06 x15 springs on an 06 x18?

I was wondering would an 06 x15 rear spring fit on a 06 x18 rear spring. My bro got a new x15, and when i got on it, me being 155lbs it felt like it didn't bottom out. Unlike my x18 or is there a way to tension the spring on the x18? to be a lil tighter so it won't bottom out when i sit on it?

Thanks

x18 shocks suck, what I did to mine,"if you don't mind spending about 40 bucks" is I took a x12 shock cut the spring down to size heated the last coil bent it as close to the next coil as it would go and ground it flat, also the shafts can be removed and in the x12 shock they have a hard rubber insert, what I did is cut a piece of that to fit in the x18 shock which eliminates bottoming out. Its kinda hard to explain the process if you never seen a x12 shock because they are a couple inches longer than a x18 shock, but with a little commen sence you can modify your x18 shock with a x12 shock.