Body shell at rear sub frame

[Tinkley]

Are there any drawings or photos of what the body shell is supposed to look like without the rear sub frame attached? I can find some shots of the area but as mostly rewelded ones, none that show a pristine new one. Just how does this assembly work? Is there an internal plate welded in? What relationship is the thick pad the subframe bolts through to the body shell? How much light should be visible through this area if any?, I have rather a lot.... I don't remember seeing any in Kitch's nice blue metallic beast, which was being restored. What sort of thickness is the original, I'd guess 2mm (SWG 14) as the bottom plate part?, maybe 3mm.

Reason being, having removed the trim cover and duck tape, there is not a lot holding the rear most bolt from waving in thin air!. In fact the thin bodyshell part is slightly bent at one corner so I assume it has been flexing. The bolts should come off OK, bit naughty the guy who had it before, dropping the frame and welding the boot floor but neglecting this bit. In fact I would say negligent as less safe than the boot floor collapsing.... Although I can 'bodge' this, it is not simple or easy. In fact I am actually reluctant to even drive the car (as is) having seen the extent of the rust and holes. It is actually pretty hard to discern how the original assembly of bits fitted and/or was welded together.

Surprisingly it is only around that bit that there is dangerous corrosion, some of the rest of it is better than expected, maybe a spot or two around the bump stop, but the sides were only surface rust. The upright flange is not bad either.

Hopefully, I can save this beast from 'doing a Jaba' because perhaps I have caught it in time.

Just checked the other side! No daylight visible, obviously a semi closed box construction. Some light rust but obviously original. It now becomes clear that someone previously has tried to 'repair' the o/s and put a mild steel piece in complete with special hole (about 25mm diam) to let even more salt water into the semi sealed box construction. Great, that makes me glad I did not take it to Lake Garda (towing boat) last week, it would definitely have collapsed either on the pave, or in the Alps.....

The replacement 'steel' piece ressemble mille feiulle layers of lovely brown stuff. Now I have a few options. 1. Leave it where it is, on drive and get mobile welder to have a go at it, with subframe dropped a few centimeters. 2. Run it somewhere it can be welded properly and get a set of rear pipes, drop all the rear and sort out properly. 3. Better not detail, but can 'repair' myself, believe me it would be stronger than what is there and possibly than original but may not meet approval!.

Anyone know a half decent mobile welder on the Surrey/Hants border?

[Dollywobbler]

Might be worth having a natter with Rob Moss at Chevronics. IIRC he's even got some repair sections for the rear subframe mounts, though I don't know the specifics.

[citsncycles]

I have this job to do this job on 2 of mine - should have taken pics when we dropped the subframe on that TRS estate Tim resued a while back

There's a heavy renforcing angle that goes up the inner sill and then under the box section that the mounting bolts go into. It's damp getting in between this angle and the panelwork it's attached to that's causing us all these problems now. It seems that many cars are coming to the end of the life of this area at the moment as in the last few months we've had several postings about this area. It's well hidden, so an MOT may not catch it. If they're like earlier Citroens, there may be drawings of this area in the bodywork chapter of the factory manuals.

As for home repair, the only part that would be difficult to make exactly would be the reinforcing angle, although yours may be salvageable - there's been discussion on this topic a few weeks back regarding one of the cars in Mothman's collection. What you need is a competent welder / fabricator, as I suspect that most welders who come and patch your car up for an MOT wouldn't want to tackle properly a job this big.

[Tinkley]

Thanks Mike. The vertical upright seems to be moderately solid, thank goodness but there is only a stub left of the angled brace part (your heavy reinforcing angle) left, mostly on the most inboard end. I would estimate that at least 30% of the 'base plate' that the mount bolts go through is open at the rear. TBH getting the nuts off should be relatively easy as the threads are actually clean and seem to have a light oil coating. The washers are finished etc (I'll use S/S penny ones on reassembly), but just how the guy attached the 'repair' is not obvious. Perhaps this was the 'boot floor' repair job?. Pigs ear of it I would say.

