Originally Posted by Heavy B
Can anyone explain the differences in the inserts for tunability? Kind of a Foams 101 or Foam Inserts fo Dummies type advice. For instance, I normally race on a loose, dry, clay track but will be going to a big race; how should I adjust for that. Or, normally I run indoors but going to start racing at an outdoor track; should I change to a different insert and why? You know that sort of stuff.
The following is taken from http://users.telenet.be/elvo/2/14.html
Full size cars use rigid carcass materials and air pressure, R/C cars use foam inserts. IMHO, the latter is more interesting. I'm just saying that because air pressure works the same in all directions; either it's high or it's low. A shaped foam insert can put high pressure on some part of the tire and little or no pressure on another. Opinions aside, foam inserts are very important.
Wide versus narrow is probably the easiest matter. Wide is good. The foam has to be at least as wide as the tire. 10 to 20% wider seems to work well.
Hardness: this is always a compromise. Firm is good because it supports the tire carcass better. (The tire doesn't fold all over itself in corners) Firm is bad because it takes away some 'bump absorbency'. (I don't think that's a word)
So, for smooth tracks, it's easy: go very firm. For bumpy tracks you'll have to compromise. Too firm and the car will bounce all over the place, too soft and it will slide in the corners.
I have found there's another very important factor: inner diameter of the foam. The foam has to fit very tightly over the rim, so it doesn't move around when driving.
The most mysterious factor of all is height. Or in other words, the 'air gap' between the insert and the tire. There are basically 2 systems that work. System 1: firm, wide, but low insert. This way, the tire carcass is still well supported, but at the top (contact patch), there is an air gap. So the part of the tire where the contact patch is, can move around. This way, you make the carcass work, like an on-road tire. You get 'internal slip' in the rubber. It shouldn't surprise you that this works very well on ultra-hard, no-grip surfaces. The surface is practically asphalt.
This system is a very good way to generate grip on slick, no-traction surfaces. But, on the downside, it can make the tire feel a little floppy, and inconsistent in throttle transitions.
System 2: really big, firm insert. This is the exact opposite of system 1: you lock the carcass, and let the pins do all the work. The oversize insert stretches the carcass like an elastic band. As a result, the pins are really standing tall, and resist bending very well. Which is nice. Downside: doesn't absorb bumps very well. But overall, the traction generated with this system is very high and very, very consistent.