I've spent the last couple of days making a first attempt at a dumb iron repair. I was successful in that I've gone from a very rotten dumb iron with no crush braces left inside to a lot of new metal welded in and new internal floor and supports. I also learnt a lot, which is all part of working on my Land Rover for me. All that said I wouldn't do it this way again and I'm making plans for a quicker and better job on the other one.
First, the pictures. What follows after that is probably too much information for most people but if you're considering a first attempt like this I would encourage you to give it a go and read this whole post because it might help you plan ahead and know what to expect when the job gets underway.
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| Starting point.. |
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| Inside I found the base of the rail needed replacing too, and the "crush tubes".. |
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| This side had thinned out and needed replacing (see later for what I think about this in hindsight) |
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| You can see the new rail floor already welded in here and the new side being aligned for tacking. |
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| Keep checking the bumper still fits.. |
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| New top tacked in.. |
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| 4mm angle steel welded in as a crush brace.. |
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| Front tacked in square and straight |
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| Tidied up with first coat of paint.. |
What I Learned
Cutting it open and welding in repair pieces as I went resulted in a lot of awkward welds in position on the vehicle and a lot of inaccuracy later on when trying to make up the square front of the dumb iron. Your accuracy is limited when cutting new metal with an angle grinder, so planning an approach that involves as few cuts as possible and as much fabrication performed ahead of time off the vehicle is the way to go rather than building it up in place. Or you can spend a lot of money on more accurate tools or a lot of time on using a hacksaw. I almost resorted to the latter but 2mm steel is actually quite a job to cut manually and my cutting accuracy with the grinder did improve a lot during this work.
The crush tubes I tried to weld in only had a 1mm thick wall, so were very easy to weld through, would have been less resilient to time and rot, and positioned near the walls of the iron would have been less structurally beneficial anyway than the thicker angle steel I eventually welded in instead. I think I would use angle steel again rather than thicker tube. Easier and stronger and more durable, and works well as a crush piece too.
(One memory also influenced me here. I can remember to this day standing on a very muddy corner in some woods at a 4x4 meet some time in the early 90s watching someone prepare to pull a stuck Series Land Rover out the mire. To the stuck man's credit, the rescue vehicle didn't take up the slack first and just tore off instead which no doubt didn't help - but when contact was made the bumper and its bolts and all was ripped right out the front of the stuck man's dumb irons. And that's why my crush brace is placed where it is.)
When welding where something needs to fit, like the bumper, consider if the weld bead will be in the way, and how much weld you need. When I welded around the underside of the new inside floor piece the next time I checked my bumper fit (do this regularly!) it wouldn't slot in because the weld was in the way. I ground it down with some grind stone drill bits. It took about 15 minutes and was a bit inconvenient but not a major drama. When I photographed a ready-made dumb iron at the LRO show in Peterborough recently there's a 1" bit of weld just to join the inside floor to the vertical support beneath it. Here's a picture:
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| Where I got the idea for angle steel as a crush piece, although I welded mine between the bumper bolts and the direction force would be applied when being towed.. not behind. |
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| Another approach in an unfinished Marshlands chassis.. |
That 1" weld on the prefab piece connects the rail end to the support beneath it, but the support isn't welded to the spring hanger metal (at least not this side, so not both sides..), and it won't prevent water ingress into the back of the dumb iron where it will sit and pool (note also, no drain hole on the underside). So why weld so little? I can't speak for who made it but the approach ensures a customer won't be on the phone immediately with a bumper not fitting because there's weld in the way. I welded mine more completely without thinking about that and had to grind some weld down to get the bumper in. There's room to do it but it takes some work and consideration and isn't compatible with selling ready made sections that don't generate requests for refunds - despite them being less water tight and arguably less structurally sound. Anyway, in future I'll offer up the fitting piece before I weld to have a look at what clearance I've got and where I can weld as much as I like and where I might like to have a lower weld profile.
