MGF Suspension and Lowering

or "Tracking" down the suspension problems

Explanation pages: camber toe caster

 


Introduction

When ever MGF owners get together and get down to discussing problems with their cars, one of the most frustrating and potentially costly is that of accelerated tyre wear.

The reason? Incorrectly set tracking.

One of the technical highlights of the MGF at launch was its suspension system, using the innovative Hydragas suspension system. It is this system that imbues the F with its superb ride and handling characteristics- but is also the source of some of these tracking related problems.

History

Hydragas is not a new suspension system. It was originally found on BMC small car products of the early 1960s. This system's earlier guise was known as Hydralastic. Hydrlastic/ Hydragas essentially consists of four spheres- one at each wheel that are interconnected front to rear with hydra-fluid (water with a high content of rust inhibitor/ antifreeze that gives it its characteristic fluorescent green appearance). Inside each sphere is a rubber diaphragm separating the gaseous nitrogen from the fluid component of the suspension system (it doesn’t take a rocket scientist to derive the name Hydra-gas!). The valving of the movement of water from the spheres into the interconnecting tubes provides the damping effect of traditional shock absorbers, and contributes to the springing rate of the sphere. (For further information and pictures, see John Thomas' explanation page.)

Figure 1 Hydrgas sphere

Hydralastic developed into Hydragas for the Austin Allegro. The inherent damping characteristics of these spheres means that cars fitted with this system do not necessarily require additional conventional dampers.

Were it not for one car, the Hydragas system would have died with the Austin Allegro in the early eighties. That car was the Austin Mini Metro (subsequently evolving into the Rover Metro/100).

When the MGF concept was being developed, a requirement was that Rover use existing parts from their passenger car range for reasons of cost. Rover engineers eventually elected to utilise the Rover Metro/100 front sub-frame both at the front and at the rear- to carry the suspension and steering gear at the front, and carry the mid mounted K-series engine, suspension and drivetrain at the rear. The use of the Metro sub-frame brought with it that car’s Hydragas suspension, and the Metro’s unequal length double wishbone suspension geometry- ideal for a sports car. Because of the Sporting nature of the F, the Hydragas system was augmented with conventional dampers for further improved suspension control, whilst maintaining soft (for a sportscar) springing.

This intelligent use of existing technologies brought us the MGF at a far lower cost than developing all-new sub-frames from scratch. The affordable MG sports car lived again!

Problems with the Standard set up

Hydragas suspension is largely problem free after three decades of development, but there some issues that directly relate to the system’s application in the F.

Remember that the MGF uses Metro sub-frames, and the Metro is a Super-Mini sized hatchback. The geometry and suspension pick points were therefore maximised for this application. Unfortunately for us, the Metro has a higher standard ride height than the F. For better handling, the F has been lowered by the Rover development team as far as can be achieved without unduly affecting camber. The ideal ride height is said to be 368mm, plus or minus 10mm. Lowering the ride height below the lower limit of this range can lead to unacceptable camber changes. [click here for an explanation of the camber problem]

Therefore it is critical that the suspension height is properly set before the car is put into daily service. MGFs for transportation to dealers are jacked up to maximum ride height so that they will fit onto the car transporters without damage to the vehicle’s underside. They are then lowered at the dealership to the correct height before sale to the public. Unfortunately, not all dealers have all the necessary equipment to do this correctly (requires 3D laser geometry checks), and the lowering process needs to be undertaken with much care, attention and patience. The car should be allowed to settle for over an hour in a constant temperature environment (the figures below are for an ambient temperature of 17 Celsius). The car needs to be regularly gently ‘bounced’ at each corner to ensure that the equilibrium of the system is reached. The car is then lowered to 368mm 10mm (measured from the centre of the wheel to the wheel arch vertically above). Without sufficient care, the car can drop still further after suspension depressurisation, and will throw out the tracking as a result.

Problems with lowering the suspension

If you’ve followed me so far then you may be thinking, "does this mean I can’t lower my car?". The answer is yes, you can lower your car- but you must proceed with caution. And ideally you should have easy access to a hydragas pump.

