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As readers of the CMCA Forum will be aware, as an engineer, I am less than impressed with the suspension of most of the low-price end of the fifth wheeler industry - particularly US imports. I am not alone in this. At least one Member has totally rebuilt his, and I know of others modifying theirs to obtain some semblance of Australian road-going acceptability. (Whilst less relevant to most CMCA Members, the issue is true of many locally built cheaper conventional caravans too).
The problem, that also seriously affected an otherwise excellent range of US-designed heavy trucks, is that almost all US roads are well surfaced and, unlike our roads, have next to no camber (and hence weight transfer). Their vehicles are designed accordingly. |
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American RV usage too is different. Typically, US ‘travel trailer’ users move all but exclusively from trailer park to trailer park. For good reasons, free-camping (‘boon-docking’ in US-speak) is rare. As a result, US trailers rarely encounter anything other than flat bitumen, flat concrete, smooth grass and the occasional short gravel driveway. |
Another factor is the seemingly inexorable US trend to ‘biggerising at Walmart prices’. This results in cost cutting to sometimes extreme lengths. Good suspension costs a lot more than barely adequate suspension. You get only what you pay for, so the inevitable result is suspension that is only just adequate for its intended usage. There’s no reason to expect better.
Usage on roads not built to US standards will result in a harsh ride that may damage things within the trailer, may cause fatigue failure of internal and external panelling, premature wheel bearing and/or stub axle failure, and wheel nuts working loose.
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There is also a safety issue. Heavy braking on bumpy roads still restrains the wheels, but to less effect if they are not fully in contact with the road surface.
I need to stress that not all fifth wheelers are like this, but many low-end products are. Airstream for example continues to make totally superb products – but following the effective deregulation of the market, general US RV quality is claimed (including by many Americans) to have gone downhill from what was not previously even that noteworthy a plateau. |
This article includes a picture of about the lowest possible cost and equivalent effectiveness of a cheap twin axle trailer suspension system. That pictured (deliberately) is of our own under-$1500 box trailer bought for things like carting cow manure and taking stuff to the local tip. It’s good enough for that, but only just. Look under almost any imported low-priced fifth wheeler and what you’ll find there is virtually identical.
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Were roads to be totally smooth, there would be no need for sprung suspension. In practice this is not realistically achievable. As Stephenson learned the hard way, even trains running on smooth rails need to cope with track irregularities.
More so than railway lines, roads (and especially non-US roads) are far from totally smooth. As a wheel rolls over a typical road it must rise and fall over bumps and depressions without the resultant shocks being transmitted to the rest of the vehicle.
The effect of such road imperfections is related to speed, and as with many aspects of heavy things that move, the forces involved are proportional to the square of how fast they do so. A trailer towed over a rough road at 60 km/h therefore gets clobbered four times as hard as at 30 km/h – not just twice as hard. At 100 km/h those blows are over ten times as hard. That clobbering is more severe than many suspect.
At a modest 60 km/h, one travels at 16 metres a second. A bump one metre across is thus traversed in one sixteenth of a second. The effect on the vehicle is not a gentle rise and fall – even at 60 km/h the effect is like a giant hammer belting the wheel upward from below. If that road is corrugated, the suspension is belted hard, (about 500 times a kilometre) more than half a million times if you cross the Tanami!
Inserting a spring between each wheel and the rest of the vehicle enables some of the energy imparted by that upward force to compress the spring. Rather than the shock loads affecting the whole vehicle, they compress that spring instead. The lighter the wheel and spring assembly, and the heavier the rest of the trailer, the greater the amount of road shock absorbed by the spring (this is one of the few good arguments for alloy wheels).
So far so good: a spring with a heavy axle and road wheel attached beneath it is compressed as it encounters a bump. The rest of the trailer will still be subject to some road-imparted shock, but a great deal less.
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At this point of what happens, the low-end of the trailer industry seemingly feels it has done enough. Add crude springs; problem fixed.
