Roothy’s 40-series Toyota, Milo, suffers metal fatigue

Back in March we had a look at the sort of damage copped by the front panel of a 40-Series Land Cruiser that’s used and abused on corrugated roads.

I may have mentioned we’d be hooking in and replacing all that pretty soon with some new trick engineering. Yes, and then Chooka showed up looking for a counter lunch and the job slowed down.

Life is like that, but with the school holidays on and the lads keen to help (keen to eat more like it!) we got back out to the shed and got stuck in. I’m pretty lucky like that because my sons have reached an age where they’re really useful. When they were younger they’d come out and spill paint everywhere and drop my spanners down the open drain just so they could see the splash!

Metal fatigue is what kills many old trucks and sees bits falling off in the dirt seemingly for no reason at all. Most people with some bush experience of machinery are well aware of it but, if you’re not, well, you probably will be soon unless you live in cotton wool.

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My son Joe has worked out a way to make brushing the rear floor more interesting – he’s listening to podcasts! Nothing knocks a hole in a lot of work quicker than a few helping hands.

The best way to explain it is to grab a piece of wire – a paper clip is good for this – and bend it up and down. Two things happen: it gets hot at the bend, it gets harder to bend and then eventually it’ll snap. Hang on, that’s three things. And you thought the only thing I could count was a shout.

Right, what’s happening is that the metal is changing at an intergranular level. One surface is being compressed, the other stretched, and then we bend it back and reverse the process. Engineers call this fatigue loading and, like Fat Kevvy after too many prawns, the result is that eventually something’s got to take a little lie down.

In the case of metals – and all metals have different properties here, which is why you don’t see too many die-cast aluminium shock absorber mounts – elasticity is lost and cracks are the result. Fatigue loading invariably means that cracks start on the edge of the metal first – this is what we call a stress fracture.

There’s a classic example of that in Milo’s mudguard inner panel. You can see the metal’s copped plenty of damage – it’s been flexed so much that it’s cracked in three places all around the mounting bolt hole without cracking the hole itself. Why’s that? Because there were thick washers on each side of the hole, which held it secure. Given time, that bolt, washers, the hole itself and the nut would have all dropped off together, leaving the mudguard free to beat up the next bolt down the line.

The original Toyota 40-Series engineers realised fatigue loading and that consequent stress fractures might be a possibility in a vehicle designed to be used on rough roads most of its life. So they took several steps to slow it down. You’ll note I said “slow it down” and not stop it, because nothing stops metal fatigue apart from leaving a vehicle in an hermetically sealed bag and never using it.

They used thick, mild steel panels for starters. At about one millimetre in thickness, they’re three times as thick as modern panels, but strength means weight and that means they had to mount them to allow for some movement. Hence the hinge you’ll see down the front of the front panel on an old Tojo.

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The front panel we talked about a couple of issues ago isn’t going to jump out on its own, despite being more bent and battered than a banana fritter. Locking the mudguards and inners to the chassis through a hinge – it’s kind of important there’s some structural integrity.

For the same reason, the chassis on the old jiggers were always riveted together rather than welded – rivets allow a little flex. In fact, a long-wheelbase 40-Series chassis can be jacked up on one corner and you’ll get about 50mm of flex across its whole length.

This flexibility was even more important in the days when suspension was merely a development of the stuff they used under horse-drawn wagons. Yes folks, the introduction of coil springs and decent shock absorbers was possibly the largest step forward in the battle against metal fatigue on off-road vehicles.

Land Rover first introduced them in the 1970s. But by 1988, when Nissan released the GQ Patrol, metallurgy had caught up – processes such as coil scragging and shot peening were used to reduce surface tension on the coils that led to reliability problems such as breakages. Manufacturers everywhere realised you could have strength with suppleness. Real suspension was born.

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With holes cut for the intercooler and hoses down below, there’s not a lot of the old front panel remaining. It’s been strengthened with angle iron along top and bottom sections, but the rest is more bent than a council full of real estate agents.

But it was way too late for Milo and, while I have thought of doing a coil-over conversion, adding complexity to something that’s simpler than the average axe has always put me off.

Okay, so now you can appreciate why good suspension is such a critical thing for 4x4s that get used seriously off-road. You see, the more of that constant corrugation pounding you can absorb in the suspension, the less that gets transmitted to the chassis and then the body of the truck itself.

So if you want to save money you’ve got to spend on good suspension. Or spend plenty of time in the shed like my lads and I do.