Suspension

For starters, I would like to say that springs and dampers are a bit of a black art, and I am by no means an expert.  The best advice I can give is do as much research as possible before spending your dollars.  Choose a set of wheels you want to race with, buy some great tyres, then start to look at developing the rest of the car.  Like I said, suspension is not my area, but the following is information that I put together and it makes sense to me.

The first two Gemini models introduced, the TX and TC, were sold before it was considered that anti-roll bars (sway bars) or radial tyres would be a valuable step forward with respect to performance and safety.

1979 saw the introduction of the TD model and Radial Tuned Suspension (RTS) which took advantage of sway bar and newer tyre technology to improve the roll stability of the car, whilst not sacrificing passenger comfort.  Basically, the car can be driven around a corner with less body roll, delivering a more stable feel to the driver.  From what I have read, I would also tend to believe different suspension components such as lower control arms can be found before and after the transition to RTS.

Springs and shocks are a bit of a grey area.  Their 'stiffness' depends on a lot of factors, but it should always be noted that the spring and shocks should be matched units.  See more below in the Spring and Shocker section.  However, to get the best handling from your Gemini, there are several areas which should be paid attention;

Centre Of Gravity

The ride height of the car generally dictates the centre of gravity.  The lower this centre of gravity, the less the body roll.  Less body roll will improve high speed handling.  So making the car lower than how it left the factory is desirable, however there is a limit.  Good suspension geometry can only be maintained with a ride height decrease of 50-75mm.  If you go past this point, the geometry of the upper and lower control arms messes up the vehicle's wheel alignment as the suspension goes through its range of motion.  

Lowering of the car should be done with a new set of springs.  Obviously if buying new springs, its a good idea to look at options with different spring rates, these might be stiffer or softer.  Either way, the lower you can get your centre of gravity, and still retain acceptable suspension geometry, the quicker you can corner, providing your tyres can handle the extra loads.

Springs & Shockers

Springs support the weight of the car, whilst shockers dampen the compression and rebound of the spring as the vehicle "bounces".  You should get shocks that are compatible with the spring rate of your springs, so when buying new springs, its a good idea to think about shocks as well.  

There is a general theory which I subscribe to, and that is springs should be designed to allow the tyres to follow the contours and bumps in the road, all the time keeping contact between the two.  If this contact breaks, things can get nasty.  Some people feel that springs should be made very stiff to reduce body roll around corners.  I feel this is the job of a sway bar. 

Certainly, handling improvements can be found with stiffer springs, but this also reduces ride comfort.  If comfort is not important, then spring rates can be increased (stiffer), but if too stiff, the car might skip and jump across the tops of bumps, removing the tyre contact from the road, which is bad.  In a perfect world, the springs would be chosen to suit the smoothness of the track, and the amount of suspension travel allowed by the body and suspension components.

Generally speaking, the stickier the tyre, the stiffer the suspension, as the tyres grip the road, the centrifugal force will be trying to roll the car.  This will transfer weight to the outside tyres.  If the suspension is too soft, things can bottom out.  

Factory springs are quite soft, which is good for keeping tyre contact with the road, providing the suspension has enough travel, and the shocks are valved correctly, but obviously the factory ride height is quite high, giving a high centre of gravity, contributing to a large amount of body roll.  The general fix is to cut the springs, but this effectively makes the suspension stiffer, in some cases, too stiff for effective handling and/or hard launches from a standing start or out of corners.  Also cutting the springs only makes them shorter, which means they might literally pop out of their location when travelling over bumpy roads for example, leaving you with no suspension and a dangerous ride on your hands.  You should make sure the suspension cannot travel far enough for this to happen, often it is the role of the shocker to limit the total amount of suspension rebound that occurs.  This is determined by the overall extended length of the shocker.

The alternative to cut springs is shorter (for lowered ride height) but relatively soft springs.  Coil springs are relatively cheap, and there are a lot of manufacturers that can offer a package to accommodate the type of driving you want to do.

