Re: active suspension (no Audi content)

[dwall@sisna.com]

What your were watching was an air-ride suspension at work.  Air ride 
suspensions are probably the most widely used type of suspension on large 
class 7 and 8 trucks today.  Those large cans are actually large rubber 
air bags, of which alot are made by a Volvo-White, which also makes entire 
trucks.  The air bags are used to vary the load capabilities of the axle, 
which directly affects the ride.  When the trucks is empty (or 
trailer-less), the driver (or the control unit on some of the most 
advanced units) lets most of the air out of the bags, effectively lowering 
the spring rate of the suspension, giving the truck a better ride.  When 
the truck is loaded, the driver (or the control unit) raises the pressure 
in the bags to raise the load carrying capacity of the suspension.  The 
latest versions of these suspension (Volvo is one of the leaders) can 
actually measure the load on the suspension and compensate the pressure 
accordingly. 

These suspension aren't "active" like you would consider in an automotive 
suspension, that is, they don't change the pressure based on road 
conditions or cornering/braking/acceleration loads.  The reason the action 
of the air bags didn't seem to match the suspension loads at the time 
(extending on acceleration when you think they should be compressing) is 
that the tandem axles are tied together by what is called a "walking 
beam".  Basically it works like an anti-sway bar turned sideways.  When 
the front tandem axle hits a bump, the travel of this axle upwards pushes 
the rear tandem down, trying to lift the truck in the process.  So when 
the truck accelerates the front tandem will squat, like the rear axle on 
an automobile would, but the rear tandem actually goes the opposite 
direction.  There are a lot more factors to a walking beam type suspension 
than I could explain here (like what happens in a corner, the wheelbase of 
the truck actually changes!)  To avoid WOB, If anyone is interested, I 
could give you references on where to read all about large truck 
suspension theory off the list.

I work for Mack Trucks (until next week at least, a whole other story), so 
this stuff is old hat.  Now if I was just as good at working on my Audi as 
I am trucks, maybe I could actually provide some Audi related information 
to the list.

Darren D. Wall
dwall@sisna.com
darrenwall@netzero.net
1985 Audi 4000 Quattro
1969 911 Turbo-Look 2.7Liter  **For Sale**


I was following a brand-spanking-new trailer truck, sans trailer. So,
like, my line of sight was this rear axle (funny, no leaf springs),
connected to the ladder frame via two huge cylinders (think ultra-large
coffee cans).

What first caught my eye was these two big "coffee cans" have "Volvo"
written over it. So like, I was watching 'em in this annoying stop-and-go
rush traffic (jam).

While watching the odd suspension, it began to occur to me the compression
& expansion of the said cans don't really go together with the bumps and
dips of the road. In fact, I thought the peculiar suspension is somewhat
slow in reacting to road condition -- sometimes even downright weird and
totally out of this world.

Then it finally occurred to me the suspension is an active one! Whenever
the truck begin to accerlate, the said coffee cans would extend themselves
about 3 inches. As such, when accelerating, the truck would "tilt" to the
front. Conversely, when the truck is coasting or slows down, the cans
would retract those 3 inches (and more during complete stops). Totally
cool! Although I don't think I'd like the "rocking boat" feel of a car
fitted with that sort of active suspension... Good for racing, I suppose.