Another Human Powered Vehicle! Part 3 - Interconnected Suspensions

This article was first published in AutoSpeed.

Last week, in Part 2 of this series, we introduced air bag springs. Much like the air bags used in trucks and buses (but lots smaller!), airbags springs have advantages over other springing media in weight, deflection, and rate characteristics. But, best of all, the behaviour of airbags can be radically changed by inter-connecting them and/or linking them with extra reservoirs.

That sounds fine in theory, but how well does it work? Especially when on a Human Powered Vehicle you don’t have the luxury of a high pressure reservoir filled with air via an on-board air compressor!

This week we look at interconnected suspensions in general and then next week we apply those ideas to HPV airbags.

Interconnected Suspension

These days, linking the suspension action of different wheels in cars is rare and is typically confined only to expensive cars that use electronic control of their suspension. This is usually achieved through damping but sometimes also through springs. But in the past, although it was still rare, quite sophisticated linking of the suspension movement of wheels was done on some quite cheap cars.

But before we go any further, some definitions are needed.

• Roll is the rotation of the vehicle around a longitudinal axis. A vehicle rolls when it is cornering: one side of the vehicle dips lower than the other.

• Pitch is the rotation of the vehicle around a lateral axis. When a vehicle nose-dives under brakes, or squats under power, it is pitching. However, pitching can also occur when the front (or rear) wheels meet a bump and that end of the vehicle rises or falls.

(Yaw is the rotation of the vehicle around a vertical axis. A vehicle that is understeering is not yawing sufficiently. But yaw is not really important to interconnected suspension systems.)

The definitions of pitch and roll may seem pretty simple, but when you start talking interconnected suspension, you need to have them instantly available to your thought processes.

Interconnected Front/Rear Suspension

One car that ran interconnected front/rear suspension was the 1960s Austin 1800. It used rubber cone springs (very interesting in themselves, but a red herring in the current discussion) and the fully independent suspension systems were hydraulically interconnected front/rear on each side of the car.

So how does it work?

When the front right wheel hits a bump, that wheel’s Hydrolastic suspension unit is compressed, so sending fluid pressure to the right rear wheel. (Remember, the springing is provided by the rubber; the fluid movement is just a signal as it were.) The fluid movement causes the right rear wheel to push down harder, so lifting the right rear of the car and preventing pitching.

When cornering, roll tries to simultaneously compress both the front and rear suspension units on the one side of the car. (And extend the other side’s suspension as well.) The fluid has nowhere to go and so actively resists roll.

I own an Austin 1800 and I think that the suspension is brilliant. The Austin tends to flow along the road rather than with the jolting that comes from today’s fashionably firm damping. Bumps are met with a ‘heaving’ motion: both the front and the rear rise together. This is much more comfortable than pitching. (And I might add, even with 75 series tyres, the Austin’s not at all shabby about going around corners. Perhaps in part because of its wide track and relative lack of roll, a cornering 1800 is stable and composed.)

So in summary:

Front/Rear Interconnected Suspension (eg Austin 1800 Hydrolastic)
Pitch	Resisted by corresponding up/down motion of the suspension at the other end of the car on the same side
Roll	Resisted by compression of both front and rear suspension on the one side of the car
Two wheel bump	Generally no more resistance than non-linked suspension
One wheel bump	Generally no more resistance than non-linked suspension

There are some further subtleties to be considered. If the pipes that connect the front and rear suspensions are small, or restricting orifices are placed in them, the pitch compensation that occurs will vary with suspension deflection speed. That’s because a bump taken slowly will move the suspension slowly, and there will be time for plenty of liquid (the Austin uses just water and anti-freeze!) to flow through the orifice. But at high speed, the dynamic resistance of the small orifice will be greater, so less pitch compensation will occur. The pitch resistance caused by the constriction of the pipes is needed - otherwise the car would have no resistance to pitch caused by braking or acceleration (except that provided by the suspension geometry).

Interconnected Side/Side Suspension

Suspension systems that are linked laterally across the car are much more common – any car with an anti-roll bar has one. An anti-roll bar is a torsion bar spring, connected to the suspension by means of cranked arms.

An anti-roll bar (sway bar, stabiliser bar, etc) tries to make the wheels at the one end of the vehicle go up and down at the same time. So if (say) the front-left wheel rises, the torsional spring that is the anti-roll bar tries to make the front-right wheel rise as well.

Take a car that is cornering to the right and so leaning to the left. The left-hand wheel is in compression; the right-hand wheel in extension. The anti-roll bar is twisted and so tries to drag the right-hand wheel upwards, in turn pulling the body more level. Remember, it does this by trying lift the inside wheel, so too stiff an anti-roll bar will actually lift the inner wheel off the ground.

An anti-roll bar most strongly resists roll (one wheel in droop while the other is in bounce) but it also resists the motion of just one wheel (eg a one wheel bump). Because of the way in which it is mounted, an anti-roll bar has no affect on bumps that affect both wheels in the same way - so a two wheel bump or droop is not resisted by the anti-roll bar. This also means the anti-roll bar has no affect on pitch.

So in summary:

Side/Side Interconnected Suspension (eg anti-roll bar)
Pitch	Not resisted
Roll	Resisted by twisting of anti-roll bar
Two wheel bump	No more resistance than non-linked suspension
One wheel bump	More resistance than non-linked suspension

Lessons

By comparing the two tables, you can see that a suspension system linked front/rear has clear advantages over a suspension system linked laterally. A front/rear linked system resists both pitch and roll without harshening one-wheel bumps. Furthermore, because such front/rear linkages are usually by fluid connections (but they don’t have to be: the Citroen 2CV ran the pictured mechanically linked front/rear suspensions), it’s easy to vary behaviour with suspension speed and/or suspension displacement.

But – and here’s a critical point for HPV design – a front/rear interlinked suspension system only works in the way described above if the vehicle has four wheels! If it’s a three wheeler, one of the major advantages of front/rear interconnection disappears – namely, the ability of the machine to resist roll. So while initially it looks like front/rear interconnection would be the way to go with airbag suspension on a trike HPV, that’s not the case.

So is it possible to gain major advantages by interconnecting suspension systems from side to side, not just with an anti-roll bar but also with a fluid-based system?

Next week: bench testing airbag interconnection

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