In recent days I have received comment about the size of airlines  
linking axles in air suspensions. In February 1976, as a graduate  
engineer in the then NSW Dept of Motor Transport, I wrote the  
following in response to a NAASRA study that recommended loadsharing  
suspensions:

"The appears to be a misconception in the industry in that  
loadsharing suspensions are believed to 'equalise' under dynamic  
conditions. A typical loadsharing tandem suspension is incapable of  
the reaction times needed for the load equalisation system to be  
effective. TRRL Report LR581 indicates there is a speed range  
(20-60km/h) in which the loadsharing suspensions studied apply  
significantly greater dynamic loads than an unlinked suspension. This  
is especially the case with rear axle loads and it is likely that  
load from the front axle is transferred to the rear axle just as the  
rear axle reaches the bump (ie resonance)...
Effective viscous damping would reduce unfavourable dynamic responses  
in loadsharing suspensions...
Some loadsharing suspensions exhibit load transfer under braking and  
these are prone to axle-hop and premature wheel lock-up....Of course,  
the best method of preventing load transfer under braking is to  
incorporate adequate radius rods to provide reaction to braking  
torque...
The main benefit of loadsharing suspensions is that they allow for  
changes in chassis inclination and permit vehicle to slowly traverse  
dips and humps without undue changes in axle load..."

TRL Report LR581 "Dynamic behaviour of two linked twin-axle lorry  
suspension systems" by J Page shows some classic resonances at 20,  
40, 60 80 and 100km/h in mechanically linked suspensions travelling  
along a road with a 2.5mm (that dot is not an error!) amplitude  
sinusoidal profile. The peak loads were double the static load with a  
"Hendrickson" style suspension and 1.3 times for a short rocker  
suspension. No air bag suspensions were available at that time  
(c1970) but it was found that strong viscous dampers all but  
eliminated the resonances. This is not unexpected  with a two-mass  
dynamic system (which is what a loadsharing mechanism is creating  
within the axle group).

In addition to the brake reaction problem, I also pointed out a  
serious road safety concern with loadsharing: in the situation of a  
tyre blow-out the last thing you want is for the load to be shared  
with the flat tyre and yet that is what a loadsharing suspension will  
do. Independent axles will put more load onto the sound tyre. That is  
the main reason why I opposed a mass limit advantage for loadsharing  
twin steer suspensions and I still oppose such a concession (it had  
nothing to do with my horror when I saw a "prototype" loadsharing  
twinsteer suspension for the first time!).

Speaking of horrors, I finally realised that people did not  
understand the mechanics of loadsharing when hinged-drawbar "pig"  
trailers (single axle group) appeared in Australia. The main  
resistance to pitching of the trailer body provided by these trailers  
was that the loadsharing mechanism reached the end of its travel. In  
fact, earlier I had toyed with a definition of loadsharing along the  
lines that it offers no pitch-resistance and behaves like a single  
axle when vehicle attitude changes.

So, in summary I realised there is no such thing as "instant"  
loadsharing and even a small (inevitable) delay in the transfer of  
load has the potential for damaging resonances. Until a practical  
active (pre-emptive) suspension system is invented that is able to  
predict the arrival of a bump or dip and assist the wheel to follow  
the uneven profile of the road then there is no point in having axles  
within a suspension group "loadshare" at speeds above 10km/h.  
Although air suspensions do have good viscous damping I see  
advantages in restricting the rate of airflow between axles in the  
group at normal road speeds.

Has there been any recent research into dynamic wheel loads and the  
rate of flow of air between air suspensions within an axle groups?

Regards
Michael Paine
PS The issues I raised in 1976 were too technical to get through the  
Australian committee process but I understand that they did help  
initiate further research on suspension system dynamic performance by  
people like Peter Sweatman. Others might have a slightly different  
recollection of these times but at least I still have some type- 
written pages that make enjoyable reading.








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