prrjim posted: ...snip... Finding such springs is very hard. I have done it a few times from my parts box. I know of no commercial ones.
No, actually it is quite easy. Just save the springs from retractable ball-point pens, cut off the end and about two or three (depending on how much tension you want or need) windings. You only need to do one truck, the other should remain a little loose. That allows the car/truck to negotiate rough track and not rock excessively.
