Like i said before , you cant
just copy a spring rate and expect the same feeling. In my opinion when u take a short and a long spring the starting rate is the only thing u can make the same. But as the spring compresses the rate of a shorter spring will change more quickly vs a longer spring. And therefore will give a different feel on track.
In offroad kyosho have made some blue and orange springs. Doesnt matter what car u put them on, they nearly always work. The same with onroad and hpi springs, doesnt matter what car u put them on, they just work great in most conditions.
For me personal the HPI springs were THE springs for asphalt racing on the T3. So when i drove the T4 for the first times the car was ok , but i never have had the same dialed feeling on asphalt as i had with the T3. Most of all the steering didnt feel right. So after we had contact with DS motorsport he designed these shocktowers for us. So for me, its a improvement of the car as the silver hpi springs give me the same nice feel as they did on the T3. And like mentioned before, my fastest lap didnt go down by that much (u can drive the fastest lap with a crap car) but i could drive the faster times more consistant and longer during my run.
I have pretty much every short spring there is for the T4 and after some back to back testing , again for me
, the car is much nicer with just normal springs.
Offcourse this is not something every t4 owner needs, but for those always look to improve or just simply still dont like the feel of the car with the short springs its a nice addition to have as option part.