Actually the coils are often kept the same length if going for more rate at exactly the same height (bigger wire or less coils achieves this). In the image below, note how the top of the spring has some coil close together, this is a lowering spring. This is done to ensure they are kept captive. With weight on the car the top part coil binds once full weight is put on the spring. You get less bump travel but it ensures the spring won’t dance around at full droop. If you did put shorter shocks in, then let the spring manufacturer know how much less drop there is. Spring like in the picture are quite a bit heavier than really needed as a lot of steel just does nothing much more than stop travel.
I did fail to mention that on a FWD a limiting the drop of the wheel with shorter shocks/dampers can work really well. Remember weight that transfers diagonally. So if a inner rear wheel comes off the ground all that weight is going to the outside front wheel. That’s the one that is loaded. Only the outer rear wheel needs to be on the ground to stop the car falling over.
400-450lb/inch works on a Mira. With the trailing arm being a lever rate at the wheel ends up perhaps half that. I actually changed the seats for a coil over type 2 1/4" base (you can then use an adjustable base also - this is not just to adjust height but also for corner weighting). That way I could use a heap of off the shelf springs a mate has. Another way to help figure out rate would be to measure coil diameter, number of coils and lengths and put the data into an online rate calculator. Then start eliminating coils. I do this but putting a “packer” or “rubber” in the coil. Drive it and note the change. Recalculate based on having one less full coil and see what the rate is. Then put another in and repeat until you get near a rate you like. Here’s some examples.
The above shows packers that have eliminated three complete coils which will have bought the rate up a heap (like three times more).
These ones are what NASCAR uses and they can pull them out in seconds during pit stops to change the balance of the car a track, weather and the car itself changes.
Where would you get these for yours? Just some big lumps of timber that fit between the coils. With no shock in the middle of the spring just poke it right through the middle. Do drill some holes and put some big zip ties through. Be careful and aware that this is for “testing and tuning” and is not permanent. For legal reasons, I am going to say do this on a close private road.
I hope this has not confused things.
I like an LSD on the road. However, I don’t do a lot of traffic and not in suburbs much. My street is 20-30 tight corners to get up 4-5km with a gain of 300m in altitude. If it rains you can get stuck if the diff is open. Not sure about the D-sport, but with the Cusco you can alter the clutches to reduce chatter and get an action either progressive or aggressive. But it does mean pulling the diff/gearbox in and out. I wouldn’t get a 1 way. The 1.5way gives a better connection between the front wheels under braking which can reduce one wheel locking up.