Accuracy is important. Land Rovers and their 2mm chassis steel can tempt us into thinking "close enough". The trouble with that is when a number of inaccuracies collect together you can end up being far enough out to give you problems with either fit, squareness, or welding up - or all three. Chassis repairs are not precision engineering, and a good square, level, and ruler marked up to at least mm should suffice if you actually use them. You don't strictly need a band saw or pillar drill if you practice and learn to use your angle grinder and hand drill. I can cut good shapes with straight edges with my grinder now by taking a good measurement to the nearest mm, scribing it on my sheet metal, and then cutting it by first letting the angle grinder walk gently along the line I want to cut, just on the surface. If you hold the angle grinder the right way it will want to travel the cutting surface along the line going away from you. Walk it gently a few times and then you can cut that straight line with your eyes shut because the grinder will follow the furrow you've started to plough. Then turn the metal to point the next cut away from you and do the same. I am not yet equipped to cut curved lines. If I can't grind down to a curve I need from a starting straight edge I will need another tool. I haven't found any need for curves yet though.
Fabricate as much as possible off the vehicle. A simple example is what I did instinctively with the rear tub outriggers. They have brackets welded on connect the chassis to the rear tub. I clamped the new outriggers to the chassis and put the fixings through the bracket into the mount points in the tub and clamped the bracket to the outrigger and tack welded the bracket to the outrigger. Then I removed it all to the garage and welded the bracket where I was more comfortable and it was easier to get at. It applies more so for fabrication, but this first time on a dumb iron I didn't do it. The front of the dumb iron is a cube. The inside floor is the base of this cube, extending further back inside the dumb iron. The sides are short but the shape of the spring hanger mounts could be cut out the cube once it was made, and the top is a straight face up until where it starts to curve, but even that can be shaped when it's time to fit if you're replacing more than the very front of the top. A cube is very easy to make, quite square, with corner magnets, so this could be done off the vehicle very quickly and comfortably, and then transferred to the vehicle to replace what's been cut out, lined up with the other dumb iron as a single piece, tacked on, check bumper still fits, then weld it up. Much easier than cutting and aligning and welding and checking every single piece of the dumb iron as you build it back up.
Next Time
I want to be very sure a new bumper will fit when both dumb irons are completely repaired, so I will buy a new bumper before doing the second dumb iron. Then if something is out by a few mm I can fix it during fabrication, rather than being stuck with newly repaired dumb irons that only fit my battered old bumper perfectly, but not a new one.
I need one or more tools to allow me to cut out the rot from inside the dumb iron without resorting to cutting through the spring hanger to get to it prior to welding the "incisions" back up. It's as strong as it was before, being welded both sides, but it's just more welding and more cleanup. A reciprocating saw would get the inside floor out really well, and either a multi-tool with cutting disc or small angle grinder would get the lower support out, and also allow me to get at the underside of the rail. Thinking about it, possibly one really good multi-tool and cutting discs would allow me to do all three things. A cut off tool would be ideal but I'm not equipped for air tools and don't plan to be any time soon.
A second trolley jack would be useful and this is not the first time I've wished I had one. I've used a scissor jack as a second before when doing suspension work, and that actually toppled and dumped the Land Rover on me sideways up against the garage wall. I was incredibly lucky that day and vowed never to use a scissor jack for anything but changing a car wheel again in future. In this case, two jacks would mean I could use one to level the front crossmember as a reference (my floor is not entirely straight and my front N/S spring needs renewing), and the second to support the front shackle so I can take all load off the dumb iron while I work, which means I can cut as much out, even the main floor, as I need. A second jack means I could get it perfectly balanced so the shackle pin can just be slid back in at the end of the session - unlike on this first attempt where I drifted it out at a bit of an angle and then spent an hour at the end of the day trying to get it back in without dismantling the whole front end..
I will fabricate the new top, sides, and rail bottom, as one piece, off the vehicle, possibly even with the top side corners as bends instead of welds. I'll have a better tool for removing the old rot as drilling a line of holes and then abusing your best cold chisel is at best not very time effective.
I may replace the main (underside) floor with the top off so I can do the first welds from above, on the inside, and then run a straight bead around the edges from underside, which will let me move quicker when I'm upside down knowing the penetration is already mostly achieved from the other side.
If I have the right tools and the pieces I need already cut and ready to go in; the new main floor section, the top and sides with bumper holes already drilled, the angle steel crush brace, and the inside floor and lower support.. I should be able to do the whole thing in a single session of a few hours, rather than the three sessions this one took me. Now I know what's involved I can do almost all the prep before starting the job which will also allow me to work on getting ready in short sessions as I get time and then take a day at it to execute when I'm ready. We will see.