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But it’s not!
What we have now is a spring that contains most of the energy resulting from a 30-50 kg axle and road wheel being hurled upward against a fair-sized spring in less than one fifteenth of a second (at 60 km/h). That is a lot of energy. At 100 km/h it’s a great deal more.
The instant that wheel has passed over the bump, the spring instantly lets go. Its pent-up energy catapults the wheel and axle assembly downward, releasing the energy in that spring. Unless suitably restrained, it does this with a force little less than that which thrust it upwards in the first place. It smashes that wheel and tyre onto the road surface like a demented jackhammer.
And that’s what’s happening underneath most low end trailers travelling over anything other than smooth roads.
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All this was realised around 1800, as indeed was the solution; if friction were to be suitably introduced, the energy imparted to the spring could be dissipated as heat.
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Most vehicles back then had long and supple multiple-leaf springs and some friction would result as the spring leaves slid slightly over each other when they were compressed (as when passing over a bump). The brighter end of the trade realised that whilst this introduced friction on the upward compressive travel, it did not work at all on the vital downward movement, because the spring leaves were then no longer pressed together. To partially overcome this, some carriage builders and owners bound the spring leaves together with strong leather thongs. Vintage sports car drivers still do. Another crude attempt at exploiting downward moving friction is a clamp fitted to the upper leaf that vaguely holds the spring leaves together.
Less crude was the Hartford friction damper, wherein friction disks clamped together were caused to rotate against each other. A later semi-rotary version used hydraulic damping (oil forced through small holes). All converted the energy in the pent-up spring into heat that was dissipated by atmosphere. Such methods helped, but what was needed was to enable the downward movement to be damped more heavily than the upward movement. This became essential with the advent of coil springs and was resolved by de Carbon and his telescopic hydraulic dampers - still used in various forms to this day.
The entire need however for controllably releasing the pent-up spring energy is ignored by a substantial number of trailer makers. Many ignore or deny the implications of totally basic physical laws of nature, arguing that their products are somehow immune.
As a direct result, the underpinnings of cheap trailers (including very many fifth wheelers and caravans) literally remain at the level of low-end 1800-era horse-drawn carts. I emphasise carts because by 1800, most carriages had surprisingly sophisticated suspension.
Even two centuries ago, many a humble cart was better sprung than some trailers of today. Figure 2, for example, shows a baker’s cart of that time: note its long supple springs.
This is not a global phenomenon. Figure 3 should put many a US and local trailer builder to shame. I took this picture in Uzbekistan in 2006. It is a 30 year old trailer built following then-common Russian trailer practice. It has a central tubular chassis, soft, long travel, independent suspension and huge shock absorbers. It has carted close to a tonne of rock some 50 km daily during those 30 years, without needing any replacements except tyres. Given a decent coat of paint, it seemed good for another 30 years.
Figure 4 shows an ultra-cheap Finnish box trailer from the local equivalent of Bunnings. It has long supple springs and more than adequate shock absorbers.
Figure 5 shows a Swedish built trailer recently imported to WA. It is superbly built in every detail and has twin Alko rubber suspension.
Australia’s laws of defamation make it unwise to show identifiable low-end fifth wheeler suspension in the context of this feature. But if you doubt the above comparison with my cheap garbage shifting box trailer, check a few out yourself.
The reality is that many trailer makers continue to produce products using a form of suspension identified as deficient two centuries ago, and trailers back then travelled at less than one tenth of today’s speeds.
No global car company in its right mind sells products to Australia without major upgrades to suspension. Our RV buyers deserve the same. I also suggest that reviewers of these products stop glossing over the only too obvious suspension deficiencies.
To avoid putting a scare into existing owners, I tentatively suggest, but do not professionally advise, that the low-end products described are probably safe if limited to travelling on well maintained main bitumen or concrete highways. But I would hesitate to bring one down our relatively good 10 km dirt road into Broome. Even once.
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