What do the helper springs do on a hydro mini?

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Cheers Dave.

They stop the trailing arms from dropping down. If unrestrained the arm would drop far enough to let the displacer strut pop out.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

So they have no bearing on the actual suspension rate of the setup, unlike additional shocks.

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Cheers Dave.

I suggest that thinking that they hold the back down (by holding the rear suspension up) is incorrect; rather they actually hold the front up.... They act as a static load to counteract the greater front end mass pushing all the fluid to the rear.

Without them, this mass would force the fluid to the rear until the front settles on the bump stops.

While they may well contribute (marginally) to spring rate that's not their purpose. Mini suspensions are separated into "dry" and "wet". However, both depend on a rubber "spring" to accomodate varying loads (ie bumps) and, by definition, have a spring rate. Fluid is basically non compressible and the "hydro" part of hydrolastic is somewhat analagous to the shock absorber in a less sophisticated suspension.. Have a think about the operation of "competition" bump stops ...

Front shocks (on hydro car) act like uprated or adjustable shocks on a dry car. Maybe it helps to imagine that springs control how far the suspension moves while shocks control how fast the suspension moves...

They say a little knowledge is a dangerous thing ... perhaps someone who really knows what they're talking about can straighten us out...

Cheers, Ian

I agree, Ian. Perhaps bad wording on my part if it was my post your are commenting on. The helper spring holds the trailing arm up but I didn't mean to imply that they "hold the back down". The main outcome is as you say, leveling the car by keeping the front up.

The page below was in the BMC Partner Magazine January 1964. It doesn't mention the rear spring and its functions. I'll look elsewhere as it would be nice to get the factory description.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

An Old and Bold Mini racer (Bob Holden) once told me he liked to raise the rear end of his hydro cars by several inches (3 I think) to improve his times....

Didn't say how he achieved this...

And.. following on from Win's comment.. I had a look at my 67 (UK) BMC Workshop Manual which has a hydro "How it Works" section but the helper springs don't rate a mention (other than to say to remove them when dismantling the rear suspension)...

Cheers, Ian

There's a similar discussion here - viewtopic.php?f=2&t=99515

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

The springing medium in both hydrolastic and dry suspension is the rubber cone. In hydrolastic suspension, the rubber cone is deformed by the incompressible fluid under pressure acting on its lower surface. The fluid of course also pushes down the piston on its interconnected unit in a manner which we all know.

In most cars with mechanical springs, the spring rate is linear. This is not the case for the rubber cones. The spring rate increases as its deformation increases. Issigonis went this way with the Moulton rubber cones because he needed a suspension system that would take up minimal space, yet accommodate a light car whose handling and ride had to be consistent (or nearly so) between empty and full laden with passengers and luggage – a change in load far greater in relation to the kerb weight than most other cars. As the payload increases, the rubber springs essentially become stiffer and so the change in the suspension’s geometry is minimised.

The problem with this arrangement is with what is called attitude change from front to rear. This is particularly noticeable when the headlights aim at the sky when you put two people in the back and a boot full of luggage. You can also see the effect if you attempt to set the ride height of a hydrolastic car before putting the battery and fuel tank (full of fuel) in the boot. You’ll never get it right. Window glass and back seat also have to be fitted before any pressure adjustments are made.

The helper springs load up the rear springs so that they sit in a more compressed state at the vehicle unladen condition. This means that when load is carried, the rear springs are sitting in a stiffer part of their response they would ordinarily be and so the deform less, and the back doesn’t sink down unacceptably.

The same problem occurs in an Austin 1800 utility where in that car, torsion bars were added for the same purpose – not to assist in the load carrying capacity, but to artificially load up the rubber springs to a stiffer part of the rubber spring operating range so as to reduce attitude change.

Tony Cripps