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Cracks on Rear Chassis

When the MOT inspector‘s report was read out to me I nearly wept 'cos on seeing the rust I thought “It’s a long job”

I was right and my welding engineer told me what it entailed: 

1.     Dropping the whole rear axle and exhaust.

2.     Making up new sections.

3.     Welding them on.

4.     Putting everything back

 2 and 3 are the easy bits and so is 4.

But 1 is a B (that is the only letter I think the Web Bosses will allow of that Word)

The main bolt that goes through Big Isolation Rubber Bush into the captive chassis nut is the main bugbear as it can seize solid but luckily mine came out with Plus-gas and perseverance by the guys at the garage!

I couldn’t take any photos while they were doing the job but they kindly let me take photos of the welded job before they painted and finished the bolting up.

Right hand side

The Problem

OK! So it appears that it is a combination of design and materials specification that has caused the problem. (Open to debate!)

Where the Main bolt goes through the Big Isolator Rubber Bush into the chassis is where the it starts.
The metal used had been assumed to be thick enough, because at that point it is double skinned and there is a captive nut also welded inside.  My Welding Engineer reckons it was about 20 Gauge if you were lucky. It should have been a gauge thicker, (at least) according to him.

They repaired it with a section fabricated from 16 gauge galvanised steel.

With the suspension flexing all the time a lot of strain gets put on this bolt, even with the two smaller bolts in front giving it a bit of back up.

Over time a stress crack starts from the bolt hole towards the back end. This allows the water in and starts to rust the chassis member from the inside, also creating a lot of rust on the thread end of the bolt eventually make it virtually impossible to unscrew it.

This problem does occur on other makes of car (certain Mercedes! Believe it or not!! And Cavaliers) although the later are not deemed to be worth doing as a whole section of chassis that curves back around the wheel arch has to be fabricated and their job will cost about £400 per side!! OUCH!
 

Left hand side

On other modern cars, there is another bush mounted on the outboard side and the bolt goes through a bush horizontally, thus taking a large amount of stress from the bush that is causing us problems. The large bush mounted with the bolt vertically attributes to the stress as the torque on is in effect a side to side movement and not around the bolt.

Right hand side different angle

Another factor to bear in mind is that these cars have a large carrying capacity (especially the Estates). They may have been dragging trailers and caravans incorrectly balanced, which also may have put undue stress on the back end and you are unlikely to notice it because most or all of the Estates and some Saloons have self levelling suspension.

Right hand side From below


£260 the whole job 1.5 days!!
Good price to the Trade!
It may cost you more.

If you use your Estate to carry very heavy loads, or you think it has been overloaded in the past, this may be happening to your chassis right now.  As you cannot see where the bolt enters the chassis you never see it start, you only see the devastating after effects of the rust and eventually on your wallet!

Words and phone pix by Bob Lavens 14/04/2005 - thanks Bob!

NOTE

This above is the opinion of a welding engineer reported by Bob.  I have my own engineer who checked my estate today, and found absolutely no evidence of corrosion or cracking on any part of the rear chassis.  I strongly suspect that Bob's car was actually grossly overloaded at some point, either by towing or load, resulting in a flexing of the chassis. The subsequent damage would have been aggravated by the action of the self-levelling suspension.  An overload on the luggage deck would have acted against the fulcrum caused by the lifting action of the levelling suspension - the resultant strain on the main chassis rails would have been colossal. 

If this had been a design fault or inadequate metal, as is alleged,  then this would have been a common sight by now, and very well known in the trade.  My own MOT man has never seen this on any Ford estate, so I think Bob has just been unlucky.

EricR