If there was a better way of designing caravan suspension, I'm sure a manufacturer would have come up with something by now.
Do you really believe that? Please read a few books on suspension design.
Have a look at the third last and last post on this site.
http://caravanersforum.com/viewtopic.php?f=2&t=22776&p=522647#p522647Those long flat springs that can be found under all leaf spring cars have been designed that way in order to change the angle of the rear axle in relation to the centre line of the car in corners. If you turn to the left for example the axle also turns slightly to the left. That is roll understeer and has been known by the auto industry since pre war days. It considerably enhances stability and handling.
The same feature can be found in coil and air suspended cars.
The short stiff high arc caravan and trailer springs do the opposite and either don't turn the axle at all or turn it the opposite way in corners which is the last thing you want. Why do the van and trailer manufactures do it this way? Most likely because it is cheaper and easier and most buyers would not have a clue anyway.
Australians used to accept average handling cars right up to the '60 s and early '70s' then the Europeans turned up. In no time Holden was making a big fuss with its improved handling "Radial Tuned Suspension" which was launched with a huge TV campaign. Ford and Chrysler soon followed.
Many people hopelessly overload the rear end of their car and naturally enough the back goes down too far. The"ëxpert"advice on forums is the stock springs are no good and the car must be lifted up with something decent like air bags or heavier springs. That completely stuffs up this vital stability feature.
The following are three posts from the same man.
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I've come from a background in auto industry where generally ride frequencies were around the 60 cycles per minute range, sometimes for sporty stuff they'd creep up to 75-80 cycles per minute [and I'm ignoring tyre effects here: I guess light truck tyres are around 1200 lb/inch so they don't have much effect]. I had a look at the coil springs on my inhererited 1340kg poptop van & the installation effects and worked out the ride frequency was around 180 cycles per minute! Totally unnecessary, causing who knows what damage quite unnecessarily to both van, contents, and tyres. I'm going to scout arond a few wrecking yards & see if I can find something to bring the ride frequency down to 75 cycles per minute: this will not require an anti-roll bar because at .75g lateral [desperate for a caravan] I figure roll angle will only use upabout 24 mm wheel travel.
I figure this will allow me to put a domestic split system air conditioner in the van [much cheaper] and the flat scree TV might last a little longer.
By the way I'm pulling the ball load right down to around 40kg [currently around 140 kg]with a new long A-frame & relocated water tank, similar to my car carrying trailer that has run at 15-25 kg ball load for the last 30 years with a 960kg car on it [gross trailer 1460 kg] with trailer spring rates around 75 lb/inch: it cruises at 110km/h behind a 1645 kg sedan & you wouldn't know it's there. The trailer does have 10% roll understeer at loaded condition & I believe that has a lot to do with it's impeccable manners.
I strongly suspect the too stiff springs matter has to do with "covering up" adverse roll steer effects by limiting suspension travel: a case of caravan chassis builders not having any understanding at all of roll steer effects & the geometric shortcomings of short , highly cambered leaf springs.
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Interesting comments, but I looked at the cruisemaster suspension system & there's no detail on spring rates, only the suspension package load capacity. So I still have no idea of spring rate, and therefore ride frequency for the van.
I wasn't impressed with the cruisemaster design detail: too many welds directly loaded in tension rather than shear [poor fatigue life & weld quality sensitive], poor bracket design (see the dampers upper mounts: could so easily be more robust with better design), tubes not boxed for stiffness, no gussets to brace the main suspension mounting crosstube which has large loads & torque input [due to the offset distance of the contol arm pivot to the cross tube] from the control arms. Add to that a narrow base to the control arms so the suspension is less stiff to lateral loads from the wheels [longitudial compliance is good for ride dynamics, but lateral stiffness should be as high as possible], and so prone to compliance steer effects. I might add these criticisms can be equally applied to nearly all trailing arm suspension systems that I've seen, and I've seen quite a few. I think I'll need to build my own.
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Just a light note on the ball mass issue. I've seen & chased big vans on autostradas & autobahns in Europe, often travelling in groups, all with big sedans [BMW 7 series, big Mercs, nothing else] all with no WDH, all cruising along around 120 to 140 km/h, with absolutely no apparent drama. So low ball mass is ok, but the rest of the rig needs be designed, not just cobbled together by someone who's been at it for years. Someone else quoted Bailey caravans in UK who have sponsored some interesting independent research: they don't have trouble with low ball loads, and some are as low as 54kg for 1750 kg vans.
I'm messing around with an old poptop, 1340kg, with independent trailing arm suspension, and found the toe setting was about 2mm toe out. I've corrected this to 4mm toe in & the difference in stability is significant, just as I expected. Interestingly I've measured the compliance steer & it's terrible: apply a lateral inwards force to the tyre & it toes out. The trailing arm bushes are in good condition, but the geometry of the arms, with the bushes close together, makes it laterally soft [ie large deflection for low applied lateral load]. Next temporary cure is more toe-in while I look at how to redesign the system for toe in on lateral load. And yes, I'll watch the tyre temps as toe in increases...
Maybe a proper roll understeering beam axle [beam has almost zero compliance steer effect] is better than a crude independent suspension? I think the Australian van industry has a long way to go....
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The technology is there for van and trailer manufacturers to make a vast improvement to their products. All they need is buyers to demand it.
This link will show what else can be wrong with caravans, it is not just suspensions
http://caravanersforum.com/viewtopic.php?f=2&t=12286The European van makers aren't big over here because they just don't make a better van....or a true offroad one.
That is what many people used to say about Japanese cars in the early 1960s until the Landcruiser, the shovel nose Toyota Corona'and the Datsun 1600 turned up. The rest as the saying goes is history.