I suspect you are right, as the cars approach 22/23/24 + years old, and as many winters with salt down, some inevitable corrosion will show up.

Thanks Dolly, Chevronics have some pre fabricated pieces that they cut and shut into this part of the BX. They are shown on Chevronics Facebook page. Whilst I don't think this car is as bad as Des's St Tropez, she is not too good either. there is a bit of Box rot on the U section leading to the towing eyes at the rear o/s - you can put your finger straight through if you want too.... But not yet a fail condition, as not structural...

As long as I can get a good surface to key too, I would not be too fussed about brazing or even silver solder to get something on it. It would at least be a lot safer.... .... than fresh air...

[citsncycles]

I've never been any good at brazing, although I know it is strong. The only think is the MOT man will not be happy if he finds it, although I agree, it's better than nothing!

This brings up an interesting question should the values of our vehicles continue to rise. They may reach the stage that other cars (such as 2CV's and Beetles) have gone through, where a quick tart up and spray to make a car look pretty may return a decent profit, while fixing them properly wouldn't. With the examples I've given I've seen people fall into the trap of spending 2 or 3 grand on a car that looks nice and has an MOT, but that has been badly bodged structurally. Maybe we need to put together either a buyer's guide thread, or even a complete section on what to look out for when buying a car (such as the reinforcing for the rear sub frame mount being in position and not just cut out and the holes plated with thin steel!) I'll give it some thought and possibly post up a new thread on the subject tomorrow, so as to not highjack this one any more than I have done (sorry).

[Tinkley]

Good thinking Mike. It's pretty hard to nail these BX down as they can look quite good and be dogs under the skin. This one was a quick buy as I had just had my previous one written off by a young lady. As an identical model, I jumped and tbh she has done OK for 300 quid. I got 40k out of her before the release bearing on the clutch failed...

This is a (I hope) cureable car, others will not be without a lot of work, which is OK if you can do it yourself. However the cost to many if it needs to be done properly will be too great for others. Don't worry about highjacking the thread, we probably need some sort of discussion on the subject. I don't want a minter but a sound working everyday car that I can have confidence in. Others will want more of a showpiece and don't need to use it daily.

I like brazing, it just flows so much more easily, and silver solder with nice scarfed joints is a peach. I used it on one of my old bike's handlebar clutch assemblies and it never gave any trouble over at least 40k.

Note the MOT testers guide is something along the lines of, if you are not sure of a repair (in regards fibreglass) but it is solid, then it should be passed but with an advisory. I doubt brazing would cause any problems in that subframe mount area, loads of area for it to flow and plenty of support for the load bearing mount plate. Might be interesting as I will have at least about 300 X 300mm of er, er replacement materials, some within the 300mm of suspension mounts - but none load bearing!.

MIG brazing is considered acceptable but gas brazing not.

[rutter123]

Access is the main issue for welding here, in hindsite after doing mine i think next time i will fabricate a whole section to weld in place rather than do it in 4 or 5 pieces, the axle really needs to come right out to tackle this, and the tank, this will also give a better idea of how far rust has gone.

[paddy1995]

i did one of my subframe mounts not long ago and did it in 4 sections and it turned out quite well and is really solid and the subframe still fits nicely i can post a few pictures if you like i did spot weld on the front and rear of the box sections as they are originally factory spot welded and don't have much stress on them and i seam welded the main structure parts as they need to be solid
(i removed the fuel tank and rear subframe and then you can weld quite easily). ps i have pics of most of the process

[paddy1995]

dont know if this is any help but its a picture of the subframe mount before i replaced it

[mat_fenwick]

I'd consider the risk of galvanic corrosion if you're brazing, but that's the only practical downside I can see. Once protected, it should be indistinguishable from a permitted i.e. welded repair anyway. I wonder if the reason for only allowing welded repairs is for ease of assessing the quality of the repair, rather than strength, as I know I've seen the steel in brazed joints fail before the braze...

