Equipment Tests > Under the Pump

Issue: February 2007

Words and photos by Robert Pepper

4WD Equipment - Central Tyre Inflation (CTI)
* Armies swear by it
* Around since the '40s
* A 4WDer's dream

Imagine never having to manually inflate and deflate your 4WD's tyre again - welcome to the world of Central Tyre Inflation.

Tyre pressures are a lot like gear ratios. Which gear you select depends on what the vehicle is doing, and that varies from moment to moment. The optimum tyre pressure changes almost as frequently too. At high speeds, heavily loaded, you want high pressures. As soon the road turns to dirt you'll want to drop, and further again for low range work. Then maybe there's an extra-steep hill where you'd want another few psi out, then quickly back in, or a short bitumen transit. Perhaps you're about to hook up a heavy trailer. Or maybe it's silly o'clock in the morning after an extended daytrip and you just want to get home and not stop to air up in the cold rain.

The problem is that tyre pressures aren't easy or quick to change, so we tend to compromise and only change them occasionally. We bounce around in the rough for short distances, or wear out tyres on the bitumen when they're set low because it's too much trouble to air up or down for a short drive in different conditions.

Now that excuse is out the window with CTI (Central Tyre Inflation), the term used to refer to systems that allow the driver to control tyre pressures from the cab, without stopping. CTI has been around since the 1940s and is widely used by the military and trucking companies that drive on a mix of soft and hard roads. It's also available as an option on large vehicles like Unimogs.

CTI, hasn't really been accessible to 4WDers until now, with kits available from AIR CTI. The setup is pretty simple; you need an air compressor, hoses to each wheel, a tyre pressure gauge where the driver can see it and a control box to inflate or deflate. The only tricky part is getting the air to the tyre. Exactly how that's done depends on the vehicle, but for many applications the wheel is removed, and a base plate added behind with an extension that pokes through the centre of the rim (the rotator). Then it's just a case of joining the rotator to the air hose from the compressor, and to the existing tyre valve. No modifications to the rim are required, and any competent home mechanic could install it. The system also works at full flex and maximum lock, and can maintain air pressure with a smallish puncture.

The system equalises pressure on the tyres, or pair of tyres if it's set that way. However, if a vehicle is on a sideslope some CTI systems permit air to flow to the upside wheel. AIR CTI has special isolator to prevent this. The big worry for 4WDers is going to be, "I'd rip those hoses off where I go". Maybe, maybe not. Chet Cline, owner of AIR CTI is a keen offroader and doesn't have a problem with damage to the system, and neither do all the logging trucks that use it. You could always disconnect the short hose around the wheel itself and then reconnect it later on once you're through the tight bit.

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So much for theory. To see how well it worked we took AIR CTI's Vitara for a test drive along some bitumen and dirt roads. Initially the pressures were set to highway (all of 20psi in the light Zook with oversize tyres) and when onto the dirt at speed the shortie felt, as you'd expect, skaty and nervous, needing judicious amounts of opposite lock around corners. Then I turned the control to deflate to 12psi, and I have to say it was an eerie sensation as with every corner I could feel the car become grippier and absorb rather than skip over the bumps. We all know the handling benefits of lower pressures on dirt, but feeling it as it happens was a first for me.

Back on the bitumen with the tyres low the car felt soggy in the corners and wanted to understeer. Just a flick of the dial, back up to 20psi and it's a road car again. Absolute magic. And then we went for a play on Chet's private testing ground, with the pressures dropped down to 7psi, which allows the little Zook to pretty much climb walls. It's a great experience to be able to look at an obstacle and dial in exactly what pressure you want, and the effect is a more capable offroad vehicle.

After that experience we have to say this is one impressively useful device. The cost for the kit is around $2300 plus compressor, or fitted with everything it'll be $3000-$3500. The system can be installed to manage all four tyres at the same time, adjust individual tyres or split front/rear, which will be best for 4WDers. AIR CTIs plan to release an electronic display as an option to complement the analogue gauge, and it can warn if there is a pressure loss.

Now you could argue a CTI system isn't necessary, and you'd be right, in the same way that five-speed gearbox isn't necessary, and nor are variable-speed screenwipers. But necessary or not, once you've tried adapting the tyre pressures to the terrain as easily as we can change gear you realise it's not a gimmick, it's a bloody good idea.
For more info visit www.aircti.com or phone (03) 5127 6128

Tyre pressures cost money
How long do you think it'd take to change the pressures of a six or eight-wheeled large truck? Too long, so truckies can't take the time and for those that drive on a variety of surfaces, like logging trucks, that's a problem. They need lower pressures for the soft stuff, and harder for the roads. With a CTI system they can have both, and there's instant savings for the company; significantly less direct road maintenance costs (as in hundreds of thousands of dollars), increased tyre life and one company even sold a D4 dozer they had used exclusively for towing out bogged trucks as they no longer needed it. In that sort of work the CTI systems pay for themselves very quickly.

 

 

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