elkay
You can get good safe cars with both methods of construction. It all comes down to how it is designed. There are separate chassis utes today with the maximum possible safety rating and no shortage of mono cars with the same rating.
All makes of separate chassis utes regardless of whether they are dual, extra or single cabs have been known to bend chassis. You will find no end of examples in Google. Have a close look at this dvd on utube and you will start to see why.
If you push down the rear end of a chassis, it will pivot on the rear axle and try and lift the front end. In the case of this Ford truck, the driver is towing with a cable attached to the top of the hoist. This has pulled the rear end of the chassis so far down that the front wheels have lifted of the ground. Few if any chassis have been designed to withstand that and this one is no exception. It has bent at the pivot point which is the front hanger on the rear springs.
The problems associated with all the Hilux, Triton, Navara size utes is much the same i.e the rear goes down and the front goes up. This will not happen though if the ute is correctly loaded and that is where the problems lie. Many people will say their car was a couple of hundred kilos under GVM but the chassis still bent so it is too weak. One particular late model dual cab ute that I am familiar with has a carrying capacity of 930 kgs. If you are going to take it up to its maximum weight then every factory designed load bearing point must be fully utilised. You would start at the front seats and put two people of around 120 kg each in them. Then put a couple more at around 100 kg each behind them with a fifth person weighing about 60 kg in between them. You now have 500 kg in the cabin. Filling the fuel tank with diesel will add about another 70 kg. This leaves 360 kg to go into the little tub body at the back. With the heaviest items placed as far forward as possible, the car will now be at GVM and should be sitting low but level with enough suspension travel left at each end to suit highway and around town driving. That is the way the factory intended it to be under maximum weight. The factory will also tell you their maximum advertised carrying and towing capacities are for good conditions only and should be reduced for off road conditions.
If you try loading it with say only two people of about 100 kgs each in the front seats and try putting the remaining 730 kg in the tub and fuel tank, the rear end will be sitting down on the bump rubbers. That is not what the factory had in mind.
To solve this problem you could place a few hundred kilos in a small box trailer. The car would now be evenly loaded and way under its maximum carrying and towing capacity. That will not suit many people so they turn to heavier after market rear springs and worse still, air bags between the axle and chassis. Alarm bells are now ringing for the chassis.
Heavier rear springs and excessive rear end loads can easily result in cracked chassis and diff housings as well as damage to wheel bearings and wheel studs. Check the damaged in these photos.
http://www.mtdare.com.au/index.php?option=com_expose&Itemid=3 Use the little red arrow below the pictures to go to the third page and click on "Vehicle & Trailer Breakdowns". Unless those cars have been trying to set new speed records on Outback roads, you can bet those broken chassis and diff housings etc. have been caused by overloading.
The air bag is the real killer though. It is not a linear spring. It gets harder as it compresses and can become very hard as it nears maximum compression. With too much weight behind the rear axle, you will need a fair bit of pressure in the bags. They then lift a huge amount of weight off the springs and become the new pivot point. As the back goes down, the chassis rocks on top of the bags and tries to lift the front. This becomes far worse with a camper trailer and a spare wheel or two on the back. The tow ball on these type of utes is usually around 1200 mm behind the rear axle. That is a hell of a long lever to be pushing down behind an air bag. All that weight behind the axle is stratic weight only. When the wheels drop into a depression in the road, it comes down with a force far greater than its stationary weight. Try holding a brick in one hand and drop it a few inches into the other and you will see what I mean. It is not hard to see why so many chassis bend under these circmstances.
The problem of bending chassis above the rear axle is not confined to separate chassis cars. I have seen it happen to a few early model Holden Commodore sedans. Once again it was not the fault of the car. The cars had a coil spring rear suspension. This meant all the heavy body sections designed to support the rear end weight were located above the axle. There was not much in the way of heavy reinforcement back near the rear bumper like prevoius leaf spring models. The factory designed tow bar bolted onto the rear of the body but had a long brace that extended forward and up into the reinforced area above the axle. Some of the cheaper non genuine bars did not have the brace. If you used one of those, you ran the risk of bending the rear end of the body. The weight on the tow ball combined with whatever was in the boot, forced the body down behind the rear axle just like it does in a separate chassis ute. The unbraced body would now bend down above the axle causing ripples in the rear quarter panels extending down from the lower corners of the back window to the top of the wheel arches.
If you use any car the way it was designed and be prepared to tow instead of carry if you have too much gear to go into the back, you won't have any problems.