Firstly, there are many F’s that have unnecessarily high ride heights. Essentially any ride height above 368mm is regarded as acceptable, and most dealers will not touch your car to lower it even if the ride height is over 395mm (like mine was!) because of the problems associated with the lowering process. But suspension this high is detrimental to handling.You can, therefore, lower your car within the manufacturers' limits to the benefit of handling (lower centre of gravity) and appearance without excessive geometry change or insurance implications so long as the car remains above 358mm. Any back street garage that services Metros can alter your car’s ride height. The Hydragas valves (that look suspiciously like tyre valves) are located in the front bonnet under the black shroud [for pictures, click here]. With due care with a friendly mechanic, you can lower the car for as little as a fiver. ALWAYS have the tracking rechecked once the ride height has resettled (leave at least 3 hours, ideally after a short drive over some speed bumps and a cooling off period!). It is not uncommon to find that one side has been lowered more than the other has, so you may have to head back to your friendly mechanic friend to get the car pumped back... I’d warn you that this is a tricky process, and is easy to get wrong!

Due to manufacturing tolerances, you may find that the camber is out by up to a degree even with the ride height within the normal range. On my car @ 360mm (17C) the rear camber is out between 30 minutes and one degree; so far after approx. 8k miles, there is no discernible uneven tyre wear. [TIP: always park with full steering lock- not only does it make it more difficult for a thief to tow your car away, it also means you can always observe tyre tread casually and regularly]

If you want to lower your car further than the minimum 358mm then you either accept radically increased tyre wear (I know of one car that regularly goes through a set of tyres every 5-6k- which is the annual mileage of that particular example), or you spend some serious money in correcting one of the weaknesses inherent in the Metro suspension design- namely the lack of camber adjustment.

One of the MGF Cup team Racing engineering companies, Techspeed, is currently undertaking development of just such a suspension system, from racing derived components. The system is likely to be issued as two levels of kit that have to be fitted by the company; I shall put the details here when they become available. This system should allow for very low suspension settings (320mm) with manufacturer’s tracking specifications.

Methods of Lowering

There are two methods of lowering a F. The first has already been discussed, namely extracting some of the fluid medium from the suspension system. But depressurisation in this fashion will have effects both on the damping characteristics and on the spring rates of the Hydragas spheres. Put simply, the lower the pressure of the fluid, the softer the damping- ultimately reducing suspension control. This is why companies that lower MGF suspension systems in this way (like Motobuild) strongly advise the fitment of up-rated dampers.

An alternative is to use a lowering knuckle, as marketed by Mike Satur and Moss International. The knuckle is a simple mechanical rod that connects the Hydragas sphere with the upper wishbone. Putting in a shorter link or knuckle will lead to a correspondingly lower suspension height, but with the advantage that the pressures within the spheres, and therefore spring and damper rates, are unaltered. Lowering the suspension in this manner will lead to a dramatically lower chance of damaging the internals of the sphere that can result from depressurisation.

Given the relatively low unit cost of lowering knuckles, and the potential longevity benefits to the Hydragas spheres, I’d certainly recommend anyone who is seriously looking into lowering their car to take this route.

Conclusion

The suspension of an MGF is a marvel of productionisation that will give many thousands of miles trouble free service with excellent ride and handling characteristics. But it is essential that the tracking be set up correctly to avoid unnecessary tyre wear.

Lowering the MGF is a surprisingly simple process that even can be done easily by a DIY novice, but unfortunately the pitfalls are many, and are equally easy to fall into. ALWAYS get the tracking checked after your car has been lowered.

When lowering the car, two rules apply:

1. When lowering the car within the manufacturer’s specified range, no suspension modification need necessarily be considered. Lowering knuckles may be a good idea, to the benefit of the Hydragas sphere longevity. Always ensure exacting depressurisation methods and checking of tracking after any suspension height change.

2. When lowering the suspension further than the recommended range (lowest, 358mm), one should always use lowering knuckles, up-rate dampers and seek out a method of camber adjustment (hopefully soon to be made available). If no camber adjusters are available then you will have to accept increased rates of tyre wear. Your insurance comapany will certainly want to be informed!


 

Competition Note

Lowering the car to within manufactures specified limits IS allowed under the standard class regulations for the Abingdon Trophy and the Moss International Speed Chamionship. Similarly the fitment of non standard dampers are allowed, so long as they are non adjustable. Lowering knuckles also appear admissible. Any other modification are unlikely to pass scrutineering. Current ride height limit is four inches as measured from the ground to the bottom of the cill (4/5/99).