Monday, September 19, 2005

Offroader 101

First day of work after two weeks' vacation, plus I bought a new bed today and had to assemble it - no stamina left for a blog post, so here's another one of my car articles.

The concept of personal transportation is uniquely appealing to human kind. A car affords you the luxury of leaving, and who of us doesn't have a desperate desire to leave every once in a while? Although roads were invented long before Gottlieb Daimler had a brainstorm, the car was predestined to be developed for accessing more remote areas, or allowing movement in conditions where roads weren't really available. In the First World War the tank did the job fine, being quick enough to keep up with the infantry troops it was supporting, able to run on most hard surfaces and occasional ones where a man's feet failed, and admirably efficient at delivering hellfire to enemy forces. Not so useful for peacetime application though, and while tank-style traction was popular between the two great conflicts, the mighty Willys jeep set the blueprint for offroaders from the 1940s forwards.

Of course today the spirit of the all-terrain runabout has been diluted by SUVs, lumbering hunks of metal that don't seem to be able to do anything well. The remnants of manufacturer conscience see trucks fitted with a low gear and usually some manner of getting the torque to the wheels in sticky (or rather un-sticky) situations, and while this allows an intelligent driver on good tires to take the vehicle off the beaten path, it does not an offroader make. I was impressed with how a V8 Explorer handled a potholed dirt road, but in retrospect my 30-year old Volvo would probably be able to do the same, being fitted with no more "lifestyle" paraphernalia than a sump guard.

The most important technical aspect of an offroader is differential locks. No less than two. A differential is a device that splits torque from an input shaft to two output shafts and does this variably, allowing the outside wheel in a corner to turn quicker than the inside (since travelling at an outer radius, it needs to cover more distance in the same time). In full-time all-wheel-drive cars there is also a central differential, dividing twist between front and rear axles so expensive things don't break when one set of rubber is on asphalt and the other suffering from severe traction issues. Now, with free differentials torque travels via the path of least resistance, which is precisely the idea if you have four wheels on a sealed surface, but once you leave the road, things get tricky. In real offroading - and I mean snow, mud and unlikely cambers - you will very quickly find yourself with one or more wheel up in the air. Without a diff lock, all the torque from the engine will go to the lifted wheel, spinning it uselessly. Electronics have been called upon in softcore SUVs to apply the brake on the freewheeling culprit, in the hope that this will force twist in other directions. This works if you've parked on an ice patch and can't get out, but in saucier circumstances pads will fry long before you build up enough forward movement in the single wheel that is touching hard surface. The solution is to lock the differentials into a rigid 50/50 torque split irregardless of road circumstances. Turning becomes complicated, but you can't go very fast on a deer path anyway. A central differential lock is quite common on American SUVs, but you really need to have at least the rear diff locking as well if you want to reach that secluded forest lake. A diff lock can be engaged via a lever or the push of a button, but make sure that your next mud cruiser does in fact have *mechanically locking differentials* before forking over your hard-earned cash.

Other properties of an offroader are pretty much negotiable. You will probably want a big, torquey engine, although perhaps the most impressive specially prepared machine I have seen was a tiny three-door Suzuki Jimny running on a 1.3 petrol four-pot. Diesel is preferable as it provides lots of pushing power at low revs. All wheel drive is a must, obviously, but whether it be full-time is up to you. If most of your driving is done on clear, dry tarmac, you can do with a vehicle that is RWD under normal conditions and engages the front axle via a rigid link once you're on slippery surfaces (no central differential then). If you want the security of each wheel pushing forward under everyday circumstances, that's great - you'll need a "4-HI" or some similar option on the transmission mode selector. Low-gear is pretty much a given on anything designed to go offroad: this is a special cog in the 'box that trades RPM for lb ft. You'll need to rev the engine and the car will not move very fast, but you'll have gobs of twist. Automatic or manual depends on how hard you're planning to use your muddy beast: a separate clutch pedal and assurance that the electronic brain won't change up at the worst possible moment is an asset, but a lot of autos are smart enough to get the right message from the 4wd low-gear engagement. If you have a shift-your-own option on your automatic, use it and keep the car in first while on the trail.

Many people would argue that a true offroader has a ladder frame and beam axles, and they have a very valid point. The good thing is that these stand up quite well to running into things, which you absolutely will do sooner or later. However there have been some very good offroaders among the recent crop of luxury SUVs. The last generation Mitsubishi Montero (the car that has dominated the Paris-Dakar desert race for as long as anyone can remember) has independent suspension and wheel travel is up from its more primitive predecessor; the Volkswagen Touareg and Porsche Cayenne have air tanks instead of springs and shock absorbers, which allows them to increase road clearance to unholy amounts; you won't have any damping in the top setting, but you can do without. A 313bhp diesel Touareg will hold its own against a Toyota Land Cruiser, but you wouldn't want to give it more than the occasional workout: replacing smashed suspension components will cost you a bundle.

Winches, sump guards, snorkels, lift kits and big tires are all things you should investigate when building a proper offroader, but that's for a specialist more knowledgeable than me to discuss. I'm after the answer to a different question, namely: So which is the prototypical offroader, the greatest mud monster money can buy? The Land Rover Defender is extremely capable, but crude and unreliable. The original Hummer is too wide to drive through a forest. The Jeep Wrangler Rubicon is underpowered and hasn't really progressed much since the days of the Willys. No, I believe the Greatest Living Offroader is the rap star's one-time favorite, the machine designed for an army but good enough to still be in civilian production 40 years later, fitted uniquely with three locking differentials, big engines, beam axles, a ladder frame and a look that means business. Known among its admirers as the Gelandewagen, here it is: the Mercedes G-class.

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