[Tinkley]

I found this little guide for MOT testers, viz a viz Acceptable methods of repair.

'It is essential that repairs to corroded or damaged areas are properly carried out. This requires that suitable materials of appropriate gauge or thickness are used for repairs. Additionally:

· Any plating or welding extends to a sound part of a load bearing member.

· the repair must appear to be virtually as strong as the original structure with only continuous seam welding being acceptable for patch repairs (even if the patch extends beyond the prescribed area).

· spot welded joints are acceptable where the original panel has been replaced to an existing spot welded flange (provided the original defective panel flange has been removed). Stitch or plug welding is acceptable as an alternative to spot welding in these cases.

· MIG brazing, a combination of adhesive bonding and riveting, or amalgamations of these with other joining methods may be the vehicle manufacturers recommended method of repair. Such repairs must therefore be accepted unless they are clearly inadequate.'

Dated Jan 2012 Appendix C.

Thanks for that photo. Unlike that scenario, I do not have a hole near the bump stop side wall but the part radius part at the rear of the rearward mount bolt simply does not exist!.

[Tinkley]

Here's a couple of shots!. Just how safe do you think it is to drive? Obviously the 16mm across flats nut and washer would be replaced even if not holding onto much!. Maybe I caught this JIT.

There may be a solution, if I could drop the frame enough to replace the 'pad' with a couple of layers of 2mm steel external and one internal and create a sandwich. Damn site stronger than what is there now!.

[rutter123]

As per paddys pics that resembles the tzd when i got it, but the rust had spread further along the chassis rail well past the bump stop, the rear end of the sill non existant, had to take measurements from the o/s to make plates to build up the inner/outer sill, the chassis rail being the less challenging to weld. Tinkley i would be inclined to have a poke around to see just how crusty it is, one side seems worse than other, but ultimately it will need doing much like mine. It prob is the worst area on a bx to do, i expect many got cubed for this alone.

[Tinkley]

I rather agree with you rutter, in terms of a lot have been cubed...

Hey!. it is still driveable!. believe it or not. I also think the MOT guys were a bit 'negligent' not saying partly off record 'that bit needs sorting, mate,even though we will let it through!.' at last years MOT!.

I have quite afew options, after all 12mm+ carbon and glass would be 2 maybe 3 time stronger than original if bedded onto clean steel. My ROT for carbon is 3-3.5 stifer than steel in real world (8 X in theory in perfect conditions), but the glass would give the toughness required. However mypreference would be a quality steel repair whether welded or brazed - the latter may be preferred for specific reasons. Hard to decide. Motor has done 73k, gearbox 156k, there is a whine probably CV joint at 40 mph or final drive bearings. Otherwise OK, I still think the BX is one of the most comfortable cars I have ever driven - full stop. Please note Citroen UK - it is better than the C4, just drive 400 miles non stop - case proven. Better than the Skoda Fabia 2 (which is pretty good car btw) especially for leg room to left of clutch pedal.

I think the ergonomics of the BX are excellent for my physiology - and I am a reasonably standard 52 metric jacket size. At 180 cm and 82 Kg not a tricky shape. Probably why I like these cars so much, and great styling by Gandini.

[Mothman]

My TXD is zakly the same as this. However, i have managed to get a set of the subframe mounts off Rob Moss. Fingers crossed Malcolm is managing to cut off the old ones and reweld the new ones in the correct position otherwise the rear tyres will just wear to shreds in no time. Darent give Malc a ring about this as the motor is such a good drive and i really dont want to scrap her. Did have to get a mate to drill the holes though for the 2 bolts as the plates didnt come pre drilled.
I think we all should be putting rust killer and lubricant here as they were buggers to get the nuts off.

Andy