Good shockers cost a bit more than springs, but you can get some great shockers off the shelf, and some are adjustable to alter the amount of damping to fine tune your vehicle.  If you plan on some racing but want to fit a cheaper pair of shocks, you will probably find oil filled shocks work better than air shocks. 

Sway Bars

Sway bars should be up-rated to heavier units to decrease body roll if high cornering speeds are desired.  Again, this is providing your tyres can handle the extra loads.

TX and TC models that utilised the Borg Warner differential did not have sway bars fitted to the rear as standard factory equipment, but it should be possible to retrofit the rear sway bar from a later model Gemini by welding anchorages to the axle tubes, and securing anchorages for the sway bar bush brackets to the floor pan.

Off the shelf Sway bars can also be bought for both the front and rear, in adjustable blade type or simply larger units that utilise the factory mounting locations.  My race car is a TD and I have fitted a larger front sway bar, leaving the factory rear bar in place.  The resultant feel is less body roll as would be expected, as well as additional over-steer.  Its typical that if you stiffen up the front end, you will get over-steer, and if you stiffen the rear end, it promotes under-steer.  I would prefer over-steer though as it allows some steering control.

Wheels & Tyres

Tyres.  The better they are, the more grip, the faster you can go around corners.  Old or simply hard rubber will not grip the road very well, and will slip.  Good rubber, especially the rubber compounds of racing tyres can provide a level of cornering grip similar to a train.  Lower profile rubber and most race specific tyres will have a squarer shoulder than the average road tyre, meaning they provide better steering response, and often a greater contact patch meaning more grip.

Good tyres are probably the first purchase I would make if I wanted to start participating in some racing events, its the only contact you have between your car and the road, and if this is an area which needs improvement, any other suspension modifications won't work to their potential.

I haven't tried all combinations, but I would bet that a car with stock suspension and soft racing tyres will out perform a car with fancy "racing" suspension and hard old tyres.

Wheels also have a big impact as the wheel track of the vehicle is measure across the car from the centre to centre of the tyres.  The greater the wheel is offset to the outside of the vehicle, the greater the stability of the car, meaning you can corner faster with the same tyre loads.  It gives similar benefits to lowering the centre of gravity.  Another way to think about it is to stand with your feet together, and get someone to try and push you over from the side (i.e. pushing against your shoulder).  You will easily get pushed over, but if you widen your stance so that your feet are just outside your shoulder width, you body becomes more stable and its harder to be pushed over.

Suspension Geometry & Wheel Alignment

Before chasing the best suspension components on the market, it would be wise to get a wheel alignment performed on your vehicle, to determine if any corrective work is needed.  Geminis are quite weak in the stub axle, chassis and floor pan department, so it is common for the stub axles to be slightly bent or the front chassis rails to be pushed back into the firewall and floor pan after mild collisions.  This has the effect of altering toe, camber and particularly caster as the lower control arm will be located further back than optimal, giving negative caster.

The suspension geometry itself should be checked and adjusted as necessary during the process of a wheel alignment.  If the geometry is out, the handling will be poor.  You can position the top ball joint to give some negative camber which will help cornering grip.  You can also modify the mounting holes for the lower control arm bushes which can increase your caster, again helping cornering.  You should speak to your wheel alignment man about this, as you need to conduct before and after measurements to make sure you are on the right track.

Any work performed should either be done by a suspension professional, or under their guidance, but generally, for extra positive caster, the lower control arm's rearmost bolt hole in the bracket welded to the floor pan can be elongated to physically move the bolt outward to the side of the car.  This has the effect of moving the lower ball joint forward, as the lower control arms front bolt location is still fixed.  The end result is an increase in caster.

For those chasing negative camber for increased traction for the front tyres when cornering, a common practice is to slot the bolt holes for the upper ball joint.  Moving the ball joint inward tilts the wheels and tyres in at their top, towards the centre of the vehicle.  It should also be noted that the ball joint themselves are not symmetrical.  A different amount of camber will be had when mounted in either of the two possible ways.  From memory, the flat edge of the ball joint flange must face to the outside of the car for maximum negative camber.