - 08:48 AM

I'd guess the shims are around 0.8mm.  Nowhere near 2Kg.

 

I also was concerned about alignment, but one of the bolts screws into a captive nut and I recall my conclusion was that failing to align correctly was not possible.  On the other hand, with the weight and awkwardness of the subframe, it's not possible to adjust the position either.



#56 fezzasus

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Posted 06 October 2015 - 08:57 AM

The shim is just a mild steel pressing that fits between the subframe and the chassis.  

 

Just to clarify, it's a zinc coated mild steel pressing. The zinc coating offers moderate protection against galvanic corrosion*.

 

*zinc retards galvanic corrosion, but doesn't stop it. Wet conditions, especially if salt is present, will increase the rates of corrosion. Therefore 10 years of everyday use will cause a lot of corrosion.



#57 FLD

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Posted 06 October 2015 - 01:26 PM

An old school engineer told me you should use a chrome barrier between steel and alloy to stop corosion like this.  I haven't looked up the electrode potentials as I had no inclination to chrome plate my shims I just duralaced the fook out of them when I had mine in bits.  Might be worth looking into if someone is more inclined to do so than me.



#58 haggi961

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Posted 06 October 2015 - 01:30 PM

1-2k seems a lot I would change them for about £750.

#59 fezzasus

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Posted 06 October 2015 - 02:05 PM

An old school engineer told me you should use a chrome barrier between steel and alloy to stop corosion like this.  I haven't looked up the electrode potentials as I had no inclination to chrome plate my shims I just duralaced the fook out of them when I had mine in bits.  Might be worth looking into if someone is more inclined to do so than me.

 

https://en.wikipedia...ial_(data_page)

 

Looking at the electrode potentials, Cr should perform as well as Zn



#60 FLD

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Posted 06 October 2015 - 02:23 PM

 

An old school engineer told me you should use a chrome barrier between steel and alloy to stop corosion like this.  I haven't looked up the electrode potentials as I had no inclination to chrome plate my shims I just duralaced the fook out of them when I had mine in bits.  Might be worth looking into if someone is more inclined to do so than me.

 

https://en.wikipedia...ial_(data_page)

 

Looking at the electrode potentials, Cr should perform as well as Zn

 

 

Hmm, the only benefit maybe that it lasts a bit longer?  I'm not sure how quickly chrome corrodes.







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