How silly of me, I posted a topic similar to this recently...should have just looked here. In any case, I wanted advice on re-felting on two fronts. First, what material do you use? I've found some soft but firm spongy stuff and I put a piece of scotch translucent tape over it to make it glide (and I don't cut it too thick). Second, I broke a few of the metal springs since I removed them to clean the residue and am thinking of getting some 0.1 mm copper sheets to cut new ones (as close approximations as possible). Have you had any of the springs break on you?
Yes, I've had springs break or deform/pop out. If the cart is important I sometimes remove a spring (there were two different types of spring used over the few years they were made btw... one type is a little more robust*) from a dud cart that didn't work after re-felting and carefully insert that into the important cart. It's fiddly but sometimes that's enough to rescue the content. I use the 8-track felt mentioned elsewhere. The adhesive is really good and once stuck they require a lot of force to remove. It's all good fun isn't it!
* I'm not sure which spring type came first, or if they were both used at the same time, simply dependant on what was available or different manufacturers of the actual spring part. I did originally think that the more robust type was the earlier type, but on the hundreds of carts I've seen with Abacus v1.0 they have so far all had the more widespread flimsier spring.