Lowering and stiffening Hydragas suspension HydragasHydragas Register
Thanks to DPR Motorsport

This page was kindly provided by DPR Motorsport, based on the work done on their racing Metro. DPR Motorsport no longer carry out Hydragas work but have let us use the information on this page thanks to high demand. This is intended as a guide for lowering and stiffening Rover 100 and Rover Metro, but can be applied to any car with Hydragas suspension.

Lowering and stiffening

The DPR Rover Metro GTi at Snetterton's Russell Corner

The Hydragas suspension on the Metro is a development of Hydrolastic, started with the BMC 1100 in the early 1960s. Hydragas continued on the MGF, although the latest MG TF and recent MG models have reverted to conventional coil damper units.

The picture on the right shows the Hydragas unit. From the engineering perspective, the analogy with conventional suspension is as follows:
  • Nitrogen Chamber = Coil Spring
  • Hydrolastic fluid= shock absorber

In standard form there are four Hydragas units, one at each wheel, that are interconnected in pairs front to rear with a special fluid. Inside each sphere is a rubber diaphragm separating the nitrogen gas from the fluid component of the suspension system. Due the balancing of the nitrogen gas against the fluid, the pressure on either side of the rubber separator should be equal but as fluid pressure increases, the nitrogen is compressed like the spring on a conventional suspension unit.

The valving of the movement of fluid from the spheres into the interconnecting tubes provides the damping effect of traditional shock absorbers. and contributes to the springing rate of the sphere.
Hydragas unit
Hydragas Modification

The basic objective is to lower and stiffen the action of the suspension. You can lower the car by reducing the pressure of the hydrolastic suspension but, at lower pressure, the suspension travel becomes less stiff - exactly the opposite of what is required!

Before you consider these modifications you should already have done the following:

Most of the work is DIY for someone who is reasonably competent but note that the DPR Motorsport Metro is used solely for racing. Don't try this with your road-going Metro unless you are confident in your own technical abilities - if in doubt, use a trained technician or specialist garage.

In terms of the special tools, the one essential is a Hydrolastic pump so as to be able to work on the suspension. Either buy a pump or search for local garages or individuals willing to carry out a pump-up service.

Below are the possible modifications in order of difficulty!

Section each unit

The standard hydrolastic units have a front to rear link that cleverly 'anticipates' humps such as provided by sleeping policemen of the road safety variety. The road holding, although not comfort, can be improved by running an individual valved section pipe to each unit.

Individualising Rover Metro suspensionAfter depressurising the system (!), the front to rear pipes can be removed and replaced by individual section pipes each with a valve. The two X's on the left picture show where these valves are mounted on the front bulkhead of the DPR Motorsport Metro.

The pictures below show the detail of each valve's location on the front bulkhead (click images to view full size).

Metro valve RH   Metro valve LH

The rear valves can be located in the spare wheel well (right)Metro valve rear
Lowering the car
Hydragas loweringHydragas lowering

The picture above shows the rear displacer unit with the rod and aluminium fitting to the rubber diaphragm. The end of the rod (opposite to tapered end) can be shortened. In the DPR Motorsport Metro, this rod has been shortened by 15mm and the ride height set to 280 mm between the wheel centre and the wheel arch.

The best approach is to measure the ride height at the hydrolastic pressure that provides the normal ride height , depressurise & shorten the rod by a fixed amount, e.g. 5mm. Then refit, repressurise to the same pressure and measure the ride height to see how much the car has dropped. For road cars, 5mm off this rod may be enough. You can then estimate the amount to remove to get to the desired ride height - but err on the lower amount as it is difficult to put back metal. You will need to bounce the car around to allow the suspension to reach a stable ride height.

The picture above omits the helper springs - provided that the droop stop modification below is made, these springs are not required but if refitted they may need shortening.

Once you have one side at the correct ride height, you should shorten the other side to exactly the same length. For the front suspension, the DPR Motorsport Metro uses the MGF displacer fitting that is 15mm shorter.

Stopping the droop

Metro droop stopMetro droop stopMetro droop stop

Stiffening the rear suspension is a compromise between increasing the fluid pressure and gaining ride height. For racing use, a wider droop stop can be fitted to stop the ride height increasing above the desired maximum. Please note that this does mean that the rear sub-frame is at risk of bending in certain situations especially when the new droop stop is made in aluminium rather than the standard rubber faced mount!

However, this modification does mean that fluid pressure can be increased thereby stiffening the suspension without raising ride height above the desired maximum..

Modifying the displacer
The 'ultimate' modification is to fit an air valve into the end of the Hydragas unit as shown on the right. On the DPR Motorsport Metro, we are experimenting with filling the air chamber for the front displacers with up to 50% liquid and running the pressure at 70 psi - the equivalent of a shortened & stiffer spring. This modification is probably best limited to the front.

Ideally the air chamber should be filled with nitrogen to avoid heat causing pressure changes but for racing use, an ordinary air pump can be used if some increase in ride height is allowed for.

A less 'ultimate' but probably more reliable modification is to fit the MGF Cup displacers supplied by Rover Sport that have a fixed lowered air pressure. Stock is limited so you need to order these quickly.
Hydragas unit with air valve Hydragas unit with air valve

Note that lowering the ride height below standard can lead to unacceptable camber changes. (click here for an explanation of the camber problem on Rob Midgely's MGF site).

Changes CAN be carried out successfully, but it is a process that takes time and patience. The workshop manual states that the car should be allowed to settle for over an hour in a constant temperature environment prior to any geometry checks. Then the car needs to be regularly ´┐Żbounced´┐Ż gently at each corner to ensure that the equilibrium of the system is reached. The car's ride height can then be adjusted to the required ride height. Without sufficient care, the car can drop or rise further after suspension depressurisation or repressurisation respectively, and will throw out the height readings as a result.


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