Fluids and solids

On Hydrolastic and live leaf spring suspensions

- MARK ORTIZ

Q I very much enjoyed your latest newsletter on suspension dynamics and it made me think of the BMC Hydrolastic suspension employed on many of their models from the ’60s, such as the Mk II Mini Cooper and Austin / Morris 1100 sedan. These cars had displacer units in place of the Moulton rubber springs (aka donuts) that were plumbed diagonally RF/LR and LF/RR.

The ‘Hydro’ units were not considered the best for racing and most race Minis (mine included) were converted to dry suspension. But homologation requirements meant earlier racers had to keep the ‘Hydro’ suspension, so they installed valves inside the car to ‘tune’ the displacement from one corner to the other, or shut it off completely so most of the suspension was in the tyre carcass.

I was just wonder if you had any thoughts on the BMC Hydrolastic suspension? The MG 1100 had a very comfortable ride for a small road car.

THE CONSULTANT

A Hydrolastic was an antisynchronous connection of front and rear wheel on the same side. It’s essentially equivalent to the torsion bars on Packards, or the coil springs on 2CVs. It produces a suspension that is relatively soft in pitch and warp, and firm in roll and heave.

You could connect diagonally opposite pairs, but you’d want an anti-oppositional connection, an anti-roll / anti-pitch device.

This does bring up one undesirable aspect of warp soft suspension – that it can be a bit problematic for a car that routinely corners on three wheels.

Rough rider