Hey Ben
Sorry but you've got to take time with me lol. When you say std GP??? And are you saying that that these springs are or are not compatible with t jets.
Each spring has a 'spring rate'. This is the force needed to compress it a specific distance. For us older ones, it was rated in lb/in, so an example might be 150lb/in, meaning it needs a force of 150 pounds to compress the spring one inch. I'd expect everything now to be in kg/cm or similar.
The spring absorbs the bump, by compressing. What we call the shock absorber is not really, it is a damper, to prevent the spring continuing to bounce after each bump.
A soft spring gives a soft gentle ride, but allows the car to move a lot. Too much and it will be difficult to maintain a straight line, and passengers will get sick. Too little movement makes the ride harsh, increases shock loadings into the car, can cause it to skip over bumps, but tends to help directional control.
The vehicle manufacturer has spent a lot of time to choose the spring rate, to give a good compromise between comfort and handling. For most cars, we value our comfort. Sporty models are often lowered, to gain handling benefits, but always comfort will be reduced. Again, the manufacturer has spent a lot of time and expense finding a setup that works.
Any metal object struck, will continue to oscillate/vibrate for some time. A spring compressed and then let go has a tendency to extend and contract over several diminishing cycles before coming to rest. This is undesirable in a car, so we fit dampers. These restrict the movement of the spring, so hopefully allowing it to compress, then rebound, and stay, ready for the next bump. The rate of compression damping can be different from the rebound rate, so dampers need to be matched to the springs and the car.
If you fit stiffer springs, without the matching dampers it may be a little bouncy as the spring overcomes the damper. Conversely, softer springs with stiffer dampers may prevent the spring recovering fully before the next bump, so making the next bump harsher, and in some circumstances can 'pump down' until little or no compression is available. (Rare these days)
Whilst The GP and Corsa were developed together, the fine tuning of spring and damper rates were individual to each manufacturer, so whilst parts will physically fit, the results may not be as you'd expect or wish. The weight of the car, and the weight distribution are also important here. If fitting Corsa springs and dampers, the results may be disappointing, but for safety, should be all four corners together. Other parts, such as top mounts and bearings should be identical.
Lowering springs need to still absorb the shock, not transmit it to the car body. Too stiff can lead to body damage such as cracking where the forces are felt, or generally just the whole car creaking as all the welds are under stress.
When the car is lowered, the spring rate needs to increase. The standard spring is selected to allow full movement to absorb the biggest bumps in normal use. This is why a Land Rover is different to your GP, as it expects bigger bumps. So a stiffer spring is needed, but the sums are difficult, as full movement is less, but the bumps on the road the same.
In your case, the T-jet is set lower than a standard GP, so rides a little stiffer. As there are fewer of these, there is less choice in the aftermarket parts supply, as tooling up to make these is less profitable, if at all. Fitting standard springs will set it at the same height as less sporty versions, which may appeal, or be acceptable to you, but may affect value later (if the next buyer notices). If you fit standard springs, you MUST do all four, and dampers as well, or your car could be considered dangerous.
Aftermarket lowering kits can work well if the manufacturer has done his sums, and those kits will be expensive as a result. Cheaper kits are fine if they just mimic the factory 'sport' settings. Others will be harsh at best, dangerous at worst. Most are somewhere in between, just being uncomfortable, but at least the discomfort encourages slower driving.
What is cutting spring interest of learning??
A cheap way to lower the car is to cut the spring to make it shorter. When found by the MOT tester this is a fail. If found by a policeman, it is an offence being in a dangerous condition.
If you shorten the spring without increasing the compression rate, it is likely to bottom out on lots of bumps. When it hits the bump stops, you are no longer in full control of the car, you may lose control and crash. Also more forces are being transmitted to the car body, which as said above, will lead to it trying to fall apart over time. An older car has a head start of course!
The spring ends are sometimes ground flat, to fit their locations, or sometimes left as wound and fit into shaped seats. Unless cut very carefully this is unlikely to be replicated, and a spring not seated properly is always trying to escape.
Hopefully our contributor with the cut springs is saving as fast as he can for some proper lowering springs.
Any modification from standard must be advised to the insurance company. Fitting lower springs and dampers form a sporty model is likely to be more acceptable than an aftermarket set. Advising the insurance of cut springs will render the policy void. Not advising them will also void it, but only after you need to claim. Then comes the prosecution and claims from the other parties.
Sorry for the length of the post, hopefully I've covered everything important. If you're still awake at this point, feel free to ask more.
